The Science of Cryogenic Processing
The following information is taken from ASM Handbook Volume 4 Heat Treating, ©1991 and last revised in 2001:
"Cold Treating and Cryogenic Treatment of Steel" from ASM Handbook Volume 4 Heat Treating, p203-206.
Cold treating of steel is widely accepted within the metallurgical profession as a supplemental treatment that can be used to enhance the transformation of austenite to martensite and to improve stress relief of castings and machined parts. Common practice identifies –84 °C (–120 °F) as the optimum temperature for cold treatment. There is evidence, however, that cryogenic treatment of steel, in which material is brought to a temperature of the order of –190 °C (–310 °F), improves certain properties beyond the improvement attained at cold-treatment temperatures. This discussion will explain the practices employed in the cold treatment of steel and will present some of the experimental results of using cryogenic treatment to enhance steel properties.
Cold Treating of Steel
Cold treatment of steel consists of exposing the ferrous material to subzero temperatures to either impart or enhance specific conditions or properties of the material. Increased strength, greater dimensional or micro-structural stability, improved wear resistance, and relief of residual stress are among the benefits of the cold treatment of steel. Generally, 1 hour of cold treatment for each inch of cross section is adequate to achieve the desired results.
All hardened steels are improved by a proper subzero treatment to the extent that there will be less tendency to develop gindi9ng crack and therefore they will grind much more easily after the elimination of the retained austenite and the untempered martensite.
Hardening and Retained Austenite
Whenever hardening is to be done during heat treating, complete transformation from austenite to martensite is generally desired prior to tempering. From a practical standpoint, however, conditions vary widely, and 100% transformation rarely, if ever, occurs. Cold treating may be useful in many instances for improving the percentage of transformation and thus for enhancing properties.
During hardening, martensite develops as a continuous process from start (Ms) to finish (Mf) through the martensite-formation range. Except in a few highly alloyed steels, martensite starts to form at well about room temperature. In many instances, transformation is essentially complete at room temperature. Retained austenite tends to be present in varying amounts, however, and when considered excessive for a particular application, must be transformed to martensite and then tempered.
Cold Treating vs. Tempering
Immediate cold treating without delays at room temperature or at other temperatures during quenching offers the best opportunity for maximum transformation to martensite. In some instances, however, there is a risk that this will cause cracking of parts. Therefore, it is important to ensure that the grade of steel and the product design will tolerate immediate cold treating rather than immediate tempering. Some steels must be transferred to a tempering furnace when they are still warm to the touch to minimize chances of cracking. Design features such as sharp corners and abrupt changes in section create stress concentrations and promote cracking.
In most instances, cold treating is not done before tempering. In several types of of industrial applications, tempering is followed by deep freezing and retempering without delay. For example, such parts as gages, machineways, arbors, mandrils, cylinders, pistons, and ball and roller bearings are treated in this manner for dimensional stability. Multiple freeze-draw cycles are used for critical applications.
Cold treating is also used to improve wear resistance in such materials as tools steels, high-carbon martensitic stainless steels, and carburized-alloy steels for applications in which the presence of retained austenite may result in excessive wear. Transformation in service may cause cracking and/or dimensional changes that can promote failure. In some instances, more than 50% retained austenite has been observed. In such cases, no delay in tempering after cold treatment is permitted, or cracking can develop readily.
In some applications in which explicit amounts of retained austenite are considered beneficial, cold treatment might be detrimental. Moreover, multiple tempering, rather than alternate freeze-temper cycling, is generally more practical for transforming austenite in high speed and high-carbon/high-chromium steels.
Hardness Testing. Lower than expected HRC values may indicate excessive retained
austenite. Significant increases in these readings as a result of cold treatment
indicate conversion from austenite to martensite. Superficial hardness reading,
such as HR15N, can show even more significant changes.
Precipitation-Hardening Steels. Specifications for precipitation-hardening steels may include a mandatory deep freeze after solution treatment and prior to aging.
Shrink Fits. Cooling the inner member of a complex part to below ambient temperature can be a useful way of providing an interference fit. Care must be taken, however, to avoid brittle cracking that may develop when the inner member is made of heat-treated steel with high amounts of retained austenite, which converts to martensite on subzero cooling.
Residual stresses often contribute to part failure and frequently are the result of temperature changes that produce thermal expansion and phase changes, and consequently, volume changes.
Under normal conditions, temperature gradients produce non-uniform dimensional and volume changes. In castings for example, compressive stresses develop ion lower-volume areas, which cool first, and tensile stresses develop in areas of greater volume, which are last to cool. Shear stresses develop between the two areas. Even in large castings and machine parts of relatively uniform thickness. The surface cools first and the core last. In such cases, stresses develop as a result of the phase (volume) change between those layers that transform first and the center portion, which transforms last.
When both volume and phase changes occur in pieces of uneven cross section, normal contractions due to cooling are opposed by transformation expansion. The resulting residual stresses will remain until a means of relief is applied. This type of stress develops most frequently in steels during quenching. The surface becomes martensitic before the interior does. Although the inner austenite can be strained to match this surface change, subsequent interior expansions place the surface martensite under tension when the inner austenite transforms. Cracks in high-carbon steels arise from such stresses.
