Effect of intercritical annealing and quenching plus tempering heat treatments on microstructure of Ni added powder metallurgy steels

In this study, intercritical annealing, which is usually practiced to low alloy ingot steels, and quenching + tempering heat treatments were applied to low alloy powder metallurgy steel. For these purposes, atomized iron powder (Ancorsteel 1000) was mixed with 0.3% graphite and Ni ranging 0–2% (wt.). The mixed powders were cold pressed at 700 MPa and sintered at 1200 °C for 30 min under pure Ar gas atmosphere. Some of the sintered specimens was quenched from 890 °C and tempered at 200 and 400 °C for 1 h. The other sintered specimens were annealed at intercritical heat treatment temperatures of 728 and 760 °C and quenched in water. The experimental results showed that a typical tempered martensite microstructure was obtained in the specimen without Ni addition after quenching + tempering, whereas the dispersion of Ni-rich areas in the microstructure of tempered martensite in Ni-added specimens was seen. With increasing tempering temperature, the volume fraction of Ni-rich areas slightly decreased. While ferrite + martensite microstructure was produced after intercritical annealing heat treatment, Ni-rich areas in the centre of martensite islands in ferrite matrix were seen in Ni-added specimens. Also, with increasing intercritical annealing temperature and Ni content, martensite volume fraction increased and ferrite ratio decreased.