Surface treatment with a high-concentration plasma jet is a promising method of improving the service characteristics of tool materials. 1 ' 2 The wear resistance of the cutting edge greatly increases as a result of the formation of a zone with a disperse structure and high hardness. However, as shown by practice, ~ 30-50% of tools (especially hardalloy tools) fail as a result of brittle fracture and chipping. Microcracks, formed at the cutting edge under the effect of normal tensile and tangential stresses, increase the degree of wear, thus greatly increasing the cutting force and, consequently, the probability of fracture. Therefore, when developing new tool materials or methods of hardening them, it is very important, in addition to analysing the conventional criteria of efficiency (hardness, heat resistance), to use the criteria of short-term strength cracking resistance on the basis of the brittle fracture mechanism.As regards the steel tools, the effect of surface hardening on dynamic cracking resistance (critical stress concentration factor K°, impact toughness KC and its component) can be evaluated in impact tests on specimens of standard dimensions with a special notch using the dynamic fracture diagram. 2 For sintered hard alloys these tests in practice are highly complicated, because of both the problems in producing specimens and the consumption of a large amount of material. It is therefore promising to use the microtests to determine cracking resistance under local loading. For hard and brittle materials, local plane strain with the formation of a half-disc crack occurs during indentation into the surface of the specimen of an indentor made of a harder material, such as a Vickers diamond pyramid (Fig. 1).Using a general solution of the axisymmetric problem for a disc crack in an infinite medium with tensile stresses at the edges of the crack, it was possible to derive correct 2 Diagram showing position of the hardened zone on hard-alloy sheets of triangular (a) and square (b) shape.relationships linking the critical stress concentration factor K lc of the material of the specimen with the critical load for the stage of crack nucleation F c and crack length 1 for the stage of its propagation: [1] Orc) 3/2 tan 1 Diagram of formation of cracks under local loading with a Vickers pyramid.where c = a + 1 (see Fig. 1, 2a -diagonal of the indentation); 2/? = 136° -angle between the opposite edges of the Vickers pyramid.This method is simple and universal. Tests were carried out simultaneously and HV hardness is measured by standard hardness meters. The effect of surface treatment can be evaluated on one specimen by consecutive loading in the hardened zone and initial metal. The correlation of the resultant values of K lc with the values determined by conventional methods was determined for a wide range of hard and brittle materials, including tool hard alloys. This method can also be used for steel specimens with hard and brittle surface coatings, for example in depositing carbides or borides and in nitridi...

At the present time, a great deal of experience has been accumulated in laser and electron beam treatment of fast-cutting steels. The effectiveness of plasma hardening has also been established. However, sufficiently complete data is unavailable in the literature on the effect of plasma heating on the strucntre and functional properties of fast-cutting steels for different combinatiolu of bulk and plasma heat treatment. Accordingly, in this work we have investigated the properties of R6M5 steel after plasma surface hardening.We know [1] that the performance rating (attainable cutting rate) of a tool made of fast-cutting steels is determined mainly by its heat resistance. This is responsible for the high wear resistance of the cutting edge of the tool on the front and rear surface. A frequent reason for failure of a tool is brittleness of the steel (> 62 HRC), which leads to micro-and macrofracture of the edge upon one-time or cyclic impacts [2]. Consequently, a criterion for choosing the optimal heat treatment for fast-cutting steel has been attainment of high hardness and heat resistance with sufficient crack resistance. In this investigation we used metallographic and microfractographic analysis, measured the hardness, and tested the samples for crack resistance