Cold and Cryogenic Treatment of Steel

Revised by F. Diekman, Controlled Thermal Processing

COLD TREATNG OF STEEL is widely During hardening, martensite develops as a rather than alternate freeze-temper cycling, accepted within the metallurgical profession as continuous process from start (Ms) to finish generally is more practical for transforming a supplemental treatment that can be used to (Mf) through the martensite formation range. retained austenite in high-speed and high-car- enhance the transformation of austenite to mar- Except in a few highly alloyed steels, martens- bon/high-chromium steels. tensite and to improve stress relief of castings ite starts to form at well above room tem- Hardness Testing. Lower-than-expected and machined parts. Common practice identi- perature. In many instances, transformation Rockwell C hardness (HRC) readings may indi- fies –84 C (–120 F) as the optimal tempera- essentially is complete at room temperature. cate excessive retained austenite. Significant ture for cold treatment. There is evidence, Retained austenite tends to be present in vary- increases in these readings as a result of cold however, that cryogenic treatment of steel (also ing amounts, however, and when considered treatment indicate conversion of austenite to referred to as deep cryogenic treatment, or excessive for a particular application, must be martensite. Superficial hardness readings, such DCT), in which material is brought to a temper- transformed to martensite and then tempered. as HR15N, can show even more significant ature on the order of –184 C (–300 F), Cold Treating versus Tempering. Immedi- changes. improves certain properties beyond the ate cold treating without delays at room temper- Precipitation-Hardening Steels. Specifica- improvement attained at cold treatment tem- ature or at other temperatures during quenching tions for precipitation-hardening steels may peratures. This discussion explains the practices offers the best opportunity for maximum trans- include a mandatory deep freeze after solution employed in the cold treatment of steel and pre- formation to martensite. In some instances, treatment and prior to aging. sents some of the results of using cryogenic however, there is a risk that this will cause Shrink Fits. Cooling the inner member of a treatment to enhance steel properties. cracking of parts. Therefore, it is important to complex part to below ambient temperature ensure that the grade of steel and the product can be a useful way of providing an interfer- design will tolerate immediate cold treating ence fit. Care must be taken, however, to avoid rather than immediate tempering. Some steels the brittle cracking that may develop when the Cold Treatment of Steel must be transferred to a tempering furnace inner member is made of heat treated steel with when still warm to the touch to minimize the high amounts of retained austenite, which con- Cold treatment of steel consists of exposing likelihood of cracking. Design features such as verts to martensite on subzero cooling. the ferrous material to subzero temperatures to sharp corners and abrupt changes in section cre- either impart or enhance specific conditions or ate stress concentrations and promote cracking. Stress Relief properties of the material. Increased strength, In most instances, cold treating is not done greater dimensional or microstructural stability, before tempering. In several types of industrial Residual stresses often contribute to part fail- improved wear resistance, and relief of residual applications, tempering is followed by deep ure and frequently are the result of temperature stress are among the benefits of the cold treat- freezing and retempering without delay. For changes that produce thermal expansion and ment of steel. Generally, one hour of cold treat- example, such parts as gages, machineways, phase changes, and consequently, volume ment for each 2.54 cm (1 in.) of cross section is arbors, mandrils, cylinders, pistons, and ball changes. adequate to achieve the desired results. and roller bearings are treated in this manner Under normal conditions, temperature gradi- All hardened steels are improved by a proper for dimensional stability. Multiple freeze-draw ents produce nonuniform dimensional and vol- subzero treatment to the extent that there will cycles are used for critical applications. ume changes. In castings, for example, be less tendency to develop grinding cracks Cold treating also is used to improve wear compressive stresses develop in lower-volume and therefore they will grind much more easily resistance in such materials as tool steels, areas, which cool first, and tensile stresses after the elimination of the retained austenite high-carbon martensitic stainless steels, and develop in areas of greater volume, which cool and the untempered martensite. carburized alloy steels for applications in which last. Shear stresses develop between the two the presence of retained austenite may result in areas. Even in large castings and machined Hardening and Retained Austenite excessive wear. Transformation in service may parts of relatively uniform thickness, the sur- cause cracking and/or dimensional changes that face cools first and the core last. In such cases, Whenever hardening is to be done during can promote failure. In some instances, more stresses develop as a result of the phase (vol- heat treating, complete transformation from than 50% retained austenite has been observed. ume) change between those layers that trans- austenite to martensite generally is desired prior In such cases, no delay in tempering after cold form first and the center portion, which to tempering. From a practical