Phase transformation process and toughening mechanism of 12Cr1MoV heat-resistant steel for high-pressure boilers by external shower and internal spray heat treatment

To optimize the heat treatment process for 12Cr1MoV heat-resistant steel, this study designed a novel process of external shower and internal spray (ESIS), followed by air cooling. This new approach was simulated in the laboratory. The study focused on the variation in impact energy and the microstructural changes in the experimental steel subjected to different processes, specifically the T890 and T740 processes (where the numbers represent the termination temperature of ESIS). The phase transformation processes and toughening mechanisms of the steel after these treatments were thoroughly investigated. The theoretical analysis revealed that the microstructures of T890 and T740 steels consist of quasi-polygonal ferrite (QPF), granular bainite (GB), and pearlite. The QPF in T890 steel forms at a higher temperature compared to T740 steel, leading to a lower dislocation density and carbon content in QPF during the subsequent cooling phase. This results in the QPF in T890 steel having a hardness of 1.02 GPa lower than that of the QPF in T740 steel. The higher hardness of QPF in T740 steel correlates with reduced toughness and resistance to crack propagation. Under impact loading, the QPF in T740 steel exhibits insufficient crack propagation resistance to counteract the effects of microscopic defects within the tensile stress region of the fracture. Consequently, cracks preferentially propagate within the QPF, creating a relatively flat propagation path. In comparison, the fracture of T740 steel demonstrates a straighter propagation path in the tensile stress region than T890 steel, resulting in significantly lower impact energy for T740 steel.