Considerable investment is made when purchasing a batch integral quench (BIQ) furnace. These popular furnaces need specific care and maintenance to keep them in prime operating condition. In this informative article by Ben Gasbarre, president of Industrial Furnace Systems at Gasbarre Thermal Processing Systems, learn how you can protect your BIQ from avoidable downtime.
This original content article appears in Heat TreatToday’s Air and Atmosphere’s February 2021 magazine. When the print edition is distributed, the full magazine will be accessible here.
Ben Gasbarre President, Industrial Furnace Systems Gasbarre Thermal Processing Systems
The batch integral quench furnace, or sealed quench furnace, is one of the most popular pieces of equipment in the heat treating industry. The core benefit is its versatility as it can easily adjust to changes in load weight, configurations, and heat treating processes. This makes
it a highly efficient and profitable piece of equipment for both captive and commercial heat treaters.
With all the good that is done in these furnaces, the downside comes in the maintenance of the equipment. By nature, these furnaces are hot, dirty, and have many moving parts, including multiple doors, load handlers, elevators, fans, quench agitators, and pumping systems; this furnace has it all! Although there are many areas of an integral quench furnace, understanding the subassemblies and having a good maintenance program can ensure the equipment operates safely and maintains its highest level of performance year after year.
Maintenance Safety
The discussion on maintenance of any piece of equipment begins and ends with safety. Prior to any work being done on the equipment, safety measures need to be considered based on the work being performed. Certain maintenance activities must be completed while the equipment is in operation; in these cases, proper personal protective equipment must be considered for work being done around hot surfaces, high voltages, elevated work, and potentially hazardous gases. If work is necessary while the equipment is offline, additional safety procedures must be followed, including lockout/tagout of all major power sources, special atmospheres, and natural gas supplies to the furnace.
Integral quench furnaces are considered confined spaces. Prior to entry into the quench vestibule, furnace chamber, and even quench pit, confined space procedures must be followed; hard stops must be in place for doors and elevators. Technicians need to ensure proper oxygen levels and air circulation prior to entry. The buddy system is always recommended when someone is entering the furnace. Prior to returning the furnace to operation, it is important to ensure all necessary safety and maintenance equipment has been removed, all supply lines are receiving designed gas pressures, and proper startup procedures are followed.
For furnace safety during shut down periods, it is wise to review furnace interlock systems and safeties to ensure proper operation. This includes items such as high-limit controllers, solenoid valves, burn off pilots, and other components critical to emergency situations. Additionally, per NFPA 86 requirements, valves and piping should be leak-checked periodically.
Reporting and Metrics for Optimum Performance
Image Source: Gasbarre Thermal Processing Systems
While Industry 4.0 is a popular concept in today’s manufacturing environment, the basic concepts behind the technology are what is important to any good maintenance plan. First, having an asset management system that enables engineers, operations, and maintenance personnel to access maintenance records is critical to ensure they can troubleshoot issues and perform maintenance activities more efficiently. Asset management tools are readily available and can range from well-established cloud-based software systems to simple Excel spreadsheet records. Ensuring important information, such as alloy replacements, burner tuning, or control calibration information, can help operations and maintenance personnel as they plan and assess future equipment needs.
The second concept is preventive or predictive maintenance plans. While these are not interchangeable concepts, the goal of implementing either is to reduce the likelihood of significant unplanned downtime, which can be costly to an organization. Preventive maintenance is a schedule of planned maintenance activities on a piece of equipment using best practices that give the best chance to catch a problem before it arises.
Predictive maintenance uses data and analytics from equipment operations that can be used to predict when problems are likely to occur. There are considerations for either approach, and the evaluation criteria for preventive versus predictive maintenance plans could be an article in and of itself.
Integral Quench Furnace Maintenance
As stated previously, breaking the furnace down into a series of subassemblies is the easiest way to develop an overall maintenance plan for equipment that has many sections and components. Discussed items will include mechanical assemblies, the heating system, the filtration system, atmosphere controls, temperature controls, and furnace seals. Each has its own importance to ensuring reliable equipment performance.
Mechanical Assemblies
Typical load transfer system alignment.
The mechanical system includes the load transfer system, recirculation fans, quench agitators, door assemblies, and elevator system. There are many exterior items that can cause abnormal equipment operation, including position sensors, rotary cam switches or encoders, and proximity switches, that if not operating properly can interrupt or cause failure within the furnace. Position settings should be logged for future reference, and sensors should be inspected regularly. Belts that may be used on recirculation fans and quench agitators should be inspected regularly for damage and excessive wear. Vibration of these items should be monitored as excess vibration can be an indication of damage or wear to the fan or agitator bearings, shaft, or blades.
The largest item of concern in this system is the alignment of the load transfer system. Unsuccessful load transfer due to misalignment or obstruction can cause significant furnace damage and create unsafe conditions within the furnace. Internal alloy components should be evaluated for integrity and alignment every six to twelve months. Elevator alignment should be reviewed to ensure smooth operation during the same period. Frequent visual inspection through sight glasses, quench time monitoring, and motor load data can give valuable information of future potential transfer issues within the furnace.
Heating Systems
Whether your furnace is gas or electrically heated, well-maintained systems can have significant impact on the operating efficiency of a furnace. For gas-heated systems, proper burner tuning and combustion blower filter cleaning can ensure optimum gas usage and can also improve radiant tube life. Burners, pilots, and flame curtains should be cleaned at least once or twice a year to ensure proper performance.
Electrically heated systems typically require less general maintenance and have fewer components that are susceptible to failure. Regular checks of heating element connections and electrical current resistance can help to identify upcoming element failure.
The largest and most critical components of reliable process performance are the radiant tubes. A crack or leak in a radiant tube can cause part quality issues. Changes in your furnace atmosphere gas consumption or troubles from controlling carbon potential can be signs of tube leaks. If the radiant tube failure is unexpected, it can also cause significant downtime if replacement tubes are not available. Cycle logs and run hour timers are the best metrics for preventive or predictive maintenance on radiant tubes.
Filtration Systems
Filtration systems are recommended for most integral quench applications. They help to eliminate build up and contamination in the oil recirculation system that flows through the heat exchanger and top/atmosphere cooler on the furnace quench vestibule. Filtration systems typically are comprised of a pump, dual filters, and an alarm system to alert users when it is time to change filters. Maintenance on your quench oil can vary by composition. Quarterly analysis of the quench oil performance is common. However, it is recommended to consult with your quench oil supplier to ensure safe and effective performance.
Atmosphere Controls
Integral quench furnace atmosphere systems can vary both by manufacturer and in overall gas composition. The most common being endothermic gas, nitrogen/methanol, along with options for ammonia or other process gases. Although these items may vary, maintenance remains consistent. Users need to ensure the integrity of the piping system including regulators, solenoid valves, and safety switches.
Endothermic gas lines should be cleaned out at least once or twice a year. Many furnace atmosphere problems can be traced back to endothermic gas generator issues, so it is important to have a well-maintained atmosphere generator to ensure peak performance in your integral quench furnace.
Typical integral quench furnace atmosphere system.
Recent technology allows for automatic burn-off of carbon probes and automated atmosphere sampling. However, probes should be burned off once per week if they are manual. Probes will require calibration and periodic replacement, and they can be rebuilt to like-new specifications. Controllers or gas analyzers that support carbon potential control should be calibrated quarterly, biannually, or annually depending on heat treat specification requirements.
Updates in the automotive CQI-9 specification will require calibration of all atmosphere flowmeters on a periodic basis. Users will need to be aware of this requirement and understand how their gas flowmeters should be calibrated. In some cases, control upgrades may be required.
Temperature Controls
Temperature control maintenance typically follows AMS2750 or CQI-9 specifications. This would relate to thermocouple replacement, system accuracy test procedures, and controller calibrations. Depending on the age of the equipment and specification requirement, these items may need to be done as frequently as once per quarter or annually.
Temperature uniformity surveys (TUS) follow similar specifications for frequency. However, a TUS can diagnose areas of the furnace that may need maintenance attention. Having a baseline TUS to reference will help identify changes in furnace performance. Changes to a TUS can indicate burner or element tuning requirements, an inner door leak, refractory damage, fan wear, or radiant tube failure.
Furnace Seals
Integral quench furnace seals can be a source of heartache for any maintenance technicians working to troubleshoot a furnace. Typical seal areas include the inner door cylinder rod, elevator cylinder rods, inner door seal against furnace refractory, outer door seal against quench vestibule, fan shaft(s), and an elevator seal if there is a top atmosphere cooler.
Typical sealing of cylinder shafts are glands comprised of refractory rope and grease. Greasing of these areas should be completed weekly. Outer door and elevator seals are typically fiber rope and may have adjustment built in as they wear, but ultimately will need to be replaced. Frequent inspection of these areas will help identify early issues. Using a flame wand or gas sniffer can help find leaks in unwanted locations. Small furnace leaks can cause part quality issues, and larger leaks can also create safety concerns within the furnace.
Additional Maintenance Items
Other key maintenance items include a bi-monthly or monthly burn out of the furnace heating chamber. This requires the furnace to have air safely injected into the chamber at or slightly above process temperature to allow the carbon to burn out of the furnace. Doing this process on a regular basis will help improve refractory and alloy component life as well as helping to maintain good process control.
Example thermal camera image
Another helpful snapshot of furnace health is using a thermal camera to take images of the equipment. It is recommended to do this on a monthly or quarterly basis. Thermal camera images can identify hot spots on the furnace outer steel shell that may indicate refractory deterioration or a furnace atmosphere leak. Thermal images can also identify potential issues with motors or bearings on fans and agitator assemblies.
Conclusion
In the end, all furnaces have different nuances that require different maintenance approaches. This could be based on the manufacturer, types of processes being run, or utilization of the equipment. By consulting with your original equipment manufacturer or other furnace service providers, a strong maintenance plan can be developed and implemented. This can include support and training from experienced professionals on that style of furnace. Broader cost benefit analysis should be done as it relates to spare part inventories, resource allocations, frequency of preventive maintenance activities, or investments into predictive maintenance and asset management technologies and how those activities can maximize utilization of each piece of equipment.
About the Author: Ben Gasbarre is president of Gasbarre’s Industrial Furnace Systems division. Ben has been involved in the sales, engineering, and manufacturing of thermal processing equipment for 13 years. Gasbarre provides thermal processing equipment solutions for both atmosphere and vacuum furnace applications, as well as associated auxiliary equipment, and aftermarket parts and service.
All images provided by Gasbarre Thermal Processing Systems.
What do gas nitriding, hot isostatic pressing (HIPing), black oxide coating, and high pressure gas quenching have in common? They all are key processes in heat treating firearm components.
Written by Rob Simons, manager of metallurgical engineering at Paulo, this in-depth Original Content article covers Paulo’s perspective on the thermal processing of firearms components and best practices for handling and lot traceability.
Check out more of Heat Treat Today’sTechnical Tuesday articles by searching “technical tuesday” in the search bar.
Rob Simons Manager of Metallurgical Engineering Paulo
While many industries are continuing to reel from the ongoing coronavirus pandemic, the firearms industry in the United States is booming. Over the past decade, the sector’s strong growth has only accelerated in 2020, fueled by consumers’ response to the pandemic and ongoing civil unrest. According to the NSSF’s 2020 Firearm and Ammunition Industry Economic Impact report, the firearms industry is responsible for well over 300,000 American jobs—a figure that has doubled since 2008.
Consumer demand for firearms also drives the need for heat treatment services for this highly regulated industry. Proper thermal processing is critical for safety and also plays a key role in delivering the quality finish that manufacturers want and consumers expect. In this article, we’ll share our firearms heat treatment expertise, delving into the common processes, specifications, and considerations of servicing this thriving industry.
Key Heat Treatment Processes for Firearms Components
Gas Nitriding
Nitrided 17-4.
Gas nitriding is used to case harden parts that must retain softer, more ductile cores. Because it is carried out at a lower temperature, gas nitriding helps prevent the part distortion that can sometimes occur as a result of conventional heat treatment. In addition to hardness characteristics, parts are often nitride coated for cosmetic purposes and to enhance corrosion resistance. Nitriding results in a beautiful, highly durable black coating that is often used in place of black oxide coatings or other blackening processes. Nitride coating is much harder than the alternatives, so the black finish will stand up to heavy use significantly longer.
Components and Materials Commonly Treated With Nitride Coating
Nitride coating is typically performed on outwardly visible components (and those you’d see while cleaning the gun) such as handgun slides and barrels, as well as muzzle devices such as flash hiders, muzzle breaks, and compensators.
These components are usually manufactured using the following materials, which can be nitrided to achieve the desired black finish and case hardness:
4140 steel. AISI 4140 is a chromium-, molybdenum-, and manganese-containing low alloy steel (usually referred to as chrome-moly steel). It has high fatigue strength, abrasion and impact resistance, toughness, and torsional strength.[1]
Grade 416 stainless steel. Grade 416 steel is a free-machining, martensitic stainless steel with a machinability of 85%. [2]
Grade 410 stainless steel. Grade 410 stainless steels are general-purpose martensitic stainless steels containing 11.5% chromium. Grade 410 steels have good corrosion resistance properties which can be further enhanced by hardening, tempering, and polishing.[3]
Grade 420 stainless steel. Grade 420 stainless steel is higher incarbon than 410 with a minimum chromium content of 12%.
4340 steel. AISI 4340 alloy steel is a low alloy steel containing chromium, nickel, and molybdenum. When heat-treated, it exhibits high toughness and strength. This material is considered extreme duty and is typically used for higher-end firearms.[4]
17-4 stainless steel. 17-4 stainless steel is an age-hardening martensitic alloy combining high strength with the corrosion resistance of stainless steel. It is relatively cost-effective and more weldable than other martensitic alloys.[5]
Nitriding Specifications
The most common measurement that firearms manufacturers specify in desirable nitriding results is intermetallic depth. Typical specifications fall between four and 25 microns of white layer depth. The amount of allowable porosity within the case depth is also commonly specified, and while there are varying ranges, less than 50% porosity is a typical target.
While porosity is often regarded as an undesirable characteristic, there are advantages to some porosity in the finished material. These microscopic voids can hold oils and enhance corrosion resistance. The resulting porosity in nitrided materials allows the coating to last dramatically longer than phosphate- or black-oxide-coated steels.
