Reheat cracks

Weld procedure qualification tests for submerged arc welding of the main vessel shell and cladding destructive tests metallographic techniques to check that Heat Affected Zone (HAZ) reheat cracks are absent. 

Delayed Reheat Cracking (DRC) was the main mechanism responsible for the cracking problem at 500 C to 520°C in the weldments of the PFR reheater and superheater shells. This is also the case for cracking in secondary pipework weldments in Phenix and it was a major contributory factor in two craqking incidents in PFR secondary pipework which contained notch-like features. All of these incidents have involved the Ti-stabilised Type 321 stainless steel. 

Examples of reheat cracking (left-hand) and Type IV cracking (right-hand) in butt welds. 

Investigation of the problem indicated that the cracks were initiated by delayed reheat cracking and grew by a high temperature brittle intergranular mechanism driven by the residual stress field. 

The condition of the vessels became of increasing concern in the final years of operation and the need for regular examination significantly extended shutdown times. The basic problem was one of choice of material a problem of availability of material during fabrication led to the use of a stabilised stainless steel and this resulted in a susceptibility to delayed reheat cracking. 

Q. Auzoux, Reheat cracking of austenitic stainless steels — influence of work hardening on intergranular damage, MINES ParisTech and CEA Sacaly, 2004. 

Effect of rare earths on grain refinement 6.3. Reheat cracking 38... 

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