Effect of Heat Treatment on the Microstructure and Hardness of a Newly Developed Plastic Injection Mold Steel

Abstract

The microstructure of a new plastic injection mold steel in the annealed condition and effect of quenching-tempering process on the microstructure and hardness of this steel were investigated by means of OM, SEM, XRD and a digital hardness tester. The microstructure of the annealed steel consisted of ferrite and secondary particles which were identified as a FeCr intermetallic phase based on EDS and XRD analysis. Strong segregation exists in the steel. The microstructures of the quenched steels consisted of fine martensite, a small amount of blocky ferrite and secondary particles except quenching at 1150°C, where the microstructures of the quenched steels consisted of coarse martensite and large number of blocky ferrite. The segregation occurring in the annealed steel can be eliminated completely after heat treatment at the temperatures above 1050°C for 30min. The hardness of the quenched steel continuously increases with quenching temperature up to 1100°C and then drop observably. Hence, the suitable quenching temperature of the steel lies between 1050 and 1100°C. After tempering at 200 and 300°C, the hardness of the steel decreased due to the formation of tempered martensite and increased slightly after tempering at 400 and 500°C owing to secondary hardening, whereas this value decreased markedly after tempering at 600°C due to the formation of tempered sorbite. The amount of secondary particles gradually increased with tempering temperature.