Machinability of cast iron
The machinability of cast iron is an important technological property of cast iron . Cast iron has excellent castability , but poor formability , so it is difficult to forge , bend or roll . Shaping is therefore mainly carried out by casting and subsequent fine machining by machining ( turning , drilling , milling, etc.). The exact properties depend on the type of cast iron. The particularly frequently used cast iron with lamellar graphite can be machined very well. The cutting forces and tool wear are low, the chips are short and cannot get caught, and the surface quality that can be achieved is good.
The machinability depends heavily on the structure and the exact formation of the carbon. Cast iron types that contain large amounts of the iron-carbon compound cementite (" white cast iron ") are very difficult to work with. Other grades are mainly made of ferrite or pearlite and are easier to work with. In particular, because of the embedded graphite, they are easier to machine than steels with a comparable structure (see machinability of steel ), since the material is interrupted by the graphite and thus has a lower strength , which leads to lower cutting forces and easy chip breaking. In addition, the graphite develops a lubricating effect on the rake face and forms a protective layer, so that the service life is very long.
Cast iron is divided into white cast iron, which consists mainly of cementite, but which can be converted by heat treatment ( malleable cast iron ) and gray cast iron , in which the carbon is in the form of graphite, which is designed as lamellae, worms or spheres .
White cast iron
White cast iron is formed when the melt cools down quickly and consists largely of cementite. In this form it is called chilled cast iron . If the cementite is transformed into a softer structure by heat treatment , it is called malleable cast iron.
Chilled cast iron
Chilled cast iron consists almost exclusively of cementite, is hard and brittle and is very difficult to machine. Hard metals are used as cutting materials , with higher hardness also oxidic mixed ceramics or boron nitride of group H. With boron nitride a cutting speed about three to four times higher is possible compared with hard metal; However, tools made from boron nitride are prone to breakage. The higher the hardness of the material, the lower the cutting speed and the intervention parameters should be selected. The tool setting angles are usually 10 ° to 20 ° and the rake angles at −5 ° to + 5 °.
Malleable cast iron
When white cast iron is subjected to a heat treatment (so-called tempering ), malleable cast iron is produced . White malleable cast iron is annealed in an oxidizing atmosphere, the surface layer is completely decarburized and converted into ferrite. Carbon still remains inside. The structure then consists of perlite and a graphite mesh. Black malleable cast iron is annealed in a non-oxidizing atmosphere, the structure then also consists of pearlite and graphite at the edges. With rapid cooling, martensite can also form.
The machinability of black malleable cast iron is cheaper, but only because the ferritic surface layer of white malleable cast iron causes problems such as long chips, built-up edges and sticking with the tool. Otherwise, malleable cast iron is easy to machine - significantly better than steels with a comparable structure, which is due to the embedded graphite. This leads to lower cutting forces, a lubricating graphite layer on the tool that reduces friction and also acts as a protective layer, as well as short-brittle chips.
Uncoated and coated hard metals, cermets , oxide ceramics and boron nitride of groups P and K are mostly used as cutting materials . Thanks to the heat treatment, a consistent structure can be achieved even with larger lots. Malleable cast iron is therefore well suited for optimizing the cutting conditions.
Gray cast iron
In the case of gray cast iron or gray cast iron, the carbon is not bound in the cementite, but rather as graphite. Gray cast iron is produced at very slow cooling rates. Since the cooling rate is finite in practice, some cementite is usually still present, but the formation of graphite can be promoted by adding silicon to the alloy. The basic structure of gray cast iron can be varied within wide limits. It ranges from ferritic to ferritic-pearlitic to pearlitic, which is the most common case. A purely ferritic structure only arises after a very long glow. In general, gray cast iron can be machined more easily than comparable steels with similar strength and hardness, since the structure of these is not interrupted by the graphite. Most cast materials therefore achieve good machinability with short chips, low wear and tear and good surface quality.
Gray cast iron is divided according to the shape of the graphite into
- Cast iron with lamellar graphite
- Cast iron with vermicular graphite and
- Cast iron with nodular graphite.
