cryogenics a brief history of cryogenic tempering process

A Brief of Cryogenic Processing Excerpt from Battelle Report

BACKGROUND

From time to time over the past 40 years or so, reports have appeared, both in the United States and Europe, of substantial benefits that can be realized by treating steel tools at a low temperature, usually near that of liquid nitrogen, -320 F (-196C). Factors of 10 or more. More recently within the United States, claims for improvements have been expanded to include copper, some high temperature alloys, carbides, plastics, and nylon (ref. A1:".

Despite the fact that several investigations conducted in the 1940’s (Ref. A2, A3) failed to confirm any substantial benefits of cold treatments when used as an integral part of the thermal treatment cycle, reports of large beneficial effects in production operations continue to appear. Today, the emphasis is on using cryogenic treatment AFTER the conventional heat treatment has been completed. Also, cryogenic treatment usually is accomplished by the tool user rather than by the tool supplier.

In the United States, the treatment that has evolved is the following (See Figure A1): (1) slowly cool (i.e., without thermal shock) the already heat treated part to approximately –320F (-196C), (2) hold at low temperature for about a day or two, (3) reheat without thermal shock to room temperature, and (4) reheat to a moderately elevated temperature, 300 to 600F (150 to 315C) for about 1 hour, to "temper" the part for the stated purpose of reducing its brittleness.

In the Soviet Union, on the other hand, cryogenic treatment employs shock cooling to obtain essentially the same benefits for hardened steels that are claimed in the U.S., namely, improved tool life (Ref. A4). The steel part is immersed in liquid nitrogen, held there only long enough to achieve temperature equilibrium, and then is allowed to return to room temperature in ambient air.

Yet another version of the process has been practiced in the United Kingdom; parts are pre-cooled in a wire basket suspended above a liquid nitrogen bath prior to complete immersion in the bath for up to 10 hours (ref. A5). Claims of property improvements in the United Kingdom are similar to those in the United States.

The origin of cryogenic processing as practiced today almost certainly can be traced back 50 years to the occasional use of low-temperature treatments in the hardening of steel. Very briefly, hardening of steel involves heating to an elevated temperature where a crystalline phase termed austenite is stable. If the austenite is cooled at a sufficient rate, it will transform to a much harder and stronger phase known as martensite.

In many steels, the transformation of austenite to martensite is complete when the part reaches room temperature. (I,e other steels, however, including many tool steels, some of the softer austenite phase is retained). Subsequent cooling to a lower temperature can cause additional transformation of the soft austenite to hard martensite. However, it is possible also to transform all (or nearly all) of the retained austenite in the steel by appropriate elevated-temperature tempering treatments that carry the added benefit of reducing the brittleness of the martensite. Thus, cold treatments are not the only way to achieve reduction of the retained austenite content.

In spite of the fact that cold treatments are not essential in transforming austenite to martensite, they have been used for many years in some applications to ensure that the retained austenite is at a minimum level. This is especially true of high-precision parts in which superior dimensional stability, is required, because transformation in service of even a small amount of retained austenite can produce damaging dimensional changes.

In addition to promoting dimensional stability, cold treatments were reported to have beneficial effects on tool performance as far back as 1937. Gylyaev, a Soviet investigator, found that a cryogenic treatment (shock cooling) of high-speed steel permitted the use of higher cutting speeds in certain instances (Ref. A6). In 1942, researchers at the Massachusetts Institute of Technology found that a certain favorable combination of properties could be achieved only by including a cold treatment in the processing cycle of a tool steel (Ref. A7). Several years later, moderate to large improvements in tool steel performance were reported when cold treatments were used (Ref. A8, A9).

The scientific basis for several of the details of cryogenic processing as practiced within the United States, is still a matter of some conjecture. The purpose of slow cooling is simply to avoid cracking from thermal shock, though the risk of this occurrence presumably is not great, judging from the Soviet’s use of shock cooling by immersing directly into liquid nitrogen.

The use of liquid nitrogen temperature, rather than a lower or higher temperature, probably is dictated by two considerations: (1) the stabilization of retained ausenite against transformation to martensite when held for extended times at room temperature (see Ref. A7), requires greater sub-cooling to restart the transformation to martensite than is available from dry ice, a cooling (2) it is easier and less costly to achieve liquid nitrogen temperature than it is to achieve much lower temperatures, such as liquid hydrogen, or liquid helium temperature. The tool steel performance results shown in Figure 1 of the proposal indicate that cooling to liquid nitrogen temperature was significantly more effective than was cooling to dry ice temperature.

