Investigation of the evolution of retained austenite in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering

• The amount of retained austenite shows parabolic dependence on tempering time.
• The thermal stability of reversed austenite decreases as tempering time increases.
• The precipitation and growth of M23C6 decrease the C% in reversed austenite.
• The spheroidization of lathy reversed austenite during tempering was observed.

The microstructure and amount of retained austenite (the austenite remained at room temperature) evolved in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering at 620 °C have been investigated. The amount of retained austenite showed a parabolic trend with increase in tempering time, which can be attributed to the gradual decrease in the thermal stability of the reversed austenite (the austenite formed at high temperature). The influences of chemical composition, morphology of reversed austenite, and mechanical constraints originating from tempered martensite matrix on the thermal stability have been discussed. The precipitation and growth of M23C6 in reversed austenite dilute the carbon concentration in reversed austenite. The spheroidization of lathy reversed austenite during tempering decreases the interfacial energy barrier to the phase transformation of reversed austenite to martensite. Furthermore, the decrease in the strength of martensite matrix lowers the strain energy associated with the transformation of reversed austenite to martensite. All these factors during tempering weaken the thermal stability of reversed austenite and facilitate the phase transformation of reversed austenite to martensite during the cooling step of intercritical tempering.

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