Casting of iron
Cast iron is an iron-carbon alloy, containing usually between 2.4 and 4 % carbon. The minimum carbon content is 1.8 %. Silicon, manganese, sulphur and phosphorus are also present in various amounts. Special grades of iron are produced which contain various levels of nickel, chrome and other metals. Due to its high carbon content, cast iron has a low melting point and a good casting ability as compared to steel. Its ductility is low and does not allow rolling or forging. Variations in properties can be achieved by varying the ratio of carbon to silicon, by alloying, and by heat treatment.
Depending on the concentration and form of the carbon (lamellar, spheroidal or compact), various types of cast iron may be defined:
- lamellar iron: carbon in the form of flakes
- nodular iron: carbon in spheroidal form
- compact graphite iron: carbon in bonded form.
The classification of cast iron is often made according to its material properties:
- grey iron: iron with a grey fracture surface. Although this applies for lamellar, nodular and
compact graphite iron, the term is commonly used as a synonym for lamellar iron
- ductile iron: cast iron with an increased ductility. This is one of the effects caused by
nodularisation, but it also applies to malleable iron. The term is commonly used as a synonym for nodular iron
- malleable iron: iron that is capable of extension or of being shaped under the hammer. This
property is related to a low carbon content, which leaves most of the carbon in bonded form.
Cast iron can be melted in the cupola furnace, induction furnace (generally of coreless type, but very occasionally can be the channel type) or in the rotary furnace. The electric arc furnace is only very rarely used for the preparation of cast iron. Figure 1 gives process flow diagrams for the melting and metal treatment of cast iron in the three different furnace types. The process generally consists of melting – tapping – metal treatment – pouring. The various aspects of melting and metal treatment are discussed in the following sections. Metal treatment involves various steps such as desulphurisation, nodularisation, inoculation and deslagging. The desulphurisation step in cupola melting may also be incorporated into the nodularisation, e.g. by using a nodularisation process which simultaneously takes up the sulphur, such as the core wired process.
Figure 1: Process flow diagrams for the melting and metal treatment of cast iron [32, CAEF, 1997]
The cupola is the leading device for re-melting iron in Europe. It is responsible for some 55 % of the tonnage of iron castings produced in Western Europe. Nowadays, the cupola is increasingly facing major challenges to its market domination. This is partially due to its fluegas quality, which requires treatment. Faced with the possible financial burden of investing in, and then depreciating, a stack gas treatment installation, many small and medium sized units have turned to electric or oxygas melting units. Thus the number of cupolas used in foundries is falling, but their average size is increasing. There have been major changes in the market for cupolas in Europe in recent years, particularly due to the restructuring of the coke industry, leading to a decreased number of suppliers and a need to import coke into Europe. Another major change is the smaller number of cupola manufacturers, with one German firm having a quasi-monopoly in the hot blast type.
The majority of repetitive iron castings are made in green sand moulds with resin-bonded cores. The cold-box amine and hot-box techniques are most widely used. The ‘Croning resin shell’ moulding process is used where a high precision and good surface finish are needed. The Lost Foam process is used to a limited extent, for repetition castings. Castings made in smaller numbers are made in chemically-bonded sand moulds. Special sand processes, such as vacuum moulding and full moulding are used for certain iron castings. There are also a few permanent moulding (die-casting) foundries making iron castings, but the short die-life of a mould limiting it to making only a few thousand components has restricted the use of ferrous die-casting. [156, Godinot, 2001], [174, Brown, 2000]
Source: European Commission, Reference Document on Best Available Techniques in the Smitheries and Foundries Industry, May 2005, p.15-17