standpoint, how- treatment is permitted, or cracking can develop transforms last. ever, conditions vary widely, and 100% trans- readily. When both volume and phase changes occur formation rarely, if ever, occurs. Cold treating Process Limitations. In some applications in in pieces of uneven cross section, normal con- may be useful in many instances for improving which explicit amounts of retained austenite are tractions due to cooling are opposed by trans- the percentage of transformation and thus for considered beneficial, cold treating might be formation expansion. The resulting residual enhancing properties. detrimental. Moreover, multiple tempering, stresses will remain until a means of relief is Cold and Cryogenic Treatment of Steel / 383 applied. This type of stress develops most fre- and also results in additional growth after cemented carbides, and some plastics (Ref 3– quently in steels during quenching. The surface machining. Aside from transformation, no other 5). Use of the process on metals other than steel becomes martensitic before the interior does. metallurgical change takes place as a result of produces similar affects as with steel. Results of Although the inner austenite can be strained to chilling. The surface of the material needs no the process include relief of residual stresses match this surface change, subsequent interior additional treatment. The use of heat frequently (Ref 6); reduced retained austenite (in hardened expansions place the surface martensite under causes scale and other surface deformations that steel); the precipitation of fine carbides in fer- tension when the inner austenite transforms. must be removed. rous metals (Ref 7, 8); and increased wear Cracks in high-carbon steels arise from such resistance, fatigue life, hardness, dimensional stresses. stability, thermal and electrical conductivity, The use of cold treating has proved beneficial Equipment for Cold Treating and corrosion resistance (Ref 9). in stress relief of castings and machined parts of What are cryogenic temperatures? The scien- even or nonuniform cross section. Features of A simple home-type deep freezer can be used tific community generally defines cryogenic the treatment include: for transformation of austenite to martensite. temperatures as temperatures below 150 C Temperature will be approximately –18 C (238 F, or 123 K) (Ref 10). This, admittedly, Transformation of all layers is accomplished (0 F). In some instances, hardness tests can is an artificial upper limit. Temperatures used when the material reaches –84 C (–120 F). be used to determine if this type of cold treating presently in cryogenic treatment are generally The increase in volume of the outer martens- will be helpful. Dry ice placed on top of the –185 C (–300 F, or 89 K). These are tempera- ite is counteracted somewhat by the initial work in a closed, insulated container also is tures easily reached with liquid nitrogen. Some contraction due to chilling. commonly used for cold treating. The dry ice work is being done with liquid helium at tem- Rewarm time is controlled more easily than surface temperature is –78 C (–109 F), but peratures down to –268 C (–450 F, or approx- cooling time, allowing equipment flexibility. the chamber temperature normally is approxi- imately 6 K). The expansion of the inner core due to trans- mately –60 C (–75 F). Cryogenic treatment was made easier to formation is balanced somewhat by the Mechanical refrigeration units with circulat- achieve and more successful by the develop- expansion of the outer shell. ing air at approximately –87 C (–125 F) are ment of microprocessor-based temperature con- The chilled parts are handled more easily. commercially available. A typical unit has these trols in the 1960s and 1970s and by the The surface is unaffected by low dimensions and operational features: pioneering research by Randall Barron of temperature. 3 3 Louisiana Tech University. Research into the Parts that contain various alloying elements Chamber volume, up to 2.7 m (95 ft ) process has been accelerating. The Cryogenic and that are of different sizes and weights Temperature range, 5 to –95 C (40 to Society of America maintains a database of can be chilled simultaneously. –140 F) peer reviewed research papers (Ref 11). Load capacity, 11.3 to 163 kg/h (25 to 360 lb/h) Advantages of Cold Treating Thermal capacity, up to 8870 kJ/h Cryogenic Treatment Cycles (8400 Btu/h) Unlike heat treating, which requires that tem- Although liquid nitrogen at –195 C (–320 F) One distinct difference from cold treatment perature be controlled precisely to avoid rever- may be employed, it is used less frequently is that cryogenic processing requires a slow sal, successful transformation through cold than any of the previous methods because of drop in temperature in order to reap all benefits treating depends only on the attainment of the its cost. of the process. The ramp down in temperature minimum low temperature and is not affected usually is on the order of 0.25 to 0.5 C/min by lower temperatures. As long as the material (32.5 to 32.9 F/min). The object of this slow is chilled to –84 C (–120 F), transformation ramp down is to avoid high-temperature gradi- will occur; additional chilling will not cause Cryogenic Treatment of Steels ents in the material that can create harmful reversal. stresses, and to allow time for the crystal lattice Time at Temperature. After thorough chill- Cryogenic treatment, also referred to as cryo- structure to accommodate the changes that are ing, additional exposure has no adverse effect. genic processing, deep cryogenic processing, occurring. In heat treating, holding time and temperature deep cryogenic treatment (DCT), cryogenic