430LNb - A New Ferritic Wire for Automotive Exhaust Applications 2000-01-0314

The increasing use of ferritic stainless steels (AISI 409, 439, 436 and 441) in automotive exhaust systems, especially for manifolds and catalytic converter canning, has led the authors to develop a new ferritic welding wire, designated 430LNb. This new material is recommended for the GMAW and GTAW processes and provides better metallurgical compatibility with the ferritic base metals, in terms of both thermal expansion and microstructure.

The composition of the new welding wire has been adjusted in order to guarantee an entirely ferritic structure in the welds of ferritic sheet materials, together with good resistance of the welds to both wet corrosion and high temperature oxidation, corresponding to the conditions encountered respectively in the colder and hotter parts of the exhaust line. This is achieved by limitation of the C (<0.02%) and N (<0.02%) contents, stabilisation with Nb, such that Nb > 0.05 + 7 (C + N) and Nb < 0.5%, and a Cr content of 17.8-18.8%.

GMAW weldability tests on 1 and/or 1.5 mm thick sheets of AISI 409, 436 and 441 grades, using 1 mm diameter 430LNb welding wire, gave good quality beads (shape, structure, tensile properties, bending, intergranular corrosion resistance), with a welding speed of the order of 2 m/min. The quality of the weld seams obtained was at least as good as that of welds made on the same sheets using austenitic filler materials (308LSi and 307Si). Tests specific to the automotive exhaust application have been undertaken in the laboratory in order to compare the service behaviour of the welds with that obtained using austenitic filler wires. These included fatigue endurance tests performed in the tension-compression mode between 300 and 950°C, thermal fatigue tests involving cycling of restrained specimens between 250 and 900°C, and dip-dry corrosion-oxidation tests simulating the combination of salt attack and high temperature corrosion. All the results obtained confirm that the 430LNb welding wire is at least as good as the austenitic filler materials most commonly employed in Europe. Tests carried out on real components by exhaust system manufacturers also confirm the high productivity and good quality of the welds.