Dissimilar Welding of 25Cr-35Ni/30Cr-48Ni Cast Heat Resistant Steels and Evaluation of Joint Properties In this study 25Cr-35Ni/30Cr-48Ni (HP/HV) cast heat resistant alloys dissimilar joint was investigated. This joint is commonly used in reformer tubes in direct reduction unit of steel making industries. The investigation by SEM equipped with EDS elemental analysis system indicated that HP alloy in as-cast condition contains M 7 C 3 ، M 23 C 6 and NbC primary eutectic like carbides embedded in an austenitic matrix. Also in HV alloy which has 5% tungsten, includes M 23 C 6 and M 6 C carbides embedded in an austenitic matrix. HP and HV alloys have appropriate ductility and relatively good weldability in as-cast condition. But during 8 years ex-service aging at 1050 °C, chromium-rich carbides grow gradually and their morphology changes to blocky and angular; as a result, ductility of these reformer tubes decreases dramatically and thus repair welding of aged HP/HV dissimilar joint may be at a risk. For economical saving by rewelding of aged reformer tubes and increasing the weldability propertices, partially solution annealing heat treatment at 1200 °C for 2, 4 and 6 hours was carried out. Tensile test results showed that by partially solution annealing heat treatment at 1200 °C for 6 hours can increase the elongation of aged HP alloy increases from 3% to 15% and HV alloy from 2.6% to 3.4%. Also study of Cr-rich carbides by EDS line scan analysis indicated that during partially solution annealing heat treatment, Cr atoms diffuse from outer parts of carbide to the austenite matrix and lead to raising of tensile strength at solution annealed condition. Also constitutional liquation cracking mechanism of HAZ carbides at service aged condition was fully evaluated and analyzed. In the HAZ of aged HP/HV welded joint , the cracks around the locally melted carbides , were initiated and propagated during solidification step of remelted carbides at cooling cycle of welding. This causes a decrease in ductility of aged HP and HV alloys. The mechanism of cracking around the locally melted carbides is similar to solidification cracking of weld metal. Varestraint weldability test showed that susceptibility to hot cracking and constitutional liquation cracking, decreases with partially solution annealing heat treatment at 1200 °C for 6 hours. Also this test results showed that solution annealed 25Cr-35Ni base metal has least susceptibility to hot cracking while aged 30Cr-48Ni-5W base metal has most susceptibility.
