Abstract
Countermeasures against hydrogen induced disbonding at the interface between 21/4 Cr-1 Mo steel and overlaid stainless steel weldment in reactor vessels which are operated under high hydrogen pressures at elevated temperatures were investigated from the viewpoint of base steel and overlaid stainless steel respectively.
It is known that the disbonding is caused by formation of carbide after post weld heat treatment and accumulation of hydrogen during cooling at shutdown at the interface. As for base steel, 21/4Cr-lMo steel plate with surface layer 2-3 mm thick of low carbon concentration could prevent the disbonding under ordinary operation condition for direct desulphurizer of heavy oil. The thickness of the low carbon layer could be decreased by decreasing the carbon concentration. It was considered that prevention of the disbonding was realized at carbon concentration less than 0.050.06% at the interface.
As for overlaid stainless steel, the effect of the amount of δ ferrite on the disbonding was investigation. It was found that more than 18% of δ ferrite after post weld heat treatment was required to prevent the disbonding. δ ferrite seemed to be effective because of decreasing hydrogen content at the interface by increasing mean diffusion rate of hydrogen in the overlaid stainless steel.
