Growth behaviour of creep cracks in the simulated heat-affected zone of modified 9Cr-1Mo steel

It is not clear whether the growth behavior of creep cracks is the same in the type IV cracking area as in other areas. We investigated the creep rupture strength by a uniaxial creep test, using specimens simulated the welding thermal cycle at different peak temperatures. The creep rupture strength was high when the austenite transformation didn't occur during the thermal cycle, with a rupture time exceeding 10,000 h. In this case, the creep cracks propagated along grain boundaries with repeated oxide formation and fracture. By contrast, the strength was low when the austenite transformation occurred during the thermal cycle, with a rupture time shorter than 1000 h. In this case, the creep cracks were formed by the interlinking of voids from prior austenite grain boundaries, then propagated until fracture. Therefore, the peak temperature of the thermal cycle is a crucial determinant of the growth behavior of creep cracks.