Cast iron with lamellar graphite
In cast iron with lamellar graphite (abbreviation GJL), the graphite is in the form of thin lamellae that are finely distributed in the structure and reduce the strength. It is the most widely used cast iron material.
GJL can be machined excellently. The graphite has a lubricating effect on the tool surface and thus reduces the cutting force and extends the tool life. Furthermore, the graphite lamellas lead to short-brittle chips, mostly in the form of flakes or crumbling chips. The chip formation takes place via shear chip formation or tear chip formation . A special feature is the formation of a protective manganese sulfide layer on the flank and the rake face. This layer is created at temperatures that prevail at cutting speeds of 200 m / min and more, develops a lubricating effect and also has a protective function. Above all, it inhibits diffusion, which is a major wear mechanism at higher cutting speeds.
The surface quality that can be achieved depends on many factors, the manufacturing process, the cutting conditions and the structure. There are no burrs, as is the case with steel, but edge breakouts due to the brittleness of the material.
The cutting speeds can be higher, the more ferrite and the less pearlite there is in the structure. With a pearlite content of 10%, up to three times higher cutting speeds are possible with a constant tool life than with a structure with 90% pearlite. Hard inclusions such as cementite or steadite reduce the applicable cutting speed considerably.
The edge zones of the castings are usually much more difficult to machine than the core structure. This is due on the one hand to other types of structure directly below the surface and to the so-called cast skin, which contains scaling and non-metallic inclusions. The surface layer is therefore mostly machined with reduced cutting values.
As a cutting material, high-speed steel is only used for certain tools such as drilling, reaming or thread cutting. Hard metals for cast iron with lamellar graphite are from Group K. Cermets are also used for fine machining. Coated hard metals and boron nitride enable significantly higher cutting values.
Cast iron with vermicular graphite
In cast iron with vermicular graphite (GJV), the graphite is in worm-like structures. Compared to cast iron with lamellar graphite and cast iron with nodular graphite (GJS), it offers numerous better performance properties: higher strength, higher toughness, better oxidation resistance and better resistance to temperature changes. It also has excellent castability.
However, the machinability is relatively poor compared to other cast iron materials, which limits the use of GJV. The main difference is that GJV does not form a protective layer made of manganese sulphide, since the addition of sulfur has to be dispensed with in order to maintain the characteristic structure. At cutting speeds below 200 m / min, at which the protective and lubricating manganese sulphide layer does not form even with GJL, there are hardly any differences in service life. Titanium as an alloying element forms very hard carbides, which also lead to high wear in tungsten carbide hard metals.
Chip formation is not continuous at many cutting speeds, which leads to fluctuating cutting forces. This is due to the inhomogeneous material made of ferrite and pearlite as well as the graphite shape.
Coated hard metal is used as the cutting material for GJV. Ceramic made of aluminum oxide is also favorable. However, both cutting materials have a certain cutting speed range in which they are each advantageous. Coated carbide is more suitable for low and normal speeds, ceramics for high-speed machining. At low speeds, the alternating stress due to the discontinuous chip formation is decisive, so that the higher toughness and fatigue strength of the hard metals are advantageous here. At higher cutting speeds, the higher hot hardness and the higher chemical resistance of the ceramics pay off. With boron nitride, higher cutting speeds are possible compared to hard metal and ceramics. However, at only 300 m / min, these are well below the 1500 m / min possible with GJL.
Cast iron with nodular graphite
In cast iron with nodular graphite , the graphite is in a spherical form. It is therefore also called nodular cast iron. It has high strength and toughness. The structure can consist of ferrite, pearlite or a mixture thereof, each with embedded graphite balls. The higher the pearlite content, the higher the abrasive tool wear and the higher the cutting force. Uncoated and coated hard metals as well as oxide ceramics of group K are used as cutting materials. The surface roughness can reach values of Ra = 1 µm. Most of the time, shavings are formed. Flow chips are only created with very sharp-edged cutting edges. The cutting speeds are only slightly below those of steels of comparable hardness and strength.
Individual evidence
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 307.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 310.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 308 f.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 310 f.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 312-314.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 314-316.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 316 f.