Eric Ford, Vice President of Sales and Marketing, Graphite Metallizing Corp.
In this Technical Tuesday original article, read how an automotive manufacturing plant is able to solve high-temperature bearing failures by upgrading to bearings that use a self-lubricating material that can operate in extreme temperatures. Author, Eric Ford, Vice President of Sales and Marketing at Graphite Metallizing Corp., shares how these bearings decreased the need for unplanned and costly maintenance of parts in the case study that follows.
An automotive engine manufacturing plant in the Midwest upgraded the bearings in their gas nitriding ovens after encountering numerous failures with rolling element bearings.
An example of a flame curtain in an industrial setting (Photo source: Graphite Metallizing Corp)
This large manufacturing plant runs automated gas nitriding furnaces for treating their various engine components. A flame curtain, at the entrance to the furnace, produces a vertical stream of combustion products to minimize both the infiltration of room air into the furnace chamber and the disruption of the furnace atmosphere inside. The bearings for the conveyor rollers, closest to the flame curtain, are subjected to intense heat for a short period of time, about 30 seconds, which is enough to cook the grease in the bearings and degrade their performance.
In many automotive plants, these machines are running 24/7 for up to six months at a time. Any breakdown of this equipment has serious consequences in terms of profitability and delivery schedules.
Excessive Downtime
The plant was having trouble with the repeated failure of the rolling element bearings, located just prior to the furnace’s flame curtain. These bearings were failing within six months, causing unscheduled maintenance and downtime. Though there was an automatic grease system, temperatures of approximately 300°F resulted in the grease being cooked away rapidly, resulting in conveyor roller seizure.
When the bearings seized, production on the line stopped. The furnaces then needed time to cool sufficiently for maintenance personnel to be able to access and replace the bearings. Starting the system up again wasted yet more production time.
The conveyor transporting the parts has bearings to support the load and convey the product through the furnace. (photo source: Graphite Metallizing Corp)
It was taking three people about four to six hours to replace the bearings and start the furnace again each time the bearings failed. These unscheduled shutdowns cost tens of thousands of dollars in production loss, labor, and materials. In addition to the expense of the downtime, there was also the added safety risk of handling parts when unloading the furnace and performing maintenance on the equipment, which was still hot.
Successful Trials
At a heat trade show during this time, the production manager of the plant learned about Graphalloy bushing materials; Graphalloy is the name for a specific family of proprietary graphite/metal alloys developed by Graphite Metallizing Corp of Yonkers, NY. Its featured qualities include non-galling, corrosion resistant, dimensionally stable, and can operate at temperatures from cryogenic to higher than 1000°F (538°C). These materials work very well in severe environments and services due to their self-lubricating properties – no grease or oil is required. There are more than 100 grades of these high-temperature bushings which are designed for specific conditions.
Flange Bush 845 (photo source: Graphite Metallizing Corp)
Soon after the show, company representatives went to the plant and proposed a simple drop-in replacement for the current greased bearing flange block assemblies. The production manager agreed to test a few of the company's 4-bolt flange blocks with copper bushings, and they were installed a few weeks later.
The target was a difficult one: The production supervisor said that a doubling of the lifespan of the roller element bearings would enable the plant to stick to its twice-annual scheduled maintenance intervals. By achieving this goal, unscheduled maintenance shutdowns would be avoided.
During the one-year trial period, the high-temperature bushings were a success. Based on the positive result, the production manager installed additional bushing assemblies of this brand type during subsequent scheduled maintenance dates, until all furnaces had been converted to new self-lubricating bushings.
The original bearing assemblies, installed over six years ago, have been operating without a single failure or showing any appreciable wear.
By replacing the metal bearings with newer graphite bushings, the automotive company eliminated at least two unscheduled shutdowns and dozens of hours of maintenance work per year. According to the production manager, using this has saved this automotive giant hundreds of thousands of dollars to date.
Automotive part designs and heat treating processes have undergone many changes over the years, especially the powertrain. By looking back at the progress of these changes, we can learn more about emerging trends in automotive heat treating today.
