Improvement of Microstructure Stability during Creep in High Cr Ferritic Heat Resistant Steel HAZ

摘要

Mechanism of type IV failure has been discussed from a viewpoint of microstructure stability. In fine grained heat affected zone (FGHAZ) of the conventional 9Cr ferritic heat resistant steel Gr.92, the prior austenite grain (PAG) and block boundaries are not covered by sufficient M_(23)C_6 precipitates after post weld heat treatment (PWHT). In addition, lath microstructure disappears by forming equiaxed subgrains during PWHT. This is caused by segregation of M_(23)C_6 forming elements at the ghost PAG and block boundaries, which were former boundaries in the base metal. Homogenisation of the alloying elements prior to the weld thermal can improve the creep property of FGHAZ, because a lot of M_(23)C_6 precipitates are formed at PAG and block boundaries during PWHT. This indicates that lack of the boundary strengthening is a main reason to cause type IV failure. A small amount of B addition (B steel) is effective in preventing type IV failure. During rapid heating of the weld thermal cycle, the original austenite is reconstituted through martensitic a/y reverse transformation and the original martensite is reconstituted during cooling in B steel HAZ. This allows M_(23)C_6 to precipitate at the same grain and block boundaries of the base metal during PWHT, even if segregation exists. The lath microstructure also disappears in the B steel HAZ after PWHT. However, creep property is much improved in the welded joint, since the block boundary is covered by a lot of precipitates in HAZ. This result suggests that HAZ creep property is mainly determined by stability of the block microstructure during creep.