How long have you been heat treating automotive gears? Which thermal processing techniques do your operations gravitate towards? In this best of the web article, uncover some of the common heat treatment functions and the properties they create in gears. Let us know what you think of this general overview of the world of heat treating gears in our Reader Feedback form!
Contact us with your Reader Feedback!
Additionally, when you read to the end of the article, future trends that we can anticipate for heat treaters in the automotive industry are offered; as one might guess, they include digital and energy-saving technologies.
An excerpt: “Automotive gear heat treatment (process) includes two aspects: firstly, conventional heat treatment such as annealing, normalizing, quenching, tempering, and quenching and tempering; secondly, surface heat treatment, which encompasses methods like surface quenching (e.g., induction quenching, laser quenching) and chemical heat treatment (e.g., carburizing, carbonitriding, nitriding, nitrocarburizing).”
What process holds a soft spot in your heart? Tempering or annealing? For Valentine's Day, turn up the heat -- errr heat treatments -- with this look at the differences in tempering and annealing! Heat TreatToday has resources for you to spark some thought and learning on these processes.
Sentiments and strong feelings can certainly be heightened this Valentine's Day. While tempering and annealing may not lend themselves easily to the holiday, we hope you enjoy a bit of a nod to the day in our headings below. Make use of the Reader Feedback button, too, and keep us in the loop with questions and comments on what heat treatment you love.
Problem with Annealing? Get to the Heart of the Issue
An automotive parts manufacturer was running into problems with cracking parts. The variable valve timing plates were returning from heat treatment with this problem. To determine why those parts were cracking after the annealing process, an investigation was launched by metallurgists at Paulo.
The presence of nitrogen combining with the aluminum already present in the particular steel being used was forming aluminum nitrides. What could be done? Read more in the case study article below to find out a workable solution that allowed the annealing to create a crack-free product.
Induction, Rapid Air, Oven and Furnace Tempering: Which One do You Love?
Contact us with your Reader Feedback!
This article gives some perspectives, from experts in the field, on what kinds of tempering are available and for what the processes are used.
Hear from Bill Stuehr of Induction Tooling, Mike Zaharof of Inductoheat, and Mike Grande of Wisconsin Oven with some basics and background information on tempering. Those reasons alone make this resource helpful with information like this: "tempering at higher temperatures results in lower hardness and increased ductility," says Mike Grande, vice president of sales at Wisconsin Oven. "Tempering at lower temperatures provides a harder steel that is less ductile."
More specific in-depth study is presented as well. The Larson-Miller equation is considered, and the importance of temperature uniformity is emphasized. Read more of the perspectives: "Tempering: 4 Perspectives — Which makes sense for you?"
Cast or Wrought Radiant Tubes in Annealing Furnaces - is Cheaper Really What to Fall For?
Marc Glasser, director of Metallurgical Services at Rolled Alloys, takes a look at radiant tubes. He particularly discusses the cast tubes and wrought tubes. For use in continuous annealing furnaces, there are several factors contributing to choice of radiant tube type.
Marc says, "Justification for the higher cost wrought alloy needs to take into consideration initial fabricated tube cost, actual tube life, AND the lost production of each anticipated downtime cycle as these downtime costs are often much more than material costs." He probes into areas that may not be considered when thinking of all the costs involved. Read more of his article "Radiant Tubes: Exploring Your Options."
Tempering Furnaces: Improvements are Thrilling
The expert behind this piece shows the importance of tempering, particularly in automotive fastener production. Tim Donofrio, vice president of sales at CAN-ENG Furnaces International Limited examines what's working in the tempering furnaces. The products are meeting and exceeding expectations.
To wrap up this Technical Tuesday post on tempering and annealing, head over to this additional resource to round out the scope of each process. "What is the Difference: Tempering VS. Annealing" gives a summary perspective on the heat treatments discussed above.
