Sub-zero treatment of steel can alter the phase structure to help achieve improvements in quality and finished parts performance. The hardness, wear resistance and stability of tool steels and components depend upon the underlying composition and molecular structure of the alloy.
Batch & Continuous
Hardened steel alloys and parts that resist mechanical wear and thermal cycling are in high demand for diverse industries, from automotive and aerospace, to metalworking and many other manufacturing processes. Extended service life can be a critical design factor in transportation applications. For example, hardened steel alloys can reduce maintenance cycles and downtime in manufacturing. Extended wear also is important for the tool-and-die industry, where a single die may be used to form tens of millions of injection-molded parts or tens of thousands of metal parts.
The need for sub-zero treatment will likely grow along with the demand for high performance materials for precision parts, tools and a growing number of metal parts and assemblies. In some cases, this treatment also can help simplify the production of existing high quality, tempered-steel components to make manufacturers more competitive. Sub-zero treatment also can be used to improve the strength and life of non-ferrous metal parts and components, including age-hardened aluminum alloys.
This well-known and proven technique normally involves lowering the temperature to between -94 and -166°F (-70 and -110°C). However, the temperature can be dropped even lower to further improve the performance of the steel — even to temperatures approaching that of liquid nitrogen at -320°F (-196°C). These ultra-cold temperatures are particularly beneficial for high alloy steels such as tool steels.
Hardening and Strength
Many minor metals, including chromium, tungsten, molybdenum, titanium, niobium, tantalum and zirconium, are used as alloys because they form strong carbides that increase the hardness and strength of steel. Such steels often are used to make high speed steel and hot-work tool steel.
Austenite is the soft phase in steels that is stabilized at high temperatures. By rapid cooling, this phase can be converted to the hard martensitic phase, supersaturated with carbon. For many tool steels, conversion to a full martensitic structure requires sub-zero treatment after quenching.
Sub-zero treatment promotes the transformation of retained austenite to martensite. Cryogenic treatment with nitrogen at temperatures below -184°F (-120°C) creates conditions for the subsequent nucleation of very fine carbides in higher alloy steels. Material performance often can be further improved with even colder temperatures.
Figure 1 shows the low level of retained austenite after heat treatment and quenching and after cryogenic treatment at -297°F (-183°C). Figure 1 compares samples heat treated at two different austenitizing temperatures. A much lower volume of retained austenite was achieved in the sample that was heat treated a full 144°F (80°C) cooler than the 2012°F (1100°C) sample. Results after cryotreatment were also more stable.
Austenite has a face-centered cubic (FCC) structure that changes into a body-centered tetragonal (BCT) crystal structure (martensite) with tempering. It is a diffusionless transformation that results in a rapid rearrangement of atomic positions. Martensite crystals are needle-like, and the shear strain determines the shape of the plates. Tempering allows the supersaturated carbon to form transition carbides, which relieves micro-stress in the martensitic matrix and prevents cracking of the part.
Sub-zero treatment offers a high degree of control compared to quench hardening. While it is possible to convert the retained austenite in a tool steel to martensite through multiple tempering stages, sub-zero treatments in the -130 to -238°F (-90 to -150°C) range can reduce the number of tempers (figure 2). This can represent a significant time and cost savings in the hardening process.
When martensite undergoes a deep sub-zero treatment, dislocations form nucleation sites where fine carbides precipitate after the steel is tempered. The formation of coherent transition carbides throughout the hardened steel can result in significant improvements in wear life (table 1).
Table 2 shows tool-life improvements using sub-zero treatment for various types of tooling. Progressive dies, for example, achieved an improved wear ratio of 6.25 times, with an average life of 250,000 hits after sub-zero treatment vs. 40,000 hits before this treatment.
Some hardened precision-steel components such as gears, bearings and rollers are subject to high mechanical stresses throughout their service life. Under operating conditions, small dimensional changes in the component can occur over time, and these can be critical to performance of parts with extremely precise tolerances. Deep sub-zero treatments can help eliminate a cause of such distortions at the outset to greatly improve dimensional stability. At room temperature, retained austenite is unstable and slowly decomposes over time. Sub-zero treatment can transform virtually any retained austenite present in the microstructure to martensite.
Depending on the alloy, cold-processing conditions and desired design goals, repeated tempering may not be necessary to produce a fully stable structure ready for machining. However, when precision tolerances are required, multiple cycles of cold treatment and tempering may be required to achieve the highest levels of microstructural stability.
Cold Treatment and Cryotreatment
Cold treatment is applied after quenching and before tempering. The final processing step must always be a temper to transform any newly formed, untempered martensite. Cold treatment changes retained austenite to the martensitic structure, which, upon reheating, results in the precipitation of a finer distribution of carbides.
Cold treatment in the -94 to -184°F (-70 to -120°C) range, and typically to a greater effect, cryogenic (below -184°F) or deep cryogenic treatment (-310°F [-190°C]) can significantly improve material performance. Table 1 compares the percent increase in wear resistance after cold treatment at about -112°F (-80°C) and after deep cryogenic treatment at -310°F on several types of steel alloys: high-carbon/chromium steel (D2), tungsten high-speed steel (T1), oil-hardened cold-work die steel (O1) and bearing steel (52100). Some alloys respond better than others and at different treatment temperatures. For another high speed tungsten steel (T2), the percentage improvement after cold treatment at -110°F (-79°C) was only about half that of T1 steel. Cold treatment temperatures can be achieved through direct or indirect cooling methods. Liquid nitrogen (LIN) is used in both direct and indirect treatment processes. In the indirect cooling method, the cryogen chills a refrigerant used for a mechanical freezer. However, direct cooling is the most efficient means to achieve the lower cryogenic and deep cryogenic treatment temperatures for controlled processing of cold-hardened steel, and LIN can deliver much colder temperatures.
In direct cooling, when the nitrogen is released into a cold treatment chamber or tunnel freezer at normal pressure, the circulating cryogenic gas comes in direct contact with the steel components and removes temperature (BTUs) directly from the part as well as from the surrounding atmosphere.
The cryogenic treatment and deep cryogenic treatment processes use holding times that are dependent upon the composition of the material and the part’s mass and shape. The microstructure is essentially set when the temperature throughout the part is uniform. This means smaller parts can be processed faster, often with holding times of just 1 to 2 hours. Larger or thicker components may require 4 to 6 hours or more.
Numerous factors impact how sub-zero treatments (cold, cryo and deep cryo treatment) affect a metal alloy. Processing factors such as time, temperature profile, number of repetitions and tempering practice, in conjunction with material parameters such as prior heat treatment and alloy composition will alter the final results.
Metals expand when heated and contract upon cooling. Cryogenic shrink fitting is a convenient technique to assemble parts when the tolerances between pieces are very close or there is an interference fit (negative tolerances). Generally, one part is cooled in order to decrease its dimensions. It is then assembled with the other part and allowed to warm to room temperature. When it comes to room temperature, it returns to its initial dimension and fits with the other component.