What is an austempering plant used for?

An austempering plant is a heat treatment plant that completes the hardening phase at a temperature above the “Martensite Start”, below which martensitic transformation takes place.

Types and use of austempering plants.

As a general criterion, the quenching medium is not binding, provided that it causes the transformation of the material at the desired temperature. In fact, not only must the requirement of transformation at a temperature above the “Martensite Start” be met, but the quenching medium must have a heat extraction capacity proportionate to the casting thickness (drasticity), so that the transformation temperature is not too high for the desired structure to be obtained.

In principle, quenching using fluid beds of sand has been hypothesised and made, limited to thin objects subjected to heat treatment.

The quenching medium almost universally adopted is the salt bath, in different configurations characterised by the ratio of the quantity of salt to the mass of the charge, stirring of the bath, and the possible addition of water in dynamic suspension in the salt.

A distinctive element of the plant is the way in which the charge is transferred from the austenisation chamber to the quenching bath, with particular reference to the safety and repetitiveness of the operations.

A further distinctive element of the plant is the configuration of the austenitiser (in an endogas/nitrogen/air atmosphere or in a high temperature salt bath). Salt bath austenisation requires special care to prevent the quenching salt from being polluted with austenisation salt.

The plants also stand out for their operating mode, either continuous for large batches of products of small size and thickness, or with a charge bucket (“batch”) for universal use.

“Batch” plants can also be configured with a single door variant for the inlet and outlet into/from the austenitiser (typical solution) or with two doors, one for the inlet and another for the outlet (“push”), providing for more than one stage for raising and holding the austenisation temperature (solution suitable for processes with a short quenching time).

Zanardi Foundries’ batch austempering plant.

a) Pre-heating furnace

The heat treatment cycle includes a pre-heating phase with natural gas combustion products. The purpose of this phase is to remove moisture from the charge and homogenise the temperature of the charge to the pre-heating value.

b) Purging pre-chamber

Using the transfer wagon, the charge is transferred from the pre-heating station (on the left, not shown in the following diagram) to the austempering unit.

Outward journey: from pre-heating to pre-chamber and finally to the austenisation chamber
Return journey: from the austenisation chamber to the pre-chamber to salt bath quenching

As it passes over the salt, the charge stops in the purging pre-chamber (left and right doors both closed) for the time required to burn off the oxygen contained in the pre-chamber itself, in the final phase saturating the pre-chamber with endogas. Having completed this operation, the door to the right of the pre-chamber opens and the charge is pushed into austenisation chamber A.

After the completion of the austenisation phase, when the charge has reached the desired conditions, the door to the right (in the diagram) of the purging pre-chamber opens, where the charge is transferred. Then the door closes, the endogas in the pre-chamber is burned off, and in the final phase a washing cycle with nitrogen atmosphere is performed.

This operation is necessary for safety reasons, as contact between endogas and salts must be prevented.

Then the door to the left (in the diagram) of the purging pre-chamber opens and the charge is transferred over the salt bath.

c) Austenisation chamber in endogas

Between the two phases in which the purging pre-chamber is involved, the austenisation phase is carried out.

The austenisation chamber, which works with radiant tubes whose combustion air is pre-heated by the flue gases, is equipped with endogas atmosphere recirculation fans.

The operation of the chamber and the arrangement of the castings in the charge bucket are a critical aspect of the process.

In fact, the temperature must be achieved with the certainty that the entire charge, down to the core of the casting in the central position, is at the austenisation temperature. It is necessary to bear in mind that the thermocouples from which the signal is taken measure the temperature outside the charge bucket, but say nothing about what is happening in the core of the casting in the central position. This condition is controlled by monitoring the process parameters and checking the results on the structure of the castings in the different positions of the bucket. 

Having passed the eutectoid, with the formation of austenite, the carbon diffusion phase from the spheroids to the metal matrix also begins.

This process continues until saturation, keeping the charge at the austenisation temperature for the required time.

The diffusion of carbon, hence also the time required for saturation, depends on the chemical composition and size of the spheroids and can be measured using differential dilatometry techniques.

If the austenisation temperature is held for longer than the time required for saturation, the grain starts swelling, a result that is sought in some intercritical grades.

d) Molten salt bath

As in the hardening of steels, the component is quickly immersed in the quenching medium consisting of a mixture of nitrites and nitrates melted at a temperature above the “Martensite Start”.