Some manufacturers utilize blanket aerospace specifications such as AMS 2757 or AMS 2959/12 because they encompass the desired porosity and case hardness depth for nitrided firearms components.
Vickers hardness testing is our preferred method for evaluating intermetallic depth in nitrided components. While 850 HV is typically the top achievable hardness for stainless steel, our team has consistently achieved 2000 HV with our nitriding processes. Higher-end hardness is beneficial for firearms components because it enhances wear resistance in components that slide against each other.
Casting of unknown material showing consolidation of macro pores through HIP.
Hot Isostatic Pressing (HIP)
With hot isostatic pressing, parts are heated to very high temperatures in a sealed chamber capable of generating very high pressures in the presence of inert gas. During processing, heat and pressure combine to close the voids that formed during part manufacturing, eliminating weakness in the parts. Most firearms components respond well to standard coach cycles for HIP, which commonly run at 2050–2200°F and 15,000 psi.
Firearms Components and Materials Commonly Processed With HIP
HIP is especially well suited for removing porosity from metal injection molded (MIM), additively manufactured (AM), and investment cast parts.
MIM is a fast, cost effective way to produce fire control components such as hammers, triggers, and safety selectors, especially for AR-15’s. Traditionally, MIM components have had a reputation in the industry for being inferior to those manufactured with conventional machining because they have been known to fail early in the field.
Common MIM grades used in the manufacturing of firearms include 4140 steel, 17-4 stainless steel, and…
FL-4605. FL-4605 is a low alloy steel with prealloyed manganese, molybdenum and nickel content for enhanced hardenability.[6]
420 stainless steel. 420 stainless steel is relatively high in carbon with a minimum chromium content of 12%, which gives it the highest hardenability of stainless steel grades.[7]
Additive manufacturing has not yet been widely adopted in the firearms industry, but we have seen it increasingly used in the manufacture of suppressors. These components—commonly known as silencers—trap the expanding gas as the weapon is fired to reduce noise, and are used for military sniper rifles. AM is an ideal method of manufacture for these components due to their complex geometry that is difficult or even impossible to achieve with traditional machining.
Investment casting is another process we see used in the production of handgun frames, specifically in the M1911 pistol.
MIM, AM, and investment castings all have one thing in common: these manufacturing methods leave voids behind in the internal structure of parts. HIP eliminates unwanted porosity in these parts, increasing their toughness, gross strength, and fatigue life which allows firearms components to withstand being repeatedly subjected to high impact.
MIM 316L before (top) and after (bottom) HIP. Porosity has been consolidated, but there are solid inclusions in the material.
HIP Specifications
For additive manufactured firearms components, we encounter two primary specifications:
ASTM F3301, a specification that outlines standards for thermal post-processing for metal parts made with powder bed fusion. ASTM F3301 identifies hot isostatic pressing as an acceptable means to stress relieve additive manufactured components.
ASTM F3055, the standard specification for additive manufacturing nickel alloy with powder bed fusion. In this specification, HIP is required for Class B, C and D components and is considered optional for Class G.
In both specifications, components must be processed under inert atmosphere at no less than 100 MPa within the range of 2048 to 2165°F (1120 to 1185°C). Parts must be held at the selected temperature within ∓27°F (15°C) for 240 min ∓60 min, and cooled under inert atmosphere to below 797°F (425°C), or to parameters as agreed upon between the component supplier and purchaser.
Black Oxide Coating
Black oxide gives firearms a sharp black appearance, enhances corrosion resistance, and minimizes light reflection. Unlike paint, black oxide doesn’t add any additional thickness to gun components. The desired result in the black oxide process is creating magnetite (Fe3O4), an alloy of iron and oxygen, on the surface of the metal. The black oxide process enhances corrosion resistance by adding rust preventive oils to the metal part.
While it doesn’t last as long as gas nitride coating, black oxide is still a popular, cost-effective option to give visible gun parts the perfect black look. Finding a partner that can provide heat treatment and black oxide under one roof can reduce your transportation costs, speed up turnaround time, and simplify your overall process since one supplier owns the final results.
Firearms Components and Materials That Use Black Oxide Coating
Like nitriding, black oxide coating is used on outwardly visible components like slides, barrels, and muzzle devices including flash hiders, muzzle breaks, and compensators. It can be applied to any carbon steel component, but it will not adhere to stainless steel.
Specifications for Black Oxide Coating
Unlike nitriding and HIP, we rarely encounter standard specifications when it comes to black oxide results. However, as a best practice we work with customers to establish boundary samples for each part number treated with black oxide so we can compare our results to what both sides agreed upon as a desirable appearance.
High Pressure Gas Quenching
High pressure gas quenching can be performed in a vacuum furnace as an alternative to oil quenching for any firearms components that are near net shape or completely finished—or where distortion is a chief concern. In high pressure gas quenching, parts are austenitized under vacuum. Then, the chamber is backfilled with inert gas, which is heavily agitated by powerful motors.
High pressure gas quenching results in cleaner parts than oil quenching, but it has other benefits that can prove highly valuable for firearms components. This process can take a conventional 4140 alloy and make it achieve the same properties as a vacuum arc remelted (VAR) 4340, a much higher quality nickel-based material. This can allow firearms manufacturers to see similar hardness and strength results in everyday components as those they would expect from an extreme duty material like 4340.
Handling and Traceability for Firearms Components
No heat treater should make a habit of losing any type of parts, but the implications for serialized firearms components are more severe than any other mass-produced components. Serialized components are what the U.S. government considers the firearm—it refers to the part that features the serial number, usually the lower frame assembly and sometimes the barrel or slide.
Firearms components to be treated with gas nitriding that have AMS 2757 or AMS 2759/12 identified as the standard are also subject to the recordkeeping guidelines outlined in the specification. AMS 2757 requires that documentation includes the equipment and approved personnel’s identification, date of processing, number of parts, alloy, lot identification, and actual thermal processing times and temperatures at a minimum.
Proper handling of firearms components by heat treaters is essential to keep the supply chain running smoothly. Improperly heat treated parts will either wind up in the scrap bin or require reprocessing, and lost parts can result in an ATF audit or investigation. At our company, we’ve engineered our process to prevent issues from occurring in the first place. Here are a few examples of how we do it:
Electronic tracking. Each lot of parts is assigned a barcode that links to electronic records of all relevant information about the job—process parameters, specifications, shop orders, etc. The process parameters on the parts’ barcode are integrated with equipment, so when parts are scanned for processing, the furnace will be automatically set to the proper parameters according to the parts’ recipe. This helps us prevent parts from being improperly heated or subject to the wrong process altogether.
Secure storage. In Paulo plants that process firearms, we use locked cages and secure vaults to protect serialized components. All access to these areas is monitored and recorded to maintain accountability and, if applicable, adhere to AMS specifications.
Specialized handling. To give our firearms customers more peace of mind and to safeguard against errors in our process, we’ve also engineered secure fixturing for many components that allows them to remain locked throughout the entire heat treatment and finishing process.
In addition to a Federal Firearms License (FFL), heat treaters should also have a documented quality management system in place. Choosing an ISO 9001-certified supplier can help give manufacturers confidence in a heat treating partner’s ability to maintain quality operations. Maintaining other certifications such as IATF 16946 and CQI-9 is also a good sign that your partner is well equipped to handle firearms work.
Conclusion
The firearms industry relies on its thermal processing partners to sustain its growth. Proper heat treatment and metal finishing results in better performing, longer lasting firearms for our military and law enforcement, which helps keep our country safe. In uncertain times, the firearms industry represents a bright spot in the U.S. economy that we are proud to support.
About the Author: Rob Simons is manager of metallurgical engineering at Paulo where he leads the commercial heat treating industry’s largest in-house metallurgy team. Rob continuously spearheads research and innovation at Paulo that lead to advanced capabilities and better results for the company’s customers. Rob holds a bachelor’s degree in Metallurgical Engineering from the Missouri University of Science & Technology.
About Paulo: Founded in 1943, Paulo is one of the largest providers of thermal processing and metal finishing solutions in North America. Headquartered in St. Louis, Paulo operates six divisions servicing the United States and northern Mexico.
In this episode, Heat TreatRadio host Doug Glenn talks with Joe Powell of Integrated Heat Treating Solutions in this fourth and final episode about bringing heat treating into the 21st century. This episode covers Direct from Forge Intensive Quenching – forge shops, listen up!
You are about to listen to the 4th and final episode in a series on rethinking heat treatment, with Joe Powell, of Integrated Heat Treating Solutions. You can find the previous episodes at www.heattreattoday.com/radio.
Below, you can either listen to the podcast by clicking on the audio play button, or you can read an edited version of the transcript.
The following transcript has been edited for your reading enjoyment.
DG: Joe, if you don't mind, take us on a 30,000 foot overview of what you've been doing at Integrated Heat Treating Solutions.
JP: What we've been doing for the past 23 years at Integrated Heat Treating Solutions and the last 75 years at Akron Steel Treating is applying heat treatments to parts made by others. We had over 1200 customers on our customer list at Akron Steel Treating and they use various materials. We kind of grew up in the shadow of the Cleveland market, which is the largest market for heat treaters, and there is the largest number of commercial heat treaters in the Cleveland market. This was possibly outnumbered by Detroit at one time, but I still think that we're probably the number one market for heat treating in this part of the country.
What has happened over the last century, in the 20th century, is that heat treating has become very, very good. New equipment has been developed like controls, thermocouples, oxygen probes, vacuum furnaces, vacuum quenching, high pressure vacuum quenching, oil skimmers, new quenchants made with reverse solubility polymers - all of these things have come together and made heat treating very, very good. However, as part of that, there has been a commoditization of heat treatment. That means that heat treating became so good that parts rarely crack or distort unacceptably, and companies have devised methods for correcting the distortion through hard turning, grinding, straightening, flattening, you name it. And the part makers and the heat treaters got along, in a kind of peaceful coalition, to get the parts out the door to the end user.
However, in the 21st century, that is just not good enough. In lean manufacturing, you have to offer an integrated solution for what you're doing. The entire value chain for making a product has to be on the same page; they have to be in alignment. The processes have to be in the proper order. What we're trying to do with Integrated Heat Treating Solutions is bring the last dimension of part design, what we call the Z dimension, to the part makers, their designers, and their material suppliers, so that we present a solution that delivers the optimal amount of value and eliminates the waste from heat treatment, or forging, as we'll talk about today.
[1] Defense Logistics Agency, "About," https://www.dla.mil/AboutDLA/ [2] DFIQ FIA Technical Committee Presentation, "Evaluation of Intensive Quenching Hardening Process Immediately After Completion of Hot Forging Operations," 2018. [3] Forging Process Improvement Using Intensive Quench, 2019.DG: Right. In these four episodes we've been talking to people about bringing heat treating into the 21st century. On your website, integratedheattreatingsolutions.com, there is a good illustration table that shows what heat treating was like in the 20th century and what it is like in the 21st century. That's basically what we're talking about. Just a quick recap of the previous three podcasts we've done: It all revolves around a customized heating, but more importantly, a customized quenching of materials so that the distortion of those parts is predictable, and that the part design can be altered so that there is essentially no post heat treatment processing. In other words, you can pretty much eliminate grinding or any type of machining, straightening, and that type of thing. Once heat treated and quenched with the technologies that you're talking about, the part is essentially pack and go.
We've talked about several examples, but the two we talked about in the recent podcasts were an 18” bevel gear, which was quite interesting. Then we talked about a fracking pump valve seat, which was also quenched in this way. So today, you and I want to talk about, as you alluded to, the forging industry. We're going to talk about something called (direct from the) forge intensive quenching (DFIQ). If you don't mind, tell us what that is. For those people in the forging industry, what is direct from forge intensive quenching?
JP: It's the principle that the forging processes use a lot of BTUs of heat to heat up a billet, and then bang it into a shape and get the grain flow going in the direction that will be great for the part mechanical properties. Once that forged shape is attained and the grain flow is attained, the part is usually allowed to cool at the end of the forging trim die line, and those cooling forgings will all cool at different rates. Because they cool at different rates, you have some fast cooling on the surface, the corners and the thin sections; but you have some very slow cooling in the core. At the end of the day, the part needs to be heated a second time in a normalization process, which heats the part to a high temperature and then does a controlled cooling of the part to align the grains of the part and the size of the grains to remove the kind of mishmash of structure that is present in an as-forged part. Then, if the part is going to be hardened at some point, and usually there is a lot of rough machining that goes on to remove the scale from the forging process, machining is necessary to remove the scale from the steel mill that has basically been hammered into the surface of the forging. All of that rough machining is done to basically present a rough machine part that can then be heat treated. So, companies like Akron Steel Treating or the captive heat treats at the forging plants will then heat the part a third time to the austenitizing temperature. If the part is made out of a martensitic steel, they'll quench it, usually in oil or polymer, and then possibly temper it to stabilize the part, and present it to the part maker for final machining, grinding and whatever final processing needs to be done to turn that forging into a useful part with the desired mechanical properties.
Akron Steel Treating doesn't do a lot of forged heat treat. We do some aerospace parts for braking systems for airplanes, called torque tubes, which is basically the hub of the braking system. Those torque tubes are generally made out of forgings which we see after forging, and then see again after 50% of the material is removed. Then the part is heat treated. In those instances, direct from the forge intensive quenching is not going to work.
Direct from the Forge Intensive Quenching
This direct from the forge intensive quench (DFIQ) project came out of a desire by the Forging Industry Association (FIA), which incidentally Akron Steel Treating has been a member since 2012. We've always felt that we could create more streamlined processing as well as a better part with leaner material if we worked together with the forgers and integrated the heat treat process with the forging process. Companies like the TimkenSteel Company have come out with low alloy materials that are forged all the time, and then they do a controlled cooling where they'll actually air cool the forging. With the alloying elements that are in there, they are able to come up with mechanical properties directly from the forge after a controlled air cool. No normalization is needed and no further austenization, or third heating, is needed. Basically, the part is air quenched and tempered right there in a controlled manner from the hot forge.