Find heat treating products and services when you search on Heat Treat Buyers Guide.com
Tempering. A vitally important step in the hardening process and a process that is used extensively throughout the heat treatment industry. There are three main schools of thought on how to achieve a properly tempered part. Here we have asked three experts to share their knowledge on the specific approach they feel works best for tempering: Bill Stuehr of Induction Tooling, Mike Zaharof of Inductoheat, and Mike Grande of Wisconsin Oven. Learn how each approaches tempering and why they feel it works well for them.
Please note that mechanical properties and microstructure, in addition to hardness, need to be carefully considered when choosing any tempering process so as to help ensure the part is fit for its intended purpose.
This Technical Tuesday article first appeared inHeat Treat Today’sMay 2022 Induction Heating print edition.
Induction Tempering: Captive Heat Treating
By William I. Stuehr, President/CEO, Induction Tooling, Inc.
William I. Stuehr President/CEO Induction Tooling, Inc.
I can only speak to this subject through a lens of 46 years and thousands of induction hardening applications. That said, I have had many tempering inductor requests within the domain of captive heat treating. The commercial induction heat treaters that I service most always use oven tempering because it is accurate, economical, and easy.
Figure 1. Wheel bearing hub and spindle sectioned and etched to show the selective hardened surfaces. Source: Induction Tooling, Inc.
For the captive heat treat departments processing high volume components, the interest in induction tempering as an in-line process sparked in the mid-1970s with the production “cell” concept. This was most evident in the manufacturing of modular wheel bearing assemblies – raw forgings were fed into the cell and completed units exited. Modular wheel bearings are composed of a hub and a spindle. Within the production cell both needed selective induction hardening and tempering. The specification for the wheel spindle required a casehardened profile to provide wear and strength and for the wheel hub, the bearing races were hardened. Equipment manufacturers designed and built specialized high-volume parts handlers, integrated with the proper induction power supplies to operate efficiently within the cell. The inductors, both hardening and tempering, were designed, built, and characterized to produce a specification hardened part (Figure 1).
Figure 2. Thermal image of a wheel spindle Source: Induction Tooling, Inc.Figure 3. Truck axle and truck axle temper inductor Induction Tooling, Inc.
Induction hardening for the hub and spindle is quick – usually five seconds or less; induction tempering is a much longer heating process. Both parts required a low power soak until the optimum temperature was achieved. For the two wheel bearing components, tempering had to be accomplished either in a long channel-type inductor or several multi-turn inductors to keep pace with hardening. The long channel inductor was designed to hover over a conveyor belt. The belt would move the hardened hub or spindle at a slow, even pace allowing the precisely controlled induction energy to migrate throughout. Care was taken in the design and length of the channel inductor to assure temperature uniformity. Multi-turn inductors are circular solenoid designs that required the hub or spindle to lift and slowly rotate at three or four locations in order to complete the temper. As in hardening, the temper installation required its own induction power supply. Thermal imaging confirmed the results (Figure 2).
Truck axle shafts are another high production component that is induction hardened and tempered. Often the axle shafts are robotically loaded in a vertical or horizontal inductor. The shaft is rotated, heated, and then shuttled to a quench position. The loading robot then moves the hardened axle shaft to another inductor, usually within the same unit, specifically designed for the tempering process. A separate induction power supply controls the input energy. The temper time can be equal to the induction hardening time added to the quenching time. This will allow for the proper input of uniform induction temper energy (Figure 3).
Today, high production automotive driveline components are routinely induction tempered. Among the examples explained are CV joints, gears, and camshafts. Monitoring of the induction energy is different compared with furnace tempering. When heating parts with complex geometries, it is necessary to focus upon where the induction energy is concentrated. Heat conduction can be carefully monitored to confirm that an overheat condition does not occur at the target temper areas. Power input, soak time, and inductor characterization control these
fundamentals.
