M. Arivarasu

Hot-corrosion resistance of dissimilar AISI 4340 and AISI 304L weldments in the molten salt environment at 600°C

ABSTRACT High temperature corrosion resistance of welded components in power plant industry, hot sections of gas turbines, boilers, industrial waste incinerators, metallurgical furnaces and petrochemical installations is one of the serious necessity. The failure of hot sections of welded component is due to the deposition of molten salt which accelerates the hot corrosion reactions in the weld joint. In this research work, the oxy-chlorination of dissimilar weldments between AISI 304L austenitic stainless steel and AISI 4340 alloy steel at 600°C in eutectic mixture of K2SO4 + 60% NaCl under molten salt conditions was investigated. Corrosion kinetics for this dissimilar joints calculated from the thermogravimetric charts were represented. Corrosion on the dissimilar joint employing E309L filler in pulsed current gas tungsten arc welding process is found to be very aggressive, due to the fragmented delta ferrites at numerous locations resulting in the formation of the micro electrochemical cells, which act as sites for corrosion initiation and thus leading to higher corrosion rate. However, in the case of continuous current, the joint is able to protect itself better in this environment due to lower amounts of delta ferrites in weld zone, which is attributed to the higher temperature available in continuous mode leading to austenisation of delta ferrites in the multipass welding.

Hot Corrosion of Weldments

Hot corrosion of the weldment is the result of the reaction between oxidation and sulfidation, which affects welded joints exposed to combustion gases containing sulfate coal ash at high temperature. It is also demonstrated that welding processes, weld consumables, temperature of exposure, and mixture of salt environment are important factors in hot corrosion of the weldment. Hot corrosion is the main failure mode of welded components in the hot sections of gas turbines, boilers, metallurgical furnaces, and petrochemical installations. In this regard, the cyclic heating and cooling can mainly influence the corrosion behavior due to possible thermal stress on the weldment and their corrosion products. Hence, the maximum service temperatures of welded joints are decreased when exposed to hot corrosion environments.

Comparative studies on GTA and PCGTA weldments of AISI 4140 and AISI 316 dissimilar metals

In this study, an attempt has been made to investigate the weldability, mechanical properties of dissimilar weldments of AISI 4140 and AISI 316. These dissimilar metals were welded using gas tungsten arc welding (GTAW) and Pulsed Current GTAW (PCGTAW) employing ER309L filler wire. It was observed from the tensile test results that the fracture was occurred at the parent metal of AISI 316 in all the trials for both the weldments. On examining the SEM fractographs, the mode of fracture was found to be ductile in nature. In addition, the scanning electron microscopy and electron dispersive X–ray spectroscopy (SEM/EDS) analysis was performed at various zones of the as–welded samples of AISI 4140 and AISI 316 to reveal the structure–property relationships.