In this Heat Treat Today Technical Tuesday feature,Bill Disler, president and CEO of AFC-Holcroft, brings his familiarity with big atmosphere carburizing systems and LPC automotive cell carburizing systems and looks at how the evolution of equipment and process requests says a lot about the trends we see today in automotive heat treating.
Although many components undergo heat treatment processes, the powertrain—specifically, gears— typically requires more carburizing time than other automotive parts. Not surprisingly, the powertrain has also seen many changes in heat treatment trends.
Not only have powertrain designs gone through tremendous transformations but so has the equipment being used to process those evolved components. Having spent years on the supplier side of atmosphere furnaces, vacuum carburizing, and gas quench as well as induction systems, I find it interesting to look back at some of the drivers that have helped morph this industry’s heat treat needs.
Traditional Continuous Atmosphere Furnace
Large atmosphere pusher furnaces produced nearly all of the powertrain gears 20+ years ago. Today, cellular low-pressure carburizing (LPC) and gas quench systems carry the load, although the results have not been cost saving. Moving from high volume gas heated carburizing equipment to small batch carburizing in electrically heated furnaces did not reduce utility costs per part; instead, other areas adjusted to compensate. Eliminating the expense of hard grinding transmission gears was an acceptable rationale for this increase in both capital expense and operating costs. Eventually, streamlining the overall gear manufacturing process, combined with locating heat treat within machining lines, produced positive measurable results. Plant traffic decreased, minimizing safety risks. Cooler and cleaner furnace systems were designed. And installations were made easier. Many agreed the changes were justified.
Integrated Vacuum Heat Treat Cells
As we look back, many of these drivers for change proved valid. Others, not so much. In most cases, consumer preference for quiet powertrains necessitates hard grinding of gears. Green is in and talk of the absolute need for zero intergranular oxidation (IGO) in carburized gears has slowed. LPC/Gas post quenched parts are perceived as cleaner and leaner; however, it is often difficult to differentiate green parts from processed parts, so it has become a best practice to add part marking after carburizing and hardening to avoid even the remote risk of sending soft parts down the line to the next stage of manufacturing. Shot peening is still common for strength reasons. The ability to nest large cellular LPC systems within machining has been a success, but rarely are the installations as quick and easy as promised.
Hybrid Furnace Concepts
Conventional atmosphere furnace technology has advanced as well, although at a slower pace, in step with a renewed interest in energy efficiency, particularly in the U.S. where gas is cheap and electric is not. Combustion systems operate cleaner and at much higher efficiency than in the past. Having said that, it is curious how little interest end users have in trading cost-saving gas-heated systems for the easier to install, neater looking electric heating options. In addition, it is no longer common to use water for cooling conventional atmosphere furnace systems as end users do not want to deal with the cost and complications that accompany this option. The market is polarized over this. LPC systems rely on large water volumes for cooling, and they are small batch, electrically heated systems. At the same time, gas quench systems consume huge quantities of water and require giant 300 HP plus motors that are tough to manage in plant power systems.
Flexible and Re-deployable Heat Treat Systems
It is my observation that the automotive market is anticipating the next iteration of heat treat equipment. One type of process or equipment style will not fit all needs, yet all hope for the perfect single part flow solution—an elusive dream due to physics. The cost/time equation still does not balance, and carburizing offers the benefits many manufacturers are looking for, despite the desire to design the process out of practice. Many automotive transmission parts that were originally processed in LPC and gas quenched now use gas nitriding instead, even though gas nitriding is another long process, and nitriding introduces ammonia back into the process—something most automotive plants are not enthusiastic to have in their plants. Two steps forward and one step back.
Repackaging Continuous Furnace Systems
With the widening range of processes and solutions under exploration, as well as ever changing powertrain systems designed to accommodate supplemental electric motors, lighter weights, smaller cars, and larger SUVs, all we can be certain of is ongoing change. I believe that we have witnessed major adjustments in automotive heat treat processing as the pendulum has swung from big, multi-row atmosphere pushers with salt or oil quench to electric-heated cellular LPC and gas quench units. One surprising result has been the resurgence of salt quenching, which controls distortion of high-pressure gas at a much lower cost with less complexity. Salt, like gas, is a single-phase quench media: It does not boil in these processes like oil does, and it can be used at temperatures that support martensitic quench with far less thermal shock and much higher heat transfer than the options. Older processes carry the baggage of tarnished past reputations, but I no longer count them out. Today’s automation, process control technology, and innovation can provide the foundation for brand new concepts, repackaging of older ideas, and hybrids of multiple technologies. Together, these create building blocks that heat treat equipment suppliers will use to meet changing trends in automotive carburizing and heat treatment. It will be interesting to be involved in the journey as these changes take place.
