Subcritical intergranular crack growth rates and thresholds of Fe and Fe + Sb
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I.
INTRODUCTION
PREVIOUS observations 1'2 with straining electrode tests of Fe and Fe + Sb demonstrated that both sulfur and antimony segregation have a dramatic effect on the fracture mode of iron tested at cathodic potentials in 1N H2804. Sulfur was shown to cause a fracture mode transition in iron tested at - 0 . 6 V (SCE) from ductile to intergranular at a grain boundary concentration of 0.13 monolayers while it was estimated that a grain boundary antimony concentration of 0.025 monolayers would produce the same fracture mode transition at - 0 . 6 V (SCE). These results were based on straining electrode tests where ductile failure occurred for all grain boundary sulfur concentrations when tested in air. The fracture mode transition results in iron can be interpreted as embrittlement from the combined effects of grain boundary sulfur plus hydrogen embrittlement, enhanced hydrogen uptake along grain boundaries caused by grain boundary impurities as proposed by Latanision and Opperhauser, 3 or a combination of both. Subcritical crack growth rate measurements in the presence of cathodic hydrogen is one method to evaluate which of these mechanisms is operating. A change in the rate of hydrogen uptake induced by grain boundary impurities may manifest itself as an increased subcritical crack growth rate. Also, the effect of impurities on embrittlement can be compared by their effect on fracture toughness. Therefore, subcritical crack growth rate measurements at cathodic potentials combined with fracture toughness measurements can be very useful for determining the role of grain bourLdary impurities in hydrogen embrittlement. The purpose of this paper is to report on the effect of sulfur and antimony segregation on the subcritical crack growth behavior of iron at cathodic potentials: Tests were conducted on the same materials used for fracture mode transition tests reported previously, l'z but compact tension samples were used for the present work. The fracture mode was I00 pct intergranular for fast fracture in air and subcritical crack growth rates at cathodic potentials. Therefore, fracture mode changes were not involved, and the effects of
impurities and hydrogen on 100 pct intergranular crack growth rates were determined.
II.
EXPERIMENTAL PROCEDURE
Samples for subcritical crack growth measurements were made from the same material used for the fracture mode transition studies except that plate material was produced from 2.5 cm diameter bar stock for this study, and 1 cm square bar material was used in the previous study. 1,2 The alloys were produced by induction melting electrolytic grade Glidden A101 iron as reported previously. 1'2 A complete description of the impurity concentrations was reported previously 2 while a partial listing is given in Table I. Sample blanks were annealed for 0.5 hour at 800 ~ followed by an anneal at 600 ~ for 240 hours in a vacuum furnace at a pressure of 10 -3 Pa. The grain sizes in the rolling plane of the plate samples were 450/~m and 270/xm for the Fe and Fe + Sb samples, re
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