Variations in material properties of grey cast iron and its impact on tool wear

Sammanfattning: Among cast materials, grey cast iron and ductile iron makes up about three quarters of all cast parts. This makes the manufacturing and machining of grey cast iron an important economical factor for the automotive industry. In a world with finite resources and growing concerns for the environment the importance and interest in recycling grows larger. This has led the Volvo Trucks Corporation to use a higher quantity of recycled metal in their casted parts. The increase in recycled metal causes an increase in the quantity of impurities and trace elements, which could potentially lower the machinability of the grey cast iron used in the engine blocks of Volvo trucks. Currently, the Volvo Trucks Corporation experience a high unpredictability of the tool wear when machining engine blocks, this study investigates how the properties of the workpiece material differs between manufactured series of engine blocks and how this impacts the degree of tool wear. This was done by collecting and examining of cutting tools from a rough milling operation in one of Volvos production lines as well as examining material samples from engine blocks machined by said cutting tools. The evaluated workpiece material properties were the chemical content, the graphite structure, the hardness, and the concentration of hard inclusions. The material properties were put in relation to the degree of tool wear. The results shows that there was considerable variations in the degree of tool wear between the studied series, ranging from virtually undamaged to severe fractures. Severe degree of tool wear had a strong correlation with a high concentration of inclusions. The concentration of inclusions were in turn higher in samples with a high content of trace elements. The result also shows that a large number of material samples had an undesired graphite distribution of type D and E instead of type A. The hardness of the workpiece material was fairly consistent in all samples but showed a weak correlation between high hardness and graphite distribution of type E or D. Harder materials were in general easier to machine, which probably can be explained by a lower concentration of inclusions.