The effects of phosphorous segregation on the properties of steels containing two types of globular cementite: (1) preferential distribution at the grain boundaries, and (2) It is known that a high concentration of impurities such as P, S, and As produces embrittlement in steels [1]. This is manifested primarily by decrease in the impact strength and change in the nature of the fracture [2], since the impurities raise the ductile -brittle transformation temperature. Most authors associate the embrittlement produced by the above impurity atoms with their segregation to grain boundaries, which produces a decrease in the effective surface energy 3' [1, 3, 4].It has been shown for single-phase Fe-P alloys [5] that the embrittling effect of phosphorous segregation is due to a decrease in the work of intercrystalline fracture, l The microfracturing stress Rmc is used to characterize brittle strength. This is a constant which specifies the force conditions under which a crack of critical size becomes unstable, and is unambiguously related to the dimensions of the structural components [6]. The objective of the present work was to continue investigations of the effect of phosphorous on the brittle fracture of iron alloys containing particles of a second phase (globular cementite) in various distributions.The investigations were carried out using unalloyed steel containing 0.3, 0.4, 0.7 % C, and 0.035 to 0.5 % P (see Table 1). Two types of structures were obtained by different quenching and tempering procedures, one with a mainly grain boundary distribution (Fig. la) and the other with a uniform distribution (Fig. lc) of carbide particles. In order to obtain grain boundary segregation of the phosphorus the steels were annealed at 600°C for 10 h. The concentration of phosphorous at the grain boundaries was determined by Auger microprobe analysis of specimens fractured directly in the analyzer under a vacuum of better than 1.33.10 -14 Pa. Analysis of the structure and fractures was performed on a scanning microscope. The sizes of the grains dg and cementite globules d c were determined by statistical treatment of the microscopic data. The standard mechanical properties and minimum brittle strength Rmc (from the condition S K ---%, xI, _--_ 0) for both distributions of the second phase were determined by uniaxial tensile tests in the temperature range 77-293 K (Fig. 2). Comparative impact tests of the 0.7% C steel with a uniform distribution of cementite particles were carried out on 3 mm diameter non-standard wire specimens having a 1 mm deep annular notch with a root radius of 0.03 ram.The principal results of the structural analysis and mechanical tests at 293 K are given in Table 1. Fractographic analysis indicated that the nature of the brittle fracture changes with increasing phosphorous concentration. At as little as 0.1% P an appreciable amount of intercrystalline component (IC) is observed (Fig. 1). At 0.3% P the fracture is predominantly intercrystalline (Table 1), although the amount of transcrystalline fractur...
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