Hydrogen assisted ductile fracture of spheroidized carbon steels
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R. GARBER, formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University, is now with Climax Molybdenum Co., Ann Arbor, MI 48105. I. M. BERNSTEIN and A. W. THOMPSON are Professors, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted October 24, 1979.
processes were not studied directly: their contribution was inferred by elimination. The purpose of this paper is to describe additional experiments conducted to study in more detail and in a more direct manner the effect of hydrogen on the substeps of void initiation, growth, and link-up. "Initiation" is defined here to mean the process by which the matrix separates from a particle (usually a carbide) to form an observable microvoid. Initiation may occur (in general) over a wide range of strain, from the yield point to well past necking; additional nuclei may also form during the final link-up process. 5,6 In this paper, "initiation" will refer to events prior to link-up unless otherwise noted. Similarly, microvoid growth can occur over the whole range of post-initiation strain, and may be considerably accelerated during link-up, but "growth" in this paper refers to the pre-link-up event. Finally, "link-up" refers to the (poorly-defined) process which terminates plasticity-controlled growth and causes the microvoids to become connected together in a relatively short interval of strain, for example by localized shear. 6,7 Figure 1 is a schematic showing the behavior expected for both the number of microvoids, and their size, with increasing strain. The rather rapid increase in these quantities at a strain near the fracture strain may not correspond to any new physical process, but (partly for experimental reasons) we find it convenient to call this relatively small range of strain the regime of link-up and final fracture. In the present work, initiation and growth were studied directly by an experimental technique involving the quantitative metallographic study of longitudinal sections of the neck centers of initially-unnotched round tensile specimens deformed plastically to strains between necking and fracture. The sectioning procedure has been described in detail recently by Van Stone and C o x : Plotting of the metallographic parameters as a fraction of strain (or stress) allows for an accurate analytical description of the processes of void initiation and growth. For example, the density of microvoids is assumed to reflect initiation, while the average size of a
1SSN 0360-2133/81/0211-0225500.75/0 METALLURGICAL TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, FEBRUARY 1981--225
LtJ N
Table I. Composition of Steels (Wt Pct)
> t'l
INITIATION AND GROWTH
[-INK-U
Steel
C
Mn
Si
S
P
Eutectoid* AISI 1018
0.79 0.16
0.56 0.79
0.16 0.03
0.019 0.016
0.023 0.005
0
>
* Corresponds to AISI 1080.
y
0 0
:E u0
nLU rn Z
NECKING
FRACTURE
PLASTIC STRAIN
Fig. 1--Schematic illustration of the expected
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