Study of microstructure evolution for 405 stainless steel under high temperature

The phase transformation temperature of ferrite to austenite for SA240- 405 stainless steel was measured by applying thermodiatometry method and calculated using the Thermo-Calc software. Meanwhile,the effect of temperature and soaking time on the microstructure evaluation was studied by different quenching and tempering treatment. The results demonstrated that the transformation of the ferrite to austenite of this steel started between 795- 832 ℃,and finished during910- 925 ℃. When the specimens annealed at temperature higher than 1 050 ℃,austenite would transform into ferrite gradually,consequently the content of as-quenched martensite decreased with the increasing temperature. Moreover,when the specimens quenched between 950 ℃ and 980 ℃,the microstructure of duplex phases composed of ferrite and martensite was obtained. The relatively high HRB values could be obtained when the quenching time was kept for 30- 60 minutes,and then the hardness would gradually decrease with increasing quenching time. Tempering at 730 ℃ resulted in ferrite and tempered martensite,and no obvious residual austenite was observed. Meanwhile the hardness gradually decreased with the increasing tempering time.