About the Author: Bill Disler is president and CEO of AFC-Holcroft, part of the Aichelin Group located in Vienna, Austria. He is a member of the Board of Trustees -Metal Treating Institute (MTI), and a member of the Board of Advisors at Lawrence Technical University, College of Engineering in Southfield, Michigan. This article originally appeared in Heat Treat Today’sJune 2019 Automotive print edition.
Many heat treat processes require protective or process gases. These gases often require careful monitoring. One of the protective and/or process gases used in many heat treat applications is an endothermic atmosphere which is made up largely of CO, CO2, H2, and N2. This article is about the creation and proper monitoring of endothermic atmospheres.
In an atmosphere furnace, the proper mix of these gases can help facilitate changes in the metal such as proper hardness and strength, resistance to temperature, or improved tensile strength to mention a few. Without careful control of temperature, time and atmosphere, metals can experience unwanted changes in properties such as hydrogen embrittlement, surface bluing, soot formation, oxidation, and decarburization. With such critical outcomes in the balance, it is necessary to control the endothermic gas.
An excerpt:
“In order for the required metal treatment to be a success, you must control and monitor the gas composition with extreme care. The concentrations of gases, CO₂, H₂O, CH₄, N₂, H₂ and CO, that make up the endothermic gas atmosphere should be measured in order to aid the prevention of unwanted reactions and ensure that the endogas generator and the furnace are operating normally.”
During the day-to-day operation of heat treat departments, many habits are formed and procedures followed that sometimes are done simply because that’s the way they’ve always been done. One of the great benefits of having a community of heat treaters is to challenge those habits and look at new ways of doing things. Heat TreatToday‘s 101 Heat TreatTips, tips and tricks that come from some of the industry’s foremost experts, were initially published in the FNA 2018 Special Print Edition, as a way to make the benefits of that community available to as many people as possible. This special edition is available in a digital format here.
Today, we begin an intermittent series of Technical Tuesday posts of the 101 tips by category, starting with Atmosphere Control.
Atmosphere Control
Heat TreatTip 5
Out of Control Carburizing? Try This 11-Step Test
When your carburizing atmosphere cannot be controlled, perform this test:
Empty the furnace of all work.
Heat to 1700°F (926°C).
Allow endo gas to continue.
Disable the CP setpoint control loop.
Set generator DP to +35°F (1.7°C).
Run a shim test.
The CP should settle out near 0.4% CP.
If CP settles out substantially lower and the CO2 and DP higher, there’s an oxidation leak, either air, water or CO2 from a leaking radiant tube.
If the leak is small the CP loop will compensate, resulting in more enriching gas usage than normal.
Sometimes but not always a leaking radiant tube can be found by isolating each tube.
To try and find a leaking radiant tube, not only the gas must be shut off but combustion air as well.
So you just ran a batch and the parts are bad. Now what? According to Jim Oakes at Super Systems Inc., here is a good checklist to use to start isolating the problem. While not exhaustive, this list can at least take you through a progression of steps to help start identifying the culprit.
Step 1: Review the process data for abnormalities. Did the setpoint for temperature and atmosphere get set properly? Does the process chart show good control of the temperature and atmosphere? Was the time at heat correct? Was the quench and temper processes run properly?
Step 2: Check the generator to make sure it was pumping out the right atmosphere.
Step 3: Check the furnace atmosphere. Even if the generator is working, there may be leaks in the furnace.
Step 4: Check carbon controller to make sure it matches furnace atmosphere reading. Verify probe accuracy and adjust carbon controller.
Step 5: Do probe troubleshooting. And if all else fails . . .
Step 6: Replace the probe or call Super Systems for help.