Induction tempering is sometimes attempted using the hardening inductor. For some very low volume parts, depending upon the part geometry and induction power supply frequency, the results may be acceptable. Careful power control and timing along with thermal imaging is needed to confirm the results. Again, since tempering takes longer, output will be much slower. Experience has demonstrated that a part specific tempering inductor coupled with a dedicated induction power supply works best.
About the Author: Bill Stuehr is the founder and president of Induction Tooling, Inc, a premier heat treat inductor design and build facility. The holder and partner of many induction application patents, Bill shares his expertise and generously donates his time and facility resources to mentor young students entering the heat treat industry.
By Michael J. Zaharof, Customer Information & Marketing Manager, Inductoheat
Michael J. Zaharof Customer Information & Marketing Manager Inductoheat
Induction tempering is the process of heating a previously hardened workpiece to reduce stress, increase toughness, improve ductility, and decrease brittleness. A medium-to-high carbon steel (i.e., 1045, 1050, 4140, 5160) heated above the upper critical temperature causes a high-stress shear-like transformation into very hard and brittle martensite. This untempered martensite is generally undesirable and too brittle for postprocessing operations such as machining and can pose a concern for poor performance in high fatigue applications. Therefore, tempering is needed to reduce internal stresses, increase durability, and reduce the possibility of cracking.
In most cases, induction tempering occurs in-line and directly after the induction heating, quenching, and cool-down operations. Traditionally, workpieces are moved to a tempering spindle or separate machine after hardening. Once moved, the part is then inductively heated and often force cooled to ambient temperature. The induction tempering process itself generates temperatures on the workpiece (typically) well below the curie point (248°F-1112°F/120°C-600°C – solid blue line in Figure 1). This phenomenon is referred to as “skin effect,” where the current density is highest at the surface of the material. Therefore, a lower inverter frequency is most desirable in order to increase the electrical reference depth.
However, while most cases reflect a secondary/separate station for induction tempering, this is not always the case. Recent advancements in power supply technology permit “real-time” frequency and power adjustments. These next-generation induction power supplies have brought tremendous flexibility into the market and have allowed induction hardening and tempering to occur at the same station, on the same induction coil. Using such a novel approach with induction heating often speeds up production while reducing the number of part movements. Induction tempering is a preferred method for many manufacturers as it offers several notable advantages. In production applications, it is viewed as a fast-tempering method, as the parts are heated quickly, cooled, then moved on to the next operation, reducing potential bottlenecks.
There is no need to collect the parts, place them into batches, and wait for long subsequent processes to finish before moving them down the production line.
Figure 1. The induction tempering process itself generates temperatures on the workpiece (typically) well below the curie point. Source: Inductoheat
Induction is a clean process and does not rely on combustible gases or chemicals that may be harmful to the environment. Additionally, it is also a very efficient process as induction power supplies are only powered on when needed compared to batch processing (like those requiring an oven). Ovens must be preheated prior to use and can often stand idle for long periods between batches, as the pre-heat/cooldown cycles can be lengthy. Induction heating equipment is also physically smaller in most cases and occupies much less real estate on the manufacturing floor.
Individual part traceability and data collection are possible when utilizing induction tempering. If paired with a quality monitoring system (QAS), data can be evaluated in real-time and compared to a known good “signature” for the part during the induction tempering process. This allows precise control of the process and the ability to reject parts that deviate outside of established metrics. It is also an effective tool for detecting process issues early when a variation occurs minimizing potential scrap and helping to prevent delivery of “bad” parts to the end customer.
Induction tempering offers many advantages over other methods of tempering and is an effective choice in many applications. Due to the benefits of speed, efficiency, repeatability, and environmental cleanliness, induction technology is widely accepted and is being used throughout many industries today.
References:
[1] “In-Line Tempering on Induction Heat Treating Equipment Relieves Stresses Advantageously,” by K. Weiss: Industrial Heating, Vol. 62, No. 12, December 1995, p. 37-39.
