Crack propagation at root toe using GTAW and SMAW Process

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Dear All Master welding,I have a project of welding DN300 thk 40mm A420 WPL6 (LTCS) 90 Degree elbow to a DN300 thk 40mm A333 Gr 6 straight pipe. The process of welding is GTAW(root using Hyundai ST-50G 2.4mm) and SMAW(hot pass and capping using welding rod oerlikon supercito 3.2mm). We have cut out and re-weld the joint as much as 3 times due to crack found at root toes of elbow side. The crack propagate from time to time.  Even our welder have control the welding in such a way like pre-heat 95 degree before welding, dye pent/MPI after root or layer of hot pass to check for surface crack (found no crack) but after capping. PAUT scan after cooling, it found the crack at the root toes. What is the possible causing of HAZ crack? Is it material properties, stress or welding process problem?

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Originally Posted by jkkt87

Dear All Master welding,I have a project of welding DN300 thk 40mm A420 WPL6 (LTCS) 90 Degree elbow to a DN300 thk 40mm A333 Gr 6 straight pipe. The process of welding is GTAW(root using Hyundai ST-50G 2.4mm) and SMAW(hot pass and capping using welding rod oerlikon supercito 3.2mm). We have cut out and re-weld the joint as much as 3 times due to crack found at root toes of elbow side. The crack propagate from time to time.  Even our welder have control the welding in such a way like pre-heat 95 degree before welding, dye pent/MPI after root or layer of hot pass to check for surface crack (found no crack) but after capping. PAUT scan after cooling, it found the crack at the root toes. What is the possible causing of HAZ crack? Is it material properties, stress or welding process problem?
Reply:Did you do this weld to a qualified welding procedure?ChrisAuction Addict
Reply:Hi ttoks,Thanks for your professional opinion and yes you are correct, i am a production engineer. You are really experience who will know whether someone is welder or not through the language they speak. Salute. i should buy you a drink. haha.
Reply:Yes. Our welder weld as per approved WPS.