Effect of tempering temperature on low-temperature impact toughness of 35MnB steel

Medium carbon boron steel is a cost-effective, high-strength steel used for bolts. This paper aims to provide a theoretical foundation and data support for optimizing heat treatment and improving performance in wind turbine bolts by systematically analyzing the synergistic effects of boron segregation, dislocation density, high-angle grain boundaries (HAGBs) and other factors during tempering on the low-temperature impact toughness of 35MnB steel. The form and distribution of boron in the steel were characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS) combined with transmission electron microscopy (TEM). The relationship between crystallographic orientation and low-temperature impact toughness was analyzed using electron backscatter diffraction (EBSD). The results show that when the tempering temperature is between 350 °C and 410 °C, numerous large M3(C, B) at the prior austenite grain boundaries (PAGBs) lead to intergranular fracture in the radial zone of the impact fracture. However, the adverse effects on low-temperature impact toughness are mitigated by the decrease in dislocation density, effective grain size, and the increase in HAGBs. As a result, the low-temperature impact energy increases slowly from 19.5 J to 24.2 J. When the tempering temperature is between 410 °C and 470 °C, the size and quantity of boron phases at the PAGBs decrease, leading to a rapid increase in impact energy from 24.2 J to 86 J. The rolling texture gradually weakens and eventually disappears after tempering, resulting in anisotropy. Therefore, boron segregation is the primary factor affecting the low-temperature impact toughness.