[2] “Induction Heat Treatment: Basic Principles, Computation, Coil Construction, and Design Considerations,” by V.I. Rudnev, R.L. Cook, D.L. Loveless, and M.R. Black: Steel Heat Treatment Handbook, G.E. Totten and M.A.H. Howes (Eds.), Marcel Dekker Inc., Monticello, N.Y., 1997, p. 765-871.
About the Author: Michael Zaharof is a customer information & marketing manager at Inductoheat in Madison Heights, Michigan. He has been with the company since 2011 and has worked in the sales application, digital media, outside sales, and engineering departments. Michael has a bachelor’s degree in computer science in information system security.
By Mike Grande, Vice President of Sales, Wisconsin Oven
Mike Grande Vice President of Sales Wisconsin Oven
Tempering (also known as “drawing”) is a process whereby a metal is heated to a specific temperature, then cooled slowly to improve its properties. It is commonly performed on ferrous alloys such as steel or cast iron after quench hardening. Quenching rapidly cools the metal, but leaves it brittle and lacking toughness, which is a desirable characteristic that represents a balance of hardness and ductility. After quenching, the material is tempered to reduce the hardness to the required level and to relieve internal stresses caused by the quenching process. The resulting hardness is dependent on the metallurgy of the steel and the time and temperature of the tempering process. Tempering is performed at a temperature between approximately 255°F (125°C) and 1292°F (700°C). In general, tempering at higher temperatures results in lower hardness and increased ductility. Tempering at lower temperatures provides a harder steel that is less ductile.
Draw batch ovens: the high-powered workhorses of the tempering process Wisconsin Oven
Tempering is performed in a convection oven using a high volume of air circulating through and around the load of steel being tempered. The air is heated in a plenum separated from the load, then delivered to the load at high velocity through distribution ductwork using a recirculation blower. Since the air is the medium used to carry the heat from the source (a gas burner or heating elements) to the load, it is important that the blower recirculates a high volume of air through the heating chamber. Further, since air becomes significantly less dense at higher temperatures, the recirculated air volume must be higher for ovens operating at higher temperatures in order to provide sufficient mass (pounds or kilograms) of air to transfer the heat from the source to the load.
For example, a typical batch tempering oven designed to process a 2,000 lb. load with dimensions of 4′ x 4′ x 4′ might have a recirculation rate of 10,000 cubic feet per minute (CFM). At this airflow volume, the oven recirculating system operates at 156 air changes per minute, which means all the air passes from the recirculating blower through the heating chamber 2.6 times per second. At a temperature of 1000°F (538°C), for example, the weight of the air being recirculated is 290 lbs. (132 kg) per minute, or 17,400 lbs. (7,909 kg) per hour. It is this high volume of air that provides good heat distribution to the load being processed and ensures tight temperature uniformity within the load during tempering.
The higher the mass of air being recirculated, the tighter the temperature uniformity will be. The temperature uniformity (±10°F or 6°C, for example) defines how much the temperature is allowed to vary within the load being tempered. If the oven operates too far outside of this tolerance, the parts may not be tempered uniformly, and the hardness might vary among different parts in the same load. It is important that the temperature uniformity of a tempering oven be verified (“certified” or “qualified”) by testing, and that this is repeated periodically, as well as after any changes or repairs are made that could affect the uniformity.
About the Author: Mike Grande is the vice president of Sales at Wisconsin Oven with a bachelor’s degree in mechanical engineering and over 30 years of experience in the heat processing industry. Over that time, he has been involved with convection and infrared technologies, and several industrial oven energy efficiency design advancements.
The next type of tempering we’d like to address is rapid air tempering. This process involves “any tempering technology taking advantage of rapid heating methods combined with shortened soak times at temperature based on those predicted by use of the Larsen-Miller calculator.”1 Here “rapid heating” is defined as “any heating method that accelerates conventional furnace heating.”2
Table 1.3 Thermal profile of conventional tempering and vertical rapid air furnaces
Rapid air tempering takes advantage of the use of a higher initial heating temperature (i.e., the use of a so-called heat head) to drive heat into the part more quickly. Additionally, rapid air tempering shortens soak time at temperature (from the more conventional furnace tempering times).