Some folks in India and Japan have tried several times to do direct from the forge liquid quenching using oils directly from the forge. What they found is that the oil quench catches on fire, and if they can keep it from catching on fire by enclosing the quench under an inert atmosphere, they're still going to have the problem of the very high heat, like 2000°-2200° F, creating a steam blanket of hot oil, or in the case of polymer water, a steam blanket of polymer water mix around the outside of the part. This then produces an inability to uniformly quench the part because the thin sections will very quickly quench out, the thick sections will sit there under a blanket of gas and essentially those two mixes of nucleat boiling - very fast evaporative cooling in the thin sections and a full-blown gas blanket on the thick sections - create a nonuniform shell around the outside of the forging. As that part cools under that nonuniform shell, it is also going to thermally shrink in a nonuniform way. Also, when it cools to the martensite start temperature, it's going to start transformation and face change in a nonuniform way in that shell.
The successes of direct from the forge quenching didn't happen until this project we started in 2015 with the Defense Logistics Agency (DLA), which “manages the global supply chain – from raw materials to end user to disposition – for the Army, Marine Corps, Navy, Air Force, Space Force, Coast Guard, 11 combatant commands, other federal agencies, and partner and allied nations,”and the FIA tech committee members who sat down and asked: “Do you think we can do this in water?” If we can do it in water, we obviously eliminate the fire hazard, but how do we eliminate the boiling hazard, or the boiling issue in the nonuniformity? And that's where we had, at that time, 15 years of experience in applying the intensive quenching process or intensive quench process.
Luckily, John Tirpak, who was then working with the DLA and the FIA as a technical advisor, saw the benefit in giving it a try. We had done lots of parts that people had said, over the years both at Akron Steel Treating and Euclid Heat Treating, couldn’t be done. And we did it. We applied it in the case of the valve seat to ductile iron to replace an 8620 carburized seat. So, we have this great flexibility, we have this great new tool, we just need to use it, or at least try it, at the forge. And that's what the DLA funded. They basically gave us a budget for the building of a prototype unit which was built and is pictured in the final report It shows the test parts that were actually quenched directly from the forge at Bula Forge in Cleveland, and then we moved the prototype unit next to Welland Forge in Canada and finally to Clifford-Jacobs Forge in Illinois.
The upshot of all of this was that once we figured out that if we could remove the film boiling from the outside of the hot forging, we could basically set the shell, and once the shell is set, we get, on most parts and most geometries, a martensite shell that is formed. That martensite shell continues to form down into the layers of the onion below the surface as the martensite temperature is reached and that martensite transformation continues by conduction, very uniformly through the mass of the part. What you end up with is a part that comes out of the quench pretty much like it went through a normalization process and then a third reheating and an oil quench and a temper. We get some self-tempering as well because we interrupt the intensive water quench before the part is fully cooled. Nonetheless, we found in the first phase of testing that parts should be tempered in a tempering furnace to develop the full effects of the tempering process, so that process is still done after the parts come out of the quench. But you eliminate the normalization process and the third reheating for an oil quench and temper that would normally be required.
Examples of DFIQ equipment (Photo source: Joe Powell)
DG: Can you tell us what parts were actually run?
JP: Yes, there were a variety of parts, and they're all pictured in that report. They ranged from a link that weighed, I believe, close to 50 pounds all the way down to a tine that was on a tiller machine (ground engaging tool) that went into a piece of farming equipment. One of the parts in between was a pintle adapter that was basically a mounting post for a machine gun for the Army. This part went through several operations. It's documented in the report, but we basically saved $13 per part to the Army by eliminating the multiple steps that took place after forging and just incorporated it into an integrated heat treating solution right there at the trim die.
DG: How did that look? Let's take the tine, for example. It's stamped out on a forge press. You've got a hot piece of metal put on a forge press stamped out. Then, one at a time, these parts are taken off of the forge press and immediately put in a quench?
JP: After they come out of the trim die, they're still pretty hot - they're still austenitic, and range in temperature from like 1900°F all the way up to 2200°F - and then they go directly into the quench. 15-45 seconds later another one comes out of the trim die and goes down into the shoot and up the conveyor and into a box to await tempering. We time the conveyor so that the dwell time in the intensive water quench is properly timed so that the core still has enough heat to self-temper, but not too hot that it over tempers the part.
DG: I'm curious about the part. After the part comes off the trim die, is it manipulated? Is there a manipulating hand that comes in and grabs it, takes it off, puts in the quench tank?
JP: In the case of the prototype, the manipulating hand was the forger. He came with tongs and provided a very 19th century placement of that part. But, obviously, all of this stuff can be automated and integrated, and with the proper equipment can be done in a way that is seamless from the time the billet is heated all the way through.
DG: Tell me this, that tine again, when the guy took it off the trim die, did he just throw it in an intensive quench tank or was it fixtured?
JP: Picture an elevator platform. It was placed on an elevator and then the elevator went down between two panels that presented water at very high flow to the part and knocked off the film boiling. I should add, the tine was the thinnest part and the enthusiasm at Clifford-Jacobs was very, very high because once they figured out that this worked, the guys on the floor said, “Let's try this part, let's try that part, let's try this part.” And of course, in the first test at Bula Forge, we actually tested at least four different alloy materials and so all of those variables would have to be integrated into the design. I call it the Z dimension of the design. You pick the right material, you have the right forging temperature of the billet, and you don't overheat it. One of the lessons learned in the four-year study is that if you overheat the forging to “help with die life” - that overheating of the forging to 2400°F (almost to the melting point) - the grains blow up. No amount of intensive quenching is going to bring them back. So, you've got to keep the temperature around 2150°F; that's about the maximum in Fahrenheit.
All I can say is that if you maintain a forging temperature uniformly around 2150°F in the billet, we can devise a quenching system that will blow the film boiling off and set that shell in the part in all but the thinnest parts in the prototype. We did about 150 tines in a row with the protype, and then the water heated up because we only had so much chilling capacity in the water tank. But as the water heated up, the quench wasn't as effective, and the tines actually exhibited some cracks when we ran another 150 - that's because there was film boiling in the mounting holes. The lesson learned was you have to have a flow, but you also have to have some pressure in order to instantly impact that part. That instant impact is key in the proprietary processes that Integrated Heat Treating Solutions is developing to bring the next version of the DFIQ unit to make it able to do the thinner parts without cracking.
DG: DFIQ, of course, standing for direct from forge intensive quench.
You've referred to a study multiple times and that study is a 2019 study called,Forging Process Improvement Using Intensive Quench. It looks like that was, as you mentioned, funded by the DLA in either 2014 or 2015. We will make that report available and people can take a look at it. Anyone that is a forger in a forge shop, or a captive forge would certainly want to take a look at that. Would forge press companies be interested in this? Could they build quenches into the actual press itself so that this process could be, more or less, in line?
JP: Yes, absolutely. Again, it is a different paradigm for them. Just like I mentioned before, all the heat treating equipment makers call themselves furnace companies and all the forging equipment makers call themselves press makers or forging die makers. The reality is the process continues and the mechanical properties in the setting of those grain flows happen in the heat treating process; the refinement of those grains happens in the heat treating process which happens in the quenching process. So, again, we need to integrate that quench into the forming equipment. Again, I have no intention, as Integrated Heat Treating Solutions or Akron Steel Treating, of getting into the business of building systems- that's not my thing. My thing is to develop a robust process that can be applied and implemented using automation and new equipment with the proper pumps and material handling that is all integrated into a seamless process.
DG: Let's talk very briefly about the benefits. We've already alluded to quite a few of them, but let's try to enumerate them here. What are the benefits to a captive forge shop in considering a DFIQ type system- why do it? What's the commercial value?
JP: We can save up to 66% of the energy that's needed to heat treat that part. The part comes off the trim die and is cooled in a box or set aside somewhere. Next, it needs to be reheated and normalized. Then, it has to be reheated a third time and austenitized before quench and temper, and that's a lot of energy. And it's also not usually done at the forge plant. It's usually done either at a captive heat treat that is integrated with the forging company or it goes to a commercial heat treat where they use huge continuous furnaces to reheat the parts and quench and temper them. I'm not going to make a lot of friends in the areas that do this, but if we're going to compete in the world and make great parts, be lean, save energy, and also have safe carbon emissions, we've got to stop heating parts that don't need to be reheated if you can avoid it. I'm not going to claim that it works on each and every part and that it should be used for each and every part. I'm just saying that there's a lot of parts that could be made a lot more efficiently if we would quench them right at the trim die.
DG: So, one of the benefits you just mentioned is potentially saving 66%, basically two-thirds, because you don't have to do a second and third heat. What else do we have?
JP: What you can have is better uniformity of mechanical properties. You can also elicit more hardenability out of a particular alloy by having this higher ability to harden with a very, very fast quench. That intensity of quench locks in mechanical properties that are unattainable in a typical oil quench or polymer water quench. One example of that is a forging that we do for a company, in fact it was one of the companies in the study. It's a 44” gear rack- it's 44 inches long, about 5 inches wide and about 4 inches thick. This gear rack is used as a piece of mining equipment and actually 10 of them are used on each side of a tower. This gear rack allows the spinning, drilling rig to go up and down and spin as it is drilling holes in the earth. This part was traditionally made from 4330 material but the end use customer, the people using this piece of mining equipment, said they’d really like to be able to replace and repair these gear racks when they get worn or a tooth gets broken.
If we could do this in the field, that would be great; but with 4330 material, we can't because we have to pre- and post-heat the weld when we replace or repair a tooth in the field. That’s just not practical in some cases, especially if this piece of equipment is on the side of a mountain and it's pretty cold outside. So, is there a way to get field repairability? That's a topic the DLA is very interested in because equipment used by the Army is often times used in very cold environments, so is there a way to repair that piece of equipment without taking it offline or bringing back for repairs?
For this particular gear rack, after they forged it to a rough shape with the gear teeth in on one side and it looked pretty much like a gear rack that was ready for rough machining, they wanted to be able to still get the same mechanical properties from a leaner hardenability steel like 4130 to replace the 4330, so that they could weld it in the field without pre- and post-heating to avoid cracking the part for the weld. They came to us at Akron Steel Treating and they asked if we could this with our 6,000-gallon batch system. We didn’t know. I took a look at the jominy curve for 4330 and the jominy curve for 4130 and said it's going to be close. The thing is 4” inches thick by 5” wide, and I just didn’t know. But I was willing to try. That has always been my favorite answer, “Let's try it.” If it blows up or it doesn't work, I'm going to learn something. You might not be happy because I blew up your part, but I learned a lot and I'm happy and we're going to move on.
So, they gave us five actual parts made out of 4130 and we heat treated them in our 6,000- gallon system. Next, we sectioned them and found that they turned out very, very uniform. They had the right surface hardness all over the part and also had the right core hardness throughout the 44” length. Then they did some field trials, and everybody was happy.
DG: So, in that case, the benefit is potentially being able to replace higher alloy parts with lesser alloy parts, field repairability, lower cost to manufacture the part, and easier to machine. You also talked about the fact that you can do significant energy savings which also potentially shortens the lead time because you're not having to go through two or three processes, but only one. The one thing we haven't mentioned, which I think probably should be mentioned explicitly, although we've alluded to it, is the elimination of some environmentally unfriendly quench media.
JP: It's a water quench. You use just a little of restorentative salt and that's it. It's water.
DG: And obviously you've got better mechanical properties which you've also mentioned.
JP: There's one more chapter to this and it ties back to podcast #2. First of all, we do these parts 15 at a time on racks in our controlled atmosphere furnace and then transfer all of them to the handling cart and quench them in our 6,000-gallon system. We noticed that when they went into the quench, they were straight, but when they came out of the quench, they were all uniformly bowed about 1 inch at the middle of the 44” length. We mentioned to the customer, that when it's time to redo these forging dies, they should bow the forging so that it comes out of the trim die with a 1” bow in the opposite direction. Once it quenches, it will quench to fit and be relatively straight and will avoid the cold straightening operation that is done after heat treat and temper to get the part straight enough so it can be rough machined.
Again, time savings as well as monetary savings and we're not imparting cold strains into the part that has been hardened after heat treat, which is a no-no, because those cold strains can find a discontinuity in the material or an inclusion, and the two combined can, once in a great while, literally blow up as it is being straightened and fly across the room into two pieces. Cold straightening is something you want to avoid if at all possible.
DG: So, again, the benefit there is that you can go back to the part designer and the heat treater.
Let's back out again to 30,000 feet. We're not talking about the gear racks anymore, just talking generally. In your concluding thoughts, what is the main message we're trying to communicate here?
JP: The integration of lean and heat treating and forging. I think bringing all that together, all of that lean thinking and applying it to the part design at the front end, and the material selection at the front end, so that we deliver the most added value with the least amount of waste in the process to the end user.
DG: I would like to wrap up by saying this too, there are a large number of people who are in the Heat Treat Today audience that I think ought to be interested in this. Basically, anybody who is a captive heat treater, manufacturer with their own in-house heat treat who is doing oil quenching, or anything of that sort, and currently doing it in batch, ought to be thinking about contacting Joe to see if they can eliminate that batch process and put the heat treat directly in line. Those are manufacturers.
Also, as we just talked today- the forging shops ought also to be interested in this. Taking forge parts of the finish/trim forge and putting them directly into a quench. But there is one other group that also ought to be interested in this and ought to be talking to you Joe, and that is the heat treat equipment manufacturers who have a stake here. They have a stake here because their current batch processes, if we continue to move down this path into the 21st century, they could be on the cutting edge of providing the type of equipment that can be potentially more inline and more quench type equipment. For what it's worth, I think that's worth mentioning.
JP: Yes. The 21st century of heat treating is moving towards induction heating and individual part by part quenches. That is really the only way to control distortion consistently, and also to effectively get the most that an alloy hardenability has to offer for the end user, in terms of strength and ductility.
DG: If these people want to get in touch with you, Joe, what's the best way for them to do that?
JP: Through the website integratedheattreatingsolutions.com or ihtsakron.com. The other person who is working with me very closely in the FIA technical committee is Rick Brown. Rick Brown is a former executive at TimkenSteel here in Canton, OH. He helped develop a supply chain for making parts out of seamless tubing that Timken made and still makes, and that supply chain included not only cutting up tubing into rings and making parts out of those rings, but also heat treatment, and in some cases, forging. Rick has a wealth of experience. He's a great guy and is one of our Integrated Heat Treating Solutions consultants who helps people at the part makers, part designers and end users get the most value out of the heat treating and forging processes. We're all working towards that goal of moving heat treatment from the 20th century fully into the 21st century.
