【正文】 s changed from 0 to % the peak rises %60 176。 , where the strength and Tso are lowest. After quenching, the structure consists of lath martensite with welldeveloped dislocation arrays. The laths are slightly misoriented with respect to each other, with an average width of ~ μ and length ~5μ, and are grouped in colonies ~5 5μ. The laths are filled with evenly distributed dislocations with a density ~1011cm2. No carbide phase was observed in the quenched steel. After tempering at 600 176。. The morphology of the carbide phases also changes. Finely dispersed rounded precipitates with a size of ~250 A are located in the junctions of the dislocation work. Along with them there are larger rounded precipitates~2 may be associated with the first of these effects. It is known [7] that with a constant grain size (or constant size of colonies of martensite laths) Tso is associated with a low yield strength σy (σ in first approximation) by the relationship εTso = σy + C, where C and ε are constants. For heat 1 Tso varies linearly with in a broad range of changes in yield strength, and only with tempering at 400600 176。, especially MC3 [8]. Precipitating evenly throughout the volume on disloca tions, lowangle boundaries of martensite laths, and highangle boundaries of colonies of laths, they strengthen the matrix and weaken (embrittle) the boundaries. It is probable that the boundaries are weakened most with the maximum density of carbides coherent with the matrix or with their precipitation, ., at secondary hardening temperatures. Such elements as vanadium, promoting refining of carbides and thus increasing secondary hardening [8], increase the embrittlement. With precipitation and coalescence of carbide phase, occurring simultaneously with polygonization of dislocations, the boundaries bee more perfect and the matrix is weakened. Both these effects lead to a drop of the ductilebrittle transition temperature, which is in fact observed. CONCLUSIONS 1. Steels of the 15Kh3MFA type are susceptible to embrittlement, which reaches a peak at a given tempering temperature. The upper limit of the peak (Tso) coincides with the temperature at which the material begins to weaken. 2. The height of the peak and the tempering temperature corresponding to it increase almost linearly when the vanadium concentration is raised from 0 to %, but they are independent of the phosphorus concentration within limits of %. 3. Temper brittleness of the steel investigated depends on the change in the carbide phase (from cementite to special carbides) that occurs with retention of the dislocation arrays preferentially in the boundaries of fragments. LITERATURE CITED . M. Utevskii, Temper Brittleness of Steels [in Russian], Metallurgizdat, Moscow (1961),p. 138. 2. P. B. MikhailovMikheev, Thermal Embrittlement of Steels [in Russian], Mashgiz, MoscowLeningrad (1956), p. 56. 3. J. Hollomon, Trans. ASM, 36, 473 (1946). 4. E. Houdremont, Special Steels [Russian translation], Vol. I, Metallurgiya, Moscow (1966),p. 455. 5. V. A. Korablev, Yu. I. Ustinovshchikov, and I. G. Khatskelevich, Embrittlement of chromium steels with formation of special carbides, Metalloved. Term. Obrab. Met., No. I,16 (1975). 6. A. P. Gulyaev, I. K. Kupalova, and V. A. Landa, Method and results of phase analysis of hlghspeed steels, Zavod. Lab., No. 3, 298 (1965). 7. J. Heslop and N. Perch, Phil. Mag., ~, No. 34, 1128 (1958). 8. V. V. Rybin et al., The mechanism of hardening of sorbitehardening steel and the possibility of determining it theoretically and experimentally, in: Metal Science [inRussian], No. 17, Sudostroenie, Leningrad (1973), p. 105. 9． L. K. Gordien