A novel approach for quantifying hydrogen embrittlement using side-grooved CT samples

Abstract

Aerospace parts made of high strength steels, such as landing gears and helicopter transmissions, are often electroplated to satisfy various engineering specifications. However, plated parts are occasionally rejected because of hydrogen embrittlement, and the industry has few means of evaluating quantitatively the actual damage caused by hydrogen. In the present article, we developed a novel method to measure the stress intensity threshold for hydrogen embrittlement (Kth) in industrial plating conditions. The method consists in plating side-grooved CT samples in industrial plating baths, and measuring Kth with an incremental step loading methodology. We validated the method with a benchmark case known to produce embrittlement (omitted post-plating bake) and we used the method on a test case for which the level of embrittlement was unknown (delayed bake). For the benchmark case, we measured a Kth of 49.0MPam0.5 for non-baked samples. This value is significantly lower than the fracture toughness of the unplated material, which is 63.8MPam0.5. We conclude that this novel combination of geometry and test method is efficient in quantifying hydrogen embrittlement of samples plated in industrial conditions. For the test case, the Kth are respectively 57.9MPam0.5 and 58.8MPam0.5 for samples baked 100 h and 4 h after plating. We conclude that delaying the post-plating bake does not cause hydrogen embrittlement in the studied conditions. Using a finite element hydrogen diffusion analysis, we argue that the side grooves on CT samples increase the sensitivity to hydrogen embrittlement in comparison to smooth samples. In smooth samples, a zone of plane stress at the surface of the specimen shields hydrogen from penetrating to the center of the specimen, a phenomenon which is alleviated with machining side grooves.