One of the great benefits of a community of heat treaters is the opportunity to challenge old habits and look at new ways of doing things. Heat TreatToday’s101 Heat TreatTipsis another opportunity to learn the tips, tricks, and hacks from some of the industry’s foremost experts.
Heat TreatToday’s latest round of 101 Heat Treat Tips is featured in Heat TreatTodayfall issue (also featuring the popular 40 Under 40).
Today’s selection includes tips from ECM USA, Carrasco Hornos, and Quaker Houghton. Each of them has provided quick steps or comments on a variety of topics ranging from furnace brazing to furnace expenses to quench performance or maintenance.
Heat TreatTip #1
How to Achieve a Good Braze
In vacuum brazing, be certain the faying surfaces are clean, close and parallel. This ensures the capillary action needed for a good braze.
A good brazing filler metal should:
1. Be able to wet and make a strong bond on the base metal on which it’s to be applied.
2. Have suitable melt and flow capabilities to permit the necessary capillary action.
3. Have a well-blended stable chemistry, with minimal separation in the liquid state.
4. Produce a good braze joint to meet the strength and corrosion requirements.
5. Depending on the requirements, be able to produce or avoid base metal filler metal interactions.
(ECM USA)
Heat TreatTip #2
How Much Lost Money Flows Through the Walls of Your Furnace
In a strict sense, heat flows through the insulating lining of your furnace wall: the lower the outside temperature in the furnace shell, the less heat is lost and, consequently, less money.
Fourier’s Law of Heaat Conduction (Source: Carrasco Hornos)
For example, an outside temperature on the oven shell of 160°F (71°C) equals a heat loss of approximately 190 BTU/hr ft2, just multiply this number by the square footage of the entire outside surface of the oven. A well-designed and well-maintained insulation can reduce the outside temperature of the shell, say 120°F (49°C), and heat losses would be close to 100 BTU/hr ft2, that’s 90% more heat lost and therefore fuel.
So, my Tip for today is: “Let’s go for the basics that don’t change, and it will always give good results.” By the way, how many furnaces are there in your plant and how many square feet do their surfaces add up to?
(Carrasco Hornos)
Heat TreatTip #4
Check Your Quench Oil
Safety – Performance – Oxidation
Safety
Water content should not exceed a maximum of 0.1% in the quench oil.
Flash point should be checked to ensure no extraneous contamination of a low flash point material (i.e. kerosene) has been introduced into the quench tank.
Performance
Cooling curve analysis or GM Quenchometer Speed should be checked to confirm the quench oil is maintaining its heat extraction capabilities. Variances in heat extraction capabilities could possibly lead to insufficient metallurgical properties.
Oxidation
TAN (total acid number) and Precipitation Number should be checked to ensure the quench oil is thermally and oxidatively stable. Oxidation of the quench oil can lead to staining of parts and possible changes in the heat extraction capabilities.
Sludge content should be checked… filter, filter, filter… sludge at the bottom of the quench tank can act a precursors for premature oxidation of the quench oil.
Work with your quench oil supplier on a proactive maintenance program… keep it cool… keep it clean… keep it free of contamination to extend the life of your quench oil.
(Quaker Houghton)
Heat TreatTip #28
Aqueous Quenchant Selection Tips
Greenlight Unit (Source: Quaker Houghton)
Determine your quench: Induction or Immersion? Different aqueous quenchants will provide either faster or slower cooling depending upon induction or immersion quenching applications. It is important to select the proper quenchant to meet required metallurgical properties for the application.
Part material: Chemistry and hardenability are important for the critical cooling rate for the application.
Part material: Minimum and maximum section thickness is required to select the proper aqueous quenchant and concentration.
Select the correct aqueous quenchant for the application as there are different chemistries. Choosing the correct aqueous quenchant will provide the required metallurgical properties.
Review selected aqueous quenchant for physical characteristics and cooling curve data at respective concentrations.
Filtration is important for aqueous quenchants to keep the solution as clean as possible.
Check concentration of aqueous quenchant via kinematic viscosity, refractometer, or Greenlight Unit. [See image: Hougton Intn’l Greenlight Unit and/or Houghton Int’l GL Display B] Concentration should be monitored on a regular basis to ensure the quenchant’s heat extraction capabilities.
Check for contamination (hydraulic oil, etc) which can have an adverse effect on the products cooling curves and possibly affect metallurgical properties.
Check pH to ensure proper corrosion protection on parts and equipment.
Check microbiologicals which can foul the aqueous quenchant causing unpleasant odors in the quench tank and working environment. If necessary utilize a biostable aqueous quenchant.
Implement a proactive maintenance program from your supplier.
Heat TreatRadio host Doug Glenn talks with Joe Powell of Integrated Heat Treating Solutions in this third of a four episode series about bringing heat treating into the 21st century. This episode covers the fascinating heat treatment of a fracking pump valve seat.
Below, you can either listen to the podcast by clicking on the audio play button, or you can read an edited version of the transcript.
The following transcript has been edited for your reading enjoyment.
Doug Glenn (DG): We're continuing our conversation with Joe Powell of Integrated Heat Treating Solutions. on rethinking heat treating. I strongly recommend that you listen to parts 1 and 2 of this series as well as today's episode. All three are fascinating. To hear the first two parts, click here.
Today, we’ll be talking about what I think, if you've listened to the first two episodes of this four part series, is a very fascinating, I think, somewhat revolutionary advancement in heat treat.
Today, basically what we want to talk about is a really interesting example of the general concept of what we talked about in session one. I want to review that first session very briefly and ask you a couple of other quick questions before we jump into the example of a fracking pump valve seat, which is where we're headed today. But first, maybe from a 30,000-foot view, Joe, tell us what we're talking about here. If you were to put this in a minute, how would you describe what it is you've been doing over at Integrated Heat Treating Solutions?
Joe Powell (JP): Integrated Heat Treating Solutions (IHTS) is a consultancy that takes 75 years of practical commercial heat treating and applies it to help part-makers make better parts by using heat treating knowledge. We also work with the material-makers who want to get more added value out of a given hardenability material. What IHTS is essentially doing is taking off from the idea that quenching causes the most problems in heating: it causes distortion, part cracking and size change that is unpredictable. That distortion engineering has been part of the ASM and other societies that have had task forces, committees, and various conferences that are dedicated to the control of distortion.
Potential factors influencing distortion (Source: American Gear Manufacturers Association, sourced by Joe Powell)
The reality is that the control of distortion has been approached by many, many people, including Dr. George Tautin, who was one of the inventors of the reverse solubility polymers when he worked for Dow Chemical and Union Carbide, and Dr. Kovosko in the former Soviet Union, who was my partner in IQ Technologies starting back in 1999. What we've discovered working with all of these very smart people is that the quench cooling rate and its relationship to causing part distortion or part cracking is a bell shape curve. In other words, if you quench very slowly in air or gas or hot oil or martemper salts, hot salts for austempering, you will not crack the part. But, if you quench faster in brine, water, or even water polymer mixtures that don't have enough polymer in them to act like an oil quench, the cooling rate will become relatively fast. That relatively fast cooling rate will give you a much higher probability of part cracking, until on some parts you'll literally crack every part you put in the quench if it's quenched in water.
If you can create a shell on the outside of the part and quench it 752°-1112° F (400°- 600° C) per second, that shell will literally hold that hot part while the hot core thermally shrinks underneath and pulls that shell under compression. As that thermally cooling shell and hardened shell of martensite goes through volume change and actually increases in volume, the grains are actually pushed up against each other under compressive surface stresses, and that compressive surface stress holds the part like a die. So, regardless of its geometry or mass, that part is going to come out of the quench having cooled by uniform conduction down to its core through that shell in a very predictable shape.
DG: That's exactly what I wanted to get to: what we're talking about here is a quenching issue. It's quenching parts fast enough so that, in a sense, what you're doing is creating a hard outer, immovable shell, if you will, pretty much instantaneously, which holds that part in place while the core cools down to the temperature that is needed.
The quenching media, in one sense, don't really matter. It can be done. The issue is getting that shell formed quickly, uniformly and then holding it at a certain temperature until the core cools.
You and I have spoken in the past, Joe, about a kind of interesting quote which I'd like you to comment on before we get to the fracking pump valve seat example of what we're talking about. Here’s the quote I'd like you to address, “Everyone knows how to heat treat. All you need is a torch and a bucket of water.”
"Every day I learn that in the 23 years that I've been working on heat treat quenching and focusing on that and controlling of distortion, there is always something new, and there is always something new in the field of, what I call, metallophysics."
JP: That's correct. Every machinist you'll ever meet, and even a machining handbook, will tell you how to heat treat a part, and do it quick and dirty. The problem is everybody thinks that it’s because they've heat treated a part in the past, that they know a lot about heat treating, and that is just not the case. There is so much to know, that all I can tell you is that every day I learn something new. Every day I learn that in the 23 years that I've been working on heat treat quenching and focusing on that and controlling of distortion, there is always something new, and there is always something new in the field of, what I call, metallophysics.
DG: Right. It brings me back to a couple of thoughts along that line. One, it's the whole idea that “a little knowledge is a dangerous thing” – we think we know and yet, we don't. You've told me a story in the past and I think it's worth our listeners hearing it, and that is just an abbreviated version of the Jack Wallace story. Again, Jack Wallace, the head heat treat metallurgical guru at Case Western Reserve University, comes into your shop and you tell him, “I can quench these things so super-fast,” and he looks at you and says, “You are a crazy man. It's not possible.”
JP: Actually, it was worse than that. Dr. Michael Aerinoff came from Russia and was telling Jack about this technology that Dr. Kovosko discovered back in the former Soviet Union. So, it had two strikes against it. Not only was it new information and contrary to the idea that the faster you quench, the more likely you are to blow up the part, but it was also contrary to the information, “Hey, we're in the United States. We know all about heat treating and metallurgy!” At the end of the day, this metallophysics twist that Dr. Kovosko put on the dynamics of the heating and cooling process is really the key to understanding and viewing metallurgy from another dimension – the dimension of residual and current compressive stresses that are affecting the part. That's what Dr. Kovosko told us about, and finally, that's what unlocked the ability of the parts that Professor Wallace witnessed being quenched and not cracking.
DG: I would have loved to have been there and seen the eyebrows of Dr. Wallace.
JP: The other two metallurgists who were in the room besides me – two owners of heat treating companies, Wayne Samuelson of Shore Metal Treating at that time and John Vanas at Euclid Heat Treating – both of them basically wrote Michael off as a crackpot because they had heard what professor Wallace had said. I was the only one dumb enough to think, “Well, come on down. If you want to demonstrate some parts, they're either going to blow up or they're not. If they don't blow up, it'll be interesting, and if they do blow up, it will be funny, so let's try it!”
DG I wanted our listeners to hear some of the other people who are now, as I say in quotes “true believers.” You've got Jack Wallace who now believes what you say is actually true. You've also got, I believe, George Tautin, who is kind of the “king of quench.”
JP: Absolutely. He's actually written a book with us. It's an ASTM book; it's publication #64, I believe, and that book tells you exactly how to build the first and second generations of IQ (intensive quenching) equipment. George also said in 2014, after he retired from making polymer quenches, that you don't really need oils or polymer quenches. You can do quenching very nicely with a properly designed quenching system and water, or water and a little bit of salt. That was a pretty strong statement from a guy who literally spent his career making those quenches better.
DG: You had mentioned one other individual, Robert O'Rourke.
JP: Yes, he is a metallurgist with over 30 years of experience with ductile iron. Bob worked with one of the industry giants, Chip Keough,* who founded Applied Process and also austempered ductile iron. Chip's company not only worked with the ductile iron society for many years, but also with Bob O'Rourke, who was one of the principals at the Ductile Iron Society; in fact, he was president back in 2015. At the end of the day, he basically said that we could take this kind of crappy material, ductile iron, and austemper it. Chip made a very good business out of austempering ductile iron at Applied Process and converted many, many parts from either as-cast ductile or even steel parts to austempered ductile iron parts.
That, to me, showed that it's possible to take a heat treating process and apply it to a material and literally create a new material out of as-cast ductile irons. Chip even said, “I know what you guys are doing. When we quench in salt, it's very uniform. There is no film boiling. There is no nonuniformity in the cooling. All you're doing is just kicking it up a notch with higher intensity and knocking off the film boiling with the intensive agitation.” And I said, “You're absolutely right, Chip.” What we did not know at that time was that it could be applied to ductile iron.
DG: Let's jump into this fracking pump valve seat. A couple basic questions. First off, we're talking about a pump that is used in the fracking industry to extract out, I assume, the fracking fluids, and things of that sort.
JP: It's actually to inject the high-pressure water sand. They call the sand a proppant. After the pump has fractured the shale layers, then they inject water and sand to hold up and prop up those cracks in the geology and allow the gas to flow out more quickly.
DG: Good. So, the point is, it is very rugged and the pump takes a beating. What was the problem that the company was having? How did it come to your attention?
JP: The frackers were having to rebuild the pumps every 40-60 hours and replace these valve seats. They had high pressure water and sand flowing through the valves. The valve would open and close under pressure at about four times a second, and that constant abrasion of the valve opening and closing and banging into the seat was causing the seat to wear out. Once the seat is worn, then the pump can't maintain its pressure, and they're not getting anywhere in terms of putting that fluid down in that well, and therefore, making it produce more oil and gas products.
DG: Essentially, you've got fracking companies who are having to replace valve seats and rebuild the valves every 40-60 hours. What was the material that was being used for the valve seat?
JP: For years, these types of seats were made of 8620 carburized steel. They usually start with a forged ring, and then they machine that ring into a valve seat with a taper and a strike face where the valve closes onto the valve seat. That part is generally carburized around 90,000th of an inch effective case step and tempered and then put into the pumps. Again, that case hardened surface is 60–65 Rockwell and wears very, very well and resists the abrasion of the sand and water. Because it's 8620, it has a ductile core underneath the strike face that absorbs the impact of the valve opening and closing on top of it every four seconds under pressure.
You have to have a combination of hard, yet ductile. And you have to have a tough part that resists wear and abrasion.
DG: These guys were using it and still having to replace it every 40-60 hours, so what was your thinking on it and how did you guys help?
