Iron-carbon alloys with a C-content of more than 1.7 % (usually 2 to 4 %) are considered iron-casting materials. They are usually shaped by casting and a final machining operation for sizing — not so often by forming.
First and foremost in this group of materials are annealed cast iron, cast iron with lamellar and vermicular graphite, vermicular cast iron and chilled cast iron. Another relatively new cast iron material is ADI (austempered ductile iron). Fig. 4-12 shows the properties and structure of various iron-casting materials.
The machining properties of iron-casting materials are heavily influenced by the amount and formation of the embedded graphite. Graphite inclusions, in the first place, reduce friction between the tool and the workpiece and, on the other, disrupt the metallic matrix. This leads to improved machinability in comparison with graphite-free iron-casting or steel materials. The results are short comma chips, small grinding forces and higher tool service lives.
Fig. 4-12. Properties and Structures of various iron-casting materials
Aside from graphite inclusions, the metallic matrix of iron-casting materials has a large influence on machinability as well. The matrix consists primarily of ferrite in materials of low strength. With higher amounts of pearlite, the material strength goes up, thus augmenting above all the tool wear. Iron-casting materials of higher strength and hardness often possess a bainitic, ledeburitic or martensitic structure.
In the case of malleable cast iron, we distinguish according to the heat treatment between white cast iron (GTW) and black cast iron (GTS). However, the tempered carbon (graphite) and the manganese sulphide included in the matrix bring about good chip breaking behaviour [N. N.83]. While in the case of black cast iron there is an even structure across the tool cross-section, the decarbonised external layer of white cast iron is purely ferritic.
In the case of cast iron with lamellar graphite (grey cast, GGL), the steel-like matrix of graphite lamellae is disrupted [N. N.15]. Due to this, short comma chips are formed. Beyond this, falling machining forces are detectable. During the machining process, no burrs are produced for the most part at the workpiece edges, but breakaways do arise.
The surface quality of the machined workpiece is contingent upon the fineness and evenness of the grey cast matrix in addition to grinding conditions [OPIT70].
The external layer of cast workpieces exhibits poorer machinability than the core. This can be derived, on the one hand, from non-metallic inclusions and, on
the other, from the altered graphite structure and microstructure directly beneath the cast rinde, as well as from scalings [N. N.15]. The result is more abrasive wear.
In cast iron with vermicular graphite, the graphite is present in the form of globular inclusions. The microstructure of cast irons of low strength and good toughness properties (e. g. GGG40) consists for the most part of ferrite. With increasing amounts of pearlite in the microstructure, the strength of the cast materials rises. In abrasive machining, this leads to increased grinding wheel strain.
The structure of vermicular cast iron (GGV) is characterised by handle-shaped graphite inclusions. Reducing internal peak stresses in the material, as found at the peaked ends of lamellar graphite inclusions in GGL, makes possible a combination of good mechanical component properties and good machinability.
The structure of ADI is composed of globular graphite and a microstructure consisting of needle-shaped ferrite and stabilised, highly carbonic austenite. It is characterised by excellent mechanical properties, especially a very high tensile strength in conjunction with high fracture elongation, high wear resistance and improved damping compared with steel of equal hardness. Significantly increased strength and ductility as opposed to conventional cast irons as well as a higher abrasive wear effect make special demands on the grinding process.
Compared with hardened steel materials, 100Cr6V for example, cast iron materials are more conducive to abrasive machining due to their graphite inclusions and their ferritic and pearlitic microstructure.