Microstructural optimization by two-step tempering treatment and quantitative analysis of microstructure effect on the toughness at −40 °C in CrNiMoV steel

Abstract

30CrNiMoV steel is widely used in engineering applications, where, in addition to high strength requirements, the demand for enhanced toughness is increasingly critical for safety. This study proposes a novel two-step tempering process that optimizes the microstructure by shortening tempering time and accelerating cooling, effectively avoiding the strength-ductility trade-off. Specifically, the intrinsic relationship between −40 °C impact resistance and microstructure were quantitatively investigated using CVN impact tests, SEM, EBSD, and TEM characterization. The results showed that the optimal new process increased the ultimate tensile strength (UTS) and yield strength (YS) from 1380 MPa to 1173 MPa – 1445 MPa and 1212 MPa respectively, and increased the −40 °C impact energy from 19.0 to 47.5 J. The ductile-to-brittle transition temperature (DBTT) was reduced from −6.5 °C to −21.5 °C. The excellent low temperature toughness achieved by this novel process could be attributed to the following reasons: Firstly, the QTwTa process optimizes the morphology and size of the carbides, significantly increasing their number density. This improves the coordinated deformation between the carbides and the matrix, reduces stress concentration during fracture, raises the crack initiation threshold σIC, and delayed the initiation of cracks, thereby enhancing fracture toughness. Secondly, the uniform and refined the block lead to a reduction in DBTT. Moreover, the more uniform and finer the block, the higher the proportion of high-angle grain boundaries, which increases the frequency of crack deflection, preventing rapid crack propagation and thereby enhancing energy absorption. In conclusion, the synergistic effect of carbide dimensions, number density of, and high-angle grain boundary and block dimensions are crucial for achieving excellent mechanical properties with the novel two-step tempering process.

Keywords

• Low-temperature toughness
• High-angle grain boundaries
• Carbides
• Block width