Cryogenics: Does it Strengthen Steel?

Article by WarAngel.

"Cryo-Treatment", "Cryo-Freezing" and "Cryogenic Tempering" are terms associated with the cryogenics industry. The concept behind cryogenics is to achieve a variety of qualities in treated metal, such as improved durability, stress relief, reduced wear and extended life through a process of "cold treating".

The process involves reducing the temperature of a metal to sub-zero temperatures to enhance their desired metallurgical properies - usually -300F and below. The treatment is performed to the entire mass of the metal rather than just the surface to make it stronger and uniform.

What features are gained from Cryogenics? According to Cryogenics International, cyrogenics can cause the residual stresses and distortions of firearm barrels that form as a result of the heating and cooling of shooting to be alleviated, thus allowing the barrels to dissipate heat quickly and efficiently. In another example, American Bladesmith Society certified Master Bladesmith Tim Hancock claims that by following up with heat treatment, cryogenics causes knife edges to stay sharp much longer and reduces micro-fracturing and edge-chipping and improving blade strength overall. Even musicians have found their trumpets and trombones sounding better!

Yet despite the praises of cryogenics, the blademaking camp is drawn to opposites. Steel blades derive their strength from heat-treating in order to achieve a hardened form of steel known as martensite. Cryo-treatment is usually, thus, performed in conjunction with the heat-treating and tempering sequence.

In high alloy steels (e.g. stainless steel, O1, D2), the rich alloy content, while providing properties such as improved wear resistance and fine grain, can actually prevent full transformation of austenite (crystalline form of steel when above critical temperature) into hard martensite, leaving the part at less than optimal hardness. Cryo-treatment can help any remaining austenite crystals precipitate to martensite. The conversion from softer autensite into martensite would result in increased hardness of a blade, which, unfortunately, would be brittle without subsequent tempering. While this new level of hardness may be acceptable in knives, problems arise in the increased length of a sword which, by nature, has greater shock-tolerance requirements than a knife. One advertisement for a Japanese-style katana, mentioned the steel used is railroad tracks that have endured the freezing Siberian winters, which some would associate with some degree of "cryo-treatment." However, such claims are just marketing hype- these temperatures are far from the sub-zero thermal levels that qualify as cryogenic treatment, and more importantly, cryo-treatment must be done after heat treating - when steel is heated for forging or hardening, it loses all the characteristics imparted by any previous treatment, hot or cold.

In most low-alloy steels, there is little or no difference achieved by cryo-treatment. This is because low-alloy steels do not retain austenite. Some bladesmiths indicate that if low-alloy steel has received good heat-treating, there will be nothing left for the cryogenic process to work with. Swordmaker Randal Graham had tested blades of various low-alloy steels and have found no significant performance or hardness gain as as result of cryo-treatment, with the only exception of AISI 52100 steel gaining an average of only one point of Rockwell hardness over five test blades which, he concluded, was not significant enough for the extra time and treatment.

There are many positive points in support for cryogenics, in general. However, much more research has to be done to provide a more solid basis for the use of cryogenics in steel blades. Thus far, the added performance of edge-retention, durability and strength can be accomplished by use of different low-alloy steels such as L6 and W2 along with proper annealment and modern heat-treating techniques such as the use of salt baths.

ABS Master Bladesmith, Howard Clark, shares the following:

"I really wish this would just go away, but obviously it is not going to anytime soon. The cryogenic (subjecting it to severe cold, as in LN2 or -290F) treatment of blades is not a new thing, and it is also not a magic thing.

"There are two potential changes that can occur. The first is dimensional stability can be improved. This is what happens to the rifle barrel. By being subjected to severe cold, the barrel now no longer changes dimension as much as it used to when it heats up from repeated firing. Gauge block standards have been cryogenically treated for years for this reason, it reduces the margin of error induced by changes in temperature.

"The second possible thing that can happen is the change to martensite if there was retained austenite in the microstructure of the material. This is applicable to blades, but only if retained austenite is present. If the heat treating is done well, then there is no retained austenite present, i.e. no benefit to the cryogenic treatment. Retained austenite can be a problem with high alloy steels (stainless steels with high carbon are particularly susceptible, but still only if the heat treatment was not optimal in the first place). Low alloy and simple steels (likely what your kukri is made from a 10xx steel) are extremely unlikely to benefit from a cryogenic treatment, as they are unlikely to have any retained austenite to convert to martensite in the first place."

In conclusion, cryogenic experiments on blades are an interesting fad, but in light of what it achieves on a molecular basis, there is really no value in cryogenically treating sword blades and thus should not be viewed as a "magical" or miracle-producing process, because it will not yield such results accordingly.

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