Stress-Induced Migration and Trapping of Hydrogen in AISI403 Steel

Abstract

This paper presents hydrogen concentration profiles in notched tensile test specimens which have been charged electrolytically with hydrogen, with and without the application of a tensile load. A standard 8mm ASTM tensile specimen, extended at one end, is employed. This extended portion serves as a cathode during electrolytic charging. In order to facilitate the application of a load during charging, the specimen is firmly held in a specially designed fixture with the help of threads that are provided on each end of the gauge section. A notch is provided, in the gauge section, to create a stress gradient. At the end of charging, 3mm-thick disc specimens are cut from the specimen and analyzed for their hydrogen content using the inert gas fusion technique. The results show that the presence of tensile stress enhances the rate of hydrogen ingress as well as the net hydrogen concentration in the matrix. In the absence of stress, diffusion down the concentration gradient controls the hydrogen distribution within the specimen. Surface area plays an important role in the accumulation of hydrogen across any section in the specimen. If the available surface area is greater, the local hydrogen concentration is enhanced. Sheathing of the charging section with a 3mm-thick jacket of pure uranium causes a significant improvement in the hydrogen concentration along the entire length of the specimen. However, the presence of a 100µm-thick coating of titanium in the charging section of the specimen did not cause any significant change in the hydrogen concentration of the specimen. The main advantage of this charging procedure is that the test portion of the specimen does not come into contact with the electrolyte, and the hydrogen reaches the test portion of the specimen via diffusion through the matrix. Hence, microstructural damage to the specimen during the entry of high-fugacity hydrogen into the matrix is avoided.