The cooling rate in hardening is known to affect substantially the susceptibility to brittle fracture, in the present work the susceptibility to brittle fracture is evaluated from the semibrittleness temperature. The relations between the experimentally determined semibrittlenes~ temperature and the time of,he isothermal hold and the rate of cooling in hardening are considered.Rotors 890 mm in diameter of steel 35KhN3MFA (Table 1) without a bore and with a bore 120 mm in diameter were subjected to oil hardening and tempering to a yield strength o02 = 650 N/ram 2 [3].Rotor steel 25KhN3MFA (Table 1) was melted in a duplex process and cast in vacuum to obtain an ingot weighing 39.5 tons. The final heat treatment of the preform consisted in hardening from 850°C in water and tempering at 610°C. Mechanical tests were conducted on tangential specimens cut from the preforms. The yield strength (002 = 800 --850 N/mm 2) and the plastic properties of the steels determined by standard mechanical tests varied but little over the cross section of the forgings.The semibrinleness temperature Ts0 of the steels was evaluated by the presence of 50% fiber in the rupture. For rotor barrels with diameters of 865 and 1135 mm of steel 25KhN3MFA T~o was determined on impact specimens of type 1 (notched with radius of 1 mm) by GOST 9454-78, and for rotors of steel 35KhN3MFA Ts0 was determined on impact specimens with preliminarily imposed fatigue cracks. For all forgings the cooling curves over their cross section in water and oil hardening were computed using special softassumed that the given cross section began to cool at the moment when its temperature decreased by 5°C relative to the temperature of heating for hardening. The mean cooling rate was determined by the formula t h -405 Vcool--TI --T 2where t h is the temperature of heating for hardening, °C; "c I is the time of the beginning of the bainitic transformation, h; "~2 is the time before the moment when the metal temperature in the cross section decreases to t = I h -5°C, h.We established (- Fig. i) that 7"5o in both steels decreases with increase in the cooling rate (more intensely in steel 25KhN3MFA).The solid lines in Fig. 2 present the variation of the semibrittleness temperature and the cooling rate over the cross section for two rotors of steel 35KhN3MFA hardened in oil. In addition, the figure presents hypothetical curves (dashed lines) of the variation of the cooling rate in water over the cross section of the rotors, which correspond to the decrease in the semibrittleness temperature determined from Fig. I. ware that allowed for the heat of the phase transformations and the effect of the temperature on the thermophysical properties. The curves were used to determine the mean cooling rates (V¢ool) from the temperature of heating for hardening to the temperature of the bainitic transformation (400°C). It was
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