Microstructures and creep rupture properties of extra low carbon 7-11Cr-4 Mo-(Ni) steels were investigated to evaluate the effect of intermetallic compounds on enhancement of high temperature strength.
The following results were obtained.
1) Precipitates were a small amount of carbides, and Laves phase, Fe
2Mo.
2) Initial hardness was higher with increasing Ni content but age hardening was observed only in steels without Ni.
3) Structures of 9Cr-4Mo and 7Cr-4Mo steels were ferrite but Ni addition increased amount of martensite.
4) Creep rupture strength of martensitic steels was higher than that of ferritic steels. Fine Fe
2Mo in the former steels contributed to enhanced rupture strength.
5) Creep rupture strength of heat affected zone (HAZ) was markedly lower than that of base metal for martensitic steels. On the other hand, the difference between them was remarkably small for ferritic steels.
6) Creep rupture strength in transverse direction was markedly lower than that in longitudinal direction for ferritic steels.
7) Though steels produced by a powder sintering and rolling process had finer grain than those produced by an ordinary hot rolling process, rupture strength was almost same or that of the former was slightly higher.
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