Rate controlling processes for crack growth in hydrogen sulfide for an alsl 4340 steel
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Since the reactions o f H2S ~ with iron have been reported to be much faster than the corresponding reactions with HEO1,H.15 and H2, t~ Stage II crack growth in H2S is not expected to be controlled by the surface reactions. Studies carried out in this environment, therefore, would allow for the examination of other rate controlling processes and provide additional insight towards the understanding of environment assisted crack growth in high strength steels. Crack growth experiments were therefore carried out on the same A I S I 4340 steel (tempered at 477 K) that had been used in the previous studies, t,8 in hydrogen sulfide, to identify the rate controlling processes in this system. Crack growth kinetics were determined at r o o m temperature as a function of pressure f r o m 0.013 to 2.71 kPa, and as a function of temperature (from 230 to 423 K) at pressures of 0.133 and 2.66 kPa. Surface chemistry experiments were carried out to determine the extent, kinetics and nature of reactions of hydrogen sulfide with this steel, and the extent of reactions with fracture surfaces. The crack growth results are correlated with and discussed in terms of measured surface reaction kinetics on this steel and published data on hydrogen diffusion, and in relation to models for transport-controlled and diffusion-controlled crack growth. MATERIAL AND EXPERIMENTAL WORK A laboratory vacuum melted A I S I 4340 steel, with extra low residual impurity content, was used in this study. Modified wedge-opening-load (WOL) specimens, with thickness = 6.4 ram, width = 52.3 m m and half-height to width ratio ( H / W ) = 0.486, were used for determining the crack growth kinetics. The material, specimen geometry and orientation, and experimental procedure (with the exception of environ-
ISSN 0360-2133/81/0511o0805500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME12A, MAY 1981--805
mental control) were identical to those used in a previous study. ] Crack growth experiments were carried out inside a modified (commercially available*) ultrahigh vacuum *Perkin-Elmer-Ultek v a c u u m bell jar, Model TNB-X.
(UHV) chamber using a " s t a t i c " environment. Hydrogen sulfide was provided from a high purity source connected as a side arm to the UHV chamber through a variable leak valve. Purification was achieved through alternate freezing and thawing, and pumping away the residual impurities (principally hydrogen, produced by the reaction of hydrogen sulfide with the container wall). Prior to each experiment, the chamber was baked out and evacuated to below 0.5 t~Pa, with final precracking of the specimen carried out under this vacuum. Hydrogen sulfide was then admitted into the chamber to the prescribed pressure. Gas pressure was monitored by a capacitance manometer, and gas purity was monitored by periodic sampling with a quadrupole residual gas analyzer attached to the chamber. Test temperature was established and maintained during each experiment by either heating the chamber with electrical resistance tape
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