Temperature changes arising from radial heat flow during hot extrusion of cylindrical steel billets

A mathematical model of radial heat flow in billets during transfer, and after upsetting in the extrusion container, has been developed. The model makes a number of simplifying assumptions, which enable computations to be carried out on a desk-top computer. The effects of different container materials, size, and temperature, and of time increments upon the computed temperature gradients are discussed. It is shown that the model predicts satisfactorily cooling curves obtained experimentally from thermocouples implanted in-steel billets, reheated to temperatures in the range 710°–1280°C. A number of different lubricants were used, including ‘Fuminite’ at higher temperatures, and either low softening point glass, or molybdenum disulphide and graphite at lower temperatures. Appropriate heat-transfer coefficients have been derived for these lubricants. The model has been used to obtain cooling curves for a range of billet sizes from 38 to 125 mm radius with container temperatures from 175° to 350°C. A number of master graphs have been derived, which enable mean temperatures to be predicted simply to within ±5 K, and surface temperatures to within ± 10 K for most practical extrusion conditions. An example of how these master curves are used is given. The derived temperatures make it possible to predict extrusion pressures, and to choose lubricants with appropriate softening ranges for specific extrusion conditions.