Cr-Mo steels, among other applications, find extensive use in steam generators (SG) of nuclear reactors because of their high resistance to creep and corrosion. Higher Cr steels (9Cr-1Mo) are generally preferred for high temperature components like superheaters and headers whereas lower Cr-Mo steels (2.25Cr-1Mo) are employed for evaporators.

Boiler and pressure Vessel code of American Society for Mechanical Engineering (ASME Code III & XI) specifies "Half-Bead" and "Butter-Bead Temper-Bead" repair welding methods for repairing Cr-Mo steels. These repair methods are allowed by the code only for steels with less than 4% Cr. Both the repair welding procedures do not need post weld heat treatment (PWHT). While these methods ensure achieving required microstructure in the heat affected zone (HAZ), they do not ensure for the acceptable levels of residual stress in the weldments.

It is known that residual stresses play a key role in the performance of welded structures particularly under fatigue and stress corrosion cracking conditions. The residual stresses can be either beneficial or detrimental depending on their sign, magnitude, and distribution with respect to the service induced stresses. Residual stress evaluation is also an important tool for process control, quality control, design assessment and failure analysis. Quantitative theoretical analysis of residual stresses in weldments is not, at present, a practical engineering design tool due to complexity of welding processes. Therefore, experimental measurements are essential to establish a quantitative understanding of sign, magnitude and distribution of the residual stresses. This would help in arriving at a most suitable repair method that gives residual stresses within acceptable limits.

In this study, residual stress analysis of repair welds of 9Cr- 1Mo and 2.25 Cr-1Mo steels have been carried out using portable X-ray diffraction system (Model RIGAKU MSF-2M). In the case of 9Cr-1Mo steel, it is found that half bead butter head method of repair generated compressive residual stresses of 80 MPa in the HAZ whereas all other methods had tensile residual stresses ranging from 50 MPa to 120 MPa in the HAZ. Also, microhardness profile showed that half bead butter bead method could achieve better tempering of the HAZ. In the case of 2.25 Cr-1Mo steel, all the methods showed compressive residual stresses in the HAZ but microhardness profile revealed that half head butter head method is better from the point of view of tempering of the HAZ. These results suggest that in the case of 9Cr-1Mo steels, if half bead butter-bead method is adopted for repair welding, failures like fatigue and SCC can be avoided in the HAZ, in spite of the fact that ASME methods are meant for steels having less than 4% Cr. In the case of 2.25 Cr-1Mo steel both methods can be used as they generate compressive stresses in the HAZ. But, in both the steels, none of the methods could bring down the peak residual stress (350 MPa tensile) at weld centre and peak hardness 400 VHN near the fusion line.