Finally, the purpose of long holding times is less clear. Transformation of retained austenite at low temperatures in tool steels generally is believed to be dependent only on temperature, not on time. Thus, merely reaching a suitably low temperature for an instant would produce the same effect as holding for several days. However, a study conducted at Louisiana Technical University (Ref. A10) indicated that holding at –310F (-190C) for longer times (20 hours, compared with 8, 10, 12, and 16 hours) produced greater improvement in wear resistance. That result probably accounts for the use of holding times of 1 or 2 days at the cryogenic temperature.

Up to this point, the discussion has centered on retained austenite in hardened steels and how low-temperature treatments can affect it. In addition, suggestions have been made by several investigators that cryogenic treatments can produce not only transformation of retained austenite to martensite, but also can produce metallurgical changes within the martensite. The change most often described is a dispersion of extremely fine carbides within the martensite phase, which was not present prior to cryogenic treatment. The reported large improvements in tool life usually are attributed to this dispersion of carbides in conjunction with retained austenite transformation.

It is difficult to rationalize the reported precipitation of carbides at very low temperatures because of the extreme sluggishness of diffusion-controlled reactions at such temperature. Nonetheless, some evidence from microscopic examination and carbide extraction studies has been presented to support this contention (Ref. A11). Additionally, a Soviet investigator, in a paper containing few details, reported, on the basis of volume changes during cooling and subsequent re-heating of tool steels that metallurgical changes were occurring within the martensite phase (Ref. A12).

While transformation of retained austenite and precipitation of extremely fine carbides within the martensite phase can, if they occur, help to explain performance improvements in tool steels, no such rationalization of improved performance of copper welding electrodes is available.

One possible mechanism for improved performance in those copper alloys that contain dispersed particles of a hard phase in a relatively soft copper matrix involves development of short range internal stresses or alteration of previously existing internal stresses. As the two-phase structure cools to cryogenic temperature, tensile stresses will develop in the copper immediately adjacent to the particles because of the greater thermal contraction of the copper. These stresses will develop even in the absence of thermal shock. If the stresses reach sufficiently high levels, localized plastic deformation can occur, which would not be reversed by re-warming to room temperature. This highly localized "cold working" possibly could lead to improved deformation resistance in service. If this is, in fact, the mechanism (there currently is no supporting evidence), long holding times would offer little benefit over very short holding times.

It should be added that the mechanism described here for copper also could be operative in tool steels that contain dispersed hard particles. Thus, internal stresses distributed in the conventionally treated steel microstructures might be altered permanently by gradual cooling to a cryogenic temperature.

At least three companies in the United States currently are offering cryogenic processing services. One of these has been in business for about 20 years and has issued royalty fee licenses to vendors in France and Canada. The process as currently practiced involves treatment of finished tools and parts that have been through their normal heat treating cycle. Thus, it is not an integral part of the heat treating cycle, but is an additional treatment. For steel tools that were originally heat treated weeks, months, or years ago, any retained austenite present will have experienced stabilization.

In summary, cold treatment originally was used on hardened steels to promote transformation of retained austenite. The reasons were to minimize dimensional changes and cracking that could occur if the austenite were to transform in service. Over the years, additional benefits of cold treating of hardened steels have been reported, namely, improved life of cutting and forming tools. These latter claims have not been as widely accepted as the former because they greatly exceed the expectations of many metallurgists and are not obtained consistently. For similar reasons, claims of improved life of copper resistance welding electrodes are not widely accepted. Nonetheless, despite the lack of widespread acceptance and the lack of understanding of how cold treatment can change properties, the process continues to be used for the purpose of improving tool performance.

CLAIMS REPORTED FOR CRYOGENIC PROCESSING

Although claims are being made for cryogenic processing in an increasing number of areas, the majority of claims apply to hardened-steel cutting and forming tools. Examples of tools that have been subjected to cryogenic treatment in the United States, the United Kingdom, and the Soviet Union include: drills, broaches, milling cutters, taps and dies, saw blades, files, cutting pliers, scissors, knives, trimmers, slitters, woodworking tools, chain saw blades, gear cutters, drawing dies, stamping dies, and punch and die sets.

The extent of reported service life improvement varies over wide ranges, from perhaps a 10 percent increase to as much as 20 times improvement. A typical figure would be a doubling of tool life from cryogenic processing. Additionally, it is claimed that cryogenically treated cutting tools, unlike coated cutting tools, continue to outperform untreated tools after re-sharpening and that less material needs to be removed in the re-sharpening process. Furthermore, the cost of cryogenic treatment is said to be less than the cost of coating, which is currently a popular method for improving tool life.

