Abstract
Long term creep rupture properties and microstructure of 12% Cr heat resisting steels have been investigated. It has been found that solid solution hardening by addition of Mo is very effective on long term creep rupture strength, but precipitation or dispersion hardening by individual addition of V or Nb is less effective. In the case of Nb, the presence of excessive amounts of undissolved NbC leads to coarsening of MX precipitates, and a marked drop in long term creep rupture strength. The combined addition of V+Nb with Mo and B leads to the superior 30000 h creep rupture strength. The parabolalike decrease in creep rupture curves at 650°C is well concerned with the microstructural changes, in particular, subgrain growth. Microstructural differences between long term crept specimens at 600°C and short term crept specimens at 650°C were discussed by considering the parameter, T(C+logt). Long term specimens at 600°C exhibit higher dislocation density substructure with intensive precipitation reactions which include both fresh precipitation in laths and agglomeration at boundaries, and exhibit intergranular fracture probably caused by stress concentration around the agglomerates at boundaries, whereas short term specimens at 650°C exhibit transgranular fracture. It is considered that the deviation of long term creep rupture time from extrapolated values is caused by differences in precipitation hardening level and fracture mode.
