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Laser beam welding of austenitic-ferritic transition joints

Transition joints between austenitic stainless steels and ferritic low alloy steels are extensively utilized in many high-temperature applications in energy conversion systems. Problems related to the use of such dissimilar metal welds (DMWs) have long been recognized, because of premature failures often occurring during service, connected to thermal stresses generated at the weld interface and to metallurgical changes (carbon migration, carbide precipitation) observed after prolonged exposure to high temperature. This paper reports the results of an investigation on DMWs carried out on plates and tubes by a deep penetration laser beam welding (LBW) process, within the framework of researching innovative welding procedures to allow for a better control of metallurgical changes and a minimization of thermal stresses. The experimental work included metallographic observations, hardness tests, x-ray diffractometry and estimation of phases on melt zones. These data are compared with the evaluation given by the Schaeffler diagram. The results show the possibility of obtaining chemical compositions and phases according to predictions. In most instances, the melt zone constitution was close to the desired one. Further trials with filler metal of more proper composition are in progress, to improve the soundness of the joint and to optimize structure of the melt zones.