Creep rupture strength and microstructure were investigated for base metals and synthetic HAZ specimens of 9Cr-1Mo-V-Nb steels with two levels of Nb and C content respectively.
Results obtained were as followed.
(1) Creep rupture strength of the synthetic HAZ specimens which were heated to the peak temperature of 900 and 1000°C was lower than that of the base metals.
(2) The difference in creep rupture strength of 10
3h at 650°C between the base metals and the syntheic HAZ specimens was minimum for the steel with 0.06% C and 0.05% Nb and was maximum for the steel with 0.10% C and 0. 05% Nb.
(3) Microstructure was tempered martensite for all specimens. Grain size of the synthetic HAZ specimens was much smaller than that of the base metal.
(4) Main precipitates were M
7C
3 and M
23C
6 and the latter carbide increased during aging at 650°C.
Amount of the precipitates was smaller in the synthetic HAZ specimens than in the base metals.
(5) Decrement in creep rupture strength of the synthetic HAZ specimen seemed to be caused mainly by grain refinement of the matrix, coarsening of the carbides, and smaller amount of precipitates compared to the base metals.
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