Many cases are reported where a certain amount of the residual austenite, often a very considerable amount, still remains in the structure of a hardened steel. in most cases this residual austenite is considered as an undesirable phenomenon, for the following reasons:
- As the austenite is much less hard than martensite, the hardness of a hardened steel containing residual austenite is well below the maximum
- As has been just pointed out, the residual austenite brings about a gradual, spontaneous change in dimensions of articles treated
- As the austenite is non magnetic, its presence in steel magnets decreases the magnetic induction and, consequently, the lifting power of magnets.
- The residual austenite remains in the steel structure because for many steels the austenite completes its transformation into martensite at temperatures well below zero. this suggests a simple method of removing residual austenite from the structure of a hardened steel, consisting of cooling it to a temperature well below zero, i.e., of subjecting it to the sub-zero treatment. this is the method proposed by Professor A.P. Gulyaev in 1937.