The Larson-Miller calculator is used in rapid air tempering to provide a comparison of hold times at various tempering temperatures and the results of tempering time change is assumed be the same (see example below); however, the interpretation of the data and results are left to the end user.
Larson-Miller Calculator
There are various reports describing the use of the Larson-Miller equation for assessing stress-relieving and tempering process conditions.4 “The relationship between time and temperature can be described as a logarithmic function in the form of the Larson-Miller equation, which shows that the thermal effect (TE) is dependent on the temperature and the logarithm of time:
“This thermal effect is also interpreted as the tempering parameter. For example, a material that is required to be tempered at a temperature of 740°F for one hour has the same TE as a material treated at 800°F for 6 minutes (Fig. 1).”5
Figure 1.5 The “TE” is a logarithmic function of time
References:
[1] Roger Gingras, Mario Grenier, and G.E. Totten, “Rapid Stress Relief and Tempering,” Gear Solutions, May 2005, pg. 27-31.
[2] N. Fricker, K.F. Pomfret, and J.D. Waddington, Commun. 1072, Institution of Gas Engineering, 44th Annual Meeting, London, November 1978.
[3] Thomas Neumann and Kenneth Pickett, “Rapid Tempering of Automotive Axle Shafts,” Heat Treating Progress, March/April 2006, pg. 44.
[4] Lauralice C.F. Canale, Xin Yao, Jianfeng Gu, and George E. Totten, “A Historical Overview of Steel Tempering Parameters,” Int. J. Microstructure and Materials Properties, Vol. 3, Nos. 4/5, 2008, pg. 496.
[5] Roger Gingras and Mario Grenier, “Tempering Calculator,” in ASM Heat Treating Society, Heat Treating: Proceedings of the 23rd ASM Heat Treating Society Conference September 25-28, 2005, David L. Lawrence Convention Center, Pittsburgh, Pennsylvania, USA, Daniel Herring and Robert Hill, eds., Materials Park, Ohio: ASM International, 2006. pg. 147-152.
Find heat treating products and services when you search on Heat Treat Buyers Guide.com
Tim Donofrio Vice President of Sales CAN-ENG Furnaces International Limited Source: Can-Eng Furnaces International
Manufacturers of high volume, high strength precision automotive fasteners have constantly faced increased product quality standards, delivery and price pressures over the last decade. These pressures force manufacturers to seek new developments and creative methods for improving their long-term competitive positions.
This Technical Tuesday article by Tim Donofrio, vice president of sales at CAN-ENG Furnaces International Limited, will discuss two developments of mesh belt heat treatment systems – innovative tempering and dephosphating systems – that have been successfully integrated and exploited by manufacturers to maintain their competitive position.
Heat TreatToday first published this Original Content article in the Air and Atmosphere 2021 print magazine. Access the digital version of the magazine here. Contact Karen Gantzer for more information on how to contribute to future editions.
Introduction
Methods for heat treating threaded fasteners have evolved significantly over the last 20 years. Earlier versions of low-capacity shaker hearth, rotary hearth, and plate belt systems have now become extinct in favor of modern, highly efficient, continuous soft handling mesh belt heat treatment systems.
Figure 1. Mesh belt fastener heat treatment system Source: Can-Eng Furnaces International
Today’s mesh belt fastener heat treatment systems (Figure 1) integrate soft handling techniques that use bulk dunnage unloading and sophisticated metering systems. These metering systems uniformly distribute fasteners across the conveyor width, avoiding any inconsistent loading that could vary the heat-up and soak times which can impact the fasteners’ mechanical properties distribution. Fasteners are conveyed through various washing, austenitizing, quenching, tempering, and post-treatment soluble oil processes, while carefully controlling critical processing parameters that ensure compliance to DIN EN ISO-898 fastener manufacturing standard and, more recently, Automotive Industry Action Group (AIAG) CQI-9 heat treatment system assessments.
