INFLUENCE EXERTED BY NONMETALLIC INCLUSIO NS IN STEEL ON THE CONTACT CONDITIONS DURING MACHINING

Under certain circumstances, non-metallic inclusions in steel are capa ble of forming intermediate layers between the tool and workpiece duri ng machining which exert a positive influence on the contact condition s. Suitable for forming such layers are modified oxide inclusions, as in calcium-treated steels, and sulphide inclusions. The material compo sition and the properties of these inclusions are governed by the stee l's alloy composition. It is, as a result, not possible to assume, tha t the intermediate layers have identical mechanisms of action for diff erent Ca-treated steels, When Ca-treated quenched and tempered steels 42CrMo4 and 38MnSiVS5 are machined, the inter:mediate layers have a po lyphase structure. This can lead to different contact conditions over the effective area. The question whether the intermediate layers have a friction-and wear-reducing effect depends essentially on their consi stency. Tests performed on free-cutting steel revealed a loss of effec tiveness during the transition from the viscous to the molten state. I t was possible to demonstrate the reduced wear of Ca-treated steels by the example of microalloyed quenched and tempered steel 38MnSiVS5. Wi thin the scope of long-time machining tests, average tool life increas es of over 400% were realised compared with conventional material of t he same quality.