Gray Iron Aging Effects As Measured Using NDT and Microstructural Analysis

Abstract

Gray (flake graphite) iron is well known to age at room temperature after casting, as evidenced by increasing strength and hardness over time, but there are conflicting theories/beliefs regarding the cause(s) of such aging response(s). Some researchers have concluded that those aging responses result from slow precipitation of submicroscopic nitrogen precipitates that are modified by interaction with titanium,1–7 but both of those elements are also known to have significant effects on graphite and matrix morphology.8–10 The results of this study correlate the logarithmic room temperature aging responses as measured by pitch and catch ultrasonic velocity testing to quantified metallography metrics and mechanical properties in providing an alternate explanation for the aging responses of gray iron. Since the positive logarithmic aging curves are characteristic of natural decay phenomena of which stress relief is an example, stress relief is a viable alternate explanation for gray iron aging effects.