Isothermal Martensitic Transformation in Sensitized SUS304 Austenitic Stainless Steel at Cryogenic Temperature

Abstract

We have investigated martensitic transformation behavior in a sensitized SUS304 austenitic stainless steel to know the stability of the austenitic phase at cryogenic temperatures. We found that the sensitized specimen exhibits an isothermal martensitic transformation when the specimen is held in the temperature range between 60 and 260 K. The time-temperature-transformation (TTT) diagram corresponding to the formation of 0.5 vol% of α′-martensite shows a double-C curve with two noses located at about 100 and 200 K. An in-situ optical microscope observation has revealed that the double C-curve is due to two different transformation sequences. That is, the upper part of the C-curve is attributed to the direct γ (fcc) → α′ (bcc) martensitic transformation and the lower part of the C-curve is due to the successive γ (fcc) → ε′ (hcp) → α′ (bcc) martensitic transformation. The direct γ→α′ transformation occurs in the vicinity of grain boundaries, while the successive γ→ε′→α′ transformation occurs near the center of each grain. The reason for appearing two types of isothermal transformation sequence in the sensitized SUS304 stainless steel will be due to the difference in concentration by sensitization heat-treatment.