JP: A whole bunch of people had tried to put tungsten carbide inserts into the strike face to make the strike face even harder than case hardened material. Then a company came out with a solid sintered tungsten carbide valve seat that costs upward of $500–800 each. You’ve got to remember that there are ten of them in the pump, and they were built as a lifetime valve seat because they actually outlasted the pump block and some of the other parts of the pump. But that was not a great solution because, at that point, you have a seat that's lasting longer than the pump block. You still had to take apart the pump anyway for other things that were worn; it's too good and it's too expensive. If you've got $8,000 worth of seats, you're not going to throw the pump block out because it's worn out, you're going to try to remove those seats.
Large Rolls on Their Way into IQ Tank (Source: Joe Powell)
Again, what they were looking for was a longer life valve seat, not necessarily a lifetime valve seat, but something that would last for all of the stages used by that pump at a certain well. They would move it at the time that the well completely fracked and started to produce and take it back and rebuild it at their shop. They were shooting for 200 hours.
DG: Right. Again, the normal was 40-60 hours with the 8620 material.
JP: Right. Having had the experience with the elongator roll and the ability to make something that was literally so hard they couldn't knurl it, we had to temper those elongator rolls back quite a bit in order for them to knurl them and then use them at the mill. I thought, if we don't temper the valve seat back and just leave it that hard, it should be carbide-like hard, because if a carbide tool can't knurl it, it's pretty doggone hard. We fired up our existing piece of equipment that we had at Akron Steel Treating, a 6,000-gallon intensive quenching tank. We heated the parts and quenched them in that big batch tank, and we got very nonuniform results.
One of the things we did not understand back in 2012 was that ductile iron, because of all the graphite particles that are in there, has a very low thermal diffusivity, meaning that in order to get the heat into it or out of it during the quench, you had to be more than intensive; you had to be, what I call, instantaneously impacting that surface with high pressure water that literally pulls the heat out at a rate that will allow you to get to the martensite start temperature, cool to the martensite start temperature, and form that shell in less than 2/10th of a second – and you have to do that all over the part surface to create that shell. This required the making of some new induction heating equipment that have an integrated quench system built into it. This integrated quench system is going way past the ability of our 6,000-gallon tank with its propellers flowing the water laminally across the surface and literally impacting the part instantaneously after the induction heat is turned off.
DG: I want to mention to the listeners that we'll put a photo of this part in the transcript that we'll have on the website so that they can get a much better sense of what the part is; there are some lips and turns and there is an inside diameter and an outside diameter. As you say, if you're flowing water laminally over this, you're going to be missing parts and you're going to be missing areas of the part, so you need to get it quenched quickly.
JP: They actually did crack in the O-ring groove and under the flange out of our 6,000-gallon tank, so we knew we had to do something different. The first thing we tried was to put in the flange and the O-ring groove after it was heat treated, but that wasn't going to work because the part-maker didn't want to have to machine it twice. We had to come up with a way of delivering that water all over the shell of that part and also keeping the core relatively ductile. We didn't want to harden it all the way through and make it brittle, so that's what we came up with while working with the folks at Induction Tooling in North Royalton.
DG: So, it was basically an induction heat and an integral induction quench, very high impact, instantaneous, probably way beyond what anybody else has seen. Describe very briefly, what kind of horsepower was needed to go into the quench.
JP: We used a 60 gallon/minute pump for the ID and a 60 gallon/minute pump on the OD. Both pumps were operating at 60 psi, so there is quite a bit of pressure and quite a bit of flow over a very, very small area.
DG: Which is exactly what needed to be done. So, talk about the results. You're hinting at them here, but what are we talking about in regards to Rockwell hardness and that type of stuff?
JP: We're getting 60+ Rockwell hardness. Again, you've got to remember that this is an apparent hardness because the Rockwell machine is fooled by the very soft graphite particles that are in the matrix. You have very, very hard martensitic iron and carbon in the surface, but you also have these little particles of spherical graphite, and that graphite acts as, what we believe, a lubricant. We haven't quantified it in the valve seat, but we've quantified it for some dies that gives lubricity that's not present in a steel part. The graphite lubricates whatever is traveling over the surface of the part. The other thing that we learned is that the compressive residual surface stresses, when tested by x-ray defraction, are about double that you get when you do carburization of the 8620 valve seat. The very high residual compressive surface stresses also hold those grains of iron carbides in place and does not allow them to abrade or erode. In the first testing, we had three seats that went out to the field somewhere in west Texas, and they lasted 166 hours. We were almost there.
So, we've modified the quenching system, we've modified our heating recipe on the induction tooling, and we made another set of valve seats which we are currently sending out for more field testing. We hope we're there and we'll see what happens. But we literally created a new material. The history of ductile iron goes from as-cast to austempered ductile iron and now, what we call, instantly quenched ductile iron or IQDI
DG: Nice. It all sounds very, very interesting, but I can see some people listening to this saying, “Ok, how much is this going to save me?” Let's talk about the ways that this process saves money. In my mind, you've got a shorter processing cycle time, you're using less expensive material, and you're getting a longer life. Are those the three major ones?
"With the valve seat, the forging and the 20 hour carburizing cycle are eliminated, and it’s machined three times faster. One customer let slip that they were saving about 66% on the material cost."
JP: There is also one other and that is ductile iron because those graphite particles machines about three times faster than steel. So your through-put in your CNC machine goes up by 2 or 3 times when you're making the part and that is no small matter. Also, because the quench is so impactful and so uniformly impactful, it sets the part and you literally get a part that quenches to fit. Once the green size before heat treating is adjusted, the part may not need much, or if any, final grinding.
DG: So, you're saving on post heat treat processing, as well.
JP: Right. And, because we use no oil, we don't have to wash the parts and we don't have to worry about disposing of quench oils or about quench oil fires. And, the process can be done in the machining cell, so it's an in-line process versus a batch carburizing process that has to go someplace for 20 hours to be carburized.
DG: Significant. I think you threw out a dollar figure when we spoke about this previously. What are the savings per valve seat?
JP: With the valve seat, the forging and the 20 hour carburizing cycle are eliminated, and it’s machined three times faster. One customer let slip that they were saving about 66% on the material cost.
DG: Wow. Significant cost savings is the point, so something worth looking into. We're going to have one more episode where we talk about another example. What do you think we'll talk about in the last episode?
JP: The integration of heat treating into the forging process.
DG: Alright super. Thanks for being with us, Joe. It’s always interesting and intriguing.
JP: The integration of heat treating into the forging process. The forging industry association sponsored a project with IQ Technologies. Akron Steel Treating is a member of the forging industry technical committee and has been for years, and we've always thought that there should be a closer alliance between forgers and their heat treaters. We're going to take the information that we gained from this 4 year project, the published final report will be on our website, and we're going to try to commercialize that for a lot of different parts.
“Successful heat treating begins by understanding the make-up of the steel that is to be treated.”
Heat Treat Today’sTechnical Tuesday feature provides an overview of the heat treatment process and the benefits wrought from heat treating in salt baths. The article also illuminates details to understand part composition and the austempering and quenching process as a whole.
The author of this Original Content article, Jerry Dwyer, market manager at Hubbard-Hall, has previously written for Heat Treat Today on the topic of polymer quenchants as an alternative to water and oil quenching. Read more here.
Heat treating is a process in which metal is heated to a predetermined temperature and then cooled in a particular manner to alter its internal structure for obtaining a desired degree of physical, mechanical and metallurgical properties. The purpose is to obtain maximum strength (i.e., increase the metal’s hardness) and durability in the material.
Numerous industries utilize heat treated parts, including those in the automotive, aerospace, information technology, and heavy equipment sectors. Specifically, manufacturers of items such as saws, axes, cutting tools, bearings, gears, axles, fasteners, camshafts, and crankshafts all rely on heat treating to make their products more durable and to last longer.1
The heat treating processes require three basic steps:
Heating to a specified temperature.
Holding at that temperature for the appropriate amount of time.
Cooling according to prescribed methods.
Understanding the Part Material
According to the ASM International’s Heat Treating Society, about 80 percent of heat treated parts are made of steel, such as bars and tubes, as well as parts that have been cast, forged, welded, machined, rolled, stamped, drawn, or extruded.1
SAE Designation. (Image source: Jerry Dwyer. Reference source #3.)
Successful heat treating begins by understanding the make-up of the steel that is to be treated. The American Iron and Steel Institute (A.I.S.I.) and the Society of Automotive Engineers (S.A.E.) utilize a four-digit system to code various types of steel used in manufacturing. The alloying element in the AISI specification is indicated by the first two digits, and the amount of carbon in the material is indicated by the last two digits. The first digit represents a general category of the steel groupings, meaning that 1xxx groups within the SAE-AISI system represent carbon steel. The second digit represents the presence of major elements which may affect the properties of steel; for example, in 1018 steel the zero in the 10xx series depicts no major secondary element. The last two digits indicate the percentage of carbon concentration. SAE 1018 indicates non-modified carbon steel containing 0.18% of carbon, while SAE 5130 indicates a chromium alloy steel containing 1% chromium and 0.30% carbon.
Carbon steel has a main alloying constituent of carbon in the range of 0.12% to 2.0%. Plain carbon steel is usually iron with less than 1% carbon, plus small amounts of manganese, phosphorous, sulfur and silicon. Carbon steel is broken down into four classes based on carbon content:
Austempering is one of several heat treatments that is applied to ferrous metals and is defined by both the process and the resultant microstructure of the work. In steel, it produces a bainite (or a plate-like) microstructure.
Typical Austempering Heat Treatment Cycle in Ductile Iron
When heated to temperatures below 730°C (1346°F), the pure metal iron has a body-centered cubic structure; if heated above this temperature, the structure will change to a face-centered cubic. On cooling, the change is reversed, and a body-centered cubic structure is once more formed. The importance of this reversible transformation lies in the fact that up to 2.0% carbon can dissolve in a face-centered cubic, forming what is known as a “solid solution.” While in a body-centered cubic iron state, no more than 0.02% carbon can be dissolved this way. The solid solution formed when the carbon atoms are absorbed into the face-centered cubic structure of iron is called austenite.
Austempering Process Steel Structuring
When quenched, carbon is precipitated from austenite not in the form of elemental carbon (graphite), but as the compound iron carbide Fe3C, or cementite. Like most other metallic carbides, this substance is usually very hard; as the amount of carbon increases, the hardness of the cooled steel will also increase.
The temperature of the quench tank is set so that the material is rapidly cooled down at a rate fast enough to avoid transformation to intermediate phases such as ferrite or pearlite and then held at a temperature that falls within the bainite region but staying above the martensitic phase. The bainitic microstructure that is formed as a result of austempering imparts high ductility, impact strength, and wear resistance for a given hardness; a rifle bolt was one of the first applications for this process.
The salt quench also provides low distortion of work with repeatable dimensional response. The materials have increased fatigue strength and is, in general, more resistant to hydrogen and environmental embrittlement.
Heat Treat with Salt Baths
Salt bath heat treatment is a heat treatment process comprising an immersion of the treated part into a molten salt, or salts mixture.2 There are numerous benefits of heat treatment in salt baths, the most prevalent is that they provide faster heating. A work part immersed into a molten salt is heated by heat transferred by conduction (combined with convection) through the liquid media (salt bath).2 The heat transfer rate in a liquid media is much greater than that in other heating mechanisms, such as radiation or convection through a gas.2
Using salt baths also helps with a controlled cooling conditions during quenching. In conventional quenching operation, typically either water or oil are used as the quenching media and the high cooling rate provided by water/oil may cause cracks and distortion. Cooling in molten salt is slower and stops at lower temperature and avoids may of the pitfalls associated with a faster quench.2
Salt baths also provide low surface oxidation and decarburization, as the contact of the hot work part with the atmosphere is minimized when the part is treated in the salt bath.2 There are additional advantages to salt heat treat:
Wide operating temperatures: 300°F -2350°F
Most of the heat is extracted during quenching by convection at a uniform rate.
Salt gives buoyancy to the work being processed to hold work distortion to a minimum.
Quench severity can be controlled or manipulated by a greater degree by varying temperature, agitation and water content of the salt.
Excellent thermal and chemical stability of the salt means that the only replenishment required is due to drag-out losses.
Nonflammable salt poses no fire hazard.
Salt is easily removed with water after quenching.
About the Author: Jerry Dwyer is Hubbard-Hall’s market manager for product groups pertaining to heat treating, phosphates and black oxide. To learn more or get in touch, please visit Hubbard-Hall’s website.
CQI-9 compliance demands adherence to the standards for the purpose of excellence in automotive heat treating. Poorly maintained quench oil can cost heat treaters in many areas.
In this Heat TreatToday Technical Tuesday feature, Greg Steiger, senior key account manager at Idemitsu Lubricants America, shares how costly quench oil issues can be addressed through proper adherence to the CQI-9 quench oil testing protocols. Let us know if you’d like to see more Original Content features by emailing editor@heattreattoday.com.
Greg Steiger Sr. Key Account Manager Idemitsu Lubricants America
Introduction
A poorly maintained quench oil can cost a heat treater in more ways than simply the cost of having to replace the oil. The costs can quickly expand to include those associated with poor quality. For example, costs associated with part rejects, or rework and downstream costs for shot blasting, or third-party inspection are often the cause of poor quench oil maintenance. Dirty or poorly maintained oils can affect part cleanliness, surface hardness, and surface finish. For instance, it is well known that a heavily oxidized oil may create surface stains that must be shot blasted to remove. High molecular weight sludge or excessive water can create surface hardness issues. Many of these issues can be addressed through proper adherence to the quench oil testing protocols established by CQI-9.
How can CQI-9 help?
CQI-9 is designed as a tool to help heat treaters produce consistent parts. Using a CQI-9 compliant quench oil analysis can also be a very powerful tool in a heat treaters tool kit. Just as the level of carburization is influenced by the carbon potential of a carburizing atmosphere, the cooling speed of the oil influences microstructure formation and microstructure composition along with mechanical properties such as hardness as well as tensile and yield strength. Furthermore, the cooling speed is dependent upon the viscosity of the oil, the amount of sludge, moisture level, and oxidation of the oil. All of these are tested on a regular basis under the requirements of CQI-9, ISO TS 16949, and most quality systems adopted by modern heat treaters. All of the tested parameters required under CQI-9 will be addressed individually later in this paper.
What is CQI-9?
