Tooling by Design
Sub-Zero Treatment of Tool Steels
A reader recently inquired about the benefits of sub-zero treatment of tool and die components after heat treatment. He also wondered if the sub-zero process was the same as a cryogenic treatment.
Cryogenics is not new. During World War II, scientists found that metals frozen to low temperatures exhibited increased wear resistance. And, NASA engineers observed the cryogenic effect on metal parts recovered from the extreme cold of space—in particular, aluminum parts, which exhibited increased dimensional stability after exposure to extreme cold.
Sub-zero treatment processes are grouped into three broad categories, each impacting the treated steel in different ways:
1) Shrink fitting—reduces the diameter of a steel shaft so that it can be readily assembled with other components;
2) Cold treatment—completes the metallurgical phase transformation of austenite into martensite during the hardening of steels, via quench and temper heat treatment; and
3) Cryogenic treatment—at liquid-nitrogen temperatures, creates conditions for precipitation of very fine carbides in higher alloy steels.
Sub-Zero Treatment of Tool Steels Process
Overall contraction of metals when cooled allows tight assembly of parts
-70 to -120 C (-90 to -190 F) until metal is cold throughout
Temporary change in size
Complete martensitic phase transformation
-70 to -120 C (-90 to -190 F) for 1 hr. per 3 cm of cross section
• Transformation of retained austenite to martensite
• Increase hardness
• Dimensional stability
Cyrotreatment temperatures can create sites to nucleate fine carbides that improve wear resistance in tool steels
-135 C (-210 F) and below for 24 hr. or longer
Improved wear resistance through carbide precipitation
Source: Linde AG, Hollriegelskreuth, Germany
Cold treatments and cryogenic treatments are additional steps in the heat treatment and hardening process.
The heat treatment of tool steels includes heating the tooling component to austenitizing temperature, followed by quenching—or rapid cooling—to transform most, but not all, of the austenite into the higher-strength martensite structure. The newly formed untempered martensite is supersaturated with carbon, which makes it unstable, brittle and likely to crack. Tempering allows the supersaturated carbon to form carbides that help relieve stresses in the martensite matrix and prevent cracking. Most tool and die professionals are familiar with this heat-treatment sequence.
Quenching usually occurs at room temperature. Most medium-carbon and low-alloy steels will undergo complete transformation to martensite at room temperature. However, high-carbon and high-alloy steels will retain austenite at room temperature. To eliminate retained austenite, the temperature must be lowered.
Cold-treatment processes tend to operate in a temperature range of - 90 to -190 F. This process completes the transformation of retained austenite in the microstructure to the stronger and harder martensite structure. The resulting steel hardness correspondingly increases with the increased percentage of martensite in the microstructure. This also increases the steel’s strength and wear resistance.
To summarize, the primary purpose for cold treating tool steel is to promote the further transformation of retained austenite to martensite, to increase the hardness, strength and wear resistance of the treated steel.
Cryogenic treatments occur at temperatures below - 238 F. Since cold and cryogenic treatments both occur at sub-zero temperatures, it would not be appropriate to distinguish one or the other as a “sub-zero” process, because both are sub-zero treatments.
With hardened tool steels, exposure to cryogenic temperatures eliminates most of the retained austenite in favor of the more wear-resistant martensite. More importantly, it also initiates the precipitation of wear-resistant carbides from the martensite phase, enhancing the steel’s overall durability and toughness.
For machine shops and metalformers, cryogenic treatments also can increase the life of drills, taps, mills, cutters, saw blades and broaches by 200 to 400 percent, according to one recent study. And in another study, an automaker reported resistance-welding electrodes running six times longer after cryogenic treatment.After cryogenic treatments, tool steels will require at least two tempering cycles in a tempering oven. These steps can be quite time consuming—as long as 20 hr.—but they are a necessity, not an option. The entire process can take 3 to 4 days, so plan accordingly. MF
Cryogenic treatments require a slow cool down, followed by a hold at cryogenic temperatures, and a slow warm up to ambient temperatures. Research is just starting to show that there are optimum hold times that should be used for different metals and other materials. Cryogenic treatment can be used on virtually all metals besides hardened steels. Treatment of carbide and plastics has shown excellent benefits. Huge wear resistance benefits are seen in pearlitic cast iron such as used in brake rotors where there is no martensite or austenite. Research papers on cryogenic processing are available at http://www.cryogenictreatmentdatabase.org/, a database created by the Cryogenic Society of America with some assistance from the Cryogenic Processing Sub-Committee of ASM International.