Typical cryogenic treatment consists of a are critical. In cold treatment, materials of dif- tempering, and deep cryogenic tempering, is a slow cool-down from ambient temperature to ferent compositions and of different configurations distinct process that uses extreme cold to mod- approximately –193 C (–315 F), where it is may be chilled at the same time, even though ify the performance of materials. (The use of held for an appropriate time. Hold periods each may have a different high-temperature trans- the word tempering is a misnomer, because this range from 4 to 48 h depending on the material. formation point. Moreover, the warm-up rate of is not a tempering process.) At the end of the hold period, the material is a chilled material is not critical as long as The process is differentiated from cold treat- brought back to ambient temperature at a rate uniformity is maintained and large temperature ment by the use of lower temperatures, the of approximately 2.5 C/min (36.5 F/min). gradient variations are avoided. presence of distinct time/temperature profiles, The temperature-time plot for this cryogenic The cooling rate of a heated piece, however, and its application to materials other than steel treatment cycle is shown in Fig. 1. By conduct- has a definite influence on the end product. For- (Ref 1). Cryogenic treatment has been in exis- ing the cool-down cycle in gaseous nitrogen, mation of martensite during solution heat treat- tence only since the late 1930s, making it a rel- temperature can be controlled accurately, ing assumes immediate quenching to ensure atively new and emerging process. The late and thermal shock to the material is avoided. that austenitic decomposition will not result in development of the process is mainly due to Single-cycle tempering usually is performed the formation of bainite and cementite. In large the fact that cryogenic temperatures have been after cryogenic treatment to improve impact pieces comprising both thick and thin sections, available in useful commercial quantities only resistance, although double or triple tempering not all areas will cool at the same rate. As a since the early 1900s. cycles sometimes are used. result, surface areas and thin sections may be Cryogenic treatment can provide wear-resis- It is worthy of note that most time-tempera- highly martensitic, and the slower-cooling core tance increases several times those created by ture profiles have been empirically developed. may contain as much as 30 to 50% retained cold treatment with hardened steels (Ref 2). Some research is being done to optimize the austenite. In addition to incomplete transforma- The process is not confined to hardened steels, profiles for individual steels. For instance, some tion, subsequent natural aging induces stress but also shows results with most metals, research indicates the holding time for AISI 384 / Steel Heat Treatment Processes

now is turning to determine why the results are seen and how to maximize those results.

Uses of Deep Cryogenic Treatment

Deep cryogenic treatment is used in many ways to reduce wear. It is in common use to control distortion of metal objects, modify the vibrational characteristics of metals, increase fatigue life, reduce abrasive wear, and reduce electrical resistance. It is safe to say the applications for this process are extremely broad. The process is in commercial use for high- speed steel (HSS) and carbide cutting tools, knives, blanking tools, forming tools, and more. Research as far back as 1973 indicates that deep cryogenic processing results in over three times life increase in end mills, 82 times life improvement in punches, over two times the life in thread dies, six times the life in copper resistance electrodes, six times the life in progressive dies, and over four to five times the life in broaches (Ref 2). Research estimates a 50% reduction in tooling costs with H13 and M2 steels that have been deep cryogenically treated (Ref 14). Fig. 1 Plot of temperature vs. time for the cryogenic treatment cycle. Tempering may or may not be necessary, Other studies have shown that DCT increases depending on the material treated. Some materials require multiple tempering cycles. Some companies are abrasion resistance of cast iron. Cast iron brake now treating materials down to –268 C (–450 F, or 6 K) rotors consistently show a three to five times life increase when tested to SAE2707 brake dynamometers (Ref 15). This has been validated against real-world experience in passenger cars, racing cars, trucks, and mining vehicles. ¼ ðÞ = T42 steel should not be longer than 8 h Nd N exp Ed kT Deep cryogenic treatment also has been (Ref 12). In contrast, research indicates that proven to create a phase change in cemented the hold time should be 36 h for AISI D2 (Ref where Nd is the number of defects present, N is carbide (Ref 5). A study by the National Aero- 13). This indicates there is much research to the total number of atomic sites, Ed is the acti- nautics and Space Administration (NASA) be done to optimize the process for all materi- vation energy needed to form the defect, k is proved the release of residual stresses in welded als. Research into the optimal ramp down the Boltzmann constant, and T is the absolute aluminum (Ref 6), and other studies prove times, hold times, and ramp up times would temperature. Reducing the temperature at a increases in fatigue life in steel springs (Ref maximize the effect of the process and mini- suitably slow rate drives the point defects 16) and in load capacity of gears (Ref 17). mize the time needed to accomplish its results. out of the structure to the grain boundaries. In Deep cryogenic treatment is used in the