Elimination and Control of δ Ferrite in M50NiL Bearing Steel

In this study, the effects of the annealing temperature and holding time on the δ ferrite content in M50NiL bearing steel are evaluated. The results indicate that when the temperature increases from 900°C to 1200°C, the δ ferrite content increases as a result of the increase in the Cr equivalent in the matrix, which decreases the stability of austenite and promotes the transformation of γ ⟶ δ. At 1300°C, the δ ferrite content sharply increases because the austenite transforms into δ ferrite in large quantities when heated to the γ + δ phase region. When the steel is annealed at 1200°C, the content of δ ferrite first decreases and then increases with increasing holding time (0.5–50 h). Meanwhile, the δ ferrite content reaches the lowest value of 3.5% after 30 h of annealing. The decrease in the δ ferrite content is attributed to the increase in the Ni equivalent in δ ferrite, which promotes the transformation from δ ferrite to austenite. The increase in the δ ferrite content is attributed to the diffusion of Mo and V ferrite-forming elements into grains, resulting in the nucleation of a large amount of δ ferrite.