Hydrogen degradation of spheroidized AISI 1090 steel
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I.
INTRODUCTION
T H E R E is a long history of conflicting results on the effect of hydrogen on the mechanical properties of iron and steel, with some results indicating that hydrogen softens the materials and others that it hardens them. Several reviews 1'2'3 include discussions of early work, a notable example being the suggestion by Beachem4 that softening behavior near crack tips enhances crack propagation. Kimura and coworkers 5'6 provided strong evidence that precharged or dynamically charged hydrogen softens high purity iron at low strains but hardens it at large strains characteristic of multiple slip behavior. More recently, Tabata and Birubaum performed in situ transmission electron microscopy and showed directly that low fugacity (40 kPa) gaseous hydrogen caused the enhancement of both screw dislocation velocity and dislocation multiplication in pure iron y and enhanced crack propagation by causing localized flow at the crack tip.S In a series of studies of plain-carbon steels, hydrogen was found to have little effect on the stress strain behavior for homogeneous, stable flow 9-12 in agreement with early work by Toh and Baldwin. ~30riani and Josephic ~4.~sfound slight hardening by hydrogen for spheroidized 1045, plane-stress sheet samples. On the other hand, microhardness studies of spheroidized 1090 indicated slight reversible softening below a critical hydrogen fugacity of 1.5 GPa and marked irreversible softening above that value. 16,17Also, four-point plane-strain bend tests of spheroidized 1045 steel showed hydrogen induced softening for low strains, but hardening for larger strains.~8 In contrast to the minimal effects in the stable flow region, hydrogen has marked effects on the onset of unstable flow, particularly in spheroidized steels. In plane-strain bend tests, indirect observations of microvoids aligned along characteristic slip traces, 11 the association of void configuration with plastic shear incompatibility stresses, 11 and observations of surface instabilities 12and of shear crack initiation H't2 all showed that hydrogen enhances the tendency for instability, the critical plastic strain for unstable flow being lowered by about a factor of two. Similar tests on quenched and tempered and pearlitic steels suggested a
S.C. CHANG, formerly Graduate Student, The Ohio State University, is now Associate Professor, Department of Materials Science, National Tsing Hua University, Hsinchu, Taiwan, People's Republic of China. J. R HIRTH is Professor, Metallurgical Engineering Department, The Ohio State University, Columbus, OH 43210. Manuscript submitted December 6, 1984. METALLURGICAL TRANSACTIONS A
role of enhanced instability in crack nucleation. 19,20Lin and Oriani 21 studied plane-strain bend specimens and suggested that the strain for plastic instability increased with hydrogen charging current density until currents near those for irreversible damage were achieved. However, the instability criterion was the observation of Liiders bands on the relaxed plane-stress lateral surface of the sp
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