Influência do gás de proteção na soldabilidade do aço inoxidável ferrítico aplicado a sistema de exaustores automotivos

Abstract

The automotive exhaust industry has gone through several evolutions over the last 20 years in order to comply with the stringent gas emission control policies and due to longer durability, increased engine efficiency and weight and cost reduction requirements. With that in mind, ferritic stainless steels have been progressively more used in exhaust systems, being traditionally and exclusively welded with austenitic wires to ensure good weld quality. Recently, ferritic wires have also been developed to provide good weld quality as well as significant cost reduction. However, it is necessary to conduct studies on these wires behavior, particularly regarding the shielding gas influence. Thus, the aim of this work was to assess the shielding gas influence on the ferritic stainless steel weldability, evaluating the joint microstructure, mechanical properties and susceptibility to sensitization. Two ferritic base metals (UNS 43932 and AISI 441), both used in automotive exhaust systems, five shielding gases (pure Ar, Ar+2%O2, Ar+4%CO2, Ar+8%CO2 and Ar+25%CO2) and three solid wires (ER308LSi, ER430Ti and ER430LNb) were utilized. The welding parameters were determined for butt joints with a three-millimeter gap, taking the welding current and arc voltage into account to ensure the same heat input for all conditions tested. It was concluded, for the shielding gases studied, that significant variations in the weldability only took place for the ER430Ti wire. In this case, weld ductility reduction, including martensite formation when it was welded with 25%-carbon-dioxide shielding gas and intergranular corrosion susceptibility increase occurred. For the other two wires (ER308LSi and ER430LNb), welding feasibility was verified with all the shielding gases studied, because it was not observed weld ductility variation and little intergranular corrosion. Then, because of the weld quality and the cost it is better to use ER430LNb wire using 25%-carbon-dioxide shielding gas.