Characterization of microstructural evolution with pre-strain in BG801 bearing steel: Grain, carbides, retained austenite and martensite

Abstract

The method of multiple cryogenic and tempering treatments significantly limits the microstructure regulation of BG801 bearing steel. In this work, the pre-strain was introduced in the vacuum-melted BG801 steel combing with heat treatment (HT) to adjust the microstructures. The samples were subjected to uniaxial tensile at room temperature with different strains (engineering strain of 0, 0.15, 0.175, 0.2), then heating to 1323 K and water-cooling. The results showed that higher pre-strain was beneficial to obtain finer prior austenite grain (PAG) and martensite grains, and a low fraction of retained austenite (RA) in BG801 steel. With increasing pre-strain, the PAG was elongated along the RD direction (rolling direction) without a change in size. The recrystallized grains with a size of about 5–6 μm nucleated inside the PAG during HT. Meanwhile, the high-density dislocations were introduced in the tensile samples by pre-strain, promoting the growth of micron-M6C by accelerating the diffusion of alloy elements during HT. And the evolution of RA and martensite indicated that the pre-strain accelerates transformation from austenite to martensite by transferring elements from the matrix to M6C and introducing shear bands into the matrix to promote the nucleation of martensite.