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In the process of plasma surface hardening, a surface layer that consists of zones of different sizes with different phase and structural composition is formed. The minimum grain size for plasma surface heating is determined by the initial austenitic grain. Its size depends on the dispersion of the initial structure. The rate of plasma heating at 200-1000 °C/s affects the size of the initial grain. The further growth of austenite crystallites with an increase of temperature essentially depends on the heating rate: small rates and high temperatures of plasma surface heating can lead to a significant enlargement of the grain. Increase of the heating temperature in the zone of thermal influence during welding from 900 to 1040 °C of rail steel 76F leads to the growth of the austenitic grain from No. 9 to No. 7 and individual grains – from No. 8 to No. 3. The largest austenite grains while heated at 1000 and 1040 °C form separate zones where conglomerates of large grains predominate. Stiffness of 76F steel, heat-strengthened on the troosto-sorbitol structure, deteriorates substantially when the temperature of rapid heating increases from 900 to 1040 °C. This also increases the instability of stiffness.

Comparative evaluation of austenite grain in high-strength rail steel during welding, thermal processing and plasma surface hardening