Ertensive use of die casting is limited by the short operating life of molds with c3'clically changing tetnperature attd stresses. As a result of this action there is thertnal fatigue failure of the operating surface laver of a tnold.This article considers features of the thermal fatigue failure tnechanisnz for steels 3Kh2VSF and 4Kh5MF8 used for tnold tnatmfacture.A study was made of the mold metal for die-casting aluminum alloys removed from operation after 25,000-27,000 castings as a result of thermal fatigue crack formation in the forming die working surface. The chemical composition of the test steels is given in Table 1.Initially the magnitude of surface residual stresses was determined in a DRON-1.5 diffractometer by tile sin2',I ' method, and the method of moments was used to determine the sizes of crystal blocks from whose values dislocation density ,o = d/D 2 was evaluated [1 ]. Results of x-ray structural analysis* are presented in Table 2. It can be seen that the level of residual stresses in tile surface of a die made of steel 3Kh2V8F is higher by a factor of 1.5-2 than in steel 4KhSMFS. Probably this difference is caused by the high relaxation capacity of steel 4Kh5MFS whose microhardness (300-400 H) is markedly lower than for steel 3KI~V8F (550-600 H). During operation there is crushing if crystal blocks and an increase in dislocation density in the mold surface layer which is more marked in steel 3Kh2V8F than in steel 4KhSMFS (Table 2).Impact strength was determined for a qualitative analysis of steel operating properties and microhardness was measured in nonstandard specimens 2 × 5 x 55 mm in size with a notch 0.1 mm deep. Specimens were cut by electroerosion from forming dies at a different distance L from the working surface. It is shown (Fig. 1) that during die operation there is a reduction in impact strength and an increase in hardness in a surface working layer 4-6 mm thick. In molds made of steel 3Kh2V8F the change in these properties is more marked than in molds made of steel 4Kh5MFS. Comparative analysis of data in Table 2 and Fig. 1 shows that the greater reduction in impact strength for steel 3Kh5V8F during operation is caused by a higher increase in dislocation density and the level of residual stresses which is the reason for the high microhardness of this steel. It should be noted that the level of cyclic external temperature and force effects during the operation of molds made of both steels was about the same since the design and production parameters of casting do not differ markedly in both cases.The structural changes which occur in the working surface layer of a forming die during operation were determined. It is established that the original highly dispersed sorbite-troostite structure in steel 3Kh2VSF is transformed into a less fine sorbite structure (Fig. 2a, b). As a result of an increase in the diffusion mobility of carbon atoms under the influence of prolonged cyclic temperature and force effects the amount of carbide phase in the working surface layer of a steel 3Kh2V8F...
