Effects of Pre-Strain on the Microstructure of Heat-Treated Bg801 Bearing Steel

Abstract

In the present work, we elucidate the influence of pre-strain on microstructures of BG801 bearing steel after heat treatment. The samples were firstly subjected to uniaxial tensile testing at room temperature with different strains (engineering strain of 0.15, 0.175, 0.2), then heated to 1323 K and water-cooled. Subsequently, the SEM, EBSD, TEM, and XRD were selected to reveal the influence of pre-strain on microstructures of the undeformed region (UR), gauge region (GR), and necking region (NR) in a specimen. The results show that higher pre-strain leads to less content of austenite phase and smaller average grain size. The high-density dislocations were introduced in the tensile samples by pre-strain, promoting the precipitation of micro-M6C at the boundaries of prior austenite grains and recrystallization grains during heat treatment, and impeding the precipitation of nano-M2C carbides form the matrix. The martensitic lath tends to be distributed along the RD direction at 45° at low pre-strain (at GR) but tends to be disordered due to the obvious recrystallization at high pre-strain (at NR). The prior austenite grains elongated along the RD direction with increasing strain, but the size without change significantly. The recrystallized grains nucleate and grow inside the prior austenite grains at NR without destroying the original grain boundaries, it is probably the connection with the dissolution of net-shape M23C6 and the high density of defect recovery consumption of the distortion energy at the prior austenite grain boundaries.

Keywords: BG801 bearing steel, Microstructure, M6C carbides, M2C carbides, Phase transition, pre-strain