The Resistance of a Control Rolled and Aged Steel to Hydrogen Induced Cracking (HIC)

Hydrogen absorbed from environment interacts with latticedefects, precipitates and non-metallic inclusions and alters themechanical behaviour of steel. This paper examines the role ofmicroalloy carbide precipitate dispersions in a thermomechanicallytreated Ti V Nb microalloyed steel to obtain a refined grain structurefor use in oil and natural gas transmission pipeslines. The positiveeffect of microalloy carbide precipitates of Ti, V and Nb on theresistance to hydrogen embrittlement (HE) of the steel is interpreted interms of the hydrogen trap theory. Accordingly, hydrogen partitionedand entrapped by fine dispersions of Ti, Nb, and V carbonitrideprecipitates is rendered relatively harmless by causing the hydrogenconcentration per trap to remain below the critical concentration levelrequired to cause hydrogen-induced cracking (HIC). Mechanical test,scanning electron microscopy (SEM) and energy dispersive x-ray(EDAX) analytical data are presented. It is shown that appropriatethermomechanical processes designed in the light of the detailedphysical metallurgy of the microalloyed steel markedly improves thehydrogen embrittlement (HE) resistance of the steel investigated. Thecontrol rolled steel aged for a short time (90 minutes) at 600oCpossesses excellent resistance to (HIC) and (HE) and stands as anattractive economical option for application in natural gas/oiltransmission pipelines.