With the integration of soft handling conveyors and low inertia part transfers, modern mesh belt furnaces can significantly reduce the opportunity for part damage and the likelihood of part mixing. Further system efficiencies are realized through external furnace load preparation, allowing for precise presentation of fasteners to the conveyor belt, resulting in minimal empty belt gaps between lots for part traceability integrity.
Figure 2. Comparison of automotive fastener temper furnace thermal profiles following integration of technology advancements Source: Can-Eng Furnaces International
Temper Furnace Uniformity Improvements
With increased quality objectives placed upon fastener manufacturers, furnace systems must be more and more precise. One of the most critical steps in the fastener heat treatment process is tempering, which is performed after austenitizing and quenching. Tempering is performed to increase iron-based alloy toughness, resulting in the reduction of excess hardness that occurs after subjecting the fastener to temperatures below the critical temperature for a defined period of time required for transformation. As quality restriction limits are imposed, so is the need to reduce the product temperature variation to meet the desired metallurgical and mechanical properties distribution.
With advancements in tempering furnace design, modern high capacity (+6000 lbs/hr) mesh belt temper furnaces can achieve product temperature uniformities of ±10°F or better, which is half of the allowable temperature uniformity survey (TUS) limits set out in AIAG CQI-9 assessment at ±20°F for continuous tempering furnaces (Figure 2).
These improvements in performance are made possible through the use of modern computational fluid dynamic (CFD) modeling tools. CFD modeling gives engineers the ability to conduct higher level analysis and optimization of the furnace’s forced recirculation and heating systems, internal furnace geometry, and product-to-airflow relationship.
Today, users of modern temper furnaces enjoy design improvements that increase the overall process reliability, while also exceeding the quality expectations of their customers.
Integration of Dephosphate Removal Systems
For a long time, washers integrated into continuous heat treatment systems have been considered to have companion equipment status, with not much attention paid to their product quality and total cost of ownership. The importance of washer design is currently changing, mainly due to a desire to protect furnace internal components, increase uptime, and improve the quality of the final product.
Washer design configurations include rotary drum, belt, and batch bin systems. For the purpose of this discussion, we will focus on the continuous rotary drum and belt washers for integration with high-capacity mesh belt fastener heat treatment systems (Figure 3). Both systems, if properly designed, can provide suitable performance, with each system providing enhanced features depending upon the fastener size and performance expectations. Careful consideration should be taken during the application review process to identify the configuration that best suits the range of products that will be processed.
Most modern manufactured fasteners are mechanically formed from carbon and alloy steel coils and are normally coated with a phosphate lubricant which is applied to reduce cold forming friction and increase tooling life and part quality. It is widely understood that DIN EN ISO 898 Part 1, Class 12.9 requirements for fasteners specifies that phosphate lubricants be removed prior to heat treatment as phosphate elements can diffuse into the austenite during the heat treatment process and form delta ferrite, which can lead to fastener brittleness and crack propagation failures.
Figure 3. High-capacity in-line rotary dephosphating system Source: Can-Eng Furnaces International
A recent trend in the industry is the increase in demand for integrated inline pre-heat treatment dephosphating systems. Although not a new requirement to the North American fastener market, more demand has recently been recognized largely due to increased demand for 12.9 strength class fasteners and increased localization of European automotive fasteners (Volkswagen/Audi), who specify strength class 10.9 and greater be dephosphated before heat treatment.
To satisfy these demands, modern heat treatment manufacturers are often integrating inline continuous dephosphating capability as part of their pretreatment strategies. The aqueous chemical removal of phosphate can be by acid or alkaline, however due to the risk of hydrogen-induced brittle fracture, the alkaline processes are preferred. Pretreatment wash systems implement a multi-stage process that includes:
Oil removal & rinse
Dephosphate
Rinse 1
Rinse 2
Drying
Careful consideration must be taken to guarantee wash solutions are completely removed and fasteners are properly rinsed prior to entry into the high temperature furnace to ensure protection of the furnace internals and product quality concerns.