Many factors can contribute to why parts are not meeting the correct hardness readings. According to Super Systems Inc., here is a quick checklist of how to start narrowing down the culprit:
Review process data for abnormalities: The first thing to do is make sure the parts were exposed to the right recipe. Check the recorders to make sure the temperature profile and atmosphere composition were correct. Make sure all fans and baffles were working correctly. Determine if any zones were out of scope and that quench times were acceptable. If any red flags appear, hunt down the culprit to see if it may have contributed to soft parts.
Check the generator. Next, check the generator to make sure it is producing the gas composition desired for the process. If available, check the recorders to make sure the gas composition was on target. If not, check the generator inputs and then the internal workings of the generator.
Check the furnace atmosphere. If the generator appears to be working correctly, the next step would be to check the furnace itself for atmosphere leaks. Depending on what type of furnace you have, common leak points will vary; for continuous furnaces, common leak points are a door, fan, T/C, or atmosphere inlet seals. Other sources of atmosphere contamination may be leaking water cooling lines in water-cooled jackets or water-cooled bearings. More than likely, if the generator is providing the correct atmosphere but parts are still soft, there is a leak into the furnace. This will often be accompanied by discolored parts.
Check carbon controller to make sure it matches furnace atmosphere reading (verify probe accuracy and adjust carbon controller). This can be done using a number of different methods: dew point, shim stock, carbon bar, 3 gas analysis, coil (resistance), etc. Each of these methods provides a verification of the furnace atmosphere which can be compared to the reading on the carbon controller. If the atmosphere on the carbon controller is higher than the reading on the alternate atmosphere check, that would indicate the amount of carbon available to the parts is not as perceived. The COF/PF on the carbon controller should be modified to adjust the carbon controller reading to the appropriate carbon atmosphere. If the reading is way off, it may require the probe to be replaced.
Configuring your atmosphere controller to ensure the correct carbon potential readings can sometimes be tricky. We suggest you double check your atmosphere control settings to make sure they are set up correctly. Before making a change to the carbon controller, make sure the atmosphere that the carbon probe and carbon controller are reading is matching up to an alternate method of atmosphere. This can be done using a number of different methods: dew point, shim stock, carbon bar, 3 gas analysis, coil (resistance), etc. Each of these methods provides a verification of the furnace atmosphere which can be compared to the reading on the carbon controller. The COF/PF on the carbon controller should be modified to adjust the carbon controller reading to the appropriate carbon atmosphere.
It is important to make sure that the alternate method of verifying atmosphere is done properly (sampling ports, time for atmosphere exposure, sample prep, etc).
The calculation of carbon in the atmosphere using a carbon/oxygen probe is based on the output millivolts — created based on the partial pressure of oxygen in the reference air versus partial pressure of oxygen in the furnace, the temperature of the furnace, and a calculation factor referred to as COF (CO Factor), PF (Process Factor), or Gas Factor.
The carbon controller can be modified so the COF/PF value can be changed to match up with the alternate reading. A furnace calculator on the SSI website or mobile app can help determine what these settings should be. It is important to note that you should not change these values to the point where you are masking another issue such as a bad probe or a furnace/generator issue.
Again, if the reading is way off (a setting of a COF below 130, for example), it may require the probe to be replaced.
If you’re having atmosphere problems with a furnace that has been operating normally for some time, avoid the temptation to remove the carbon probe. There are several tests you can run on nearly all carbon probes while the probe is still in the furnace, at temperature, in a reducing atmosphere. Super Systems Inc. provides an 11-step diagnostic procedure in a white paper on their website, in a paper titled, “Carbon Sensor Troubleshooting” by Stephen Thompson.
Atmosphere furnace pressure should be only slightly above ambient. The range should be between 0.25 – 0.35 inches water column. Higher pressures in multiple zone pusher furnaces will cause carbon control issues. High pressures in batch furnaces will cause high swings when doors and elevators move.
