Cast iron

Cast iron is an iron -carbon alloy with a high carbon content (> 2.06% ), which distinguishes this material from steel, thus opening other applications. The term " cast iron " derives from the casting as a method of molding from; Cast iron parts are manufactured in a foundry.

Properties

Among cast iron is a group of iron alloys having a high proportion of carbon ( > 2%), and silicon, as well as other components such as manganese, chromium or nickel. A distinction is made between

• Gray cast iron ( gray cast iron), in which the carbon in the form of graphite occurs
• White cast iron in which the carbon is present as a carbide in the form of cementite.

The density of iron is about 7.2 g / cm ³ lower than the density of steel and pure iron with 7.85 g / cm ³. The material has not forge in the eutectic area with about 1150 ° C has a much lower melting point than steel, it can be but due to high carbon content. In foundries, it is usually melted in a cupola furnace.

Cast iron parts with uninjured skin cast have good corrosion resistance, which can be increased by alloying with silicon, chromium and nickel. A simple method for testing the quality of cast iron can be made with a hammer: A blow to a right-angled edge is to leave a lasting impression without the edge chipping.

Due to lack of mobility of the internal stresses of the non-uniform microstructure of gray cast iron has no discernible elasticity and must therefore be classified as rather brittle. On the other hand gives the material a graphite good thermal conductivity, and good damping properties because of the embrittlement of good dimensional rigidity. Therefore, cast iron is particularly suitable for machine tool beds and tables. There are also advantageous self- lubricating properties when cut by machining the fins and the graphite itself or in place of other lubricants can be " stored " in the cavities.

Production

Varieties

The simplest cast-iron variety is cast iron with lamellar graphite (designation according to current European standard EN 1561 " GJL " or earlier in accordance with DIN 1691 " GGL " ), in which the graphite is in the form of thin, irregularly shaped lamellae. These fins act as a tensile stress at the notches, so the tensile strength is relatively low ( " predetermined breaking point "). In contrast to the tensile strength of the transmission of the compressive stress is much better. The compressive strength is about a factor of 4 higher than the tensile strength.

Better mechanical properties of cast iron with spheroidal graphite ( ductile iron, ductile cast iron GJS designation according to current European Standard EN 1563, formerly GGG according to DIN 1693), in which the graphite is present in more or less spherical shape. This is achieved by desulfurization of the melt by the addition of small amounts of magnesium, cerium, calcium or shortly prior to casting.

Another important form of malleable, which is subjected, after solidification as ledeburite again to an annealing treatment ( heat treatment ).

• When white malleable ( GJMW ) is annealed in an oxygen -donating atmosphere, making the castings of carbon is removed (at least in the edge region ), thereby approximating the properties equivalent to steel.

• Black Malleable ( GJMB ) is annealed in an oxygen-free atmosphere. This leads to changes in the structure, which influence positively the properties. The improvements are not limited to the peripheral zone of the workpiece.

These varieties tolerate even small plastic deformations without breaking. Typical applications find such materials as malleable in pipeline construction with threaded connections.

A more recent development is the material vermicular graphite cast iron ( CGI designation according to current ISO 16112, formerly CDR). For him, the graphite is neither in form nor fins as a ball form, but as lumps that look like worms in the micrograph ( Vermiculus, Latin for worm ). The mechanical properties of this material are between the cast iron with lamellar graphite and those of cast iron with spheroidal graphite. However, its production is more difficult and requires a run to close tolerances melting treatment.