THERMAL FATIGUE RESISTANCE OF WELDED JOINTS IN STAINLESS STEEL HIGH TEMPERATURE EXHAUST APPLICATIONS

Abstract

Weld seams often constitute critical points in the thermo-mechanical fatigue design of a stainless steel automotive exhaust manifold. Therefore, a thermal fatigue test on V-shaped specimens was developed by ArcelorMittal Isbergues Research Centre to simulate the thermo-mechanical loading of such a part. Two ferritic base metals dedicated to high temperature applications, together with various filler materials (both austenitic and ferritic grades) commonly used in the exhaust market were tested with a 250-950°C thermal cycle. The results were compared in terms of lifetime, cracking mechanisms and micro-structural evolution in order to point out the best base/filler metal combinations. For austenitic weld metals, the higher thermal expansion coefficient than ferritic weld metals (50% higher) led to a more pronounced oxidation and a higher level of stresses generated at the interface between melted zone and base metal. Consequently, the cracks were localized to this interface for austenitic filler material while they appeared in the base metal and out of the heat affected zone for ferritic filler material.