Process for surface-hardening into the bainite zone of castings of ductile iron-carbon casting materials

摘要

Castings with a bainitic-austenitic structure have a higher strength and fatigue strength and better wear resistance than the ferritic to perlitic grades. In order to avoid the poorer machining properties and the delay of a bainite hardening throughout, surface-hardening of only the particularly stressed regions into the bainite stage has already been carried out, namely by inductive heating and quenching in hot oil or in salt baths. The expensive hot baths are avoided according to the invention by preheating the castings as a whole to a temperature approximately corresponding to the intended transformation temperature and rapidly heating the surface zones or casting regions to be hardened to a temperature from 880 DEG to 950 DEG C. From these, the heat rapidly flows off into the only moderately preheated interior of the casting, so that the austenitised regions have to be cooled only slightly by moving air or spray mists down to the vicinity of the intended transformation temperature. Immediately afterwards, they are transferred into a furnace preheated to the transformation temperature. Because of the short delay, machining can be carried out before the surface-hardening.