Hot tensile properties of simulated heat-affected zone microstructures of 9Cr-1Mo weldment

Abstract

The heat-affected zones (HAZ) of 9Cr---1Mo steel weldments consist of coarse-grain martensite with δ-ferrite, coarse-grain martensite, fine-grain martensite and intercritical structure. These HAZ microstructures have been simulated with respect to microstructure, hardness and grain size by isothermal heat treatment cycles in the temperature range 973-1573 K. Tensile tests at a strain rate of 3 × 10^-4s^-1 over the range 298-873 K showed that 0.2% YS and UTS values for the various microstructural conditions were in the descending order of coarse-grain martensite, coarse-grain martensite with δ-ferrite, fine-grain martensite, base metal and intercritical structure. The presence of δ-ferrite in the coarse-grain region of HAZ restricted the grain growth, the increase in strength and the decrease in ductility. This minimises the problem of cracking in the coarse-grain region of HAZ in this steel which is often encountered in low-alloy Cr---Mo ferritic steels. The regions with intercritical structure showed the lowest strength and fracture energy. Fine-grain martensite exhibited a good combination of strength and ductility in the HAZ.