No abstract

In this paper, through high-temperature tensile tests of 55NiCrMoV7 steel, high-temperature fracture behavior, microstructure evolution, and carbide distribution characteristics of both the thermal–mechanical coupling zone (fracture zone) and thermal stress zone (clamping zone) at different temperatures were studied. Intrinsic relationships between high-temperature fractures and carbide types, distribution and size were revealed, and evolution mechanisms of microstructure near cracks in 55NiCrMoV7 hot-working die steel during high-temperature deformation was clarified. Samples were stretched at different temperatures from 25 °C to 700 °C, and microscopic examinations were carried out using SEM and TEM. The results showed the following. With the increase in temperature, tensile strength and yield strength decreased, elongation and reduction of area increased, and fracture mode changed from brittle fracture to ductile fracture by transition temperature at about 400 °C. During high-temperature deformation, the grain dislocation density decreased and the tempered martensite decomposed, recovered, recrystallized, and then grain grew. M7C3 and M23C6 carbides precipitated and grew along the grain boundary, and a small amount of fine granular MC carbides was dispersed in the grain. The work done by the external force on the deformation zone would cause the temperature of it to be higher than the tensile temperature, which provides thermodynamic conditions for the redissolution of small carbides near the fracture zone and the grain growth of large carbides, resulting in a decrease in small carbides and increase in large carbides in thermal–mechanical coupling zones.

The microstructure (before and after quenching), phase (before quenching), carbide content and hardness were studied by scanning electron microscopy, X-ray diffraction, optical software and Rockwell hardness tester. The results indicate that carbides of Cr5 steel have different morphology features and obvious aggregation phenomenon, most carbides dissolve in the matrix after quenching, the carbides of not dissolving with small particles distribute in the martensite. The style of carbides before quenching have M 7 C 3 、M 23 C 6 and M 3 C 2. volume fraction of carbides gradually reduce with quenching temperature increase, and the temperature of carbides dissolving in the matrix is 970C. Cr5 steel hardness increases and later decreases with the quenching temperature increases, the peak hardness (54HRC) is achieved in the 970C.