Untersuchungen zur Zerspanbarkeit von austenitisch-ferritischem Gusseisen mit Kugelgraphit (ADI) (2006)

In many applications increasing demands on weight and cost saving require the application of new materials. In this regard austempered ductile iron (ADI) offers a high potential. Cast iron materials exhibit a high freedom of shape combined with rather low manufacturing costs. In addition, ADI offers an excellent combination of high strength, high toughness and a good wear resistance. The fabrication of ADI-parts consists of a casting process and a following special heat treatment. Depending on the heat treatment parameters high strength and wear resistant or high ductile grades can be developed. Current developments focus on high-performance applications of transmission and chassis components. For these applications the ductile ADI-grades ADI-800 to ADI-1000 represent an ideal group of materials. However, high accuracy demands require basically a machining in a heat treaded condition. The demands on machinability and particularly on tool life are high.Preliminary investigations showed that the machinability of ductile ADI does not meet the required demands. Both compared to conventional ductile iron materials and to steels of similar strength a clearly lower tool life is attained based. An extreme crater wear is a characteristic tool wear phenomenon when machining ductile ADI with coated tungsten carbides. This crater wear is located very close to the cutting edge and leads to a destabilisation of the crater lip. Furthermore, a high dynamic cutting force load is present due to a heavily segmented chip formation. Due to the wear characteristics observed in the preliminary investigations the material-dependent influences of ADI on the cutting process were fundamentally examined in this work. Key investigations were the analysis of the special austenitic-ferritic microstructure regarding potential wear mechanisms as well as a detailed study of the segmented chip formation and the subsequent loads in the contact zones of the cutting edge. Depending on the following conclusions the fundamental processes in the cutting zones and its phenomena were clarified. The unusual wear mechanisms are due to the special austenitic-ferritic microstructure of ADI. This microstructure consists of a fine structure of ferrite and high-carbon austenite. The austenite can transform into martensite during a plastic deformation. The consequence is a very strong strain hardening. This effect is observed during the chip formation and leads to a very high specific mechanical cutting edge load. A segmented chip formation is characteristic when cutting ductile iron. The tendency to a segmented chip formation is much stronger when machining ADI. Rather low mean cutting forces are the consequence of this segmented chip formation, but also strong dynamic proportion of these cutting forces. In addition, the contact zone on the rake face becomes very small and moves close to the cutting edge. A further consequence of the segmented chip formation is the support of adhesive wear. The segmented chip formation occurs in a compression and a gliding phase. During the compression phase the material is pressed under high pressure on the rake face and generates a material bonding. During the gliding phase the chip segment and the adhesive bound is again worn away. Moreover, the austenitic-ferritic microstructure has a strong tendency to adhere with tungsten-based carbides. This adhesion wear strongly increases with increasing cutting speed and associated cutting temperature. Besides adhesive also abrasive wear effects are observed. This abrasive wear is caused by the hardness of the austenitic-ferritic microstructure and segregation-effected hardness fluctuations. The hardness of austenite is much higher compared to stainless steel due to the crystal structure changed by the high carbon content. Furthermore, the abrasive tool wear is affected by the quality of the casting structure. A main reason is the alloying with manganese or molybdenum. These alloying elements segregate to the grain boundaries and lead to the precipitation of martensite and carbides. For the development of cutting tools the respective causes of the wear phenomena must be considered. In this regard, an important role for the improvement of tool wear behaviour plays to the design of the cutting edge. It is important to find an optimal compromise between a sufficient stability and a good sharpness of the cutting edge. On the one hand breakages should be prevented and on the other hand the influence of the segmented chip formation on the adhesive wear phenomena should be reduced. The same applies to the selection of the cutting tool materials and coatings. An optimal cutting tool material should combine a high abrasive wear resistance and very good edge strength. On the one hand, the high dynamic cutting forces require a good coating adherence. On the other hand, the adhesive wear of the tungsten carbide substrate must be prevented.