Abstract
High-manganese steel is used in the rocket, space, and defense industries due to its low specific gravity, stable austenitic structure over a wide temperature range, and ability to strengthen under mechanical deformation. However, the wider application of this steel is hindered by such factors as: instability of mechanical properties in the initial state, tendency to thermal embrittlement and very poor machinability. Therefore, the work is aimed at solving the problems of eliminating the above-mentioned negative factors. Conducted studies of the effect of temperature-time parameters on the structure and properties of steel made it possible to establish the mechanism of thermal embrittlement of 9Г28Ю9МВБ steel. During slow cooling from hot deformation temperatures with speeds of the order of 0.03 º/s in steel, a large number of gray-blue coarse phase particles are released, which are located on the grain boundaries, which causes a sharp decrease in impact viscosity. It was established that in the temperature range of 500-800 ºС, the solid solution disintegrates with the release of particles of the strengthening K-phase - (Fe, Mn)3 AL, Cx. It was determined that the simultaneous decrease in hardness and impact strength in the temperature range of 750-950ºС provides a significant improvement in machinability. The lowest values of mechanical properties are achieved during aging at 700 ºС, the improvement of machinability in this case is maximal compared to exposure to other temperatures. So, for example, with an increase in the temperature of isothermal holding at 650ºС for 35 hours. machinability improves by almost 1.7 times, and for the same time at 700 ºС – by 2.4 times. When the holding temperature is increased to 950ºС for 35 hours. the machinability of steel increases only 1.6 times, which is due to the less intense decomposition of the saturated solid solution. Significant anisotropy of mechanical properties is formed as a result of the development of liquid chemical heterogeneity and its imitation during further thermomechanical processing. The value of the coefficient of anisotropy in sample bars of different grades of initial steel reaches values from 1.5 for a bar with a cross section of 105x105 mm to 3.9 units for a bar with a diameter of 120 mm. The study of the influence of the annealing temperature on the homogeneity of austenitic steel made it possible to develop a mode of heat treatment of austenitic steel 9Г28Ю9МВБ, which consists of heating to 1250°С, holding for 2 hours. at this temperature and subsequent cooling in water, in order to fix a homogeneous supersaturated solid solution. After such heat treatment, the striated microchemical heterogeneity is significantly reduced, the anisotropy coefficient does not exceed 1.1 units for all types of steel assortment.
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