The Microstructure and Mechanical Properties of the Bonding Layer of a Bimetallic Ring Processed by Hot Rolling Based on a Centrifugal Casting Ring Blank

The compact cast-rolling compound forming of bimetallic rings is an efficient process for manufacturing large bimetallic rings, in which hot ring rolling is crucial for the microstructure and final performance of bimetallic rings. A 40Cr/Q345B bimetallic centrifugal casting blank was hot ring-rolled, and the effects of this process on the interfacial microstructure and mechanical properties were investigated. Compared with the casting state, the bimetallic ring exhibits an arc-shaped interface and a continuous gradient microstructure in the bonding layer (BL) after hot ring rolling. Under the radial temperature gradient, the diffusion distances of C, Cr, and Mn at the middle are greater than at the ends, resulting in a BL thickness ranging from 3.04 to 3.85 mm. The outer 40Cr and inner Q345B exhibit more significant dynamic recrystallization (DRX) and grain refinement than the BL, with 40Cr showing greater refinement than Q345B, attributed to the strain gradient induced by heterogeneous deformation that promotes dislocation accumulation and DRX, improving coordinated deformation. The microhardness of the outer 40Cr increases from 257.3 HV to 380 HV. The shear strength of the BL reaches 463.5 MPa, 48.8% higher than the casting one, and the fracture mode transforms from brittle fracture to ductile–brittle fracture. Tensile strength and elongation are 667.1 MPa and 12.1%, respectively, between those of 40Cr and Q345B. During tensile deformation, strain partitioning between hard and soft zones alleviates strain concentration, delays local cracking in the hard zone, and forms a strain gradient in the BL that achieves heterogeneous deformation-induced strengthening and effectively retards crack propagation, ultimately achieving an excellent combination of strength and plasticity.