The member companies of the Automotive Industry Action Group (AIAG) encompassing automotive manufacturers and their Tier I suppliers have enacted an industry heat treating standard called CQI-91. This standard was originally a standalone standard designed and adhered to primarily by North American OEMs and Tier I suppliers as a quality tool to create consistent documented processes within the heat treating industry with the goal of producing consistent reproducible results. Since that first implementation of CQI-9, the standard has now been incorporated into the ISO TS 16949 standard and is now adhered to by most automotive OEMs and their Tier I suppliers. The full range of management responsibilities, material handling, and equipment operations of the CQI-9 standard is beyond the scope of this paper. Instead we will be discussing the used quench oil analysis requirements of CQI-9, why the tests are required, and how heat treaters need a CQI-9 compliant quench oil analysis to properly care for their quench oils.
Utilizing a compliant CQI-9 analysis and the supplier provided operating parameters for the CQI-9 required tests is the first step in the proper care of a quench oil.
CQI-9 Compliant Analysis
Most quench oil suppliers provide a quench oil analysis. Although the quench oil supplier may provide a quench oil analysis, for the analysis to be CQI-9 compliant the analysis must contain the following tests or their equivalent:
Water content; ASTM D6304
Suspended solids; ASTM D4055
Viscosity; ILASD509
Total acid value; ASTM D664
Flash point; ASTM D92
Cooling curve; JIS K2242
The frequency of the above testing must be a minimum of semiannually. A more frequent sampling interval does not violate CQI-9. In fact, the more often a quench oil is analyzed, the easier it is to use the quench oil analysis as a tool in the proper care of a quench oil. It is important to note that the CQI-9 standard does not prescribe specific test methods be used in the above testing; however, they must be performed to a traceable standard. The CQI-9 standard only states that the above values, along with a cooling curve, must be reported. The following sections will describe each test in a CQI-9 compliant analysis.
Water Content
Everyone knows water in a quench oil can be have catastrophic safety and performance consequences. However how much water is too much? That is a question that is difficult to answer. The answer depends on a variety of factors such as the quench oil used and all of the variables associated with a furnace atmosphere. A general rule of thumb when it comes to water levels is to keep the water level below 200PPM. At levels above 200PPM of water, uneven cooling begins to occur.2 It is important to remember a quench oil is not a pure homogenous fluid. Samples taken at various places throughout the quench tank will be similar but will also have differences. These differences will include water and solids levels. Therefore, in areas where the water content exceeds the 200PPM level, uneven cooling will begin. Parts coming into contact with this “localized” quench oil with high water can potentially begin to crack, have a high surface hardness, or have staining problems. Yet parts in other areas of the load continue to behave normally. For this reason, and also because water is much heavier than oil, it is imperative the oil be under agitation. In addition to the potential uneven cooling issues high water may create, a high level of water can also influence the rate of oxidation in an oil.
Suspended Solids
Because solids are typically denser and more viscous than liquids they do not have the same heat transfer properties as a liquid. Due to the inequality of heat transfer capacities between liquids and solids, it is very important to keep the solids level, especially high molecular weight sludge, at a minimum. Sludge reacts in an opposite manner of water. Where water can increase quench speed, high molecular weight sludge will decrease quench speed through uneven cooling.2 The result of the uneven cooling from sludge is typically seen in soft surface microstructures or soft surface hardness. Also, like water, sludge is heavier than oil and the lack of homogeneity in the oil means having proper agitation is paramount when sampling.
Viscosity
Changes in viscosity can lead to both faster quench rates and slower quench rates. As the quench oil is used in the quench process, it undergoes thermal degradation.3 This degradation process can be seen when the oil becomes thinner or less viscous. During this process, a small portion of the base oil and a small amount of the quench oil additives undergo a process called thermal cracking. In this process, heavier molecules are broken into smaller molecules through the use of heat. This thermal cracking creates lighter less viscous oil from heavier oils. The newer lighter viscosity of the quench oil can potentially lead to changes in the quench speed of the oil. These changes can have an impact on the microstructure, case depth, core hardness, and surface hardness on the quenched parts.
As an oil is subjected to the high temperatures of a quenching operation, oxidation is a natural occurrence in the oil. As the oil oxidizes it will begin to increase in viscosity until it reaches the point of forming an insoluble sludge. Therefore, an increase in viscosity typically means the oil is oxidizing. Just as an oil that becomes thinner and less viscous may have a change in cooling properties, an oil that becomes thicker and more viscous may see a change in cooling performance. A thicker oxidized quench oil may affect surface hardness, microstructure, case depth, and core hardness. In severe cases of oxidation staining may result. Such stains typically require post quench and temper processing such as shot blasting.
Total Acid Value
The Total Acid Value, or TAV, is a measure of the level of oxidation in a quench oil. The amount of oxygen in a quench oil cannot be measured without a sophisticated laboratory analysis. However, the formation of organic acids within a quench oil can be easily determined via a titration method. It is well understood that these organic acids are the precursors in a chain of chemical reactions that will eventually form sludge. As the TAV increases so will the levels of oxidation, and in turn, the amount of sludge will also increase. Consequently, as the TAV increases, the amount of staining due to oxidation may increase. The cooling properties of the oil may decrease due to the increased sludge formation as well. Figure #1 shows an example of how the acid value increases the viscosity of a quench oil due to the formation of polymeric sludge in the quench oil.2
Figure #1. Acid number vs kinematic viscosity for Daphne Hi Temp A
Flash point
The flash point of a quench oil is another check to ensure the safety of the quench oil user. As oil thermally cracks, the heavier base oils become not only lighter in viscosity, but their flash points also decrease. If left unchecked, the decrease in flash point could result in a higher risk of fire. In addition to serving as a watchdog against the results of excessive thermal cracking, a flash point is also a safeguard against human error and adding the wrong quench oil to a quench tank. High temperature oils typically have a higher flash point than conventional oils. An increase in flash point, along with no change in TAV, and an increase in viscosity could indicate a contamination issue between oils has occurred.
Cooling curve
There are many different methods of running a cooling curve. Many Asian suppliers of quench oil will use the Japanese Industrial Standard (JIS) K 2242. European suppliers will use the ISO 9950 and North American suppliers rely on the ASTM D 6200 method. All of these standards measure the same basic property, the ability of an oil to reach martensite formation. However, they differ in one basic item. The JIS K-2242 and methods used in China and France use a 99.99% silver probe that is smaller than the size of the Inconel probe used in the ASTM and ISO methods of Europe and North America. Because of this difference, it is important to note that cooling curves and cooling rates between the methods should not be compared. Figure # 2 shows the comparison between the two probes and their dimensions.
Figure # 2. ASTM D-6200/ ISO- 9950 and JIS K 2242 quenchometer probes^2 ISO/ASTM Inconel probe 12.5mm x 60mm. JIS K 2242 Silver probe 10mm x 30 mm
In addition to comparing the cooling curve against the standard for the quench oil used, the Grossman H value should also be calculated and used as an indicator of cooling performance. Unlike the old GM nickel ball test that tracked the time to cool a 12mm nickel ball to 352°C, the Grossman H value measures the severity of the quench6.
In using the Grossman H value, the lower the value, the slower and less severe the quench. For use as a rough guide in comparing the quench speed in seconds to the Grossman H value measured in cm-1 the table below can be used.
Table #1
For example, air has an approximate H value of 0.01 cm-1 and water has an approximate H value of 0.4 cm-1 compared to oil with an approximate H value of ___ cm-1
The calculation used to determine the Grossman H factor has historically been:
H=h/2k
Where h is the heat transfer coefficient of the part when measured at the surface of the part and k is the thermal conductivity of the steel. Typically the heat transfer coefficient is measured at 705°C. A steel’s thermal conductivity does not typically change according to alloy composition or temperature. Therefore, the Grossman H value is proportional to the heat transfer coefficient of the part.
Interpreting a CQI-9 quench oil analysis
Table #2
Discussion
In examining the test parameters for CQI-9, it becomes apparent that many of the test results should be compared with other test results. For example an increase in the amount of sludge or solids should also increase the viscosity of the quench oil. As the sludge increases, the level of oxidation increases, and therefore, the level of organic acids formed in the quench oil should be increasing the TAV. Finally, as the sludge increases, the cooling property of the quench oil should decline as indicated in the lower H value.
Figure #3. Total Acid Value (TAV) and Grossman H value
Likewise, as the flash point decreases the amount of thermal cracking is increasing, which should reduce the viscosity and thereby increase the H value and the overall cooling speed of the quench oil. Conversely, if the test parameters are not working in concert with each other, there may be other issues going on within the quench oil. For instance, an increase in the water content can be detected before the increased water levels begin the oxidation process thereby increasing the TAV. Or a viscosity change without a change in other parameters could be an addition of the wrong quench oil to the quench tank. The graph below for Idemitsu Daphne Hi Temp A helps illustrate this point.
Figure #4. Graph for Idemitsu Daphne Hi Temp A demonstrating viscosity change
In the graph above, it can be seen when the water H value increases and the viscosity remains stable, the likely explanation is an increase in water. When both the H value and viscosity decrease, additive consumption is the most likely reason. Likewise, when the viscosity increases and the H value decreases, the formation of sludge from oxidation is the culprit.
Having test parameters that work in conjunction with each other is only beneficial if sample frequencies are often enough. While CQI-9 only stipulates a semi-annual sampling frequency, the conditions of a quench tank can change in very short order. There are the obvious changes when water is added to the tank. However, many of the changes are more subtle, and left unchecked over time can create potential costly solutions such as a partial dump and recharge of the quench tank, poor part quality, or an increase in downstream processing such as shot blasting. For this reason, many quench oil suppliers request a minimum of quarterly sampling. In addition, if a sample is missed on a quarterly sample frequency, there is still time to sample the quench tank and remain in compliance with CQI-9.
Conclusion
Over time the condition of a quench oil will change and corrective measures will be needed to bring the quench oil back into the suggested supplier’s operating parameters. The chart below helps understand what some of the methods need to be.
With proper care and maintenance, a quench oil can last a very long time. A conventional oil should last 10 to 15 years or longer while a marquench oil should last seven to 10 years. The proper care of a quench is simple and straight forward. A quality quench oil should not need the use of additives to improve oxidation resistance or quench speed. Simply adding enough fresh virgin oil to replace the oil that is being dragged out through normal operations should replenish the oxidation protection and quench speed to within the normal operating parameters. The table below offers recommendations for treating out of normal operating parameters for the required CQI-9 tests.
Recommendations for treating out of normal operating parameters for the required CQI-9 tests
Most heat treaters make weekly quench oil additions to their quench tanks. The most popular type of filtration system is a kidney loop style where the quench oil is constantly filtered. There are two basic types of these systems. They differ in the number of filters used. For a single filter system, a 25 micron filter is sufficient for quench oil filtration. In a two-stage filtration system, a 50 micron filter is typically used in the first stage and a 25 micron filter is used in the second stage. In a two-stage filter, the cheaper 50 micron filter will be replaced more often than the 25 micron filter in the second stage.
Utilizing a compliant CQI-9 analysis and the supplier provided operating parameters for the CQI-9 required tests is the first step in the proper care of a quench oil. The next basic steps are ensuring there is enough fresh quench oil available for regular additions to replace the oil that is lost through drag out and proper filtration of the quench oil in a constant kidney loop type of a system. With these steps in place, a quench oil will offer consistent performance for years and will be one less concern heat treaters face in the operation of their furnaces.
References:
Automotive Industry Action Group, “CQI9 “Special Process: Heat Treatment System Assessment;” AIAG version 3, 10/2011.
M.A. Grossman and M. Asimov. Hardenability and Quenching. 1940 Iron Age Vol. 107 No.17 Pp 25-29.
About the Author:
Greg Steiger is the senior key account manager of Idemitsu Lubricants America for quench products. Previous to this position, Steiger served in a variety of technical service, research and development, and sales marketing roles for Chemtool, Inc., Witco Chemical Company, Inc., D.A. Stuart Company, and Safety-Kleen, Inc. He obtained a BSc in Chemistry from the University of Illinois at Chicago and is currently pursuing a Master’s Degree in Materials Engineering at Auburn University. He is also a member of ASM International.
Heat treaters have their processes down to a science, literally. But what factor can compromise your heat treated part, let alone possibly cause detrimental damage to your facility?
Greg Steiger Sr. Key Account Manager Idemitsu Lubricants America
Michelle Bennett Quality Assurance Sr. Coordinator Idemitsu Lubricants America
Heat TreatToday is pleased to present this original content article for today's Technical Tuesday. Greg Steiger, senior key account manager at Idemitsu Lubricants America, and Michelle Bennett, quality assurance senior coordinator at Idemitsu Lubricants America, describe water contamination in quench oil, the effects of this contamination, and how to test and safely remove the water from the quench oil.
Introduction
Water is an amazing substance. Water helped create the Grand Canyon and Niagara Falls. When water freezes, it doesn’t contract like most materials. Instead, it expands and creates potholes that swallow up our cars every winter. As the temperature rises, water also expands. This property allows water to heat our homes and is why steam engines work. The thermal expansion of water as it turns into steam is what can create catastrophic events in a quench oil. This paper will look at potential water contamination sources in a quench oil, what the effects of the water can be, how to test for the presence of water in a quench oil, and how to safely remove the water from a quench oil.
Sources of water contamination
There are two major classifications of potential water contamination. The first source can be classified as potential internal sources of water. These potential sources are typically a part of heat treating furnace or oil cooling system. They include water-cooled bearings, fans, doors or heat exchangers. These water-cooled components are under a contestant pressure and will eventually leak. Because the quench tank is usually below these sources of water, the water will eventually find its way into the quench tank. Water-cooled bearings and fans are located within the furnace and are often directly above the quench tank. While a water-cooled door is typically not directly above a quench tank, it is in close proximity to the quench tank. This proximity will allow leaking water to enter the quench tank. Heat exchangers are typically situated away from the furnace. However, in a water-cooled heat exchanger, the water is never more than the wall thickness of the cooling tubes away from the oil. Should a cooling tube form a leak, the water and quench oil would simply mix within the cooling stream and the quench oil water mixture would return to the quench tank.
"The greatest risk of external water contamination lies in preventable operator or maintenance mistakes, especially when the equipment is down and open for maintenance."
