Brittleness of steels at near-solidus temperatures (current state of the problem)

Abstract

1. High-temperature brittleness of steels is observed in the temperature range 0.90–0.99T melt and is characterized by a narrow temperature range (generally not exceeding 10°C) in which the ductile and strength parameters are not affected by the deformation rate. In this range the steels fracture by a grain-boundary mechanism with the formation of a characteristic molten relief. 2. The temperature of the ductile-brittle, transformationT d/b (the temperature of zero ductility) is most affected by C. S, P, and B; an increase in their content diminishesT d/b. The addition of manganese and the treatment of the melt with REM and AEM weaken the effect of sulfur and increase the temperatureT d/b by 10–100°C. 3. Most investigators attribute the appearance of the ductile-brittle transformation at near-solidus temperatures to the early melting of grain boundaries. The problems of the mechanism of formation and the chemical composition and role of the liquid phase in the evolution of grain-boundary fracture requires additional theoretical and experimental studies. 4. The available data (the “abrupt” behavior of the curves of ductile-brittle transformation, the experimentally determined liquid layers atT=T d/b) show that the temperature of grain-boundary melting can be determined using the method of high-temperature mechanical tests and can be understood as the temperature of the ductile-brittle transformationT d/b. 5. The temperature of grain-boundary melting in tool steels R6M5 and R18 coincides with the temperature ranges of heating for quenching recommended by GOST 19265-73, which should be taken into account in practical heat treatment because the melting of grain boundaries can be a cause of instability of the mechanical properties of small-size tools.