The impact toughness and mechanism of low-carbon steel fracture at low temperatures

Abstract

The goal of the work is to study the effect of severe plastic deformation (SPD) on the impact toughness and fracture mechanism of St3sp low-carbon structural steel within a test temperature range of 293 – 213 K. The issues of deformation processing of steel St3sp billets using SPD method in conditions of the equal-channel angular pressing scheme (ECAP) are considered. The results of low-temperature tests by impact bending of Charpy steel samples in various states are presented. The impact toughness decreased by ~1.3 times as a result of ECAP in 16 passes. It is shown that the temperature dependence of the impact toughness of steel subjected to ECAP differs from that for steel in the delivery condition. A fractographic study of the fracture mechanisms of the steel in the initial state and after processing by ECAP at a test temperature of 293 – 213 K is carried out. It is shown that for the steel in the initial the transition from fracture with the formation of viscous and brittle fracture zones at 293 K to brittle at 213 K occurs through successive expansion of the brittle fracture area with decreasing temperature, whereas for hardened steel, the mixed fracture area appears in the local region at 233 K and expands to the entire cross section of the sample at 213 K. The microstructure formed as a result of ECAP in 16 passes in the temperature range up to 213 K prevents pure brittle fracture and leads to a mixed fracture pattern.