Influence of fissures on tensile and fracture toughness of steels with ferrite/pearlite microstructures

The tensile and impact properties of two control rolled steels made to the X65 specification have been examined. One steel exhibited a large number of deep fissures on the fracture surface of impact specimens while the other with a similar composition and room temperature strength showed little fissuring. Tensile specimens (both notched and unnotched) were tested over the temperature range room to −196°C. Samples were taken from both the longitudinal and transverse directions (relative to the rolling direction) as well as from the through-the-thickness direction. Fissures did not influence the tensile behaviour of unnotched tensile specimens taken in the transverse and longitudinal directions. However, tensile specimens taken from the throughthe-thickness direction started to show brittle behaviour in the heavily fissured steel at about −170 and −120°C for the unnotched and notched samples respectively, which contrasted with below −196°C and about −160°C for the ‘weakly fissured’ steel. Impact behaviour was also influenced by the presence of fissures. By subdividing the Charpy specimen into smaller segments, the impact transition temperatures were lower in the fissured steel compared with the lightly fissured steel. Longitudinal Charpy specimens generally, had impact transition temperatures ∼20°C lower than the transverse Charpy specimens and were also slightly more fissured. The weakly fissured steel had less grain elongation and a weaker (200) texture than the heavily fissured steel. In the through-the-thickness direction the impact transition temperature was increased by 50–60°C and the ductile shelf energy reduced by 50% for the fissured steel compared with the weakly fissured steel. Normalising the control rolled steels removed all the fissures, produced an equiaxed grain structure, weaker (200) textures and resulted in much lower strengths. A model is proposed to account for the appearance of fissures within a defined temperature range.