Effects of Ti(C,N) additions on the microstructure and properties of the carbon-free high speed steel reinforced by intermetallic compounds

Abstract

In this paper, Fe–Co–Mo steels strengthened by intermetallic compound μ phases and Ti(C,N) particles were prepared by vacuum sintering at 1350 °C using the powder metallurgy technique. The microstructures, mechanical properties, wear resistance of Fe-25% Co–15% Mo (wt. %) steels with different Ti(C,N) additions (0, 1.0, 2.0, 3.0 wt %) were comparatively studied. The results demonstrated that the Ti(C,N) addition could significantly inhibit grain coarsening during the sintering process and make the net-like intermetallics distribute more evenly. Meanwhile, it was manifested that the additives facilitated the precipitation of μ phases during the age-hardening process, leading to the reduction of peak-aging temperature and the increase of aging hardness. These mechanisms brought about the improvement of peak hardness, bending strength, impact toughness, tempering hardness, and wear resistance of the Fe–Co–Mo steel obviously. Nevertheless, as Ti(C,N) content increased to 3.0 wt %, FCM steels presented a decline in bending strength and impact toughness, resulting from the negative effects brought by the excessive brittle Ti(C,N) distributed at grain boundaries. Among the samples, the steel with 2.0 wt% Ti(C,N) addition possessed the optimal mechanical properties with the hardness of 85.7 HRA, bending strength of 3517 MPa, and impact toughness of 11.4 J/cm2, respectively. Moreover, the precipitation of the nanoscale μ phase during aging was also investigated. It showed that the orientation relationship between the precipitated μ phase and the matrix is (010)μ//(110)α and [001]μ//[-111]α.