Improvement of Critical Intergranular Fracture Stress by Increasing Carbon Content in Tempered Martensite Steels

Effect of carbon (C) content (0.05 mass% to 0.3 mass%) on critical intergranular fracture stress of tempered martensite steel was investigated using 3 mass% manganese (Mn) steel. The critical intergranular fracture stress was obtained by calculating the maximum principal stress at fracture in a tensile test of circumferentially notched round bar specimen using elastoplastic finite element analysis. As a result, critical intergranular fracture stress of tempered martensite steel increased with increasing the C content. Therefore, the dominant factors of critical intergranular fracture stress were examined from the viewpoints of the amount of segregation of each element on prior-austenite grain boundaries and the grain size of the martensite substructure. The first result was found to be the effect of reducing the amount of Mn segregation by increasing the C content. This was thought to be because cementite acts as a solid solution site for Mn, and the amount of Mn in solid solution in the matrix phase is reduced by increasing the C content. The second result was found to be the effect of refining the substructure of martensite surrounded by high angle grain boundaries by increasing the C content. This indicates that grain size also affects crack initiation resistance in intergranular fracture by determining the stress concentration at the grain boundaries as the distance of dislocation accumulation.