Application of −140 °C Sub-Zero Treatment For Cr-V Ledeburitic Steel Service Performance Improvement

The effect of −140 °C, 17-hour sub-zero treatment and subsequent tempering on the hardness, flexural strength, and fracture toughness of Cr-V ledeburitic steel was investigated. The main purpose was to highlight the role of microstructural alterations, such as a decrease of the retained austenite content, the quantitative parameters of carbide distributions, or others, due to applied treatments. Compared to conventional treatments, the applied sub-zero treatment improves the hardness of the material significantly within the whole range of tempering temperatures used. The hardness enhancement is in the range from 197 to 137 HV10 for tempering at 170 °C to 450 °C. The flexural strength is slightly improved at low tempering conditions up to 170 °C, while there is almost no effect of the sub-zero treatment on flexural strength for higher tempering temperatures. The fracture toughness is substantially improved (by about 3 MPa m^1/2) in the untempered state as well as after high-temperature tempering. The fracture toughness is slightly worsened, by about 0.15 to 1 MPa m^1/2, within the 170 °C to 450 °C range of tempering temperatures. These changes in fracture toughness are reached with a significant increase of hardness. This steel performance is associated with enhanced fracture surface roughness and an increased ductile microvoid coalescence micromechanism presence in fracture surfaces. The results also show that treatment at −140 °C leads to a greater enhancement of the complex of mechanical properties than can be obtained by soaking the samples in liquid nitrogen.