The second classification is external sources. These sources of water contamination are not part of the heat treating furnace. Examples of external sources can be further broken down into leaks and operator or maintenance personnel mistakes. Leaks typically include fire extinguishers and fire suppression systems leaks, leaking fire resistant hydraulic systems, atmosphere leaks, pneumatic cylinders and building leaks. To prevent the leak type of contamination, routine maintenance, like a daily “Gemba” walk to spot any leaks, is the best defense against water entering a quench oil through a leak. The greatest risk of external water contamination lies in preventable operator or maintenance mistakes, especially when the equipment is down and open for maintenance.
Quite often when a furnace undergoes repairs, the quench oil is pumped out into empty totes to be reused after the furnace repair is finished. There is nothing wrong with doing this if the totes are clean. However, there have been reports of heat treaters doing this without first inspecting the totes to ensure that they are clean and free of any type of contamination. There have also been instances when the totes were not properly sealed and then stored outside, thus allowing rain water to get into the quench oil. But, the potential to add an incorrect product to the quench tank is a preventable operator error.
How water affects a quench oil
As previously mentioned, water expands as it turns into steam. At 212°F, water has a density of 0.96g/cm3.1 One gallon of water occupies 0.14 ft3. At one degree above boiling the steam from the boiling water has increased to occupy 224 ft3 and a density of 0.0006 g/cm3. The thermal expansion rate of water is approximately 1600%. What this means is the single gallon of water that was in the quench oil before it turned into steam now has a volume approaching 1600 gallons. In order for the 1600 gallons of steam to escape from the quench tank, it must displace an equal amount of quench oil. With nowhere to go, this displaced oil will find hot spots and open flames to create a catastrophic event.
Quench severity
Fig.1 Schematic of ASTM D-3520 (ref. 7)
Historically, the severity of the quench has been measured by ASTM D-35202. In this method, a chromized nickel ball is heated to 885°C and is dropped through an electronic sensor, which starts a timer, and into a steel cylinder of quench oil in a magnetic field. Once the chromized nickel ball reaches the Currie temperature of nickel at 354°C, the ball becomes magnetic and closes the timing circuit when the ball comes into contact with the cylinder. The popularity of this test has always been that it provides a number that is easily interpreted by heat treaters to “rate” the oil as fast (9 – 11 seconds), “medium” (12 – 14 seconds), “slow” (15 – 20 seconds) or marquench (20 - 25 seconds). A schematic of the test method is shown in Figure #1.
This test worked well to differentiate between different how well the quench oils cooled the nickel ball. The test really didn’t distinguish between the cooling characteristics of a quench oil. The test result in Figure #2show a time in seconds for the nickel ball to reach 354°C for three separate oils. However, when the actual cooling curves of the oils are examined there are three distinct cooling curves shown.
Fig. 2 Three separate cooling curves with the same quench speed as measured by ASTM D-3520 (ref. 7)
Because mechanical properties such as yield strength and hardness are dependent on the severity of the quench, the Grossman H value3 has become more popular over the years. In using the Grossman H value the lower the value the slower and less severe the quench. For instance air has an approximate H value of 0.01 cm-1 and water has an approximate H value of 0.4 cm-1. The calculation used to determine the Grossman H factor has historically been:
Where h is the heat transfer coefficient of the part when measured at the surface of the part and k is the thermal conductivity of the steel. Typically the heat transfer coefficient is measured at 705°C. A steel’s thermal conductivity does not typically change according to alloy composition or temperature. Therefore the Grossman H value is proportional to the heat transfer coefficient of the part.
Cooling curve
The basic cooling curve consists of three stages: the vapor blanket, nucleate boiling and convection. A basic cooling curve with the three different cooling phases is shown in Figure #3.
Fig.3 Three stage cooling curve (ref. 4)
In the vapor blanket stage, the load and the quench oil coming into contact with the load are above the evaporation temperature of the oil. An insulating vapor blanket forms around the load and no cooling occurs. Because the vapor blanket is insulating and does not allow for cooling, the vapor stage carries the highest risk of distortion.4 Once the vapor pressure decreases to a point where the oil can once again condense on the load and the temperature of the oil falls below the evaporation temperature, the nucleate boiling stage begins. In this stage, the load undergoes the most aggressive cooling. After sufficient cooling has occurred and the quench oil temperature is below the boiling temperature of the oil, a smooth transition into the convection stage begins.
Stabilization of the vapor stage
As water is dispersed throughout the oil, the viscosity of the oil changes. As the amount of water increases, the viscosity of the oil also increases.5 A careful examination of Figure #4 will also show a slight movement of the cooling curve to the left and a lengthening of the vapor stage as the amount of water increases. Furthermore the water in the oil is not uniformly dispersed, and this non-uniform dispersion creates uneven cooling rates throughout the oil. To restore even cooling, it is recommended the water in the quench oil be reduced to below 200 PPM.
Fig. 4 Cooling curve change due to water contamination (ref. 4)
Types of water found in a quench oil
In simplistic terms, water in a quench oil can be thought of as being dispersed in the quench oil due to agitation or as free water having exceeded the saturation point of the oil. As a general rule of thumb in the industry, the saturation point is considered to be 0.1% or 1,000 PPM. However, the saturation point will vary according to the temperature of the oil and the additives within the quench oil. Daphne Hi Temp A-U is a good example of a clear amber quench oil. Figure #5 shows a picture array of the appearance of the oil as the amount of water approaches and then exceeds the 1000 PPM industry standard.
Fig. 5 Daphne Hi Temp A-U appearance as the amount of water dispersed within the oil nears and exceeds the saturation point of the oil. (Used with permission Idemitsu Lubricants America)
Notice in the data above that as the amount of water increases in the Daphne Hi Temp A-U, so does the viscosity as measured at 100°C. In addition to the viscosity rising as the amount of dispersed water increases, so also does the quench severity as measured by the Grossman H value. Furthermore, the appearance of the quench oil changes as the amount of water increases as well. (See Fig. 5 for the Daphne Hi Temp A-U.) With small amounts of dispersed water—45 PPM—the quench oil is clear and there is no water that is precipitated out after centrifuging for 15 minutes at 5500 RPM. However, as the amount of water begins to approach the 1000 PPM level, the appearance of the quench oil begins to become hazy. As the saturation point is exceeded, the appearance remains hazy and water precipitates out after centrifuging for 15 minutes at 5500 RPM.
Testing for oil in a quench oil
There are two basic types of testing methods for determining if there is water dispersed in a quench oil. One of the methods is subjective and the other is quantitative. The crackle test involves heating a metal coupon to approximately 400°F and placing a few drops of the quench oil on the surface. If there is a sufficient amount of water in the oil visible bubbling within the oil and audible crackling will occur. Unfortunately, this is typically above the saturation point of the quench oil. At which point it is often too late. Figure #6 shows examples of crackle testing.
Fig. 6 Crackle test results for Daphne Hi Temp A-U
The second and preferred testing method is through ASTM D-6304 Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils and Additives by Coulometric Karl Fisher Titration6. The Karl Fisher test uses the Bunsen electrochemical reaction to calculate the amount of water in a used oil and is accurate in used oil from 1 PPM to 50,000 PPM.
Removing water from a quench oil
Removing excessive water from a quench oil can be achieved economically through several methods. Table #1 is a brief trouble shooting guide to the safe removal of water from a quench oil.
Table 1 Trouble shooting guide for removal of water from a quench oil
Conclusion
Finding small amounts of water, less than 50 PPM is very common in a used quench oil sample. This small amount could simply be condensation within the bottle and quench tank. However,when the amount of water begins to reach levels above 200 PPM, troubles can begin. At levels above 200 PPM of water, the following may occur:
Uneven cooling due to non-uniform dispersing of the water within the quench oil
Increase in viscosity
Increase in Grossman H Value
Lengthening of the vapor blanket stage
Increase in the severity of the quench
Like most materials, water expands as it changes from a liquid into a vapor. With a thermal expansion rate of 1600%, a gallon of water turns into considerable more steam. Therefore excessive water transitioning into steam in a quench oil creates safety concerns when the steam forces the quench oil from the tank. Examples of these safety concerns are:
Risk of harm and injury to plant personnel
Damage to furnaces and related equipment
Damage to the heat treat facility the surrounding plant and nearby buildings
Severe cases can result in a quench oil fire or a building fire
The importance of a “Gemba" walk should not be overlooked. Water can enter into quench oil systems through normal heat treating operations such as a leak in a water-cooled piece of equipment, others can be from preventable sources such as a building leak or other human error. No matter what the source is, if water is suspected in a quench oil, the quench tank should be sampled and tested before it is used.
References:
Handbook of Chemistry and Physics. 60th edition CRC Press, p. E-18.
ASTM International, “Standard Test Method for Standard Time of Heat Treating Fluids (Magnetic Quenchometer Method),” American Society for Standards and Materials.
M. A. Grossman and M. Asimov, “Hardenability and Quenching,” 1940, Iron Age Vol. 107 no.17, p. 25-29.
ASTM International, "ASTM D-6304 Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils and Additives Coulometric Karl Fischer Titration," West Conshohocken, ASTM International, 2016.
B. Lisic and G.E. Totten, "From GM Quenchometer Via Cooling Curve Analysis to Temperature Gradient Method," ASM Proceedings: Heat Treating, 18th Conference, 1998.
About the Authors:
Greg Steiger is the senior key account manager of Idemitsu Lubricants America for quench products. Previous to this position, Steiger served in a variety of technical service, research and development, and sales marketing roles for Chemtool, Inc., Witco Chemical Company, Inc., D.A. Stuart Company, and Safety-Kleen, Inc. He obtained a BSc in Chemistry from the University of Illinois at Chicago and is currently pursuing a Master’s Degree in Materials Engineering at Auburn University. He is also a member of ASM International.
Michelle Bennett is the quality assurance senior coordinator at Idemitsu Lubricants America, supervising the company's I-LAS used oil analysis program. Over the past 9 years, she has worked in the quality control lab and the research and development department. Her bachelor’s degree is in Chemistry from Indiana University.
In this 4-part series, Heat Treat Radio host, Doug Glenn, talks with Joe Powell of Integrated Heat Treating Solutions about bringing heat treating into the 21st century.
According to Joe, the real focus should be on the quenching portion of the process where distortion often happens. In many instances, distortion is able to be eliminated. Find out how in this episode.
Below, you can either listen to the podcast by clicking on the audio play button, or you can read an edited version of the transcript.
The following transcript has been edited for your reading enjoyment.
Doug Glenn (DG): On today’s episode, I sit down with Joe Powell, president of Akron Steel Treating Company to hear what he and his team are doing to combat heat treat distortion. Joe Powell is a veteran in the industry and carries a wealth of knowledge with him. Joe, your company has 75 years of experience working with different part makers, and after a very brief conversation with you, pretty much anyone would conclude that you’re a man on a mission to bring heat treating into the 21st century. Before we turn you loose on that topic, first tell us a little bit about Akron Steel Treating and how it got started.
Joe Powell (JP): It was founded by my father in our garage in 1943 at the behest of the Department of the Army who wanted him to heat treat some parts, and it grew along with all the tool and dye makers in Akron, OH by making machinery for making various rubber products like tires, belts and hoses . . . you name it.
DG: You’ve also spearheaded another company: Integrated Heat Treating Solutions. What are you doing with that company?
It should be “quench treating” not “heat treating.” That’s the way I look at it.
JP: Integrated Heat Treating Solutions is the culmination of 75 years of commercial heat treating experience with literally over a 1000 different part makers. What we’ve learned that if we can integrate our heat treating solutions with the part-making design and the optimal material selection, we can produce better parts. And what I mean by “better parts” is they could be lighter, they could have longer fatigue life, and they could have less distortion after heat treating. All of these benefits are brought to the table to part makers so that heat treating becomes a fully integrated part of lean manufacturing.
Once heat treating becomes a lean, integrated part of manufacturing, everybody wins. It enables the use of leaner alloy materials; it eliminates oil quenching; it eliminates long carburizing cycles and batch carburizing cycles; and we now are able to literally do the heat treating in the manufacturing cell where the parts are made.
DG: What do those two companies look like now?
JP: We have about 50,000 square feet and are currently in the process of acquiring another building to our east. We have 48 employees and there are three shifts; and again, we do salt heat treatment, vacuum heat treatment and controlled atmosphere heat treatment. Also, we are currently getting into induction heat treating with our friends at Induction Tooling.
For the last 23 years, we have been concentrating on finding the best way to quench parts and to drive the distortion out of the part-making process. The heat treat distortion has been a problem for centuries. Parts crack, they distort, they come out of the heat treat process unpredictably with size change that is absolutely necessary to get the mechanical properties, but also, if it’s nonuniform, that size change can cause major problems down the line that have to be corrected by hard turning, grinding, flattening, straightening, you name it.
Dynamics of uniform and Uniform Intensive Quenching model (Source: integratedheattreatingsolutions.com)
We’ve also delved into the science of computer modeling, finite element modeling as well as computation of fluid dynamic modeling with our friends at DANTE Solutions. What has happened from that modeling is seeing this concept: the surface of the part contains a bunch of grains, and those finite elements – if they are not quenched uniformly – will transform nonuniform, leading to nonuniform thermal shrinkage upon beginning quenched. Then they will also transform to martensite nonuniformly, which means that the thin and thick sections of a part will have different amounts of distortion and size change. In order to control that, we’ve developed what we call “quench to fit” technologies where we literally build a shell on the outside of the part, using a gas quench or a uniform salt quench or uniform water quench. Once you’ve built that shell in the first few seconds of the quench on the outside of the part, that martensite shell acts like a custom-made quench dye, and that custom-made quench dye allows the part core to cool by conduction through that shell. So, if that cooling by conduction happens by very uniform conduction through the geometry and the mass of a given part, you will have a predictable size change after heat treat. And, you will enable the part designer to go back to the initial part design and adjust it accordingly so that it quenches to fit during the quench process.
When a commercial heat treater receives the part, 99 times out of 100, that part is using a material that was selected many, many years ago, because that is what they’ve always used. Additionally, it’s going to be heat treated in legacy equipment that has always been used. For instance, case carburized 8620 steel valve seats have been used for decades now, and they last about 40-70 hours in the fracking pump, but a ductile iron valve seat can be made to last many times longer; it’s cheaper to buy the material and our heat treating equipment can heat treat it in 5 minutes instead of a 20 hour case carburizing cycle in batches. That single part flow of that new induction heat treating equipment and quenching equipment that is built into it can be built in right at the end of the CNC machines.
