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

One of the major problems of the advanced high Cr ferritic heat resistant steels is reduced creep strength of the welded joint. It is caused by type IV failure at fine grained HAZ (FGHAZ).  In the present study, mechanism and prevention of type IV failure has been discussed from the viewpoint of microstructure stability.  In FGHAZ of the conventional 9Cr heat resistant steel Gr.92, the prior austenite and block boundaries are not covered by sufficient M₂₃C₆ precipitates after post weld heat treatment (PWHT).  In addition, lath microstructure disappears by forming equiaxed subgrains.  This is caused by segregation of M₂₃C₆ forming elements at the ghost prior austenite and block boundaries, which are observed in the base metal.  On the other hand, if the prior austenite grain and block boundaries are covered by M₂₃C₆ due to homogenisation of the alloying elements prior to the weld thermal cycle, the creep lifetime is much improved to the base metal level.   This indicates that lack of the boundary strengthening is a main reason to cause type IV failure.  A small amount of B addition is effective in improving the creep property of the welded joint.  It is attributed to improvement of the HAZ microstructure.  During rapid heating of the weld thermal cycle, the original austenite is reconstituted through displacive ferrite to austenite reverse transformation and the original martensite is reconstituted during cooling.  This allows M₂₃C₆ 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 FGHAZ.  However, creep property is much improved in FGHAZ, since the block boundary is covered by a lot of precipitates.  This result suggests that the creep property of HAZ is mainly determined by stability of the block microstructure during creep.