Hydrogen assisted fracture of spheroidized plain carbon steels
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THE many forms of hydrogen assisted fracture of metals can be separated into one of two general categories: 1) the normal fracture mode is somehow accelerated by the presence of hydrogen, with no change in the microscopic features of the fracture process, or 2) hydrogen causes a shift from a normally ductile fracture mode to a more brittle fracture mode. An example of the latter effect is commonly found when high strength steels are exposed to hydrogen, with the result that the normal fracture mode involving microvoid formation and growth is superseded by cleavage fracture, ostensibly due to the relaxation of local fracture stresses brought on by hydrogen segregation and accumulation at internal interfaces, l If the same alloy were to normally fracture (in air) by intergranular cleavage due to prior embrittling effects such as by temper embrittlement, for example, exposure to hydrogen would further relax the local fracture stresses but would leave the fracture mode unchanged.2 In addition, when low and medium stength steels are exposed to sufficiently large amounts of hydrogen, for instance by electrochemical chargingp large losses in ductility can be attained. Yet, while these fractures appear to be macroscopically brittle, microscopic examination reveals them to have occurred in a ductile manner? In tensile tests, hydrogen causes fracture with less nonuniform reduction in area (RA), i.e. with a less severe neck, than in the uncharged condition, thus causing a loss in macroscopic ductility. The rest of this section contains a brief chronological survey of previous ideas on the hydrogen problem in low and medium strength steels.
H. CIALONE, formerly with Brown University, is now Research Scientist, Battelle Laboratories, Columbus, OH 4320i. R. J. ASARO is Associate Professor, Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted August 14, 1980. METALLURGICAL TRANSACTIONS A
Perspectives on the Effects of Hydrogen in Low Strength Steels Despite a long standing research effort on the problem of the effect of hydrogen in metals, the precise manner by which hydrogen causes ductility losses in low and medium strength steels has not been firmly established. As early as 1941 Zappfe and Sims 5 postulated a kind of interfacial decohesion mechanism by stating that grains are composed of "submicroscopic building blocks," and that hydrogen precipitated between these blocks would assume molecular form and exert considerable internal pressure. This idea could account for such common hydrogen-related phenomena as "fisheyes," "snowflakes" and "silver steak," as well as "checks" and tears at the specimen surface, which they observed in mild steels annealed in hydrogen or cathodically charged with hydrogen. Seabrook el al, 6,7 in 1951, made some observations on the effects of hydrogen on the mechanical properties of steels. They found hydrogen to have very little effect on the stress-strain characteristics of a pearlitic 1020 steel charged at a cathodic current density of 78 mA/cm 2. Charging increa
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