Heat treatment of cast iron castings

As with all metal materials, both ferrous and non-ferrous, the final performance of cast iron can similarly be improved by way of heat treatments. If the aim is to simply reduce the presence of residual stresses resulting from the production process, stress-relieving treatments are carried out for a few hours at a temperature in the 200-550°C range.

As with steels, heat treatments can also be designed for cast irons by analysing the TTT and CCT curves, which respectively describe the microstructural behaviour of the material as a result of isothermal treatments (with constant temperature and residence times) and non-isothermal treatments (in which the temperature gradually decreases due to the different cooling methods applied).

These diagrams are “personalised” according to the specific composition of the cast iron or steel and allow the evaluation of the material’s microstructural evolution depending on the temperatures and cooling conditions adopted in a heat treatment. A few examples of hardened (quenching + tempering) cast irons are provided, which, however, must be implemented very carefully to avoid problems associated with the brittleness of the martensitic phase generated during quenching.

Quenching, i.e., the rapid cooling of the material from high temperatures, can be done in water or oil (and, therefore, the final temperature will be ambient temperature), or in a salt bath (in this case, the aim is to cool from high temperatures up to 250-350°C to carry out an austempering treatment).

Tempering, on the other hand, is heating after quenching at temperatures normally from 200 to 650°C, thus reducing the brittleness of the martensite (that is, the microstructure resulting from the quenching process).

Rif department austempering

Several specific isothermal treatments, such as the austempering heat treatment, are much more effective, and allow optimisation of the microstructure and performance of the cast irons (see section on ADI and IDI ductile irons).

Lastly, several thermochemical treatments (nitriding, boriding) with specific cast iron surface hardening properties (even to a depth of a few tenths of a millimetre) are also reported. The nitriding treatment consists of keeping the material at a temperature of 500-600°C for a few dozen hours in an ammonia-rich environment. In these conditions, nitrogen is diffused on the surface of the material and compounds are formed (iron nitrides) with very high hardness, which thus improve the surface characteristics of the cast iron. 

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