Study on Hydrogen-Induced Cracking in the Heat-Affected Zone of Heavy Forgings Overlaid by Stainless Steel

Contrary to general knowledge, it was found that cold cracks in the heat-affected zone (HAZ) under stainless steel weld overlay on the tubesheet forging of a steam generator were generated during fabrication and not in service. The cold cracking, known as “hydrogen-induced cracking,” occurs when four factors are simultaneously satisfied:

1. existence of hydrogen in the HAZ
2. existence of tensile stress or strain
3. existence of microstructure susceptible to hydrogen embrittlement
4. low temperature (<150°C).

The conditions are still satisfied at welding of the second layer without pre-heating, even after pre-heat is applied for welding of the first layer, followed by soaking treatment. To avoid cold cracking, it is recommended that pre-heating should be maintained at least during welding of the first and second layers and until post-weld heat treatment or soaking treatment