Most of the claims made for cryogenic processing have not included estimates for monetary benefits. One exception is the estimate of a major aerospace company that was based on in-plant experience with the use of cryogenically treated M7 high-speed steel drill bits for drilling holes in titanium alloys. The estimated annual savings was $350,000 based on $1,000,000 annual expenditures for drill bits (Ref. A13). Part of the cost savings comes from reduced purchases of new tools, but a significant part also can come from reduced downtime and reduced reconditioning costs. The latter cost saving item is especially important in complex cutting tools.

A major concern in cryogenic processing is the inconsistency in benefits observed among different types of tools and different steels. For example, in a comparison study to the one reported in Reference A13, in which cryogenic treatment produced significant benefits in M7 high-speed steel drill bits, no benefits can be illustrated further by a survey conducted in the Soviet Union in the late 1970’s (Ref. A4). That survey, conducted among 204 manufacturing plants that used cryogenic treatments (shock cooling) on steel tools, found the following:

Results in Cryogenic Treatment   Percent of Plants

Life increased 2x or greater (up to 10x)                                      50
Life increased in some cases but was Not unaffected in others      18
Life increased in some cases but decreased In others                    3
No Effect                                                                                  24
Negative results                                                                          5

It is seen that about 70 percent of the plants observed tool life improvements; however, the improvements were inconsistent and, further, it was reported that improvements did not result for all types of tools.

A second survey of Soviet industry was far less complimentary to shock cooling as a method for improving tool life. Smol’nikov and Kossovich (Ref. A14) concluded on the basis of a survey of 47 companies that tool life was increased, only about 10 to 40 percent by shock cooling. Furthermore, those improvements occurred only when the tools were improperly heat treated initially.

Although no surveys have been conducted among users of cryogenic processing in the United States or the United Kingdom, where thermal shock is avoided and where cryogenic exposure times are longer, a mixture of results similar to those found in the Soviet Union would be expected.

Sorting through all of the claims for steel tools, it appears certain that tool life can be improved significantly by cryogenic processing. However, efficient utilization of the process to produce optimum results consistently points to the need for a clearer understanding of what is happening to the steel as it is exposed to very low temperatures and returned to room temperature.

Another more recent claim of cryogenic processing that was mentioned earlier related to copper alloy electrodes for resistance welding. Typically, 3 to 5 times improvements in life between re-dressings, are being claimed by the U.S. vendors. No reasonable explanation of why cold treatment should be beneficial to copper alloys has yet been offered.

REFERENCES IN APPENDIX

A1 Edward R. Busch, President, Materials Improvement, Inc., Detroit, MI, Private communication, December 1986.

A2 O. Zmeskal, written discussion, Trans. ASM, Vol.34, 1945, pp.294-307.

A3 A. H. d’Arcambal, oral discussion, Trans ASM, Vol. 34, 1945, pp. 307-308

A4 E.S. Zhmud’ "Improved Tool Life After Shock Cooling", Metals Science and Heat Treatment (English Translation of a Russian journal, Oct. 1980, pp. 701-703.

A5 J. Taylor, "Cold Plunge Gives Tools an Extra Lease of Life", Metalworking Production, May 1978.

A6 A. Gulyaev, "Improved Heat Treatment of High Speed Steel", Metallurg, Vol. 12, No. 12, 1937, pp. 65-70.

A7 P. Gordon and M. Cohen, "The Transformation of Retained Austenite in High Speed Steel at Subatmospheric Temperatures", Trans. ASM, Vol. 30, 1942, pp.569-588.

A8 S.W. DePoy, "Subzero Treatment of High Speed Steel", The Iron Age, April 13, 1994, p.52.

A9 B. Berlien, "Subzero Hardening Cycles", Steel, Jan. 10, 1944.

A10 R.F. Barron, "A Study of the Effects of Cryogenic Treatment on Tool Steel Properties", Louisiana Technical University Report, August 30, 1073.

A11 K.M. Smith, "An Investigation into the Influence of Cryogenic Treatment on the Wear Resistance of Tool Steels", Report presented for Bachelor of Science degree, Lanchester (England) Polytechnic, May 1979.

A12 A.N. Popandopulo and L.T. Zhukova, "Transformations in High Speed Steels During Cold Treatment", Metals Science and Heat Treatment (English translation of a Russian journal), Oct. 1980, pp.708-710.

A13 J. A. Boldt, Northrop Corporation, "Palmdale Production Environment Aircraft Length Drill Cost Evaluation", 3x Corporation Cryogenic Seminar, March 17, 1986, Phoenix, Arizona.

A14 E.A. Smol’nikov and G.A. Kossovich, "Cold Treatment of Cutting Tools", Metals Science and Heat Treatment, (English translation of a Russian journal), Oct. 1980, pp.704-705