The use of cold treating has proved beneficial in stress relief of castings and machined parts of even or non-uniform cross section.
Advantages of Cold Treating
Unlike heat treating, which requires that temperature be precisely controlled to avoid reversal, successful transformation through cold treating depends only on the attainment of the minimum low temperature and is not affected by lower temperatures. As long as the material is chilled to –84 °C (–120 °F), transformation will occur; additional chilling will not cause reversal.
Time at Temperature. After thorough chilling, additional exposure has no adverse effect. When heat is used, holding time and temperature are critical. In cold treatment, materials of different compositions and of different configurations may be chilled at the same time, even through each may have a different high-temperature transformation point. Moreover, the warm-up rate of a chilled material is not critical as long as uniformity is maintained and gross temperature-gradient variations are avoided.
The cooling rate of a heated piece, however, has a definite influence on the end product. Formation of matensite during solution heat treating assumes immediate quenching to ensure that austenitic decomposition will not result in the formation of bainite and cementite. In large pieces comprising both thick and thin sections, not all areas will cool at the same rate. As a result, surface areas and thin sections may be highly martensitic, and the slower-cooling core may contain as much as 30 to 50% retained austenite. In addition to incomplete transformation, subsequent natural aging induces stress and also results in additional growth after machining.
Aside from transformation, no other metallurgical change takes place as a result of chilling. The surface of the material needs no additional treatment. The use of heat frequently causes scale and other surface deformations that must be removed.
Cryogenic Treatment of Steels
The value of cryogenic treatment of steel and other materials has been debated for many years; even serious metallurgical professionals have serious reservations about its value. Notwithstanding these concerns, it is the intent of this discussion to review some of the current literature and practices of those who believe that cryogenic treatment enhances steel properties.
A typical treatment consists of a slow cool-down rate (2.5 °C/min equivalent to 4.5 °F/min) from ambient temperature to the temperature of liquid nitrogen. When the material reaches approximately 80 K (–315 °F), it is soaked for an appropriate time (generally 24 hours). Then the part is removed from the liquid nitrogen and allowed to warm at room temperature in ambient air. By conducting the cool-down cycle in gaseous nitrogen, temperature can be controlled accurately and thermal shock to the material is avoided. Single-cycle tempering is usually performed after cryogenic treatment to improve impact resistance, although double or triple tempering cycles are sometimes used.
Kinetics of Cryogenic Treatment
There are several theories concerning reasons for the effects of cryogenic treatment. One theory involves the more nearly complete transformation of retained austenite into martensite. This theory has been verified by x-ray diffraction measurements. Another theory is based on the strengthening of the material brought about by the precipitation of submicroscopic carbides as a result of the cryogenic treatment. Allied with this is the reduction in internal stresses in the martensite that happens when the submicroscopic carbide precipitation occurs. A reduiction in microcracking tendencies resulting from reduced internal stresses is also suggested as a reason for improved properties.
The absence of a clear-cut understanding of the mechanism(s) by which cryogenic treatment improves performance has hampered its widespread acceptance by metallurgists. Nonetheless, it is important to review the studies done to determine the effects of cryogenic treatment on the performance of steel in a variety of applications.
Case Studies of Cryogenically Treated Steels
Resistance to abrasive wear was investigated in a parametric study. Five tool steels were tested after conventional heat treatment, after cold treatment at –84 °C (–120 °F), and after being cryogenically treated at –190 °C (–310 °F). Figure 1 and Table 1 show the results of these abrasive wear tests. Cold treatment at –84 °C (–120 °F) improved the wear resistance by 18 to 104%, but the cryogenic treatment results show 104 to 560% improvement.
Corrosion resistance to water-saturated hydrogen sulfide gas was determioned on conventionally processes and cryogenically treated stainless steel and tool steel samples. The results are shown in Table 2. The decrease in corrosion rate ranged from a modest 1.035 to a significant 1.786. The mechanism suggested by these boundaries, which limits the diffusion of hydrogen sulfide into the metal. Type 316, an austenitic stainless steel, is susceptible to intergranular corrosion, and apparently refinement of the grain boundaries did not have as much of an effect on the corrosion rate.
These two scientifically designed studies serve to highlight the effects of cryogenic treatment. Many other case studies with varying results appear in technical journals and engineering publications. The variability of the results listed in these articles does not disprove the effectiveness of the cryogenic treatment. The need for study of any potential application should be apparent, and a careful technical and cost-effectiveness analysis should be made before embarking on such a program.
Equipment for Cryogenic Treatment
The heat-exchanger system passes liquid nitrogen through a heat exchanger, and the exhaust gas from the unit is piped into the main gaseous-nitrogen header line. The chamber atmosphere is drawn over the heat-exchanger coils by a fan. In some versions of the system, the cooling is boosted by spraying liquid nitrogen directly into the chamber.
The direct spray system sprays liquid nitrogen directly into the chamber, while
a fan circulates the gases over the work. In this system, the spent gas cannot
be recovered for use as a furnace atmosphere. The equipment design does not
permit the liquid nitrogen to come into direct contact with the work, thereby
reducing the probability of thermal shock.