The fasteners are conveyed either by independent conveyors or continuous rotary drums that transport fasteners through each stage of the washing and dephosphating process. Careful consideration and control of wash solution concentration, solution circulation, product dwell time, solution temperatures, and avoidance of contamination is integrated into the equipment design as it is paramount to successful dephosphating integration and final product quality.
The effectiveness of the removal of phosphate is determined by colorimetric analysis, also known as the “blue test.” In this test, a defined quantity of product with a known surface area is immersed into a chemical solution, which will react with any residual phosphate present to form a blue color. The intensity of the color is proportional to the amount of phosphate present.
The effective removal of the phosphate layer prior to the heat treatment is critical to the final fastener quality. Modern dephosphating systems, when properly integrated with the pretreatment and heat treating system, can provide the manufacturer with improved processing flexibility and product quality performance at the lowest cost per pound to process.
About the Author: Tim Donofrio, vice president of sales at CAN-ENG Furnaces International Limited, has more than 30 years of thermal processing equipment experience. Throughout his career, he has held various positions within the custom engineered forging, commercial heat-treating services, and custom engineered heat treating equipment industries.
Graphic of Atomic Structures (Photo Source: AHT Blog post “What’s Happening to Metals During Heat Treatment”)
For this Heat TreatToday Technical Tuesday, check out this Best of the Web primer if you are looking to share a few basic pieces of heat treat info with your trainees. These heat treat fundamentals are about what happens to metals in the heat treatment process, tracing steel heat treating back to the ancient Romans in 223 B.C. — though, Encyclopedia Britannica currently places the origins in Egypt by 900 BC. Heat treatment benefits, atomic structural transformation, and hardenability are all covered here.
An excerpt: “Not every steel reacts the same. Chemical composition can vary greatly between the different grades of steel. Certain alloying elements can greatly increase the hardenability of steels such as nickel (Ni), chromium (Cr) and molybdenum (Mo). Hardenability is not how hard a material is. Hardenability directly relates to the ability of a metal to form martensite and martensistic [sic.] structure upon quenching, which points to…”
The heat treating of constant tension bands used by automakers is a complex process, and the challenge posed to a leading heat treating company by a supplier of these bands was to determine how to reduce the risk of failure due to stress corrosion cracking.
“Improving the physical characteristics of metal components often requires fine-tuned treatments that bring them to the brink of destruction. It’s a quirk of metallurgy heat treaters contend with constantly.”
Solving the problem involved, as noted in this case study from Paulo, breaking “a few cardinal rules en route.”
Last week, we ran a news release about ThermoFusion in California expanding their heat treat capabilities to include marquenching and austempering (click here to see that release). In that short article, some comments were made about the aggressiveness of various quench methods and their effect on distortion and cracking.
Joe Powell, of Akron Steel Treating Company, Integrated Heat Treating Solutions, LLC, IQDI Products, LTD., and IQ Technologies Inc, one of the heat treat industry’s foremost experts on quenching, wrote in to help educate all of us a bit more on the finer points of quenching. Below are his comments. Joe can be reached at JoePowell@akronsteeltreating.com.
Doug,
In your recent article, you stated that Marquenching and Austempering use a “less aggressive” quench cooling rate, “and reduce distortion caused by rapid temperature change (thermal shock)” which is only half true. The main mechanism that allow a molten salt quench to reduce distortion is the elimination of mixed phase cooling – there is no slow film boiling (gas) phase cooling mixed with the high-evaporative cooing phase of nucleate boiling, but only a single phase of all liquid convection cooling. It’s the non-uniformity of cooling at the surface of the part that will distort or crack the part not so much the rate of cooling.
Joe
Joseph A. Powell, President Akron Steel Treating Company
Rapid Air Tempering
