Tempered martensite embrittlement and intergranular fracture in an ultra-high strength sulfur doped steel
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jor role in tempered martensite embrittlement. It readily segregates during austenitization and significantly weakens the grain boundaries. Recently, we showed that after certain austenitizing conditions, sulfur could also segregate to the austenite grain boundaries and cause tempered martensite embrittlement. 7 In that study the bulk concentration of sulfur in the steel was only 40 w p p m . In this paper we report a further study of the effects of sulfur on tempered martensite embrittlement and intergranular fracture. Previous studies have shown that sulfur is a very potent embrittler in iron and steel. 7,13 It will be shown that a full explanation of its effects depends on the austenitizing temperature, the fracture test temperature, and whether it is present at the grain boundaries as unprecipitated, elemental sulfur or as precipitated sulfides.* *Throughout this paper, when we refer to precipitated and unprecipitated sulfur we are referring to sulfur which is in an observable, three-dimensional precipitate and that which is not. We recognize that the unprecipitated sulfur could in fact be in islands of a two dimensional precipitate.
EXPERIMENTAL The steel used for this study had a chemical composition of 3.5Ni, 1.7Cr, 0.3C, and 0.01S. It was vacuum induction melted, cast, forged, and finally rolled into 1.27 cm thick plate. The samples were initially austenitized in argon at 1200 ~ for 3 h or at 850 ~ for 1 hr. This was followed by an oil quench. They were then isothermally tempered for 1 h at temperatures between 150 and 500 ~ Tempering was also done in argon, and these treatments were followed by a water quench. Full size Charpy V-notch samples were used to measure fracture energies. Machining and notching of the samples were done after all heat treatment had been completed.
ISSN 0360-2133/81/0211-0309500.75/0 METALLURGICAL TRANSACTIONS A 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, FEBRUARY 1981--309
Sulfur segregation to the grain boundaries was studied by using Auger electron spectroscopy. Details of these experiments have been reported previously. TM Precipitated sulfides were identified with energy dispersive X-ray spectrometry and transmission electron microscopy. RESULTS The room temperature Charpy fracture energies for samples austenitized at both 850 and 1200 ~ are shown in Fig. 2. In both cases an embrittlement trough is observed although it is deeper after the 1200 ~ austenitization. The samples austenitized at 1200 ~ had lower fracture energies than those austenitized at 850 ~ after all tempering treatments. T o assess more fully the effect of the austenitizing heat treatment, we austenitized samples at temperatures between 850 and 1200 ~ for 1 h. (Note that with this exception, all other data concerning samples austenitized at 1200 ~ refer to the 3 h treatment and not the 1 h treatment. However, there was no difference in the fracture energy and morphology of the sample given the 1 h treatment and that given the 3 h
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