The effects of austenitization temperature on the high temperature ductility of Fe-P-S Alloys
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INTRODUCTION
W H E N iron is tensile tested at elevated temperatures and slow strain rates, it commonly fails intergranularly, and fracture occurs by the nucleation, growth, and coalescence of grain boundary cavities. Recent work of George et al. J has shown that grain boundary sulfur, regardless of whether it is in the form of sulfides or elemental sulfur, is very detrimental to the ductility of iron at elevated temperatures. Some of their conclusions are the following: If the sulfur content exceeds the solubility limit at the test temperature, both iron sulfide precipitates and elemental sulfur are present on the grain boundaries during the test. As a result, cavities nucleate easily on the interfaces between the sulfides and the matrix, leading to brittle, intergranular failure. In such cases, elemental sulfur is expected to play only a secondary role in the intergranular fracture process. If the sulfur content is lower than the solubility limit at the test temperature, all the sulfur is in solution and no sulfides are present on the grain boundaries. However, other kinds of particles are usually present on the grain boundaries. In particular, since the solubility of oxygen in iron is less than 2 ppm at temperatures less than 800 °C, 2'3 oxides are almost always found on the grain boundaries, and in certain cases these grain boundary oxides act as cavity nucleation sites. It was postulated that sulfur segregation to the oxidematrix interface aids the cavity nucleation process by alterP.L. LI is with Baotou Research Institute of Rare Earths, Baotou, Inner Mongolia, The People's Republic of China. E.P. GEORGE, formerly with the Department of Materials Science and Engineering, The University of Pennsylvania, Philadelphia, PA 19104-6272, is with Oak Ridge National Laboratory, Metals and Ceramics Division, P.O. Box X, Oak Ridge, TN 37831-6093. D. P. POPE is with the Department of Materials Science and Engineering, The University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272. This paper is based on a presentation made in the symposium "Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.
METALLURGICALTRANSACTIONS A
ing the interface and making cavity nucleation relatively easy at that location. 4 If the sulfur content is reduced to a very low level, - 1 wt ppm (in the balance of this paper all impurities will be reported in terms of weight), then cavities do not nucleate at oxides and the high temperature brittleness of iron disappears. 4 In such irons, grain boundary oxides (or, for that matter, other kinds of grain boundary particles like carbides or nitrides 1) are no longer cavity nucleants because of a lack of segregated sulfur on the surfaces of the grain boundary particles. In contrast to the detrimental effect of sulfur, phosphorus 4'5 and carbong improve the ductility of iron at high temperatures; a suggested mechanism of this improvement is that these elements c
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