ON THE INTERACTION BETWEEN INCLUSIONS IN STEEL AND THE CUTTING-TOOL DURING MACHINING

The machinability of steel is considerably enhanced by such inclusions in the steel that form a wear inhibiting protective layer on the cutting tool during machining. The objective of this work was to clarify the interactions between the inclusions and the cutting tool, resulting in protective layer formation during the machining of steel. The experimental work consisted of machining tests and reactivity tests. The machining tests were carried out in order to study protective layer formation on uncoated cemented carbide tools and the related inclusion behaviour in the chips, using both conventionally made steels and powder metallurgically manufactured steel bars as workpiece materials. The reactivity tests were conducted in order to study the interactions between synthetic inclusion compounds and cemented carbides at simultaneous high temperature and pressure. On the basis of the experimental results, Literature and thermodynamic considerations, it was concluded that oxidation of the tool surface, or the presence of a suitable oxide as a tool constituent, is a prerequisite for protective layer formation during the machining of steel in the case of both oxide and sulphide layers. Oxidation of the tool can be facilitated by oxide inclusions with a low stability or suitable duplex inclusions in the steel. Inclusions with a high chemical affinity towards the oxidised tool surface then adhere to it, forming a good substrate for more inclusions to become attached to through a continuous solid state process. Layer formation is not possible if the oxidation of the tool surface results in phases less stable than iron oxides. This explains the lack of layer formation on pure tungsten carbide cobalt tools. A new qualitative model was developed to describe the behaviour of inclusions in the flow zone taking into consideration both deformable and non-deformable inclusions. The results indicated that small inclusions contribute significantly to protective layer formation, It is also known that a good deformability of inclusions is not a prerequisite for layer formation during the machining of steel. Utilisation of small, non-deformable inclusions is therefore a valid possibility for considerable machinability enhancement with no deterioration in mechanical properties.