Site competition between sulfur and carbon at grain boundaries and their effects on the grain boundary cohesion in iron
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I.
INTRODUCTION
A small amount of sulfur often causes grain boundary embrittlement of iron and steel at low temperatures ~'2as well as at elevated temperatures. 3 The grain boundary embrittlement is considered to be caused by the segregation of sulfur at grain boundaries. Several papers have been published on the grain boundary segregation of sulfur in iron alloysfl '4-9 However, grain boundary segregation of other solutes, such as carbon, nitrogen, and other impurities and additional elements, was also observed in specimens used in most of these researches. 5-8 These solutes may interact with sulfur and modify its segregation. In these researches sulfur was not added to the specimens in a controlled manner, but the sulfur present as an impurity was utilized in the investigations. Hence, it is difficult to specify the state of sulfur (i.e., in solid solution, in a precipitate, or in other bound states with other solutes), on which the degree of segregation and the properties of the specimens depend strongly. Thus, it is difficult to discuss the sole effect of sulfur on the properties of iron, e.g., the intergranular fracture, based upon the results of previous researches. The purpose of the present research was to investigate the grain boundary segregation of sulfur and carbon and the effect of the segregation on the toughness of a-iron. High purity specimens containing controlled amounts of sulfur and carbon were prepared from a high purity electrolytic iron and high purity alloying elements. The results are compared with the results of similar experiments with high purity iron-phosphorus-carbon alloys. 10 II.
EXPERIMENTAL PROCEDURE
A. Preparation of Specimens Alloys were made from a high purity electrolytic iron (Mairon-HP, Toho Zinc. Co.) and sulfur of 99.99 pct. The S. SUZUKI and K. ABIKO, Associates, and H. KIMURA, Professor, are with The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal, 980, Japan. S. TANII, formerly Graduate Student, is with the Asahi Glass Company, Tokyo, Japan. Manuscript submitted October 15, 1986.
METALLURGICALTRANSACTIONS A
main impurities in the electrolytic iron are carbon (10 wt ppm), oxygen (50 wt ppm), phosphorus (less than 1 wt ppm), sulfur (5 wt ppm), silicon (less than 10 wt ppm), and manganese (1 wt ppm). The electrolytic iron of 5 kg was vacuum melted and deoxidized with a high purity iron4.5 pct carbon alloy in a crucible lined with calcium oxide. (The iron-carbon mother alloy was made from the high purity electrolytic iron melted in a 99.99 pct pure graphite crucible under an argon atmosphere.)" The excess carbon (0.01 to 0.02 pct) from the deoxidation treatment remained in the ingots. This residual carbon was reduced by hydrogen treatment to the desired levels. The ingots were forged and rolled at temperatures between 1100 and 1300 K to square rods (6 mm x 6 mm) and round rods (4.5 mm in diameter). Specimens (4 x 4 x 55 mm 3) for Charpy-type impact tests and specimens (3.6 mm in diameter and 33 mm in length) for Auger electron spectroscop
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