This contribution addresses the effect of sub-zero Celsius treatments at cryogenic temperatures on the steel microstructure. It is shown that the formation of martensite, including the socalled isothermal, i. e. time dependent or thermally activated, martensitic product, can provide an explanation to the observations in the literature that both the temperature of cryogenic treatment and the holding time at cryogenic temperatures may have an influence on the performance of steel products in service. This review traces the most important stages in the development of cryogenic treatments along the last 95 years and describes it in parallel with the description of the current state of understanding of the kinetics of martensitic transformation in steel. In the last part of the contribution, the new insight is put into context with a practical example where various types of subzero Celsius treatments are applied.
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