2002 Volume 42 Issue 11 Pages 1309-1317
Nowadays the preferential creep damage accumulation at the Intercritical HAZ (ICZ) leading to Type IV cracking has been a great concern for various industries. The ultimate failure of the welded components fabricated from ferritic steels often takes place at this particular region. Type IV cracking has been found in almost all the ferritic steel weldments so far, from a conventional 1.25Cr-0.5Mo steel to a modified 9Cr-1Mo steel. However, the mechanism of Type IV cracking has not yet been understood equivocally. In the present work, cross-weld creep behavior of a service exposed 1.25Cr-0.5Mo steel has been examined in order to clear the feature of Type IV damage. The discussion shall be made on the important role of grain boundaries around small grains, which was transformed into austenite during welding, to promote Type IV cracking. The evident feature of grain boundary facets suggests strongly that Type IV cracking is induced by the grain boundary sliding around small grains. Significant impurity segregation, which is expected to accelerate the damage development by stabilizing cavities, was found at grain boundaries.
