In the ISO classification set up in 1958 the group K workpiece materials were classed as workpieces that when machined generated crack type chips.
For typical group K workpiece materials, such as cast iron castings, the vibration created when crack type chips are produced can easily cause the cutting edge to chip. Generally, crack type chips leaves a poor surface finish, and as a result this can cause abrasive tool wear. Note however, that due to the way in which the chips are generated, the cutting resistance is not as high as when compared to machining steels or stainless steels.
Machinability of workpiece materials
Recently, high-grade cast irons, materials included in K group as castings, have increased adhesiveness and strength. Consequently, some cast irons are now seen as difficult-to-cut materials.
For high-grade cast irons such as ductile and malleable cast iron, crack type chips are not always produced which makes chip control an important consideration.
High-grade cast irons also produce high cutting resistance, which can easily result in chipping, heat generation and crater wear. A particular feature when machining ductile and malleable cast irons is that, in comparison with general steels, the crater wear is produced at a point closer to the cutting edge. Plastic deformation of the cutting edge that causes flank wear should also be a consideration when cutting these materials.
Other cast irons available include hardened cast irons, such as solid-solution chilled cast iron, and heat-treatable pearlite malleable cast irons. Both of these materials are as hard as steel. Due to the fact that they include graphite within their structure, high frequency vibrations occur when machining. Therefore they are often considered to be difficult-to-cut materials.
Another feature of cast irons is the uneven surfaces. This is caused because the surfaces undergo thermal expansion and contraction and become very hard due to heat oxidation during the casting process. The cast surface is also referred to as surface scale and is one of the main causes of problems experienced when machining cast irons.
There are many products manufactured by casting iron in moulds made of sand. When the molten metal is poured into a sand cast there are occasions when the sand mixes with the surface of the metal. Surface scale contaminated with sand has a poor machinability level, sometimes resulting in short tool life.
General cast irons
When machining general use cast irons, crack type chips are usually produced, therefore the conventional view is that cutting resistance and chip control need not be taken into consideration. When machining, the chips developed can cause vibrations making it important to increase the cutting edge strength. However, as the use of general cast irons with higher tensile strength increases, there are cases where a cutting edge to reduce cutting resistance is used to help improve work efficiency. Each tool manufacturer offers inserts with chip breakers for cast iron machining, although not as many types as for steel machining. This is because the application demands that such inserts are only needed to prevent cutting resistance, rather than for the reason of chip control.
The manufacturing process for cast iron is a relatively simple process. The generally process is the re-melting of materials such as pig iron, steel scrap material, foundry scrap, and ferrous-alloys, this is to obtain the required composition.
The actual re-melting process can be carried out in a number of methods. One method being is the use of a cupola, in which the materials are melted. Other methods used are to melt the material in a coreless induction furnace or in a small electric-arc furnace.
Grey cast Iron. The black lines represent the graphite flakes.
Grey cast iron is the most widely used of all cast irons and it is often referred to as just cast iron. Grey cast iron contains 2.1%~6.9% carbon (C) and ~2% silicon (Si). The silicon is added to assist in changing the cementite into a graphite formation. The graphite is in the form of irregular flakes which can show up when the metal is fractured as the identifiable grey matrix. It is because of the presence of graphite within the microstructure that cast iron is brittle unless it is specially treated. The graphite within the microstructure acts as a kind of chip-breaker and lubricates the cutting edge. Also as the percentage of graphite increases so too does the ability to absorb vibration energy this leads to increasing the dampening abilities.
Grey cast iron is highly dependent on the rate of cooling of the casting and of the wall section thickness. Thin sections can have reasonable tensile strength. However as wall thickness increase it is difficult to maintain the tensile strength. The properties of grey cast iron dependent on the proportion of graphite within the microstructure. If all of the carbon has separated from the molten state (this is called full graphitization) then the grey cast iron resulting will have graphite flakes in a ferritic microstructure. If however 0.5% to 0.8%of the carbon remains in the form of Fe 3C then the resulting grey cast iron will be pearlitic and the cast iron will be stronger and harder.
The mechanical properties of grey cast iron result from the effects of the chemical composition and of the cooling history. In general as the combined equivalent of carbon and silicon is reduced the strength of the cast iron is increased. When tensile strengths above 350MPa are required, for thicker sections, alloying elements such as chromium (Cr), nickel (Ni) or molybdenum (Mo) are used.
