Analysis of thermal stresses synergy in surface layer of carburised creep-resistant casts during rapid cooling processes

ABSTRACT During the carburising processes, treated elements are transported on a specially designed technological equipment of the furnaces. This equipment works in particularly hard conditions. This is because in each work cycle, apart from the load carried by the charge, it is subjected to harmful effect of many other factors such as the highly carburising technical atmosphere of the furnace and high and rapidly changing temperature. These factors influence fatigue changes in the equipment’s parts, the observable effect of which may be cracks on the surface, propagating deeper into the material in subsequent cycles. A pivotal factor influencing the kinetics of fatigue processes in the carburising furnaces equipment are thermal stresses appearing in each operating cycle, with each temperature change. The thermal stresses in this equipment are caused by the temperature gradient across the section of the heated or cooled element and different thermal expansion of the structural components of the carburised alloy. In the paper, on the developed FEM model of the subsurface layer of a carburised rib of a pallet for transporting the charge in the furnace, analyses of stress distributions during rapid cooling were carried out. These analyses were performed with taking into account the separate impact of the above-mentioned sources of thermal stresses and their simultaneous action. In the analyses which consider the influence of the temperature gradient, the main load factor was the temperature distribution determined on the basis of the heat flow analyses carried out for the analysed section of pallets rib. The conducted analyses made it possible to assess the impact of local stresses caused by different thermal expansion of structural components of the carburised alloy on the change in the distribution of global stresses caused by the temperature gradient formed during rapid cooling. Based on the obtained results, it was assessed how the increased concentration of carbide precipitates, characteristic for the grain boundary of the carburised alloy, favours the formation of cracks observed in real castings, and whether the analysed stress sources have a significant impact on the kinetics of fatigue processes.