Deformation Behavior Of GCr15 Bearing Steel After High-strain-rate Impact

GCr15Bearing steel is a kind of thehigh carbon chromium bearing steels which is most widely used in engineering.General conditions of the bearing experience is the contact fatigue load. However,In the operation of the bearings in wind turbine gear box, the transient load acting on the bearingscaused by the wind. Which is suffer from a certain impact loading. And thisphenomenonis more apparently in the offshore wind turbine. But in the design of the wind gear box bearing, no attention is paid to the transient load, which is one of the factors that can leadthe life of the bearings is far lower than the designed life. Due to the differentunderstandingsof the failure mode and mechanism,the research of Wind Power Gear Box bearingis emphasison the contact fatigue and no consideration of failure modes under dynamic impact loading.So, it is very necessary to study its performance under dynamic impact loading.It also provides further theoretical basis for the study of formation mechanism of the adiabatic shear band(ASB) which is widely concerned in recent years.In this paper, a series of studies on the mechanical properties and microstructure of the GCr15 bearing steel under high speed impact is carried out.High strain rate impact tests of hat-shaped and cylindrical specimens were conducted in GCr15 by using the Split Hopkinson Pressure Bar. And the stress-strain curve are analysed. The formation mechanism and microstructural evolution of ASBs was investigated by using optical microscope, SEM and FIB/TEM examinations.Finally, the hardness characterization around the adiabatic shear zone was measured by the microhardness tester and the nano-indentation.The results of this study suggests that:GCr15 did not exhibit strain hardening effect under the dynamic loading. The adiabatic shearbands were formed in both the cylindrical and the hat-shaped specimens after the high strain rate impact loading.The width of the shear band is about 0.5-6μm.And cracks were formed along ASBs, so the adiabatic shear bands are generallyregarded as a failure mode of GCr15 steel under high strain rate deformation.But the cylindrical specimens have narrower and less denser ASBs than the hat-shaped specimens.The variations of the grain size and microstructure indicated that:The core of ASBs was consisted of subgrains and recrystallizedequiaxed grains which dominated by themechanism of dynamic recrystallization. While the transition regions were controlled by dynamic recovery, which wascharacterized as elongated subgrains and massive dislocation clusters.Driven by the high shear stress, process of the grain refinement is following: First, partitioning the elongated grainsby entangled dislocations which cluster at various segments; Then splitting the parent grains into smaller grains bysplitting and rotating due to a synergistic effect of shearing and twisting. Grain refinement was dominated under thedynamic recovery and incomplete dynamic recrystallization.Coexistence of the nano-sized bcc-ferrite(?) and the hcp-structure(ε) in ASBs indicated a phase transformation from bcc to hcp structure in the high strain rain deformation of GCr15.Cementite dissolution occurred induced by large plastic deformation and dynamic recrystallization in ASBs,leading to the reduction of the amount and the size of cementites.The microhardness increased sharply due to the refined grains and reached the highest at the core of ASBs.The hardness characterization of nano-indentation is consistent with the results of microhardness.