Microstructure and mechanical property evolution mechanisms of 15Cr14Co12Mo5Ni2WA aviation gear steel during cold rotary forging

Abstract

15Cr14Co12Mo5Ni2WA steel has huge application prospects in manufacturing aeroengine gears owing to its high strength and toughness. However, just because of its high strength, 15Cr14Co12Mo5Ni2WA steel is difficult to integrally deform at room temperature, which greatly restricts its industrial application. Cold rotary forging (CRF) is a local incremental metal forming process, and the small deformation force makes CRF ideal for deforming 15Cr14Co12Mo5Ni2WA steel. Thus, this paper is aimed to study the microstructure and mechanical property evolution mechanisms of 15Cr14Co12Mo5Ni2WA aviation gear steel during CRF process. The results show that grain size decreases from 26.2 μm to 16.9 μm, and the lattice orientation in each grain gradually becomes uniform during CRF process, which is mainly attributed to the combined effects of (Cr, Mo)23C6 carbide migration, dislocation movement and lattice rotation. Owing to the complex normal-external-transversal multi-directional metal flow, as-received ξ-fiber (<110>//ND) firstly transforms to single dominant Brass texture in CRF-20% condition, and then transforms to θ-fiber (<100>//ED) and Rotated-Cu texture at last deformation stage. Tensile/yield strength and Vickers hardness increase from 1049.3 MPa/1015.1 MPa and 316.6 HV in as-received condition to 1180.1 MPa/1146.7 MPa and 376.3 HV in CRF-50% condition, which is mainly attributed to the gradually increasing grain boundary strengthening, dislocation strengthening and secondary phase strengthening during CRF process.