CRYOGENIC TEMPERING

CRYOGENIC TEMPERING

ABSTRACT

          Heat treatment is a process of heating the steel to a certain definite temperature followed by coiling at some predetermined rate in order to obtain desired mechanical properties. All basic heat treatment processes for steel involves transformation and decomposition of austenite.
                        Hardening is a process used to increase hardness of the steel .In this process the steel is heated above Ac3 and Acm line followed by cooling into quenching medium. Due to hardening process, austenite is converted into Martensite (super saturated solid solution  of carbon in alpha iron.)
In case of high carbon steel (steel containing carbon percent
More then 0.6%) martensite transformation finish temperature lies below room temperature. Due to which some austenite remains in untransformed into the martensite, this untransformed austenite because of Martensite transformation finish temperature (Mf) lies below room temperature is called as retained austenite.
                                   The amount of retained austenite is undesirable in hardened steel, because it is non- magnetic and soft. During a course of time retained austenite may change into pearlite and Bainite while tempering. The pearlite and bainite thus formed causes a change in size of jobs which is undesirable in case of precision measuring instruments such as snap gauge. This undesirable retained austenite is eliminated and cooling is carried out below the room temp. This treatment is called as subzero treatment.
In India, in most of the industries, subzero treatment is carried out by quenching the steel up to – 500C by refrigeration.
But it is found that if this austenite is quenched below -190° C to - 196° and the increase in “durability” is up to 300 times more.