Heat treatment of cast iron

Heat treatment of cast iron is a set of technological procedures, which – simply put – consists in heating the cast iron to the desired temperature, keeping it there for the required time, and finally cooling it at a preset speed. Thanks to this set of methods, it is possible to obtain the appropriate properties of the casting. 

This text specifically shares methods of the heat treatment of cast iron - an alloy of iron with carbon and other elements, in which the carbon content ranges from 2% to a maximum of 3.8% to 6.7%. Want to know more about cast iron? Check out our Knowledge Base, where we describe the different grades of cast iron, the differences between cast steel and cast iron, and much more.

From this article you will learn

• what is cast iron heat treatment,
• what are the goals of heat treatment of cast iron,
• what is the classification of the heat treatment,
• how the structure of the casting is improved,

Heat treatment purposes

The technology of heat treatment of cast iron is an effective way to improve the functional properties of castings, to create a structure that guarantees specific mechanical properties or to correct a defective structure obtained at various stages of the technological process.

As a result, heat treated castings are perfectly compatible with the specific usage conditions.

Heat treatment of cast iron is used for both alloy and non-alloy cast iron castings. What's more, it plays a particularly important role in the process of ductile iron castings. This is due to the impact of the matrix on the mechanical properties of castings, much stronger than compared to, for example, gray cast iron with lamellar graphite.

Classification of heat treatment treatments

Heat treatments can be divided into three categories, namely: annealing, thermal improvement and precipitation strengthening. Their classification is presented in the table below.

Methods of cast iron heat treatment and casting structure improvement

Heat treatment of cast iron, depending on the desired parameters, may be subject to various methods and techniques. For example (and in general), these include:

1. annealing and stress relieving used to reduce or remove casting stresses without changing the microstructure and mechanical properties,
2. ferritizng and graphitizing, softening annealing, used to reduce hardness and increase machinability and improve plastic properties,
3. normalizing used to homogenize the microstructure and improve strength properties,
4. hardening and thermal improvement used to improve resistance to abrasive wear and increase mechanical properties,
5. hardening with isothermal transformation, mainly used in the technology of ADI ductile iron and gray cast iron with AGI flake graphite and AVCI vermicular graphite,
6. surface hardening used to increase the hardness and abrasion resistance of the cast top layer.

 from Odlewnictwo współczesne. Poradnik odlewnika, Tom I, wyd. Stowarzyszenia Technicznego Odlewników Polskich, 2013, s. 290, own translation

What are the different categories of iron heat treatment methods?

Annealing

Annealing is a method of heat treatment of metals consisting of heating the material to a certain temperature, keeping it at this temperature, and then slowly cooling it down. The aim of machining cast iron by flattening is to bring the material condition closer to the equilibrium conditions.

We distinguish several methods of annealing:

• homogenizing,
• normalizing,
• softening (spheroidization),
• ferritizing,
• graphitizing,
• relaxing.

Quenching and tempering

Quenching and tempering are a group of cast iron processing techniques, which are designed to increase the mechanical properties, especially strength, of cast iron. Quenching and tempering consists in heat treatment carried out in non-equilibrium conditions, especially in the case of cooling, which is fast enough to limit or prevent processes of diffusion (spontaneous mixing of substances).

Thermal improvement treatments include:

• tempering (low, medium and high),
• cryogenic processing, carried out at a temperature of 80➗150℃ below zero, thanks to which the casting gains a uniform microstructure and very good strength.
• hardening.

The latter, however, deserves a separate point.

Hardening

Hardening is a procedure involving rapid cooling aimed at obtaining a supersaturated solid solution in Fe∝, i.e. the so-called martensite, the crystallographic lattice of which is different from that of the phase from which it was formed.

In simpler terms, hardening, which is used to increase the strength of castings, consists of:

• heating the casting to the austenitizing (hardening) temperature, i.e. one that is 30-50˙C higher than the transformation start temperature (A1),
• heating the casting at this temperature,
• cooling faster than the critical rate.

We distinguish:

• isothermal-bainitic hardening,
• surface hardening.

Precipitation and dispersion strengthening

Precipitation and dispersion strengthening consists of:

• supersaturation, i.e. rapid cooling of the alloy in order to obtain a supersaturated solid solution, preceded by annealing at elevated temperature,
• aging, during which the oversaturation component is separated from the solution in the form of fine phases with a certain degree of dispersion; A distinction is made between natural aging at room temperature and artificial aging at room temperature.

Heat treatment of cast iron - effects

The final effect of heat treatment is the result of many parameters, including, among others:

• heating rate,
• temperature,
• soaking time at a specific temperature (necessary to equalize the temperature in the entire volume of the casting and the entire course of the required transformations or phenomena, e.g. diffusion),
• cooling rate (this is the most important element of casting heat treatment!).

Selected methods and goals of heat treatment are presented in the table below, on the example of thermal improvement of castings by hardening.

from Odlewnictwo współczesne. Poradnik odlewnika, Tom I, wyd. Stowarzyszenia Technicznego Odlewników Polskich, 2013, s. 297 (own translation)

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