College of Engineering
Materials Engineering Department
BS in Materials Engineering
Prof Blair London
Stainless steel medical guide wire cores, processed by Abbott Vascular, are returning anomalous mechanical properties upon strain hardening and heat treatment. As theory dictates, mechanical strength properties increase with strain hardening, and decrease with an annealing treatment. The opposite response is being observed. This is counter intuitive to fundamental materials knowledge, and a perfect materials engineering paradox. This project was designed to characterize these behaviors, and attempt to determine causation. To accomplish this, tensile testing of all processing steps mapped the mechanical property evolution of the wire. Published literature research revealed the potential of nitrogen and chromium solid state diffusion at the annealing temperature. This would result in the precipitation of chromium nitrides, which would increase mechanical strength properties by impeding dislocation movement. X-ray diffraction (XRD) was utilized for chemical composition analysis to investigate this hypothesis, looking for CrN intensity peaks, amongst the austenite and ferrite matrix. Scanning electron microscopy (SEM) and metallography was utilized for fracture analysis. A denser coalescence of microvoids after the annealing step, would dictate an increased concentration of sites to initiate fracture. CrN precipitates act as sites for fracture, therefore if the denser coalescence is observed, it will reinforce the CrN hypothesis as well. Change in response to the Adler’s Reagent etchant after the annealing processing step may indicate a change in corrosion resistance, indicating chromium depletion that occurs with CrN precipitation. Further analysis is recommended utilizing transmission electron microscopy to definitively conclude that the precipitation of the CrN phase is occurring.