What is austempering heat treatment?

Austempering heat treatment, traditionally known to be applied to steels, has been rediscovered with important advantages when applied to cast irons, particularly spheroidal graphite ones. For both steels and cast irons, the execution of the austempering treatment requires avoiding the passage through the pearlitic nose in the Continuous Cooling Transformation (CCT) curves.

Schematic CCT diagram for a cast iron illustrating the presence of IPR of Ferrite, Pearlite, Ausferrite, and Martensite in T-t space

Source: METAL CASTING, Reference Book for MY4130 – By Prof. Karl B. Rundman – Dept. of Materials Science and Engineering Michigan Tech. University

For this to happen, it is necessary to add alloying elements, capable of moving these CCT curves to the right by an increasing entity as the thickness of the casting increases (in fact, the casting cooling rate, under the same conditions imposed by the heat treatment plant, depends on the thickness). In the course of liquid-solid transformation, the elements contained in the metal are subject to segregation (Scheil’s law), obtaining a different concentration of each of them from point to point. The concentration of the elements at each point has a significant influence on the transformation speed, determining the mechanical properties, structural stability and machinability of the component. In spheroidal graphite cast irons, the segregation sequence (therefore the minimum and maximum concentrations of each element) is guided by the distance between two adjacent graphite nodules, allowing the correct execution of the austempering heat treatment on thicknesses greater than what is possible with steels, thus making austempering to foundry castings advantageously practicable.

What are the stages of the austempering treatment?

Austempering treatment steps are: preheating; complete austenitization; (or intercritical austenitization); quenching in salt bath; permanence at constant temperature until the ausferritic reaction is completed.

  • Preheating: heating of the material to about 500° C, with the aim of removing moisture from the load, reducing the time required for the rise to the austenitization temperature.
  • Complete austenitization: maintenance at a temperature higher than eutectoid AC3 (indicated in the iron-carbon diagram) with complete transformation of the ferrous structure into austenite.
  • Intercritical austenitization: maintenance at an intermediate temperature between the eutectoids AC1 and AC3 (indicated in the iron-carbon diagram) with partial transformation of the ferrous structure into austenite in equilibrium with a ferritic phase called proeutectoid.
  • Quenching in salt bath: descent from the austenitization temperature to the quenching temperature with sufficient cooling rate to prevent pearlitic transformation.

The permanence at the quenching temperature, higher than that of martensite start (indicated in the diagrams containing the Bain curves), allows the development of the ausferritic reaction, consisting in the transformation of austenite into acicular ferrite, with the release of Carbon (ferrite contains a negligible amount in its lattice) that migrates into the lattice of untransformed austenite, stabilising it.

Graph 1: graphical representation of the austempering cycle of austempered ductile iron

Why in austempering do we talk about isothermal quenching?

In the heat treatment of austempering, we speak of “isothermal quenching”. The ausferritic transformation, in fact, takes place at a constant temperature. Unlike what happens for other types of treatments, after the initial rapid cooling to quenching temperature, it is necessary to maintain it constant. in order to allow the diffusive transformations to complete.

How does the structure of ductile iron change from one stage to another?

During austempering treatment, the structure of cast iron changes radically. In the rough state of melting, or after a heat treatment of ferritization or normalisation, the structure is composed of ferrite and/or pearlite. Pearlite is an intimate mixture of ferrite and carbides. Ferrite is a body-centred cubic structure, with iron atoms at the vertices of the cubic lattice and a carbon atom at the centre of the lattice.

Pearlitic structure (100x)

Ferritic structure (100x)

The ausferritic structure, obtained as a result of the austempering treatment, consists of a body-centred cubic ferritic phase (much more refined than the one previously described), together with a face-centred cubic austenitic phase (the carbon atoms are arranged in the centre of the faces of the cubic lattice). The face-centred structure of austenite (stabilised by carbon) gives the material favourable properties in terms of ductility, toughness and behaviour at low temperatures.

Ausferritic structure – ADI1050 (x500)

The austenite (gamma iron) present in the austenitization phase is an equilibrium structure in the iron-carbon diagram and is characterised by the carbon content corresponding to the saturation curve (example value 0.8% weight in the figure).

