Microstructural features and properties of TIG melted AISI 430 ferritic stainless steel welds

Abstract

Extensive grain growth in ferritic stainless steel welds causes severe loss of ductility and other properties which limits the usage of this low cost stainless steel in many structural applications. While a low energy input and faster heat dissipation conditions have been suggested for grain growth control, the range of the process parameters that falls within these conditions is not well identified. Therefore, it has not been possible to optimize the microstructure and properties of ferritic stainless steel welds. In this work, the microstructural features of AISI 430 ferritic stainless steel welds produced using TIG torch melting at different process parameters were studied and developed a relationship between process parameters and mechanical properties. Furthermore, two new schemes were employed to refine grain structures and their influences on chromium carbide precipitation in the weld are discussed. The investigation was conducted in three phases. In the initial phase, the low energy input conditions were identified for welding the 1.5 mm thick AISI 430 ferritic stainless steel used in this work. Arc currents in the range of 70-110 A and welding speeds in the range of 2.5 -3.5 mm/s were identified as safe welding conditions for this material. Within these process parameters, the ductility of the weld was up to 45% of the base metal which is higher than the values reported in the literature. In the second phase, the new schemes to refine grain structures by the incorporation of elemental metal powders into the melt pool and cryogenic cooling of the weld were studied. These new schemes for refining the weld microstructure offered dual benefits of grain refinement and constriction in weld dimensions. The constriction in weld geometry is found to be very significant and it is beyond the range reported in any of the existing grain refinement strategies. However, the addition of metal powder provided greater benefits in terms of grain refinement and constriction in weld geometry, but it precipitated hard intermetallic particles in the microstructure resulting in low ductility. The precipitation of such hard particles was absent in the cryogenic cooling technique. The mechanical properties of welds are influenced by both the grain size and the phases present in the microstructure. In the final phase, chromium carbide precipitation in the welds under different grain refinement conditions was evaluated and found that the precipitation of carbide could be prevented when the weld was processed with an energy input less than 500 J/mm. The addition of metal powder such as a mixture of aluminum and titanium or cryogenic cooling did not facilitate carbide precipitation; however, the addition of aluminum powder into the melt pool facilitated carbide precipitation and increased sensitization in the welds. The present investigation achieved over 80% improvement in weld ductility via cryogenic cooling without affecting the sensitization resistance of the steel. This level of ductility is significantly higher than the maximum of 65% achieved with existing grain refinement techniques in fusion welding and is only comparable to those of the friction stir welding which generates ductility of over 90% of the base metal in AISI 430 ferritic stainless steel welds. Furthermore, the work developed an innovative parameter, the grain refinement index, for the evaluation of the degree of grain refinement for a given treatment condition relative to the base metal, not to the weld metal, which is the common practice in existing grain refinement techniques.