Wear Resistance: Grey cast iron has high wear resistance, and as such it is often used in applications such as cylinder bores, piston rings, where wear resistance is required.
Machinability: In comparison to other members of the iron alloy group, grey cast iron is
one of the easiest iron alloys to machine. The most difficult to machine is the pearlite cast irons.
Ductile cast irons
When machining ductile cast irons, continuous, rather than crack type chips are produced. This can generate higher cutting temperatures which can create tool damage similar to that experienced when machining steel. Therefore, choosing tool materials and geometries similar to those used for steel machining can help prevent tool damage.
The manufacture of ductile cast irons is the same as grey cast irons, up until the point prior to casting. Just before casting magnesium (Mg) is added to the molten solution. The magnesium assists the graphite to form into spheres (nodules) rather than flakes as they tend to in grey cast irons. The graphite spheroids can exist in either a ferrite or pearlite structure or both.
In comparison to grey cast irons the graphite spheroids provide improved mechanical advantages. Ductile cast iron is similar to grey cast iron in having a low melting point, good fluidity, improved cast ability, and wear resistance. However compared to grey cast iron it has improved strength, ductility toughness and hot workability that tend to make the machinability a little less that grey cast iron.
Ductile Iron (Nodular
Ferritic). The graphite can
be seen as
Malleable cast irons
White cast irons are the only member of the cast iron family that has carbon present in the microstructure only as carbide; the total carbon content is present in the cementite form (Fe3C). Without the graphite in the microstructure white cast irons have a light appearance and hence the naming white cast irons. White cast irons can be achieved by rapid cooling of the solution.
White cast irons, depending on the alloying content tend to be extremely hard and abrasive. There is very little practical use for white cast irons in terms of structural components but as they are extremely hard they tend to be used for bearing applications.
Chilled Cast Irons are an improved version of the above white cast irons. By adjusting the carbon composition of the white cast iron it is possible to obtain a situation in which normal cooling rates are used then the surface of the cast has a white cast iron composition. Whereas the core of the cast that is cooled at a slower rate achieves a structure the same as grey cast iron.
The manufacturing of malleable cast irons is carried out by the heat-treating white cast irons.
Within the malleable cast iron group there are 2 main variations:
1) Whiteheart malleable cast iron
2) Blackheart and pearlite malleable cast iron
Whiteheart malleable cast irons
Whiteheart malleable cast irons are manufactured by heat-treating white cast irons. The microstructure of whiteheart malleable cast iron will depend on the varying cooling rates of the section size. Note that the microstructure does not consist of flake graphite. In small sections it will basically contain pearlite and temper carbon in ferritic substrate. In the large sections there will be 3 varied zones:
Surface zone: Contains pure ferrite.
Intermediate zone: Contains pearlite, ferrite and temper carbon.
Core zone: Contains pearlite, temper carbon and ferritic inclusions.
Blackheart and pearlitic malleable cast iron
The microstructure of blackheart malleable cast iron consists essentially of ferrite. Whereas the microstructure of pearlitic malleable cast iron varies, according to the grade specified, of pearlite or other transformation products of austenite. Graphite is present in the form of temper carbon nodules, and should not exist as flake graphite.
Mechanical properties of malleable iron.
Malleable iron, like ductile iron, offers high ductility and toughness because of its combination of nodular graphite and low-carbon matrix. However due to the way in which graphite is formed in malleable iron the graphite nodules are not truly spherical as they are in ductile iron but are irregularly shaped aggregates.
Malleable and ductile cast iron are used for similar applications in which ductility and toughness are required. Generally the deciding factor between whether to chose a malleable or ductile cast iron is based on price and availability rather than on properties. However for certain applications such as thin-section castings malleable cast irons have the advantage. applications, however, malleable iron has a distinct advantage. It is preferred for thin-section castings:
Other applications in which malleable cast iron is used for are connecting rods and universal joint yokes, transmission gears, differential cases and certain gears, compressor crankshafts and hubs, flanges, pipe fittings and valve parts for railroad, marine and other heavy-duty applications.
Major cast iron types
[There are five types of major cast irons.