Strain controlled vs stress controlled hydrogen induced fracture in a quenched and tempered steel
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THE general phenomenon
of hydrogen-induced cracking of structural alloys has been a subject of intense study by a number of research groups in recent years. Much of this attention has been directed toward the behavior of iron and its alloys, including quenched and tempered steels having a wide range of yield stress. Perhaps the greatest area of controversy which has emerged from these studies centers around the apparently contradictory observations of the m o d e of hydrogen-induced cracking (HIC) on the microstructural level. On the one hand, a number of investigators have observed HIC to be intimately connected with the process of plastic deformation and to occur on [1101 or {1121 slip planes 1,2 or {112} subboundaries 3 in iron crystals, on {1101 planes in martensite lathes or along lath boundaries in martensitic steels, 4 or along surfaces o f maximum shear strain (the "slip-lines" of the macroscopic slip-line field) in notched spedimens of quenched and tempered steel? In other studies 6,7 it was observed that the presence of hydrogen produced plastic flow, or plastic instability, at abnormally low stresses. While the detailed mechanisms involved in this kind of HIC have not yet been made clear, its existence has now been well documented. It is apparently closely related to the process of plastic strain by dislocation motion. On the other hand, it is generally recognized that cracking of components of quenched and tempered structural steels in service in environments which contain or produce hydrogen usually occurs at least partly along prior austenite grain boundaries. A substantial amount of research on this apparently brittle, stress controlled cracking mode has shown conclusively that it occurs because of the presence of segregated metalCHARLES J. MCMAHON, JR. is Professor, Department of Materials Scienceand Engineering, University of Pennsylvania, Philadelphia, PA 19104, and Y. TAKEDA is Metallurgist, Mitsubishi Heavy Industries, Nagasaki, Japan. Manuscript submitted September 30, 1980. METALLURGICALTRANSACTIONSA
loid impurities which reduce intergranular cohesion. 8-~2 It has been found that the stress intensity needed to produce detectable amounts of crack extension decreases as the impurity concentration increases; 8,9,12 this is accompanied by a corresponding increase in the a m o u n t of intergranular fracture. 9,1~ This type of cracking has been rationalized s-13 in terms o f the widely-held concep04-~6 of segregation of hydrogen to the region of large hydrostatic tension ahead of a notch or crack and the lowering of the cohesive strength of the metal in proportion to the local hydrogen concentration. Clearly, we are faced with two apparently different modes of HIC, and this raises several obvious and important questions: What factors determine which mode of cracking will occur? What are the fundamental differences in mechanism between the two cracking modes? Are the two modes equally harmful as far as the performance of steel is concerned? In the course of an investigation of HIC in a series of l
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