Effects of alloying elements on the creep rupture time of high Cr ferritic steel weldments

The effects of precipitation strengthening by V and Nb additions and the effects of N and W additions on the creep rupture time (CRT) of heat affected zone (HAZ) in the weldment of high Cr ferritic steels were studied by welding heat cycle simulation. The CRT of HAZ was slightly shorter than that of base metal, even in the steel without V and Nb addition. This can be explained by the coarsening of grain size in HAZ. As the base metal is more strengthened by V and Nb containing nitrides (V, Nb) N, the CRT of HAZ drops more. The CRT of HAZ of stronger base metal never becomes lower than that of weaker base metal. Although the increase in nitrogen content improves the CRT of base metal, the CRT of HAZ is improved less than that of base metal. These results can be related with the state of(V,Nb)N precipitation by the local climb mechanism. The W addition promotes the strengthening effects of (V,Nb)N precipitation, by increasing the solution of Nb during welding heat cycle, Thus, when the CRT of base metal is improved by W addition, the ratio of CRT in HAZ to that in base metal is kept nearly the same as that of steel without W addition.