Variation in diffusible Hydrogen Content and Hydrogen Assisted Cracking Susceptibility of Cr-Mo Steels

In this paper results obtained from diffusible hydrogen (HJ measurement and hydrogen assisted cracking susceptibility (HAC) of three different Cr-Mo steel welds are discussed. H₀ measurements were carried out using a gas chromatograph and HAC susceptibility studies using the ur-modified hydrogen sensitivity test (UT-Modified HST). Specimens for both these studies were prepared by employing identical welding parameters. Results showed that with increase in alloy content the cracking susceptibility of the steel increased while H₀ content in the weld decreased. The decrease in the H₀ content is explained based on decrease in the apparent diffusivity and increase in the apparent solubility of hydrogen with alloy content. These are in turn attributed to increase in the density of hydrogen traps with alloy content. Regression analysis of the results showed that it is possible to predict the safe preheat temperature for the steel from the composition if H₀ content in the weld is known.