Effect of partitioning treatment on the microstructure and properties of low-carbon ferritic stainless steel treated by a quenching and partitioning process

We carried out a quenching and partitioning (Q&P) process to some samples of AISI 430 ferritic stainless steel and applied intercritical annealing treatment to others as a comparison. The microstructure of the sample after intercritical annealing treatment contained ferrite and martensite. The microstructure of samples under the Q&P process included ferrite, martensite, and retained austenite. The strength of samples treated by a Q&P process was equivalent to that of the samples after intercritical annealing treatment. The former had better ductility than the latter through optimizing the partitioning process parameters. As the partitioning temperature increased, the diffusion rate of carbon atoms increased, more carbon atoms diffused from martensite into austenite, and more retained austenite stabilized at room temperature. The area fraction change of retained austenite resulted from the competitive mechanism of diffusion and homogenization of carbon in austenite and austenite decomposition. The partitioning process had no apparent effect on the strength but significantly affected ductility. The influence of the TRIP effect of retained austenite and softening effect of martensite on ductility should be considered comprehensively. The TRIP effect mainly depended on the fraction and mechanical stability of retained austenite. The diffusion process of carbon atoms during the partitioning stage was briefly described. Part of carbon atoms diffused from martensite into austenite, part of carbon atoms formed precipitation, and part of carbon atoms was trapped by defects at grain boundaries. The relationship between the yield platform and the Cottrell atmosphere formed by carbon pinning dislocation was illustrated in this paper.