Effect of δ-Ferrite on Hot Deformation and Recrystallization of 316KD Austenitic Stainless Steel for Sodium-Cooled Fast Reactor Application

The sodium-cooled fast reactor is the most mature reactor among generation-IV nuclear reactors. A carbon/nitrogen-controlled 316KD austenitic stainless steel has been developed for the construction of pressure vessels and internals in Chinese CFR600 demonstration reactor. During their industrial production, d -ferrite is present in large-scale billets because of the combined effect of non-equilibrium segregation and low cooling rate. For large-scale billets containing d -ferrite, inhomogeneous grain-size distributions are observed in the product after hot working. Extensive studies on the recrystallization of the austenite phase in austenitic stainless steels during hot deformation were conducted. However, the effect of d -ferrite on the recrystallization behavior of the austenite phase remains unclear. In this study, uniaxial hot compression tests of 316KD austenitic stainless steels involving as-cast and homogenized conditions were conducted at 1423 K and 0.1 s- 1 using a Gleeble-3800 thermal-mechanical simulator, and the effect of d- ferrite on hot deformation and recrystallization was analyzed by SEM, EBSD, and TEM. Results showed that d- ferrite could be nearly eliminated through d- ferrite. austenite transformation after homogenization at 1473 K for 14 h, whereas austenite grain showed evident growth. The elimination of d- ferrite was a Cr-diffusion-controlled process through kinetic analysis. Plastic deformation occurred preferentially in d- ferrite and at the d- ferrite/austenite interface, and subsequently in austenite during hot deformation. The flow stress of as-cast samples was much lower than that of homogenized samples at the same strain because of the presence of soft d- ferrite. Dynamic recovery occurred easier in d-ferrite, and the resulting dynamic softening remarkably reduced flow stress with an increase in strain. Discontinuous dynamic recrystallization characterized by original austenite grain boundary bulging was the dominant mechanism in homogenized samples. However, the presence of d- ferrite promoted the occurrence of continuous dynamic recrystallization in austenite near the d- ferrite/austenite interface in ascast samples. Compared with the homogenized samples, a higher degree of recrystallization was observed in as-cast samples because of the combined effects of continuous dynamic recrystallization and discontinuous dynamic recrystallization.