I am a commercial heat treater who believes that part design should be integrated for heat treating by the part-maker. It’s a nuance, but what it really boils down to is that sometimes commercial heat treaters do it best, but sometimes the part-maker can do it better. [Side bar quote: I am a commercial heat treater who believes that part design should be integrated for heat treating by the part-maker. It’s a nuance, but what it really boils down to is that sometimes commercial heat treaters do it best, but sometimes the part-maker can do it better.]
I am a commercial heat treater who believes that part design should be integrated for heat treating by the part-maker. It’s a nuance, but what it really boils down to is that sometimes commercial heat treaters do it best, but sometimes the part-maker can do it better.
DG: So, the importance in the part design process of including the heat treater is that you can more consistently predict what the distortion will be, because if I understand it correctly, you can actually predict distortion in the part and therefore design the part with the distortion that will come consistently every time you design that part, yes?
JP: Yes. And it doesn’t matter if it’s an air quench or a hot salt quench or a uniform water quench, it just has to be very, very uniform from the initiation of the quench. In other words, you can’t take it out of the furnace and air cool it for 45 seconds and then begin a water quench, it doesn’t work that way. That shell is starting to form instantaneously when the heat is turned off. An air quench is very slow compared to an intensive water quench and so you have to introduce that quench all over the part surface shell as instantaneously, and with as much uniform impact, as possible. That’s what we do in terms of designing equipment to do the quench process.
DG: Right now, there are a lot of companies, a contractor or commercial heat treater, that send you parts to heat treat. Is it not possible that if the part designer and the heat treater talk in advance as they design the part, that some of these parts could be, in fact, heat treated in-house and not be sent out to a commercial heat treater? Is that possible?
JP: They could actually be heat treated not only in-house, but directly after the CNC machine, right in the manufacturing cell, right after the forge. It takes the proper selection of the optimal hardened ability material. In other words, part of that part design with the heat treater has to be considerations like, “Is it going to get too hard in the core? Is it going to swell up too much in the core? Is it going to be unable to build that shell on the surface without blowing it off, because the core starts to harden up?” So again, the optimal material selection and the design of the mass and the geometry of the part need to be considerations that the heat treater gets a chance to look at.
A “textbook” example of the bell curve. (Source: integratedheattreatingsolutions.com)
DG: So, if the part designer and the heat treater get together and talk about the part design before the part is finalized, or if they’ve got a legacy part, they can sit down and talk with a heat treater that understands what you’re doing over at Akron Steel and Integrated Heat Treating Solutions. If they can understand that, and if they can talk with you about how that part might be redesigned, it’s very possible that you could use lower cost materials to get the same thing, minimize the amount of time to actually heat treat, and you may be able to put that part in a single piece or at least possibly a small batch flow so that there’s not a bottleneck at heat treat, yes?
JP: Yes.
Sponsorship for this episode is Furnaces North America the Virtual Show.
DG: Joe, let’s talk about the quenching bell curve as it relates to distortion.
JP: There are many, many metallurgists and many metallurgical textbooks that indicate that the faster the quench cooling rate, the higher the probability of distortion. There is a curve that is generated that basically says that if you quench very slowly in gas, or if you increase that quench rate and go to a hot salt or a martemper bath or an austemper bath or you increase it even further with warm oil or highly agitated oil, or you go to a brine quench where you do a polymer or a polymer water quench where you increase the rate of quench cooling, there is a point at which most of the parts are going to crack and you’re going to have major distortion. It is not because of the quench speed being faster, it is because the uniformity tends to be less the faster your quenchant. In other words, you need to keep the water from film-boiling and creating a situation where the initial quench is actually done under a steam blanket, or gas, very slowly. Once the thin sections of the part quench-out under gas, then you have the thick sections that are still under that gas blanket, and you have very rapid cooling and very rapid martensite transformations that cause a shift in the size of the part where the shell now cannot contain the core swelling that’s happening underneath the surface.
Whereas 21st century heat treating practice is, what I call, a “uniform quench renewal rate” and an instant impact. In other words, you instantly impact the shell, create that shell, and once it’s created with uniform cooling, then the rest of the cooling happens by conduction through that shell. Whatever the geometry and the mass of the part is will determine that uniform conduction cooling which ends up being very predictable. Once it’s predictable, then you can morph the green size of the part before heat treating so that it predictably quenches to fit during the quench process.
(source: integratedheattreatingsolutions.com)
DANTE Solutions has a method where they use their model to model the finite elements in the part so that the thin and thick sections of the part quench uniformly. IQ Technologies Inc. and my company, Integrated Heat Treating Solutions, have gone on the other side and shown that it is really a bell-shaped curve, and that the probability of distortion goes back down if you can create that shell on the outside of the part instantaneously, and then provide a uniform quench renewal rate to the part surface so that the core can cool by uniform conduction through that shell.
DG: Let’s just put in our listener’s minds the standard bell curve. Most of the quenching and most of the textbooks that we see these days is done on the left hand side of that bell curve, and as you approach the peak of that bell curve, the probability of distortion and/or cracking occurs. People are saying – don’t quench too fast because you’ll get cracking. You’re kind of switching the whole paradigm to say that it’s not the speed at which you quench, but more so: Can you create, almost instantaneously, a hard shell because of exceptionally rapid cooling on the whole part so that that shell basically holds the part in place? If you can get that, then you can cool the rest of the part, however slow or fast, in a sense, you want, because it’s not going to distort because it’s already locked in.
JP: Right, and this is cooling by conduction which is the physics of the material. How fast will it give up the heat through its mass? It’s the difference between 100 degrees or 50 degrees or 10 degrees per second of cooling and 400 to 600 degrees centigrade cooling per second, so it’s very, very intensive. The middle of the bell curve, where most parts are cracking, is because there is not a uniform quench renewal rate. You start off with a gas quench, then you end up with a very intensive evaporative cooling quench with nucleate boiling. You then end up with water quenching without boiling, and so you have three different phases of cooling happening on different parts of the part. This is exacerbated by different parts in different sections of the batch which will have different cooling rates.
It’s almost impossible to get the full benefits of very, very intensive quenching or even very, very uniform gas quenching in a vacuum furnace unless you have staged the cooling in such a way that you create that uniform shell at the beginning of the quench, and you hit that martensite start temperature and cool to that martensite start temperature all over the shell of the part uniformly. That’s the key.
DG: There are several things that jump into my mind like questions that might arise from people. You’ve already hit on the differences in part thickness – you may have thick sections, you may have thin sections. It’s very possible to maybe get down to the martensite start temperature on the thin section right away, but the thick section may not be, and therefore you’re going to distort because you haven’t created that “frozen shell” uniformly around the entire part. Let’s talk about, not just part thickness, but part geometry in the sense of the awkward curves and turns or lips and things of that sort on parts. How would we deal with that?
JP: That’s where new 21st century heat treating equipment needs to be designed. Every furnace company that is selling furnaces to either captive heat treaters or commercial heat treaters calls itself a furnace company. The reality is, yes, heating is important and it is the precursor to getting the mechanical properties, but the heat treatment is actually done, and the mechanical properties are actually obtained, in the quenching process. It should be “quench treating” not “heat treating.” That’s the way I look at it.
Image from Smarter Everyday YoutTube video on Prince Rupert’s Drop (source: https://www.youtube.com/watch?v=xe-f4gokRBs&ab_channel=SmarterEveryDay)
For the last 23 years that’s what has been more apparent to me. My dad taught me how to quench stamps that were used for marking the inside of tire molds, and these steel stamps would uniformly blow up if you just quenched them. But if you were able to uniformly quench the marking end, you could get it hard as hell and it would last a long, long time, but you had to kind of bifurcate the quench. You had to make sure that you created that shell in the marking area of the stamp and let the rest of the stamp kind of cool much more slowly. In other words, create the shell in the face of the stamp where the lettering is, and set those letters. Then the rest of the stamp can basically cool much slower because you don’t need the hardness there; it’s not the working part of the part.
Also, the designers of the stamps had to integrate the right radius in the face of the stamp. If they had sharp corners, those sharp corners would blow off during the heat treat. So, over time, we said, “If you don’t want us to crack this stamp, you’re going to have to put a radius over here and change the design slightly.” It didn’t take much change, but it did take a recognition of the fact that this was not going to work. There’s no way to eliminate the nonuniform cooling in the shell if you’ve got a corner. Steam collects in that corner and it doesn’t quench, so you can’t create the hardened shell.
DG: Let’s take a little deviation and talk about something non-metal. Let’s talk about the Prince Rupert’s drop to illustrate residual compressive stresses.
JP: The mystery of the Prince Rupert’s drop of glass is that glass makers noticed that if they dropped a drop of molten glass into a bucket of cold water it would form a drop that has a head and then a tail – it almost looks like a tadpole. If you hit the head of that glass drop with a hammer or try to break it with a pair of pliers, you can’t do it. It is literally unbreakable at the head. However, if you snap the tail off, it instantaneously explodes. This is because there are counterbalancing tensile stresses that are below the surface in the tail that once you break the compressive stresses off, it’s like taking the hoop off a barrel and the barrel staves explode; the elements on the surface just explode. The reason they don’t explode on the drop of glass at the other end is because there are sufficiently high compressive stresses on that surface that hold the drop of glass and keep it from fracturing.
DG: This is a fascinating video where you take a Prince Rupert’s drop, actually hang this Prince Rupert’s drop and shoot it with a .38 or a .45 or a 9 mm, hitting the head of that tadpole, if you will, and it shatters the bullet while the glass remains untouched. However, if a guy just simply takes his finger, or whatever, and snaps the tail, not just the tail shatters, but the whole tadpole blows up.
JP: What we’ve been able to do with all of the research that we’ve done is to harness those compressive stresses and make them available to the part-marker for making their parts more robust, making them lighter, and making them basically carbide hard and hammer tough. They don’t chip when hit with a hammer.
DG: Let’s jump back to some of the projects you’ve done at Integrated Heat Treating Solutions. Do you have any current projects that you’re working on where this integrated solution – where you were involved with part design or improvement of part design – worked well?
JP: Yes. There are several case studies. The first case study was a punch that lasts 2 – 9 times longer than an oil quench punch.
DG: A punch for what?
JP: Punching holes in metal plates. And the other thing that has happened is that since we’ve begun working with Induction Tooling, we’re able to then bring this down to the level of thinner parts and more complex geometry parts. We’re able to get more hardenability out of lean hardenability alloy such as ductile iron. Plain ductile irons are now acting as carbides. Even the people that make the material said it couldn’t be done, but we’re doing it.
DG: Can you give an example of that?
Watch more resources at Integrated Solutions website. Click the image above to access these resources.
JP: Yes, that would be a fracking pump valve seat made out of ductile iron and heat treated with our special heating and quenching technologies.
DG: What was the performance prior to the treatment and afterwards?
JP: 40 to 60 hours and our initial testing we got 166 hours, so 2 ½ times longer.
DG: So 2 ½ times better performance on this fracking valve seat, and you were using the same material?
JP: No. Rather, we replaced an 8620 carburized steel that needed to be carburized for 20 hours in the furnace, and we did it with a 5 minute induction heating process.
DG: Of what type of material?
JP: Ductile iron.
DG: So we’ve got a punch, a valve seat in the fracking industry. What else?
JP: We have bevel gears that we do. We have worked with the part manufacturer and they’ve adjusted their CNC program so that it actually quenches to fit and doesn’t require a final grind.
DG: Expensive hard machining or hard grinding after heat treat.
JP: Right. And it saves them about $750 per gear in final grind costs. And, the gear lasts longer because it has high residual compressive surface stresses versus a standard carburization process and quenching in oil that does not have as high of a residual compressive surface stress. Especially after you grind it all off to get the final dimensions you want.
DG: Right. So you put all these nice hard stresses in, then you grind them off.
JP: Exactly.
DG: Any other examples?
JP: We have a company that wanted to have a weldable gear rack that could be welded on in the field on mining equipment that’s out on the side of a mountain. Because it might be cold up there, and they didn’t want to have to pre- and post-heat in order to weld on the gear rack, or repair a tooth on the gear rack, they wanted to have a material that had less hardenability but still wanted to have all of the mechanical properties. We were able to get the mechanical properties of 4330 from a 4130 material that doesn’t need to be pre- and post-heated to prevent it from cracking when welding it onto the machinery. They call that “field repairability.” So, we were able to enable field repairability and still maintain the mechanical properties’ requirements.
DG: In future episodes, we’ll go into some depth on some of those applications you just described, but before we wrap up things for this episode, is there a last impression you’d like to leave with us?
JP: Professor Jack Wallace* did not believe that there was a right half of the bell-curve, he did not believe that intensive quenching would work, but, again, he became a believer. It is all key to understanding the dynamics and uniformity of quenching over time. If you get the uniformity, you’re in good shape and eliminate a lot of heat treating problems.
DG: Thanks, Joe. Looking forward to you joining us for future episodes.
JP: Thanks so much.
*Professor Jack Wallace was the “Dean of the College of Metallurgical Engineering at Case Western Reserve University in Cleveland Ohio – who said in 1997, ‘Intensive water quenching would not work! – The parts will blow up in the quench!’ He became a convert once he figured out how compressive surface stresses worked during uniform quenching.” Information provided by Joe Powell.
Doug Glenn, Heat Treat Today publisher and Heat Treat Radio host.
To find other Heat Treat Radio episodes, go to www.heattreattoday.com/radio and look in the list of Heat Treat Radio episodes listed.
Heat TreatTV pulls the best heat treat videos from the web for your viewing, and today Heat Treat TV highlights the pressurizing effect of quenching.
The mystery of Prince Rupert’s Drop is a well-known phenomenon. Somehow, the glass will not break under significant pressure, but a breakage to compromise the structure of the tail of the drop leads to absolute combustion, similar to a chemical explosion.
In this video, you won’t only simply learn about what the drop is, but also why it works and where it comes from. The relationship between glass and metal is the effect that the quench process has on the structural integrity of the materials. Learn more about external surface tension and its role in heat treat in this Heat TreatRadio podcast about Rethinking Heat Treating, Part 1 with Joe Powell of Integrated Heat Treating Solutions.
