Tempering behavior of 9%Cr-1%Mo-0.2%V steel

Abstract

Metallurgical observation of 9%Cr-1%Mo-0.2%V steel tempered at 750°C for a maximum of 100 h has been made. Accompanying the recovery of martensitic structure, discontinuous changes in hardness, intensity of an X-ray diffraction peak of the matrix, the lattice strain calculated from the integral width of an X-ray peak, the amount of extracted residue and the size of M23C 6 were observed when tempering time is around 10 h. These changes are caused by the annihilation of dislocations and the coalescence of martensite lath followed by the formation of subgrains in the later stage of tempering, which is supported by electron back scattered pattern (EBSP) measurements. The ultra fine grains of the order of 0.1 μm were confirmed around martensite lath and block by EBSP, which is obviously correlated with the discontinuous changes in the hardness and the lattice strain. Precipitation of M 23C6 on excess dislocations of martensite induces a larger amount of precipitates as compared with the thermal equilibrium, which causes the dissolution of M23C6 during consequent tempering. The dissolution of M23C6 results in slower growth rate as compared with the Ostwald ripening. The observed over-all time exponent is 1/16. © 2006 ISIJ.