In this way, the “snap-like” martensitic transformation and corresponding stresses are avoided.

Since, at the temperature of the salt bath, austenite cannot exist in its natural “primary” form, as described by the iron-carbon diagram (link at temperatures above the eutectoid, a transformation reaction of the primary austenite (face-centred cubic gamma iron) into ferrite (body-centred cubic alpha iron) is triggered, with the expulsion of excess carbon from the lattice. This free carbon escapes into the lattice of the austenite that has not yet reacted and over-saturates it, therefore stabilising it in a metastable mould at temperatures below the eutectoid.

Since the reaction takes place at temperatures far below the eutectoid temperature, the ferrite takes on an acicular form (Widmanstätten ferrite) and not a grain form, which is instead typical in the case of natural cooling in the mould.

The resulting structure is therefore a mixture of acicular ferrite and metastable austenite, over-saturated with carbon and therefore stabilised by it.

The reaction described above must be imagined “in the spot”, i.e. within a small volume compared to the distance between two spheroids (or two thin sheets).

The distance between two generic graphite “wells” is another means that can be used to apply a heat treatment to cast irons that is not possible in steels. In fact, the distance between two of these elements (e.g. two spheroids) characterises the reaction “pattern” that is replicated in countless instances simultaneously.

The space between two spheroids must be imagined as consisting of a number of small volumes, each characterised by its own chemical composition, an effect of the chemical segregation produced in the eutectic system upon natural solidification in the mould.

The local chemical composition drives the local kinetics of the reaction (driving force). The reaction proceeds faster near the spheroid and slower at the edge of the grain (median area between two adjacent spheroids).

Due to the local concentration of the elements, the volume at the start of the solidification process is the one near the spheroid (First To Freeze FTF), whereas the volume at the end of the solidification process is the one at the edge of the grain (Last To Freeze LTF).

The same time sequence is repeated in the austempering reaction: the reaction is faster near the spheroid (First To React) and slower at the edge of the grain (Last To React).

A well-managed process achieves a correctly developed reaction throughout the segregative interval: in the FTF, once equilibrium has been reached, the structure must remain stable for as long as is still necessary to reach equilibrium in the LTF; in the LTF equilibrium must be reached within a time compatible with the stability of the structure in the FTF.

If these requirements are not met, the material may contain unstable austenite in the LTF and/or secondary carbides in the FTF, which would compromise mechanical properties and workability.

It is possible to check that the structure meets these requirements using the “heat tinting” method and/or a tensile test measuring the dislocation motion parameters. 

In addition to the robustness of the process in the austenisation phase, obtaining an appropriate structure requires a robust heat treatment plant also for the quenching phase, capable of ensuring the repeatability of the operating parameters:

  • Salt bath and charge mass ratio: the ratio of the mass of the salt bath to that of the charge must be large enough to contain the initial temperature rise within reasonable limits;
  • Temperature uniformity: the bath must be stirred so as to obtain temperature uniformity in all parts of the charge;
  • Quenching drasticity: adding water to the salt contributes, together with stirring the bath, to increase the quenching drasticity, reducing the quantity of binders required to avoid crossing the pearlitic nose, with beneficial effects from an economic and qualitative point of view, due to the minimisation of segregations.

Obtaining an appropriate structure requires that the material in its raw melt state is of good quality, the choice of chemical composition to avoid crossing the pearlitic nose must be appropriate to minimise the segregative effects.

The austempering temperature (salt bath temperature), chosen according to the grade (hardness) must be consistent with the austenisation temperature, in order to ensure sufficient “driving force” in the LTF.

As described above, the time in the austenisation chamber must be appropriate for the complete diffusion of the Carbon in all the thicknesses of all the castings in the charge.

During the quenching phase, the austempering time in the salt bath must be appropriate for the stability of the austenite in the LTF, while preventing excessive time from causing the formation of secondary carbides (entry into the second stage in the FTF).

e) Treatment bucket

The arrangement of the castings in the charge bucket is a critical aspect of the process, contributing to the homogeneity of temperature distribution in the charge in both the austenisation and quenching phases.

f) Washing machine

The purpose of the washing machine is to remove all residual traces of salt from the castings and dry them to prevent subsequent oxidation.