Effect of fibrous macrostructure on mechanical properties, anisotropy and fracture mechanism of roller bearing ring

Abstract

The fabrication of bearing ring with reasonable fibrous macrostructure is necessary to enable long-service period in aero-engine. This investigation demonstrates a forging method to improve fibrous macrostructure distribution via a low height-diameter ratio (HDR) bar and sufficient lubrication to control radial flow of metal during forging process. Furthermore, a combination of electron microscope and three-dimensional X-ray computed tomography (3D-XCT) technology was performed to analyze and compare the microstructure evolution after stress relief annealing (SRA) and spheroidizing annealing (SA) and tensile test. Results show that the angle between the fibrous macrostructure and the working surface decreased to 2–5°, compared to 45° under the same conditions without low HRD bar. An inhomogeneous deformation was developed during hot ring rolling (HRR), characterized by the elongated grains parallel to the circumferential direction (CD). The SA treatment imparted the required properties such as uniform microstructure and appropriate tensile properties into the hot forged 8Cr4Mo4V bearing ring. 3D-XCT studies have confirmed that the plate-shaped primary carbides distributed along the axial direction (AD) in the two-dimensional (2D) observation were partially irregularly connected morphologies in the 3D view. In the three relevant directions, the tensile strength and elongation increased with the order of radial direction (RD) <AD < CD. The aspect ratio and orientation of grain and plate martensite were the key factor for determining the strength of SRA samples. For SA samples, the orientation and proportion of primary carbides band (PCB) cause plastic anisotropy. The crack propagation model of SAR and SA samples along three directions was discussed.