Thermal effect of shaping - Volume theory and phenomena

Under current economic pressure, the forming industry must improve the quality of its products while lowering their cost price. To cope with these constraints, the practitioner must analyze and optimize not only the mechanical aspects of forming operations, but also, and increasingly, their thermal aspects. All forming operations, whether "hot", "cold" or "lukewarm", involve temperature: plastic deformation heats up the part's core; this heating is compounded on part of the part and tool surface by heating induced by the metal sliding over the tool. In hot forming operations such as extrusion, rolling or forging, the temperature gradients between the hot part and the colder tool induce surface cooling of the part and, correlatively, surface heating of the tooling; analogous phenomena, though less intense and inversely so, occur in cold forming between the tool heated by the preceding operations and the cold part. The consequences of these thermal phenomena are manifold. In the shaped part, temperature differences induce flow stress deviations ${\sigma }_0$ which tend to modify the plastic flow, with sometimes harmful geometric (dimensions, shape deviations), mechanical (residual stresses) and metallurgical (structure, grain size, cracks, etc.) consequences. Temperature closely conditions the thickness of lubricant films, by modifying the viscosity of liquid lubricants and the consistency of solid lubricants, and thus the coefficient of friction and shaping energy. It produces a marked change in lubricant properties when hot, for example when vaporization is used to eject forged parts, or for descaling in rolling mills. The working temperature of the tool (and often the temperature cycle during working, or the difference between surface temperature and core temperature) of a die, rolling mill cylinder or forging die closely conditions the applied stress regime, dimensional tolerances (expansion) and most forms of wear, either directly (thermal fatigue, creep), or indirectly (resistance to abrasion, mechanical fatigue, surface hardening embrittlement, physical evolution kinetics (structural transformation), chemical evolution kinetics (oxidation, diffusion, etc.)...)]. In short, tool life is closely linked to the thermal regime imposed by the forming operation.

As these problems are complex, numerical simulation is increasingly being used to analyze and control them. However, the development of a complete simulation is a time-consuming operation, and the responsibility of experienced specialists. The aim of this paper is therefore to present the orders of magnitude of thermal phenomena in order to discuss their nature and intensity, prior to more precise estimates obtained...