FUSION-WELDING OF A MODERN BORATED STAINLESS-STEEL

Experiments designed to assess the fabrication and service weldability of 304B4A berated stainless steel were conducted. Welding procedures and parameters for manual gas tungsten are (GTA) welding, autogenous e lectron beam (EB) welding and filler-added EB welding were developed a nd found to be similar to those for austenitic stainless steels. Follo wing the procedure development, four test welds were produced and eval uated by microstructural analysis and Charpy impact testing. Further s amples were used for determination of the postweld heat treatment (PWH T) response of the welds. The fusion zone structure of welds in this a lloy consists of primary austenite dendrites with an interdendritic eu tectic-like austenite/boride constituent. Welds also show an appreciab le partially molten zone that consists of the austenite/boride eutecti c surrounding unmelted austenite islands. The microstructure of the EB welds was substantially finer than that of the GTA welds, and boride coarsening was not observed in the solid state heat-affected zone (HAZ ) of either weld type. The impact toughness of as-welded samples was f ound to be relatively poor, averaging less than 10 J (7.38 ft-lb) for both GTA and EB welds. For fusion zone notched GTA and EB samples and centerline notched EB samples, fracture generally occurred along the b oundary between the partially molten and solid-state regions of the HA Z. The results of the PWHT study were very encouraging, with typical v alues of the impact energy for HAZ notched samples approaching 40 J (2 9.5 ft-lb), or twice the minimum code-acceptable value. The PWHT resul ts in spheroidization of the boride such that the heat-treated welds h ave microstructures and failure modes similar to the as-received mater ial. A weld process/PWHT combination that results in acceptable proper ties was identified, and the feasibility of joining these alloys was d emonstrated.