1989 Volume 75 Issue 6 Pages 972-979
Creep rupture data extending 80000h duration have been obtained from nine castings of Cr-Mo-V steam turbine steel. The data of the nine castings showed a wide range of rupture time at higher stresses and lower temperatures, and also a wide range of rupture ductility at lower stresses and longer rupture times.
The rupture time variation was found to arise from the differences in fine distribution of V4C3 and high matrix dislocation density. The V4C3 distributions and the dislocation densities of the nine castings became similar one another as stress was reduced and rupture time consequently increased, which corresponded to a narrow range of rupture time at lower stresses and higher temperatures.
All of the castings showed the rupture ductility falling at longer times, which was associated with a change from transgranular to intergranular fracture due to cavitation. Therefore the rupture ductility behaviour at longer times depended mainly on a susceptibility to cavitation, and the susceptibility was closely connected with segregation of impurities (P, S, Sn, Sb, Cu and As) to grain boundaries. Total amount of the impurities segregated to grain boundaries had a clear correlation with the reduction of area at rupture.