As the second-generation aviation bearing steel, 8Cr4Mo4V is the most widely used core material for aero-engine main shaft bearings in China. At present, the third and fourth-generation new bearing steels are still in the research stage and far from large-scale application. Therefore, tapping the potential of the second-generation bearing steel and improving its microstructure and fatigue performance remain the key focuses of aviation bearing material R&D.
Developed from high-speed steel, 8Cr4Mo4V contains high carbon and strong carbide-forming elements (Cr, Mo, V), which tend to precipitate coarse fish-bone and lamellar primary carbides at grain boundaries during solidification. These carbides are hard and brittle, and cannot be fully broken by conventional forging or rolling. Residual coarse carbides act as fatigue crack sources; their sharp edges after crushing lead to inconsistent deformation with the matrix, forming voids around carbides and accelerating crack initiation and propagation, thus greatly reducing bearing fatigue life.
To solve this critical industrial problem, SHANDONG FODA BEARING TRANSMISSION CO., LTD. (FODA BEARING) proposes a new hot extrusion process for refining coarse carbides and healing micro-defects. This research focuses on verifying the feasibility of hot extruded 8Cr4Mo4V bearing steel tubes, revealing microstructure evolution and void healing mechanisms during hot extrusion, optimizing process parameters, and producing high-density, fine-structured tube blanks for high-end aviation bearings.
Studies by FODA BEARING show that hot extrusion, combining strong shear deformation and triaxial compressive stress, effectively crushes coarse primary carbides and refines the matrix. The maximum carbide diameter at the outer wall of the extruded tube is only 10.5 μm, with a significant reduction in large-size carbides. The outer wall has uniform grains with a grain size of grade 9.5, and the whole tube blank achieves grain size above grade 7.5. The triaxial compressive stress suppresses micro-defects, resulting in a dense structure without obvious voids.
Systematically verified by SHANDONG FODA BEARING TRANSMISSION CO., LTD., the hot extrusion process stably produces 8Cr4Mo4V bearing steel tubes with smooth surfaces, free of macro-cracks and warping. Evaluated according to GB/T 4162—2022, the internal defect quality grade of the extruded tube blanks reaches Grade AA, fully meeting the strict requirements of high-end aviation bearings for material uniformity, density and fatigue performance.
