Heat treatment of cast steel

Heat treatment is a set of technological procedures, the process of 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 steel.

Want to learn more about cast steel? Check out our Knowledge Base, where you will find articles describing the different grades of cast steel, the differences between cast steel and cast iron, and much more.

In this article you will learn:

• what is the heat treatment of cast steel,
• what are the goals of heat treatment of cast steel,
• how heat treatments are classified,
• how the structure of the casting is actually improved.

Purposes of heat treatment of cast steel 

The technology of heat treatment of cast steel is an effective way to improve the functional properties of castings, and thus to create a structure that guarantees specific mechanical properties or to correct a defective structure obtained at various stages of the technological process. This makes the heat treated castings perfectly compatible with the very specific requirements of their use.

During the solidification of steel castings, certain unfavorable phenomena may occur, such as:

• uneven size of the grains formed on the cross-section of the casting, and the coarsest structure is in the middle of the castings wall,
• uneven content of elements included in the cast steel, both inside the crystals (micro-segregation) and on the cross-section of the casting (macro-segregation),
• precipitates at grain boundaries,
• high casting stress.

Therefore, heat treatment is an essential technological operation in the production of steel castings, because it contributes to the improvement of the crystal structure, reduction of dendritic segregation, reduction of internal stresses, and increase of physical and mechanical properties.

Odlewnictwo współczesne. Poradnik odlewnika, Tom I, wyd. Stowarzyszenia Technicznego Odlewników Polskich, 2013, p. 413 (own translation)

Heat treatment of cast steel - classification

The basic methods of heat treatment of steel castings are::

• homogenizing annealing (homogenization, diffusion annealing),
• complete annealing,
• normalization,
• normalization+tempering,
• double normalization with two or one final tempering or no tempering,
• soft annealing,
• thermal improvement (quenching + high tempering),
• double quenching with two or one final temperings,
• surface quenching + low tempering,
• thermo-chemical treatment.

Methods of heat treatment of cast steel and improvement of casting structure

An important and dominant group of heat treatment processes for steel castings are various types of annealing. Particular types of this process differ mainly in the scope of heating and the method of cooling. It is also common for annealing to be merely a pre-heat treatment or, in the case of normalizing annealing (both with and without tempering), the final heat treatment of the steel casting.

Below we will briefly describe the characteristics of the annealing and quenching processes.

Heat treatment of cast steel - annealing

Heat treatment of cast steel by annealing consists in the following sequence:

• heating the material to a certain temperature,
• holding it at this temperature,
• slow cooling.

The purpose of such heat treatment of cast steel is to approximate the state of the casting material to the equilibrium conditions as precisely as possible.

• homogenization: heating the casting to a very high temperature, holding it for a long time at this temperature and cooling it down - it is a typical diffusion process, thanks to which the secretions located on the grain boundaries are removed and there is no micro- and macro-segregation of the chemical composition. However, it also has disadvantages - high temperature and long heating time are conducive to decarburization and grain growth. For this reason, after this process, another type of annealing that promotes grain refining (e.g. normalization) must be applied. Due to its high cost, it is mainly used for large alloy cast steel castings.
• normalizing annealing (normalization): it consists in heating the cast steel to a temperature of approx. 30-50 ℃ above the Acз temperature (the temperature at which ferrite completely transforms into austenite), heating it at this temperature and cooling it in the air. Thanks to this type of annealing, a uniform, fine-grained structure is obtained, the amount of stress is reduced and the mechanical properties are increased.
• full annealing differs from normalizing only in the method of cooling - as part of full annealing, the casting is cooled together with the cooling of the furnace.
• softening annealing (also called spheroidizing annealing): heating the castings to a temperature slightly (approx. 10-15 ℃) below or above Ac₁ (which means the eutectoid transition point, i.e. the temperature at which the structure begins to change into austenite), and then cooling it very slowly, at a rate of 10➗20℃/h. It is worth noting that by spheroidization we mean a change in the form of cement, thanks to which the hardness is reduced.
• stress relief annealing: aims to remove residual stresses, trying not to introduce structural changes; this method of annealing castings is used, for example, after repairs of castings by welding.

Heat treatment of cast steel – quenching

Quenching is used to impart specific mechanical properties to components to ensure they are fit for use in the desired application. Quenching consists of:

• heating the cast steel to the austenitizing temperature, which depends on the carbon content,
• annealing at this temperature for approx. 2.4 min/1mm of casting wall thickness,
• cooling in such centers that will allow to obtain a martensitic, bainitic or mixed structure without causing cracks and deformation of the casting.

There are two quenching methods: volumetric and surface quenching, and within them, individual groups.

What are they about?

Volume quenching methods:

• ordinary - the most commonly used method, consisting in continuous cooling in one medium,
• intermittent - involving continuous cooling in two media, e.g. water and oil,
• gradual - consists in cooling in two medias: in the first to maintain the austenitic structure and in the second to transform austenite into martensite.

Surface quenching methods:

• inductive quenching - in which the surface layer of the casting is quickly heated with the help of eddy currents, induced by a variable magnetic field present in the inductor, through which the alternating current flows, the surface layer of the casting is quickly heated, and then this layer is quickly cooled down.
• flame quenching - i.e. those for which copper tips of burners are used, the structure adapted to the shape of the hardened surface; in flame quenching, the amount of heat applied to the surface of the casting must be much higher than the amount of heat that penetrates into the material by conduction.

Cooling

After austenization, in order to obtain the desired cast steel microstructure, the casting must be cooled at a specific rate in media characterized by the required thermokinetic properties, i.e. ensuring such supercooling of the austenite that the cast steel microstructure after quenching consists of martensite or bainite.