Investigation on mechanism of type IV cracking in P92 steel at 650 °C

Welded joints of P92 steel subjected to creep testing at 650 degrees C and 70 MPa were investigated. Type IV cracking was observed in the fine-grained heat-affected zone (FGHAZ) of the welded joints by optical microscopy. It was found that with varying creep times, the number of creep voids increased at an accelerating rate and the maximum number of voids was formed in the FGHAZ. Scanning electron microscopy observations revealed that precipitates were formed in the interior of creep voids, suggesting that the nucleation of the creep voids is related to the precipitates. These creep voids then connected with each other, isolated the grain from the matrix, and formed zigzag microcracks, leading to type IV cracking. New coarse carbides-the Laves phase and Cr7C3-were precipitated during creep. These carbides can deteriorate the creep strength and stimulate the nucleation of creep voids in the FGHAZ.