This paper considers the influence of the process of ion nitriding over the resistance against fragile destruction of ВН21 ВS4659 (3Cr2W8F-GOST) steel, tempered at 700 0 С. The resistance of the nitride steel against fragile destruction is defined at static bending by means of the critical crack opening-δ Iс. The paper discusses the influence of the parameters of the process of ion nitriding (ammonia pressure in the chamber, temperature of nitriding, time of treatment) over the critical crack

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... ening. A conclusion is drawn that after ion nitriding the resistance against fragile destruction of ВН21 goes up, which is expressed by means of the parameter δ Iс. INTRODUCTION Tools for material treatment, such as prints and press forms, working under very heavy conditions, are very demanding. The requirements towards them can be satisfied by the use of steels with high content of carbide-forming elements in them δ Ic [1,2,3]. Most often the heat treatment of hot work tool steels is carried out in salt baths or in vacuum. The reliability and exploitation durability of the hot work tools depend mainly on the following characteristics: destruction stability К Ic and critical crack opening δ Ic [1,2,14]. At present the criterion К Ic is most frequently used for comparative assessment of materials with respect to their resistance against fragile destruction [1,6,7,8]. However, its application with steels with limit of flow Rр 0,2 < 1100MPa is restricted, as the plastic zone at the summit of the crack becomes quite big. In order to avoid these additional preconditions, it is necessary to use some of the criteria of the non-linear mechanics of destruction-e.g., δ Ic. The critical crack opening δ Ic is an integral characteristic of the plastic deformation and the plastic constant of the material, which explain its susceptibility to fragile destruction. The process of nitriding is a method of increasing the hardness, wear resistance, heat resistance, resistance to tiredness and durability of the details [5,8]. The nitrided details can be regarded as multilayer materials, comprising a matrix phase and one or more phases of the thin layer, which differ in their chemical composition, in the parameters of their crystal lattice, as well as in their physical and mechanical properties. These differences lead to a state of stress, arising at the border between the matrix and the nitrided layer. In addition to the natural residual stresses of the materials, caused by compressive stresses, in the process of work also occur applied stresses, resulting from the conditions of loading. If the total stresses in an arbitrary cross-section exceed the values of the critical stresses for a given multilayer system, then the fragile destruction will start in the boundary diffusion area or in the thin layer [8]. The aim of the present work is to investigate the influence of the process of ion nitriding over the resistance against fragile destruction in ВН21 steel by defining the crack opening at static bending-δ Ic. MATERIALS FOR INVESTIGATING AND MODES OF HEAT TREATMENT Hot work tool steel ВН21 ВS4659 (3Х2В8Ф-ГОСТ) with increased heat resistance has been chosen for the investigation. The chemical composition of the above mentioned steel has been defined by equipment for automatic analysis "Spectrotest", as shown in Table 1.