Effect of heat treatment on the welded joint of X70 steel joined to duplex stainless steel by gas tungsten arc welding

The objective of this work is to study the microstructure and the texture evolution in welded joint of X70 steel with duplex stainless steel joined by gas tungsten arc welding (GTAW) process and heat treated at 200 degrees C and 600 degrees C. Then, the microstructural evolutions are correlated to their hardness mechanical property. The characterization techniques used are optical microscopy, scanning electron microscopy, EDS, Vickers microhardness, tensile tests, X-ray diffraction, and electron backscatter diffraction (EBSD) technique. Two heat affected zones were observed in welded joint with different microstructures. Heat affected zone on the X70 steel side and another on the duplex stainless steel side. A solidification microstructure was observed in the fusion zone such elongated ferritic grains and a growth direction < 100 > was found. The most microstructural transformations were observed in the vicinity of the interface which connects the heat affected zone to the fusion zone either on the X70 steel side or on the duplex stainless steel side. The effect of heat treatment on the hardness of the welded joint was noticed in the side of the duplex stainless steel. The fusion zone is the hardest zone of the welded joint (309.4 Hv) and which is not affected by the applied heat. X-ray diffraction analysis did not reveal the formation of new phases after the heat treatments applied to the welded joint. The tensile tests have shown that the welded joint treated at 200 C presents the highest value of ultimate tensile strength (667 MPa).