Considerazioni sulla microstruttura di ruote monoblocco con tracce di bainite (Considerations about microstructure of solid wheels with traces of bainite)

The aim of this paper is to clarify the effects of bainitic spots in the homogeneous lamellar pearlite structure of wheels located near the tread. Bainite is an intermediate structure, always in overlapping with pearlite, which is formed during the heat treatment of quenched and tempered steels, even though its quantity can be controlled depending also on the chemical analysis and CCT diagrams of the steel. The process management guarantees a proper level of hardness and mechanical properties in the wheel. The formation of bainite is inevitable during the continuous cooling of hypo-eutectoid C steel grades (ER7, ER8, ER8S, ER9) where the Carbon Equivalent is exceeding 0.6%. In order to mitigate the presence of bainitic islands on the tread, Lucchini RS is working since a long period toward the calibration of a special aim analysis and an adapted design. According to the hardenability of the new aim analysis, the cooling rate of the devices for rim chilling was defined, with the objective to both reduce bainitic structures and obtain hardness and toughness properties in the rim. A method of bainite content measurement was proposed and used on ER7, ER8 and ER8S wheels after the activation of all the improvement actions. Furthermore, in order to qualitatively investigate the effect of the presence of bainite structures in the wheels, 2D plain strain finite element (FE) simulations were carried out.