A Method of Predicting Hydrogen Attack in 0.5Mo Steel Equipment

Abstract

Authors investigated the conditions for hydrogen attack initiation in base metal and weld heat affected zones (HAZ) by comparing operating conditions and material properties on the basis of field surveys. Operating conditions are expressed in terms of hydrogen attack parameter Pw, derived from Shewmon's equation for determining the expansion rate of steel corresponding to methane bubble formation.
The threshold value of Pw for hydrogen attack initiation varies proportionally to massive M₂₃C₆ carbide ratio in base metal, and Pw=-7.8 is obtained as the threshold value for normalized and tempered (NT) base metal showing proeutectoid ferrite and acicular bainite with Fe₃C.On the other hand, the resistance of HAZ is correlated to hardness at HAZ. If the hardness is lowered below Hv 230 by post weld heat treatment, HAZ can be considered resistant as NT base metal is.
Authors defined the severity of hydrogen partial pressure and temperature as HAT (Hydrogen Attack Tendency), by formulating based on Shewmon's equation. Then, a nomogram named a HAT chart is developed to express the length of incubation period by comparing HAT and the threshold value of Pw. The resistance of 0.5Mo equipment to hydrogen attack can be characterized nondestructively by testing hardness at HAZ and identifying carbide morphology of base metal. And then, the remaining life of 0.5Mo equipment can be predicted by the HAT chart.