Microstructure and Mechanical Properties Relationship of Friction Stir- and A-GTA-Welded 9Cr-1Mo to 2.25Cr-1Mo Steel

Dissimilar weldment of 9Cr-1Mo (P9) to 2.25Cr-1Mo (P22) steel processed with fusion welding process must be subjected to post-weld heat treatment (PWHT) prior to operation due to the presence of martensitic microstructure in the weld. However, due to the gradient of carbon activity, PWHT results in the diffusion of carbon from low Cr to high Cr side, which results in a soft zone and hard zone formation in the weld. This would degrade the mechanical properties of the weld joint. In order to tackle this, the present study employed friction stir welding (FSW) process at two different rotational speeds via 200 and 500 rpm. The microstructure and mechanical properties of the FSW specimens were compared with that of activated-gas tungsten arc (A-GTA) weldments. Regardless of the tool speed, the FSW and A-GTA processed specimens indicated martensitic microstructure in the weldment, and hence, PWHT was carried out. The PWHT treated FSW specimen revealed soft and hard zone evolution throughout the weld matrix, whereas in A-GTA welds, no severe diffusional phenomenon was observed across the weld. The A-GTA weld joint exhibited better strength properties than that of the weld joint made by FSW process, and therefore, A-GTA welding can be suitable techniques for welding of P9–P22 steel.