Re-dissolution of VN during tempering in high chromium heat resistant martensitic steel

Abstract

Precipitation behavior of VN during isothermal tempering at 740-800°C of 7%Cr-0.4%V-0.09%N steel (% denotes mass%, hereinafter) has been studied. Initially, rapid softening takes place accompanied by the precipitation of VN and, after that, the quasi-steady state in a hardness vs. tempering time diagram is continuing for a while. After the quasi-steady state, re-dissolution of VN particles rapidly occurs followed by final precipitation of VN. Just before the peak time of the re-dissolution of VN particles, both the temporal decrease in hardness and the temporal increase in the integral breadth of an X-ray diffraction peak take place. The similar precipitation phenomenon is confirmed in 0.14%C-9%Cr-1%Mo-0.2%V-0.09%Nb steel. In both steels the re-dissolution of VN or NbC (hereinafter MX) accompanies the decomposition of martensite. The following reactions are suggested as a mechanism for the re-dissolution of MX type particles: local stresses induced by the recovery of martensite unlock the pinning of dislocations by the MX type particles and the consequent isolated particles, which become energetically unstable, are re-dissolving into the matrix.