The role of hydrogen in the ductile fracture of plain carbon steels
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In this report we consider the problem of hydrogen induced ductility losses in a plain carbon spheroidized steel. Specifically, the effect of internal hydrogen on the formation of voids from second phase (cementite) particles and t h e i r subsequent growth and coalescence was studied by careful microscopic inspection of uniaxially strained b a r s , both initially cylindrical and circumferentially notched, with and without hydrogen. Void initiation o c c u r r e d with lower strains and s t r e s s e s with hydrogen, although an equally important contribution t o the ductility loss was from hydrogen accelerated void growth and coalescence. This latter process takes p l a c e by the propagation of voids a l o n g the g r a i n , and possibly subg r a i n , boundaries which interlink the cementite spheroids. The results indicate that hydrogen facilitates interface separation, possibly by accumulating at the boundaries during hydrogenation of the specimen and lowering the cohesive strength, thereby m a k i n g void initiation and growth along them e a s i e r .
1. I N T R O D U C T I O N O F the many ways in which hydrogen can degrade the mechanical properties of materials two, in particular, seem most prevalent in steels: hydrogen e i t h e r causes a change in the mode of fracture, usually from a " n o r mal" ductile process involving void initiation and growth to less ductile transgranular or intergranular cleavage, or hydrogen enhances, without changing the character of the " n o r m a l " mode of failure. 1'2 The f i r s t instance is common in high strength steels where ductile fractures are often found t o give way to cleavage presumably brought on by a relaxation of l o c a l fracture s t r e s s (or strains) due t o hydrogen s e g r e g a tion and accumulation at internal interfaces.3 In high strength alloys that would normally display cleavage fractures, possibly due to the p r i o r embrittling effects of impurities, for example, hydrogen can have the second effect and enhance a n o r m a l intergranular or transgranular cleavage by lowering even further the stresses required locally to cause failure. 4 On the other hand, for low strength steels such as the plain carbon spheroidized steels discussed herein, conditions leading t o a change from what is normally a ductile failure mode t o brittle cleavage with hydrogen are not readily attained. Yet when these steels contain r e l a tively l a r g e amounts of hydrogen, introduced for instance by electrolytic charging, they are subject t o significant losses in ductility. This is a g a i n a case where hydrogen s e e m s to accelerate, without changing, the n o r m a l mode of fracture, which in this case is ductile. This paper contains a report on some experiments conducted on hydrogen charged spheroidized plain carbon steels, the p u r p o s e of which was to m e a s u r e the influence of hydrogen on a process of ductile fracture. The ductile fracture process we studied was quite s i m i lar t o others discussed in the literature, but w e will indicate ho
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