Impact Toughness of Low-Carbon, Low-Alloy Steel with a Ferrite–Bainite Microstructure from Multiple Test Results

To describe general trends in the variation of impact toughness and cold brittleness, we have studied two low-carbon, low-alloy, low-sulfur steels having a ferrite–bainite microstructure and similar in chemical composition. According to results of multiple impact bending tests in the temperature range of the ductile-to-brittle transition, the cold resistance (quantified by the fraction of the ductile component on fracture surfaces of specimens) of the steel containing less sulfur and carbon (0.002% S and 0.106% C) is considerably higher than that of the steel richer in S and C (0.008% S and 0.120 C). The bainite content in the ferrite–bainite microstructure of the steel containing more S and C exceeded that in the steel with lower S and C content. The formation of cleavage sites in the steel containing more S and C predominantly involved MnS. The nonmetallic inclusions in the steel containing less S and C had no effect on cleavage nucleation. Splits were observed on only a small fraction of fracture surfaces in the steel specimens containing less S and C and on almost all fracture surfaces of the steel specimens containing more S and C. Unlike in steels of the X80 strength level, the formation of splits in quenched and tempered low-carbon, low-alloy, low-sulfur steels follows the intergranular fracture mechanism. Splits were formed within bainite bands along bainite packet boundaries when the region of the highest normal stress coincided with the axial segregation region. Typical intergranular fracture sites in steel specimens containing more S and C were MnS inclusions, whereas in the steel specimens containing less S and C no effect of MnS on the formation of intergranular splits was detected.