Research on Nucleation Site of Martensitic Transformation by High Magnetic Field Application

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Effects of cooling rate from solution treatment temperature, pre-strain before sub-zero cooling and reheating before sub-zero cooling on formation of alpha'martensite in SUS 304L steel with and without an application of high magnetic fields were examined. Rapid cooling from solution treatment temperature increases the formation of alpha'martensite. Small pre-strain at room temperature is attributable to a formation of epsilon martensite at the sub-zero temperature. Pre-strains over around 1% decrease the amount of alpha'martensite and 3% pre-strain depresses the formation almost completely. Application of high magnetic fields up to 26 tesla can enhances the formation of alpha'martensite in the steel pre-strained by over 3%. It is conseived from the compairing the magnetic driving energy and depression energy by work-hardening that this depression cannot attributes to the suppression of the growth of nuclei of martensite due to work-hardening but to the reduction of the number of the nucleation site. Re-heating at around 500ﾟC before cooling suppresses the formation of the martensite. This phenomena occurs through time dependent process showing activation energy of diffusion of substitutional elements. This depression is attributable to break-down of the nucleation sites induced by quenching from solution treatment temperature. Water quenching from re-heating temperatures over 500ﾟC increases the amount of alpha' martensite but air cooling from those temperatures does not increase. From these results, it was thought that quenching from solution treatment and re-heating temperatures over 500ﾟC induces strains through thermal stress in the specimens and such strains play an important role as nucleation sites.