The enhancement of hydrogen attack in steel by prior deformation
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I.
INTRODUCTION
I T has been known for some time that the cold working of steel increases its susceptibility to hydrogen attack (HA). ~,2 Recently Ransick and Shewmon 3 have studied the effect of prior deformation on HA in a Si-killed 1020 steel. Rectangular bars of the steel were bent before being exposed to hydrogen gas at elevated temperature. Comparison of the extent of damage in bent and in undeformed regions revealed the expected enhancement of attack with cold work. Well before any evidence of HA was found in the undeformed material, fissures had opened up around the edges of pearlite colonies in the bent bars. Furthermore, a pronounced anisotropy in the damage at ferrite/ferrite grain boundaries was noted in the deformed specimens. Attack was seen most frequently on boundaries parallel to the long axis of the bars in the tensile portion of specimens and on transverse boundaries in the compressive regions. Ransick and Shewmon found that the influence of deformation on HA could be eliminated by annealing bent bars at 650 ~ for one hour before exposure to hydrogen. A directionality of damage similar to that described above has been seen in nickel base superalloys strained at room temperature and annealed at high temperature: small voids form on grain boundaries parallel to the stress direction in the case of tensile deformation, but on transverse boundaries in material subjected to compression. 4'5'6 Later investigation7 showed that the cold work produces microcracking at grain boundary carbide/matrix interfaces and that this microcracking is the process whereby the voids are nucleated. It also was found that the residual stresses at the interfaces play an important role in growth of the voids when the material is heated. 4'5'6 (The residual stresses are caused by the difference in deformability between the hard carbides and the matrix. 6) S. E CLUGSTON, formerly with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60201, is now a Rotary Foundation Scholar, University of Auckland, Auckland, New Zealand; J.R. WEERTMAN is Professor, Department of Materials Science and Engineering and the Materials Research Center, Northwestern University, Evanston, IL 60201; and E G. SHEWMON is Professor, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. Manuscript submitted June 17, 1982.
METALLURGICALTRANSACTIONS A
The goal of the present research is to see if an analogous situation occurs in the deformed steel. Does bending the bars cause microcracking at cementite/ferrite interfaces? If so, does the one hour anneal at 650 ~ which erases the effect of prior deformation on HA, also heal up the cracks? What is the cause of anisotropy in the grain boundary damage in the deformed bars exposed to HA?
II.
EXPERIMENTAL DETAILS
The experimental details of specimen preparation and exposure to hydrogen are given in Reference 3. Briefly, rectangular bars of 1020 Si-killed steel with dimensions 6.4 • 12.7 • 63.5 mm were deformed in three point bending
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