On the Microstructures of Steels at Elevated Temperatures

Abstract

Some microstructural observations at elevated temperatures in vacua (10⁻⁵ mmHg) have been studied on the A₁ transformation and grain growth of austenite for mild steel, and also on the tempering for the Si-Cr-V tool steel.
The results obtained in this study are summarized as follows:
(1) In heating of the mirror polished mild steel, the grain boundary of ferrite reveals faintly at 300°∼400°C, and then becomes clearly at 600°∼700°C. The grain boundaries of austenite can be slightly detected anywhere in ferrite grains at 730°∼750°C and then become clearly at higher temperatures following the fade-away of boundaries of ferrite grains.
(2) On the grain growth of austenite for mild steel, coalescence of some of the grains occurs with the decrease in the total area of grain boundaries. The boundaries of austenite reveal clearly in a rectilinear shape after a prolonged heating at a high temperature.
(3) When austenite grains are cooled, ferrite grains reveal at random in austenite grains below 700°C. The structure of pearlite, however, cannot be observed in this step. The reason of this behaviour is presumed that cementite will be graphitized at the surface layer of the test piece. Because there are many black spots in the metallographic polished and etched surface after cooling, and the normal microstructure can be observed in the lower parts of the test piece whose surface was removed about 0.05 mm in thickness.
(4) On tempering for the quenched Si-Cr-V tool steel, any surface changes cannot be observed up to 170°C. However, the surface relief can be detected on the etched surface at about 200°C due to the initiation of the decomposition of residual austenite. At about 550°C, this surface relief turns into a smooth wavy surface which precipitated many small carbides. These observed hot stagemicrostructures on tempering agree with the results of dilatation tests.