Welding metallurgy's putative influence on Bjork-Shiley convexo-concave valve outlet strut failures

Background and aim of the study: Multiple reports of convexo-concave valve outlet strut fractures have focused on the welds, often implicating putativ e defects of uncertain character and significance. This study differs from all others in that it systematically assesses a large number (n = 60) of in tact and fractured valves and clearly differentiates findings on the critic al, tensile-stressed, inflow side of the outlet strut leg from those on the outflow side, which are subject only to compression. Methods: Each valve was examined by scanning electron microscopy and subjec ted to multiple metallographic sectioning of each strut-flange interface. A ll fractures and selected intact valves were further analyzed with X-ray di spersive spectroscopy. Results: Fatigue striations were seen in all fractured valves, and their or ientation indicated that every fracture initiated in an area on the inflow side, spreading out progressively towards the outflow side. Data indicated that 22% of the first-to-fail leg separations and 17% of all fractures init iated outside of the weld. Element segregation areas were seen in 40% of we lds, significantly more commonly in intact valves, invariably located on th e outflow side, and typically (85%) apart from the fracture path. Microporo sity was identified in 15% of welds, usually near the outflow side, and in the same proportion of intact and fractured valves. One fracture surface ha d a 38 mu m, inflow-side void that might have been a factor in crack initia tion in this valve, which was highly stressed. Conclusions: With this singular exception, no metallurgical feature of any weld was found that appeared to have contributed, even in a minor way, to t he process of outlet fracture.