Effects of reheating after solution treatment and magnetic fields on α′ martensite formation in SUS304L steel during isothermal holding at cryogenic temperature

Abstract

Abstract Effects of reheating treatments after solution treatment and/or exposure of high magnetic fields on the α′ martensite formation in SUS304L stainless steel during isothermal holding at liquid nitrogen temperature were investigated in detail. Reheating treatments were performed at 473 to 1373 K for 7.2 ks followed by water quenching after solution treatment (1573 K×18 ks). The amount of α′ martensite first increased with reheating temperature up to 523 K, then decreased with temperature between 523 and 773 K, and increased again above 773 K. After reheating to 773 K, the α′ martensite formation was suppressed completely. Simultaneous application of a magnetic field of 20 tesla (T) during isothermal holding at liquid nitrogen temperature after reheating revealed no clear change in α′ martensite formation compared with the result with no magnetic field. Isothermal heating experiments provided the apparent activation energy for the suppression of the α′ formation around 773 K to be 194 to 245 kJ/mol which is close to the activation energy of self diffusion of Fe, Ni, and Cr in SUS304L steel. This suggests that the suppression of the α′ martensite formation around 773 K is caused by the loss in function of nucleation sites of α′ martensite through the diffusion of those elements. The reason for an increase in the amount of α′ martensite by reheating above 773 K is attributable to residual stresses and/or lattice defects induced by water quenching from the reheating temperature.