Wisdom dictates a trust-but-verify approach to your endothermic generator. Although your generator is supposed to crank out a consistent endo atmosphere, we suggest periodically verifying the integrity of that atmosphere with a dewpoint analyzer or a 3-gas analyzer. Generator control systems provide control of air gas ratio and possibly a trim system, used to maintain a dew point that could be rich (too much gas) or lean (too much air). The dew point range could typically be between 30°F and 50°F. Flowmeters are provided to maintain a base ratio (2.7 : 1) for the air/gas mixture supplied to a retort filled with nickel-coated catalyst. The gas is then passed through an air cooler (some older systems used water) to freeze the reaction so the gas can be transported through a header system to furnaces. The ratio at which the gas is generated offers a dew point that can be measured. The makeup of the endothermic gas provided by a generator is typically 40% hydrogen, 40% nitrogen, and 20% carbon monoxide. Maintaining these percentages will result in a carburizing atmosphere that is conducive to best carburizing practices.
Non-dispersive infrared analyzer (NDIR) systems are invaluable when trying to troubleshoot generator issues. The analyzer will typically measure CO, CO2, and CH4. As mentioned earlier, if we know that 20% CO is being generated, we can cross check the air/gas ratio and sticking flow meters, or determine that an adjustment of the air and/or gas ratio is required. The measurement for indication of sooted or nickel depleted catalyst can also be achieved by using an analyzer. If the indicated measurement of CH4 is higher than .5%, a burnout of the catalyst is required, using the manufacturer’s required procedures. If after a burnout the CH4 level is still high, the catalyst may need to be replaced altogether.
If you have any questions, feel free to contact the expert who submitted the Tip or contact Heat TreatToday directly. If you have a heat treat tip that you’d like to share, please send to the editor, and we’ll put it in the queue for our next Heat TreatTips issue.
Effective furnace scheduling requires the inclusion of several key elements.
“Customer” Demands: Manufacturers with in-house heat treat departments have internal customers who, like customers the world over have one thing in common, they want to provide parts to you tomorrow and have them processed and ready yesterday. These internal customers cause frustration and angst but their work is what pays the bills.
Product & Process Variables: There are numerous product and furnace process variables all of which must be considered when scheduling. Common variables include:
Material grade and chemistry
Atmosphere carbon potential
Hardening and tempering furnace temperatures
Ammonia addition for carbonitriding and the purge time required when finished
Belt speeds
Cycle times
Variable quench programs
Process changes are necessary but minimizing the degree of variation between consecutive product runs is the goal. The more significant the change, the longer the gap time required to allow the furnace to stabilize with the new furnace parameters. Gap time is an unrecoverable cost – wasted time and money.
Sample Furnace Scheduling Sheet
Sample Furnace Scheduling Sheet
Quality issues can also be caused by not allowing sufficient time between significant process parameter changes. If the proper gap time is not provided, the end of one lot or the beginning of the next may experience quality issues.
Each heat treat department must determine the balance of efficiency and customer service that works best for their operation.
Developing a close working partnership with your internal customers is beneficial for both parties. Heat treating is typically at or near the end of the manufacturing cycle and all the lead time has been utilized by the previous steps. Teach them the basics of your operation and explain the ways they can help you provide better service and delivery. By providing as much information as possible about their delivery requirements, you can schedule to meet their demands.
Rush jobs are the nature of the business and will always be with us. They are inevitable but they can be reduced. I know of one customer who provided parts at 3:00 PM and asked for impossible results for the next morning. After numerous conversations with the heat treat department, the part supplier finally understood the heat treat process and now allows one, two, or even 3 days for results. Encourage part suppliers to give you next week’s Hot List at the end of the current week.
Heat treat scheduling is never easy but it can be improved to help your operation.
About Young Metallurgical Consulting
Young Metallurgical Consulting works with in-house heat treat departments to teach the day-to-day processes necessary to manage and improve their area of operation. In-house heat treaters will learn the aspects of heat treating that are not taught in a classroom and can only be gained through direct, hands-on experience. Contact John Young at john@youngmetallurgicalconsulting.com.
“Customer” Demands: Manufacturers with in-house heat treat departments have internal customers who, like customers the world over have one thing in common, they want to provide parts to you tomorrow and have them processed and ready yesterday. These internal customers cause frustration and angst but their work is what pays the bills.
