Abstract
Two plain-carbon steels and a low-C, high-Mn steel have been extruded to round bar with an extrusion ratio of 16: 1, reheating temperatures of 680°-1 280°C, and conventional lubrication at the upper end of the range, with molybdenum disulphide grease or a low-melting-point glass being used at the lower end of the range. Extrusion pressure increases systematically with decreasing temperature, giving a large temperature rise and causing billet austenization during, or immediately after, extrusion of billets reheated below 810°C. The use of molybdenum disulphide grease lubrication causes pressures to be more sensitive to the condition of the liner, pressure pad, and die than for conventional hot extrusion, but leads to improved extrusion surface quality and tolerance. As a result of refinement in ferrite grain size, the strength and toughness of mild steel bars improve systematically with decrease in reheating temperature to 810°C, but there is little further improvement at lower temperatures because of the mixed microstructures that are developed.
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