Heat treatment techniques overview

4D high-pressure gas quenching offers numerous advantages versus press quenching.

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Editor’s note> This is the fifth in a five-part series.

In this fifth and final installment of my series on heat-treating techniques, I will discuss the pros and cons of 4D high-pressure gas quenching.

The basis for any form of heat treatment in general is to improve material properties and specifically to improve material performance at the surface. Unfortunately, most heat-treatment techniques generally produce, at some level, unpredictable dimensional changes, or heat-treatment distortion. Material (dimensional) distortion is the undesired trade-off between the development of proper mechanical properties and the necessity of rapidly quenching the material from elevated temperatures in a quenching media (i.e. brine, water, polymer, oil, gas, molten salt, etc.) to create the variation in internal microstructure that causes the bulk material properties that are trying to be achieved. Due to this compromise, users have been developing, researching, and implementing techniques, technologies, and equipment in an effort to reduce part distortion. Why, once a component is hardened, it becomes very expensive to remove material to recreate the required finished part form, dimensions, and within the required tolerances.

Quenching, at a high-level, is the rapid cooling of the surface of the gear which causes a large thermal gradient between the surface and the core. If the quench is not uniform, the resulting thermal stresses cause non-uniform transformation of the microstructure, which in-turn generates non-uniform material properties throughout the part (gear) and non-uniform stresses both from surface to core and from one aspect of the gear to another (e.g. heat-treat distortion induced runout, etc.).

When one looks at the bearing and gearing industries, materials typically are hardened via austenitizing and quenching. Not only do these components require high hardness and wear/corrosion resistance, they also require high dimensional precision to tight tolerances as well as repeatability of results. One of the most common ways to reduce material distortion when quenching is a method by which a heated component is placed in a special fixture and a steady force is applied to the component, which allows the part to resist material deformation when the quenching media is applied. This method of quenching is known as “press quenching” and requires specialized equipment, manual or robotic handling, custom die sets, and high maintenance as well as being operator dependent to achieve consistent results.

Quenching of steel is a metallurgical process which is done after a material is heated through its phase transformation into the austenitic range and then rapidly cooled. This is done to cause austenite to complete a phase transformation into martensite. During this quenching process there is a large temperature gradient throughout the physical body of the gear. If this quenching process is not completely uniform, it will induce non-uniform thermal stresses and non-uniform transformation of the microstructure. This will in turn cause non-uniform, and in some cases, large non-uniform deformation in relation to the pre-heat-treated geometry. The preceding comments apply to an individual gear or part. The adverse effect of non-uniformity during quenching is amplified in batch quenching due to the addition of the non-uniformity of the quenching stream that penetrates the batch. The collection of gears, their orientation, and collective flow path definition, causes even more uneven temperature distribution through the pack, and thus the effect of cooling, on each individual gear.

Manufacturers over the years have attempted numerous methods to reduce distortion after quenching as it is difficult and costly to remove excess material once the component has been hardened. One of the most popular methods of distortion control is the press quench technique, which does not employ batch quenching, rather it quenches one part at a time. Press quenching offers very attractive results when it comes to distortion control; however, there are unattractive aspects of the process including safety concerns (handling of hot components, open flame), environmental (oil), washing (oil removal), etc. which require special handling and equipment to quench a component after heat treatment.   

It is well known that machining after heat treatment is one of the most costly and difficult tasks to complete in the entire manufacturing life cycle. This is why an extreme amount of engineering is devoted to the prevention of distortion of a component to ease the post heat-treatment machining operations. With the ever-prevailing desire to lower the cost of raw materials and still maintain proper mechanical performance, extreme amounts of pressure are applied to the heat-treatment process to bring up the quality level of the low-cost steel. When using these low-quality steels, they are prone to high levels of distortion during the quenching process, such that they distort more than the allowable amount and either become too challenging to hard machine or are not able to be used all together. ~4 percent of the price for a hardened component is attributed to the removal of post heat-treatment material so that it meets the finished size requirements. According to Seco/Warwick, it is possible that in some cases as much as 30 percent of gear manufacturing cost is related to hard machining. When users can control distortion, they lower the overall cost of the component.