auto- There are several theories behind the effects other words, the solubility of vacancies and motive racing industry to increase life in virtually of cryogenic treatment. One theory involves other point defects in the matrix drops. This every engine component. Drive line components the more nearly complete transformation of could account for some of the effects seen such as transmission and differential gears, sus- retained austenite into martensite. This in DCT. pension springs, torsion bars, axles, suspension theory has been verified by x-ray diffraction In the past, the absence of a clear-cut under- members, and, of course, brakes are treated. measurements. Another theory is based on standing of the mechanism by which cryogenic Deep cryogenic treatment also is in commer- the strengthening of the material brought treatment improves performance had hampered cial use by musical instrument makers. Yamaha about by precipitation of submicroscopic car- its widespread acceptance by metallurgists. Wind Instruments has done extensive testing of bides as a result of the cryogenic treatment Some confusion has arisen from the fact that DCT and offers the process on its wind instru- (Ref 7, 8). Allied with this is the reduction there are a number of different effects on ments (Ref 18). There is much activity in the in internal stresses in the martensite that hap- metals, many of which cannot be seen in simple high-performance stereo industry in treating pens when the submicroscopic carbide precip- microstructural examination of the material vacuum tubes, wire, power cords, vacuum itation occurs. A reduction in microcracking with a light microscope. The lack of easily tubes, transformers, connectors, and more. tendencies resulting from reduced internal detected microstructural changes led many to stresses also is suggested as a reason for discount the process. Another reason was the improved properties. Studies also show reduc- generally accepted belief that nothing happens Equipment for Cryogenic Treatment tion in residual stresses. Another theory postu- to solid objects as the temperature drops. lates that the extreme cold reduces the Extreme cold has been available on earth only All cryogenic treatment equipment is com- free energy of the crystal structure and creates for about 100 years. Understanding of materials prised of a thermally insulated container and a more orderly structure. Another area to con- science developed with the observation that some means of extracting the latent heat of sider is the basic effect of cold on the crystal heat changes properties. Much of the early the payload to reach the desired low tempera- structure of metals. Point defects in the crystal research was centered around determining ture. In most cases the insulation is a solid structure are temperature dependent. Lower- whether or not cryogenic treatment actually material that contains small closed cells of ing the temperature of the crystal structure provided the advantages claimed. Because the trapped still air. The thermal conductivity of will cause the number of point defects in the early research and actual industry usage have such insulation essentially is that of still, non- crystal structure to change according to: proven the validity of the process, research convecting air, assuming that the solid material Cold and Cryogenic Treatment of Steel / 385 that encloses the air pockets is of thin cross sec- the cooled component while the relatively ACKNOWLEDGMENT tion and low conductivity. \Examples are poly- warm interior does not shrink. Tensile stress urethane foam, aerogel, and expanded glass induced this way can lead to cracking or the ini- Article revised and updated from E.A. Carl- foam. Fifteen centimeters (6 in.) of any of these tiation of residual stress, especially at sharp son, Cold Treating and Cryogenic Treatment will conduct approximately (15 Btu/h.ft2) edges. Reaching cryogenic temperatures by of Steel, Heat Treating, Vol 4, ASM Handbook, across a temperature differential of 204 C mechanical refrigeration for industrial size pay- ASM International, 1991, p 203–206. (400 F), which exists between the interior of loads requires multistage refrigeration. These a refrigerator at –195 C (–320 F) and ambient are very expensive machines to build and temperature of 26 C (80 F). maintain. REFERENCES These solid insulating materials are relatively Fortunately, liquid nitrogen is abundant, inexpensive and, in the case of foamed-in-place readily available, and relatively inexpensive. It 1. R.F. Barron, A Study of the Effects of polyurethane, can readily fill irregularly shaped has a boiling point of –196 C (–321 F) and Cryogenic Treatment on Tool Steel cavities. They all suffer from one important a heat of vaporization of approximately Properties, drawback: temperature cycling establishes a 150 Btu/liter. It is produced in huge industrial 2. R.F. Barron, Yes, Cryogenic Treatments temperature gradient across the insulating slab gas production facilities and delivered to the Can Save You Money!, Fall Corrugated that results in differential contraction in the facility where the expansion and phase change Containers Conference (Denver, CO), material. Repeated temperature cycles ulti- occurs, free of the capital and maintenance Technical Association of the Pulp and mately result in fatigue cracking of the insula- expense demanded by in-house mechanical Paper Industry, 1973, p 35–40 tion. Energy expenditure to sustain the refrigerators. 3. S. Kalia, Cryogenic Processing: A Study of temperature difference goes up, and tempera- Two other approaches have been tried but Materials at Low Temperatures, J. 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