A manufacturer of prestige cars asked TWI to investigate the failure of a flange-to-tube MIG weld which had occurred after approximately 3000 miles of durability testing in a prototype exhaust system.
The flange was fabricated in carbon steel with a tube made from 304 austenitic stainless steel. These components were MIG-welded with Type 316L austenitic stainless steel filler.
During test, the welded component was held rigidly at the triangular flange end, with the tube remaining unsupported. Several such samples, which had suffered similar short-life durability test failures, were forwarded to TWI for recommendations on the quality of welding and choice of filler metal, and for advice on how future failures could be avoided.
Following a full metallurgical investigation and consultation with welding engineers and fatigue experts, it was concluded that the cause of failure was fatigue which had initiated at the weld toe, at the point of maximum service stress. Cyclic stresses resulting from the road excitation vibrations were concentrated at the changes in geometry along the pipe, i.e. at the weld toe, thus generating conditions conducive to fatigue crack growth. The quality of welding did not significantly affect the service life of the flange-to-tube weld.
To improve the fatigue performance of the welded component TWI recommended that the magnitude of stress cycles be reduced by redesigning the geometry of the component or by changing the overall design of the exhaust system.
The exhaust system was modified with an additional mount and brace and subsequently passed all further durability tests.
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