The microstructure and properties of autogenous gas tungsten arc and laser welds in type 441 ferritic stainless steel

Abstract

Type 441 stainless steel (EN 1.4509 or UNS S43940) is a low carbon dual-stabilised ferritic grade with a nominal chromium content of 18%. This steel displays high corrosion and oxidation resistance, good formability, excellent high temperature strength and thermal fatigue resistance. Type 441stainless steel is used primarily in high temperature applications such as the automotive exhaust systems. The ferritic grades of stainless steel are difficult to weld successfully, especially in thicker sections, and for applications involving welding, the recommended plate thickness is limited to 2.5 mm for type 441. This investigation studied the weldability of type 441 stainless steel in thicker sections, with specific emphasis on the microstructure and mechanical properties of the weld metal and heat-affected zone after gas tungsten arc welding and laser beam welding at various heat input levels. The precipitation of intermetallic compounds (such as Laves and sigma phase) during the weld thermal cycle, carbide precipitation and grain growth in the weld metal and high temperature heat-affected zone were considered. The results indicate that the microstructures of the weld and heat-affected zone that form during autogenous welding of type 441 stainless steel are complex and strongly dependent on the cooling rate after welding (and therefore the weld heat input used). Laves phase, sigma phase, M23C6 carbides and needle-like titanium-rich carbides (with niobium in solid solution) were observed in the welds and heat-affected zones of gas tungsten arc welds. The presence of intermetallic compounds and carbides embrittled the weld metal and increased the hardness of the weld metal significantly. The fusion zones of the laser welds were observed to be mostly free of second phase particles, whereas the heat-affected zone contained partially dissolved cuboidal titanium-rich carbides and some M23C6 carbides (in the higher heat input welds). The laser welds displayed significantly higher strength and ductility, which can be attributed to the lower heat inputs utilised and the finer grain sizes obtained. Although type 441 is dual-stabilized with titanium and niobium, welding at low heat input levels resulted in chromium-rich M23C6 precipitation in the high temperature heataffected zone during cooling, effectively sensitising the welds to intergranular corrosion. Sensitisation in gas tungsten arc welds was limited to a narrow region of the HTHAZ adjacent to the fusion line in low heat input welds, but extended well into the HTHAZ and weld metal at heat inputs of 0.3 kJ/mm and 0.45 kJ/mm. Sensitisation was mostly suppressed in samples welded at a heat input of 0.7 kJ/mm. Sensitisation was observed in the weld metal of laser welds performed at 0.11 kJ/mm, and in the weld and HTHAZ after welding at 0.23 kJ/mm.