Hydrogen-assisted ductile fracture in spheroidized 1520 steel: Part II. Pure bending
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INTRODUCTION
D U C T I L E fracture has been of both fundamental and applied interest for some time. Recently, there has been significant work going well beyond the traditional attention to behavior of tensile specimens and establishing the fundamental fracture phenomena in notched and cracked specimens, as has been reviewed, tl,2,3j The same broadening of interest has also been evident with regard to hydrogen phenomena in ductile fracture, with, for example, renewed attention to hydrogen effects at notches.[4,Sl This article complements the previous article on tensile fracture [61 and draws upon earlier results [7,8,91 in addressing ductile fracture in bend bars. The goal is to clarify the process, or micromechanism, by which hydrogen alters ductile fracture behavior without changing the mode of fracture. It seems clear that this can only be done with generality if fracture is studied under varying states of stress and strain, t~-5~ which is most conveniently accomplished through the use of different specimen geometries. Although hydrogen may decrease the ductility, it is still ambiguous whether this is due to hydrogen effects on void initiation, growth, or void coalescence in ductile fracture processes of spheroidized steels. Also, there are reports that hydrogen causes enhanced plasticity prior to void initiation. Due to these disparate results, there have been difficulties in identifying the intrinsic effects of hydrogen on ductile fracture processes. The differences in results among various research g r o u p s [6'1~ may be related to (1) the presence of metalloid impurities, (2) the different specimen geometries and, consequently, different stress and strain states, and (3) different charging fugacities. These issues have been discussed elsewhere [4-6A8'19[ and are addressed in the present article also; the background on the three possibilities just mentioned is as follows. IN-GYU PARK, formerly with Carnegie Mellon University, is Group Leader, Fracture and Materials with the Research and Development Center, Korea Power Engineering Company, Seoul, Korea. ANTHONY W. THOMPSON, Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 23, 1990. METALLURGICAL TRANSACTIONS A
A. The Presence of Metalloids Since small changes in chemistry can affect surface and interfaciai energy, the initiation of voids can be significantly influenced by impurity levels or solute additions. [2~ Impurities, such as S, P, and other metalloids, are known as dellimental elements. In spheroidized steels, the considerable effect of manganese sulfide with hydrogen must be considered, based on the fact that manganese sulfides are stronger t r a p s [23'24] than carbide interfaces and on the importance of local hydrogen concentration on hydrogen embrittlement, tS]
B. Different Specimen Geometries If hydrogen effects are related to plastic instability, this effect could be more easily observed in notched specimens rather than in cylin
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