Welding metallurgy's putative influence on Björk-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 putative defects of uncertain character and significance. This study differs from all others in that it systematically assesses a large number (n = 60) of intact and fractured valves and clearly differentiates findings on the critical, 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 subjected to multiple metallographic sectioning of each strut-flange interface. All fractures and selected intact valves were further analyzed with X-ray dispersive spectroscopy. RESULTS: Fatigue striations were seen in all fractured valves, and their orientation 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 initiated outside of the weld. Element segregation areas were seen in 40% of welds, significantly more commonly in intact valves, invariably located on the outflow side, and typically (85%) apart from the fracture path. Microporosity was identified in 15% of welds, usually near the outflow side, and in the same proportion of intact and fractured valves. One fracture surface had a 38 microns, inflow-side void that might have been a factor in crack initiation 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 the process of outlet fracture.