Effect of post weld heat treatment on weld metal microstructure and hardness of HFCA-TIG welded ASTM-B670 high temperature alloy joints

Abstract

In this investigation, the effect of post weld heat treatment (PWHT) conditions on weld metal (WM) microstructure and hardness of ASTM B670 alloy joints are studied for gas turbine engine applications. This high temperature alloy was joined using high frequency compressed arc TIG (HFCA-TIG) welding process. The precipitation hardening was performed on ASTM-B670 alloy joints directly after welding (DPH); after solutionizing the welds at 980 °C (980ST+PH) and at 1065 °C (1065ST+PH). The WM of ASTM-B670 alloy showed better response to PWHT in minimizing the Laves phases and enhancing the WM hardness of joints. The DPH, 980ST+PH and 1065ST+PH joints showed 55.63%, 58.80% and 64.08% enhancement in hardness of WM compared to as welded (AW) joints owing to the precipitation of hardening phases after PWHT. The 1065ST+PH joints revealed higher hardness of WM than DPH and 980ST+PH joints It is attributed to the complete dissolution of Laves phases in WM causing more Nb available for precipitation hardening of WM. Nevertheless, the PM hardness of 1065ST+PH joints are lower than 980ST+PH joints owing to the severe grain growth. The HFCA-TIG welds showed volume fraction of Laves phase up to 5.10% in WM which is 80.38% and 36.25% lower than constant current TIG (CC-TIG) and pulsed current TIG (PC-TIG) welds. It is attributed to magnetic compression and high frequency pulsation of arc which minimizes input of heat in welding and offers greater refinement of WM.

Keywords

• ASTM-B670 alloy
• InterPulse TIG welding
• Post weld heat treatment
• Microstructure
• Weld metal hardness