Research on asynchronous quenching behavior of boron steel tubular parts with variable cross-sections in hot gas forming-quenching integrated process

The hot gas forming of complex-shaped boron steel tubular parts has an asynchronous quenching characteristic, making it difficult to control the microstructure and properties of formed tubular parts. To investigate the asynchronous quenching rule during hot gas forming, a tubular part with variable cross-sections was designed. The asynchronous quenching behavior and microstructure evolution under different process parameters were analyzed via phase transformation-based finite element simulations. Furthermore, the formed tubular parts were obtained under corresponding experimental conditions, the microstructure, hardness, and tensile strength of the tubular parts were tested and compared with simulation results. The results confirmed that the initial forming temperature and bulging pressure determine the area and cooling rate in the non-contacted zones, thereby achieving the quenching strengthening during the cooling of the tubular parts. With the increase in bulging pressure, the cooling rate of the corner zone will be accelerated, and the martensitic volume fraction will be significantly improved. Additionally, the pressure holding time affects the quenching effect in the non-contacted zones. With an initial forming temperature of 850 °C and bulging pressure of 15 MPa, when the pressure holding time increased from 15 s to 20 s, the quenched tubular part was mainly composed of martensite, as a result, the hardness and tensile strength of the part could reach 494 HV and 1486 MPa, respectively. Therefore, in hot gas forming-quenching integrated process of complex-shaped boron steel tubular parts, the cooperative regulation of microstructure and mechanical properties could be achieved with appropriate process parameter adjustment.