The cryogenic treatment of tool steels reduces the percentage of retained austenite (RA) that remains after heat treatment and increases the amount of martensite. Using a computer-controlled program, cryogenic metal treatment gradually reduces the temperature of tool steels to -300° F and keeps them at this cryogenic temperature for 24 hours. By promoting the transformation of RA into martensite and precipitating carbon particles that fill microscopic voids, cryogenic treatment increases durability, improves wear resistance, and reduces fatigue failure. This is especially important in tools steels, which are used to shape other materials.
Heat Treatment and Tool Steels
Heat treatment for steel is a controlled process that alters the microstructure of metals to impart properties such as increased surface hardness and high-temperature resistance. Tool steels are a category of carbon alloy steels that have a relatively high carbon content and are alloyed with other metals such as tungsten, chromium, vanadium, and molybdenum. The carbides that are formed by alloying carbon with these other metals is part of what gives tools steels their high hardness, resistance to abrasion and deformation, and ability to withstand elevated temperatures along a cutting edge. Examples of tool steel components include cutters, reamers, dies, drill bits, knives, and hand tools.
Austenite and Martensite
During the heat treatment process, austenite is converted into martensite. Austenite, which is softer, is less desirable than martensite, which is tougher and stronger. Although the goal of the heat treatment process is to convert as much austenite into martensite as possible, some austenite remains during tempering, the heating of steel below its melting point, as carbides form. If the retained austenite percentage (RA%) is too high, soft spots can occur. With cutting tools, the retention of excessive amounts of RA can result in stress cracking and premature tool failure. When observed with scanning electron microscopy (SEM), steels with an excessive RA% exhibit microscopic voids that can promote fracturing.
Carbon Content and Wear Resistance
All steels contain iron and are alloyed with carbon for enhanced wear resistance. Steels with a high carbon content (such as tool steels) are more wear resistant but also retain significantly more austenite after heat treatment. Because of the link between carbon and RA%, cryogenic treatment provides even greater benefits for higher-carbon steels than for lower-carbon steels. During the cryogenic metal treatment process, retained austenite is transformed into martensite and the microscopic voids in the tool steel’s microstructure are eliminated through the precipitation of called eta-carbides. These submicroscopic carbides are brilliant white or black and can be observed with SEM.
Cryogenic Temperatures and Tool Steels
Research indicates that cryogenic steel treatment is most effective at temperatures below -250° F. For some metals, even lower temperatures may be necessary for improved wear resistance. As reported in a study available on Research Gate, the wear resistance of tool steel samples that were cryogenically treated at -310° F were approximately 2.6 times greater than the wear resistance of tool steel samples that were cryo treated at -120° F. Specifically, samples of A-2 and D-2 tool steels exhibited improved wear resistance by factors ranging from 2.0 to 6.6. In addition to the soaking temperature, the soaking period and the cooling rate also determined the wear resistance imparted by cryogenic treatment.
Choose Nitrofreeze® Cryogenic Treatment of Tool Steels
The optimal cryogenic treatment of tool steels can provide significant improvements in the quality of machine cutting tools and hand tools. Nitrofreeze® of Worcester, Massachusetts (USA) is an experienced provider of cryogenic treatment services and uses proprietary processes that are clean, fast, and cost-effective. To find out if your part is a viable candidate, contact Nitrofreeze® at the phone number or email listed below. The consultation is free.
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