Relationship between Deterioration of Toughness Due to Overheating and Amount of MnS Precipitated on Austenite Grain Boundary in Ni-Cr-Mo-V Steel

Abstract

The effect of overheating on the amount of MnS(V_s) and the number of MnS(n) precipitated on unit area of ductile intergranular facets and hence on the change in upper shelf energy (vEs) was investigated in this study. The vEs decreased proportionally with an increase in V_s in steels overheated by isothermal holding. A change in vEs related to both V_s and n in steels overheated by continous cooling. The increase in vEs due to slow cooling at a cooling rate less than 2°C/min was caused by an decrease in n; despite an adequate amount of V_s. The V_s was 0.1 to 0.5 times as small as the amount of MnS precipitated on unit area of grain boundaries (V_s') which was calculated from the Mn/S solubility product and grain boundary area per unit volume. The V_s/V_s' tended to decrease when S content increased. This suggests that the increase in S content enhances the intragranular precipitation of MnS. It appears that an decrease in the onset temperature of overheating with the decrease in S content can be explained by the increase in V_s' due to coarsening austenite grain and the prevention of intragranular precipitation of MnS. The critical value of V_s' for preventing overheating was 5×10^-5 mm³/mm², which was obtained from the relationship between vEs and V_s'. Combination of Mn and S contents to get the V_s' of less than the critical value was determined.