Detection of Hydrogen Embrittlement in Plated High-Strength Steels with Eddy Currents: Is the Sensitivity Sufficient?

Hydrogen embrittlement (HE) of high-strength steels has been a long-standing issue in the aerospace industry. Detecting the presence of hydrogen in steel with a low-cost and fast non-destructive method would be useful to avoid wasting parts. In this paper, we evaluate the feasibility of using eddy currents to achieve this goal. At first, we evaluate HE and hydrogen content in a series of chromium-plated 4340 steel notched bar samples (extremely embrittled) using sustained-load tests and thermal desorption spectroscopy. The failure of the notched bars at only 20% of their maximum mechanical load is coherent with the high hydrogen concentration in the samples, which can be as high as 1400 ppm atomic (similar to 30% in steel, similar to 70% in the coating). However, a baking at190 degrees C for 23 h can desorb most embrittling hydrogen in the 4340 steel and relieve HE. Therefore, successive bakings are used to progressively desorb hydrogen from the samples and produce various hydrogen concentrations. By using eddy currents after each baking, coupled with a finite element model implemented in COMSOL, we can deduce the electric and magnetic properties of steel and chromium as a function of hydrogen concentration. Unfortunately, no significant variation of these properties can be observed, even for concentration variations as large as similar to 900 ppma. The only observable changes happen after performing bakings at high temperature, which induce microstructural changes in the samples. However, eddy currents remain a good approach to detect small changes in electromagnetic materials properties resulting from other causes than hydrogen.