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Design-of-Experiment Approach to Hydrogen Re-Embrittlement Evaluation
Scott Grendahl | US Army Research Laboratory
Solvent substitution for maintenance and overhaul operations of military systems has been a primary environmental concern for many years, and cadmium replacement in these systems has been targeted for decades. Both of these areas have a common obstacle for implementation of any potential alternate: hydrogen embrittlement. Fully understanding the potential for failure is crucial to the decision process on where and when these alternate chemicals and coatings can be safely implemented. To satisfy all of the Department of Defense (DoD) and original equipment manufacturer (OEM) requirements, and to qualify just one maintenance chemical to work with a cadmium replacement, such as Zn-Ni, on high strength steel, many tests with varied parameters are required. Under current conditions, the total amount of testing required to qualify a single cadmium replacement has proven cost and time prohibitive and leaves many questions unanswered when a failed result is achieved.
The overall objective of this project was to determine the material strength level and applications where the alternative chemicals and coatings could be safely used on high strength steel, which would lead to a consolidated test method.
A three-phase approach was developed to demonstrate the hydrogen threshold for the applicable chemicals and coatings. The first phase centered on the test material and geometry. Hydrogen susceptibility over a range of material strength, load level, and hydrogen-emitting environments was evaluated in two different material grades of 4340 steel. The five common geometries from ASTM-F519 were evaluated to discern the most susceptible to hydrogen. In the second phase, five prominently used solvent substitute maintenance chemicals were tested to mimic the industrial manufacturing and maintenance cycle. For this phase, a design of experiment (DoE) approach was used to vary parameters over a range of material strength, load level, and maintenance cleaning solution concentration for aerospace grade 4340 steel. The third phase evaluated the most prospective cadmium alternatives in combination with the resulting parameters of the first two phases. Not all chemical cleaners and not all cadmium alternatives could be characterized. The emphasis was focused on the most prevalent chemicals and coatings being used in the aviation industry, where the most impact could be made in lessening the implementation restrictions. Contrary to the existing standard, greater information was gleaned beyond the result of a pass/fail test. By incorporating the failure time, load, and stress level data into DoE failure models, predictive equations over the broad ranges were developed.
Phase one results showed that across all geometries tested, air-melted 4340 steel demonstrated more tolerance to hydrogen than aerospace grade 4340 steel, which was not expected. The DoE predictive models run in phase one suggest that if the material demonstrates no hydrogen sensitivity at a 3.5% salt concentration environment for 168 hours at a specific strength and applied load combination, then it should not be expected to fail in a lifetime of service exposure in a natural environment at that strength and applied load level.
Phase two results clearly demonstrated the differences among the test chemicals. A comparison with all models across test chemicals showed that nearly all the chemicals do not produce a severe enough environment to induce hydrogen re-embrittlement at or below the 210-ksi strength level. The sensitivity increases with material strength level, applied load, and to a lesser degree, with cleaner concentration. All of these trends are in line with traditional expectations.
Phase three results also clearly demonstrated the differences among the coatings tested.
This project developed the hydrogen threshold for the most common maintenance cleaners and cadmium replacement coatings, in combination, applied from the smallest amounts to the worst case scenario, on aerospace VAR 4340 steel utilized on critical components of the aviation industry. These data and their corresponding predictive equations and models allow the Army Aviation governing authority to relax the current risk assessments for these materials and further the implementation of prospective chemicals and coatings in an intelligent and informed manner, especially for those materials below the strength level tested in ASTM-F519. This will greatly increase the number of considered applications and materials, thus widening the use of alternatives, and reducing the use of environmentally unsafe chemicals and coatings.