Regulation of microstructure and mechanical properties of 15CrNi3MoV steel large-tube forging

The microstructural heterogeneity in thick-Sect. 15CrNi3MoV steel forgings caused by cooling rate gradients during quenching critically impacts their mechanical reliability. Combining finite element simulation and multi-scale physical simulations, the microstructure evolution was decoded, and an optimised heat treatment process was designed for a 10-t large-tube forging. Key findings reveal that the cooling rate dictates phase transformation: the surface forms martensite, while the centre develops martensite and granular bainite with metastable martensite-austenite (M-A) constituents. During tempering, prolonged holding at 650 °C drives the decomposition of M-A constituents into fine carbides, with 12-h tempering achieving optimal strength-toughness balance. Crucially, carbide uniformity eliminates property gradients across 140 mm in thickness, suppressing embrittlement risks. Moreover, in the 180-mm-thick plate, the large-sized M-A constituents formed due to incomplete quenching, resulting in the carbide aggregations after tempering, which deteriorates the impact toughness. By integrating numerical simulation with validations from laboratory-scale and pilot-scale physical simulations, the relationship between microstructure and properties can be precisely predicted. Implementing the optimised process (890 °C/8 h water quenching + 650 °C/12 h tempering) on the 10-t large-tube forging demonstrates homogeneous properties. Thus, a generic methodology was provided for tailoring heat treatment protocols in ultra-thick alloy steel components.