Robust Field Capable Assessment of Surface Modified Nickel Superalloys
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abstract
Mechanical failure of polycrystalline metal alloys is difficult to predict with confidence, in large part because the microstructure of a material is an aggregate of a variety of grains, grain boundaries, and precipitates. Depending on conditions, any one of these microstructural elements may be the materialâ s â weakest link,â causing initiation and propagation of component-level damage. For example, equipment used under high pressure, although made from seemingly identical high strength, high corrosion resistance alloys, may fail under vastly different conditions (stress, temperature, and environment). This variance happens because batch-to-batch variations of alloy microstructure yield distinct distributions of microstructural weakest links. The accuracy of current failure models is inherently limited because they do not take microstructure into account. We propose to construct vastly improved lifetime models by taking each componentâ s unique microstructure into account when predicting failure. Our approach involves four innovations: a) high-throughput, quantitative microstructure characterization using reflectance-based optical methods, b) small-scale mechanical testing to identify microstructural â weakest links,â c) microstructure-informed failure modeling, and d) the development of a field-capable tool that will enable lifetime predictions for components in service conditions. The techniques to be developed are applicable to any polycrystalline metal. To maximize the relevance of our work to QNRFâ s research priorities, we will focus on two specific model materials, namely Monel 400 and Alloy 718, and one specific failure mode: hydrogen embrittlement. Hydrogen embrittlement is a significant factor in well tool failure in the oil and gas industry, due to the elevated temperatures and pressures the tools encounter downhole. Gas fields, such as those found in Qatar, expose the tools to a hydrogen environment that leads to substantial hydrogen uptake, particularly in nickel based alloys. Because existing models and testing methods are inadequate, oil companies are forced to rely on conservative lifetime limits for tools, increasing material costs and rig downtime. Our proposal aims to develop a fast, simple tool, useable at the rig site, which enables more accurate life predictions with a minimum of downtime. By integrating reflectance-based optical methods with the failure modeling, we expect to develop a tool that rig crews could use on site to evaluate the remaining life of oil tools in high impact or stress service, eliminating the need to send specimens back to labs and increasing confidence in end-of-life predictions.
