低圧タービンローター用鋼の過熱脆化に及ぼす成分元素及び製造条件の影響

抄録

For low sulphur steels recently used for a low pressure turbine rotor in order to improve mechanical properties, it became to a problem for an onset temperature of overheating to shift low temperature with decrease in sulphur content. Effects of chemical composition and manufacturing conditions on overheating were investigated in 2.83.5Ni-1.7Cr-0.4Mo-0.1V steels. The lowest onset temperature of overheating was obtained in a steel with about 0.005% S. The susceptibility to overheating increased with decreasing the Mn content, but overheating was not observed at heating temperature of 1 350°C by decreasing the sulphur content to less than 0.001% even in a low Mn (0.02%) steel. Cerium addition of 0.024% (Ce/S=4) prevented overheating. Lowering the heating temperatures and/or the finishing deformation temperatures reduced overheating. Overheating was observed at cooling rate widely ranging from 0.5 to 300°C/min, especially remarkable overheating occured at cooling rate of 2 to 170°C/min. It appears that overheating relates to the increase in the volume fraction of MnS precipitated on unite area of intergranular facet (V_s) and that decreases in sulphur and/or Mn contents and rises in heating temperature and/or deformation temperature resulted in coarsening austenite grain and consequently increasing V_s.