Effects of austenite on the pitting corrosion in 202 stainless steel with two phase of austenite and martensite were investigated through the electrochemical polarization test. Two phases structures of martensite and austenite were obtained by reversed annealing treatment at the range of 500℃-700℃ for 10min. in 70% cold-rolled 202 stainless steel. Volume fraction of reversed austenite has increased rapidly with an increase of annealing temperature. Pitting corrosion has arisen mainly on martensite phase in 202 stainless steel with two phases of austenite and martensite. Pitting current density has decreased with an increase of volume fraction of austenite. Consequently, pitting corrosion at martensite has occurred largely with an increase of volume fraction of austenite. Pitting corrosion was affected by volume fraction of austenite.
This study was carried out to investigate the effect of thermo-mechanical treatment on the formation behavior of martensite in 316L stainless steel. Dislocation, α and ε-martensite were formed by thermo-mechanical treatment. Martensite with surface relief and specific direction was formed by thermomechanical treatment. Martensite formed by thermo-mechanical treatment were reversed to austenite with an ultra-fine grain size of less than 1 μm by annealing treatment at 700 o C for 20 min. α-martensite with a K-S orientation relationship and ε-martensite with a [110] [100]ε orientation relationship to the matrix was formed by thermo-mechanical treatment. The volume fraction of dislocation, α and ε-martensite were increased with the cycle number of thermo-mechanical treatment. In 5-cycle number thermo-mechanical treated specimens, more than 25% of the volume fraction of α-martensite and less than 5% of the volume fraction of ε-martensite were attained. The austenite grains were fairly deformed by thermo-mechanical treatment and the grain size of austenite with various orientation relationship were changed from micrometer to sub-micrometer by the 5-cycle thermo-mechanical treatment.
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