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Kinetics of bainite formation in ADI

This project was my final-year undergraduation project (July-December 1997). If you are not familiar with the Incomplete reaction phenomenon, it might be interesting to give a look here here first. This is one of Pr H.K.D.H. Bhadeshia's pages on bainite.

Aim and Methods.

The aim was to study the kinetics of bainite formation in Austempered Ductile Iron (ADI), in particular with regard to the incomplete reaction theory of bainite formation.

A thermomechanical simulator was used to reproduce the austempering process, which was followed with a laser dilatometer. The volume change of the sample was then related to the volume fraction of bainite formed as a function of time and the carbon content of the residual austenite calculated. The latter was also obtained experimentally by X-Ray diffraction measurement of the austenite lattice parameter.

ADI

ADI are a particular kind of spheroidal iron (Fe with about 3wt%C and other elements) which owes their name to the process used to produce a bainitic matrix.

austempering heat-treatment

The first step is an austenitisation done around 900 C, for a time varying between 15min to 2h. During this step, only austenite and graphite spheroids are present. The temperature and duration of the process are important because they fix the carbon content that the austenite has when the second stage is started.

The second step is the austempering stage itself, during which the material is held at a temperature around 350C for a few hours. This leads to the formation of bainite.

ADI combines excellent toughness and wear resistance together with the possibility of near net shape forming. It is increasingly used in the automotive industry to manufacture cam shafts.

Why is the kinetics important ?

When bainite forms in the austenite, the carbon content of the latter increases as the transformation occurs. If the austempering stage is too short, it is possible to form martensite in the residual austenite. If it is too long, carbide precipitation occurs. Both phenomena are detrimental to mechanical properties.

It is therefore necessary to know how long the austempering treatment should be. Validating the mechanism of bainite formation in this material is a first step towards quantitative predictions of the microstructure.

Results

It has been shown that the formation of bainite follows the incomplete reaction phenomenon: the final carbon content of the austenite is clearly that given by the T0 line.

carbon content of bainite in ADI bainite volume fraction in ADI

The carbon content of the residual austenite as a function of the austempering temperature. Note that at 350 C, three different austenitising temperature lead to the same final carbon content, but to different bainite volume fractions (see right).

The volume fraction of bainite formed during the austempering heat-treatment. The austenitising temperature effect is clear: it fixes the initial carbon content. The lower the temperature, the less carbon is initially present and therefore the more bainite can form before the residual austenite carbon content reaches the T0 line.

More

Segregation effects were also studied as they have a strong effect on the final microstructure.

If you are interested, you can download my report as a gunzipped postscript file. It takes only about 1Mb.
You can download all the dilatometry data collected during this project. More details on the download page and in the report. The icon on the left will take you directly where you can find them, but you can also give a look at the whole MAP site (right).
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