Influence of initial quenching on the microstructure and mechanical properties of quenched and partitioned ferritic stainless steels

Modern steel industry has great interest in developing new advanced high-strength steels, especially for the automotive industry. The need for stronger and more ductile sheet steels has led to development of novel heat treatments such as quenching and partitioning. The Q&P heat treatment provides an opportunity of manufacturing strong steels without sacrificing their formability. However, there is limited research conducted on the microstructure evolution of many alloys potential for Q&P such as stainless steels.

This study evaluates the selection for the optimal quench interruption temperature during Q&P of ferritic stainless steels. The paper compares different simulation models for optimizing the Q&P-process. Q&P was applied to two AISI 420-type stainless steels EN 1.4021 and EN 1.4034 to assess the simulation results. Microstructure analyses with X-ray diffraction and electron microscopy revealed that simulated values overestimate the retained austenite fractions after Q&P due to formation of Cr-rich carbides. Mechanical tests showed that Q&P is applicable to grade EN 1.4021 stainless steel, whereas EN 1.4034 fractured in a brittle manner under tensile load. Electron microscopy revealed intergranular fracture type and concentration of Cr-rich carbides at parent austenite grain boundaries in EN 1.4034. These results suggest that impurities may expose stainless steels to temper embrittlement during partition.