Press quenching is a process used when batch quenching provides too much distortion where a parts movement after quench exceeds the additional material left for post heat treatment removal. These parts consist of case- and through-hardened components (gear, rings, etc.). When components are sensitive to hardening distortion, press quenching offers a versatile way to harden thin cross sections and large geometric parts. They quench one part at a time, providing dimensional stability, controlled distortion, and are very repeatable.

One of the critical aspects of press quenching is the design and construction of the special dies. These dies are built such that they mechanically align and retain a hot plasticized part with pressure as the die restricts the desired features from distorting when quenching through its phase transformation. Oil flow across the part surface is also important in achieving the desired hardness and microstructure, and as such, the dies must be produced to balance the need for die contact and proper oil flow over with the part.

One of the latest advancements of distortion control is 4D high-pressure gas quenching (HPGQ). Both it and press quenching quench a single part at a time, but the 4D HPGQ process does not subject a part to any clamping forces or issues associated with liquid quenching inconsistencies. The 4D HPGQ process results in every single part being heated and quenched identically at surprisingly low gas pressures, thus producing extremely accurate dimensional variation with highly repeatable results. 4D HPGQ systems are easily integrated into current manufacturing environments and the process is a revolutionary advance in quenching technology which has been shown to reduce or even eliminate the need for expensive and difficult post-hardening manufacturing processes.

When discussing quenching, furnace systems will use various quenching processes / media to achieve the desired metallurgical properties. Based on a material’s ability to be hardened, more aggressive cooling rates may be needed to reach the required hardness as required for the components end use. The 4D HPGQ process from Seco / Warwick is a new technology that allows for significant improvements to the quenching process with the main focus on reduction of distortion. Distortion reduction is achieved mainly from the use of a high-pressure gas quenching system installed in the quenching / unloading chamber. The quenching platform uses a proprietary cooling manifold and chamber arrangement that surrounds the part during the quenching process. This approach ensures there is a uniform flow of cooling gas across the part geometry (top, bottom, and side).

In order to achieve this additional physical component when quenching, a gear is placed in the quenching chamber, the proprietary cooling manifold surrounds the part, and, finally, the support table rotates the component while the N2 cooling gas is directed preferentially over the surface of the gear such that the cooling rate is very uniform. The ‘secret to success’ is in the design of the dies, the holding, clamping, and positioning components of the system. This 4D is combined with both the 3D quenching approach and with part rotation. With 4D HPGQ the best, most uniform quench rate and induced stress is achieved, along with extremely consistent or part-to-part results.

There is a great deal more detail to be discussed and the technology is still as much in development as it is in optimization of deployment. To that end, there is far more investigation, information exchange, and real-world use to be explored. I encourage anyone interested in this process to contact Tom Hart or Maciej Korecki.

Acknowledgments

I would like to thank Tom Hart, product manager — Vacuum Furnaces, Seco/Vacuum, and Maciej Korecki, vice president of the Vacuum Furnace Segment in the Seco/Warwick Group, for their assistance and contributions to this article. Look for an article from Hart to be presented at the 2023 FTM conference in October. I would also like to thank all those who offered comments that have helped make my contributed articles better. I will be taking some time off, but look forward to writing again in 2024. 

References

  1. “4D High Pressure Gas Quenching — A Leap in Performance vs. Press Quenching”, by Thomas Hart, Seco/Vacuum and Dr. Maciej Korecki, Seco/Warwick S.A., AGMA 19FTM14.
  2. “Advanced Distortion Control for Heat Treated Components,” by Tom Hart of Seco/Vacuum Technologies, LLC and Maciej Korecki of Seco/Warwick S.A.
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Dr. William Mark McVea, P.E., is President and Principal Engineer of KBE+, Inc. which develops complete powertrains for automotive and off-highway vehicles. He is the Principal Engineer with Kinatech, a joint venture with Gear Motions / Nixon Gear. He has published extensively and holds or is listed as co-inventor on numerous patents related to mechanical power transmissions. Mark, a licensed Professional Engineer, has a B.S. in Mechanical Engineering from the Rochester Institute of Technology, a Ph.D. in Design Engineering from Purdue University.