Monitoring Hydrogen Release from 4140 Steel Using the L_CR Ultrasonic Technique

Hydrogen build-up and release in steel microstructure is of considerable interest in studying the mechanical performance of the material. L_CR ultrasonic waves are a potential research tool that will enable the nondestructive evaluation of this reversible hydrogen buildup since their speed is affected by stress and their travel path is through affected layer just below the surface. The present study used three blocks of 4140 steel cut from a longer sample. Frequency spectrum data also were collected. The assumed model was of hydrogen occupying the vacancies in the steel block during the high temperature high-pressure period in the autoclave, and then being purged upon removal as the block contracted, creating a residual tensile stress in the surface area of the block. As the hydrogen is purged, the tensile stress should decrease. The L_CR results confirmed this trend, although it was more strongly seen for the 11.7 MPa (1691 psia) block. Photomicrographs confirmed no cracks in the surfaces, although there was considerable decarburization. Frequency analysis showed that the decarburization did affect the frequency spectrum but did not affect the L_CR wave speed.