Ausferritic structure – ADI1050 (x500)

Source: METAL CASTING, Reference Book for MY4130 – By Prof. Karl B. Rundman – Dept. of Materials Science and Engineering Michigan Tech. University

If this structure is maintained for a long time at a temperature just below that of the eutectoid transformation (AC1), we would obtain a ferritic structure, practically free of carbon, accompanied by the appearance of a graphitic halo around the graphite nodules. In fact, at this-high-temperature (subcritical annealing) the carbon would have mobility and sufficient time to diffuse from its site to the graphite nodules. Since the austempering temperature is too low to allow carbon this mobility, the transformation takes place “in situ”. In this way, the carbon rejected by the austenite transformed into ferrite takes refuge in an immediately adjacent austenitic lattice, progressively helping to stabilise it, preventing its transformation. The maximum amount of carbon that can be contained in untransformed austenite is given by the metastable saturation curve (example value 2.0% weight in the figure). Thus, the difference between primary austenite and that after the austempering reaction consists in the different carbon content and in the different refinement, preserving the commonality of the face-centred structure.

Application of austempering heat treatment on as-cast materials

Austempering heat treatment is commonly applied to ductile irons and, more rarely, to grey cast irons and steels.

What characteristics should as-cast ductile irons have?

As-cast ductile irons (i.e. not heat-treated), must not contain primary carbides to such an extent as to impair the function of the casting, they must be characterised by sufficient nodularity in relation to the thickness of the casting and must be added with alloying elements to an extent to avoid pearlitic transformation during rapid cooling to quenching temperature.

What characteristics should as-cast grey irons have?

As-cast grey irons must not contain primary carbides to such an extent as to impair the function of the casting, they must be characterised by good shape and distribution of the lamellae in relation to the thickness of the casting and must be added with alloying elements to an extent to avoid pearlitic transformation during rapid cooling to quenching temperature.

What characteristics should as-cast steels have?

Steels must also be added with alloys in an amount sufficient to avoid pearlitic transformation during rapid cooling to quenching temperature. As with cast irons, even in steels alloying elements segregate in the solidification phase (the concentration of the element is not uniform). While in cast irons the inhomogeneity of the concentrations is repeated in the distance between two graphite nodules and/or contiguous lamellae, in steels the distance that guides the process is that between the secondary arms of the dendrites, as happens with aluminium alloys (SDAS, secondary dendrite arm spacing). This characteristic can represent a limit for the thicknesses of the steel components to which austempering can be applied, orienting the applications more towards components obtained from rough indefinite steel than from foundry castings.

Origins and brief history of austempering heat treatment

The austempering of steels, with the complete transformation of austenite into lower bainite, was a well-known treatment already in the early 70s, when the Kimme Kimmene foundry in Karkkila (Finland) started pioneering experiments for the production of large gears in austempered ductile iron castings. At the end of the 70s, Dr. Horst Muehlberger patented a process for the manufacture of machinable austempered ductile irons (ADI), a property not applicable with the first process so far. In 1982 Zanardi Fonderie signed a license agreement with Dr. Muehlberger, starting its activity in the field of austempered ductile irons. In the same year, Applied Process in the USA began the activity of heat treatment of austempering on ductile irons and steels.

Influence of time and temperature in austempering

Among the “key” variables of the austempering treatment are time and temperature that influence the process. The dominant key variable is the Manganese content, characterising the three different original technologies (Kimme Kimmene, Muehlberger, Applied Process). The austempering temperature is chosen according to the desired hardness after heat treatment. For a given austempering temperature and a given chemical composition, there is an optimal austenitization temperature. Temperatures higher than the optimal one allow a saving of the alloying necessary to avoid pearlitic transformation, but reduce the possibility of obtaining stable austenite in a reasonable time, with decay of machinability and mechanical characteristics. The austempering time must be sufficient to make the transformed austenite stable. The austenitization time, on the other hand, must be sufficient for the carbon saturation of the primary austenite.