In this month’s column, I will discuss the transformation of retained austenite, and its stabilization.
In the last column, I reviewed the dimensional changes that occur during tempering. One of the topics mentioned in the article was the transformation of retained austenite. In the column, it was shown that retained austenite transforms to bainite in the range of 230°-280°C, and transforms to martensite in the range of 400°-600°C. The product of decomposition (martensite or bainite) depends on the tempering time and temperature. Bainite transforms isothermally, whereas the martensite will form upon cooling from the tempering temperature .
Retained austenite forms in steel, when the steel is heated to the austenite region and then quenched. If the quenching temperature is above the martensite finish temperature, Mf , then the austenite to martensite transformation will not be completed. In other words, not all the austenite will transform to martensite, and an amount of austenite will remain, or be retained.
In many alloys containing more than approximately 0.3 percent carbon, the Mf temperature is below room temperature. Remember from previous columns, that the martensite start temperature can be estimated from the equation :
The martensite finish temperature can be estimated from the equation :
This relationship is shown for a series of different alloys in Figure 1 .
Since, at room temperature, a portion of the austenite is not transformed to martensite, it remains intermingled with the martensite at room temperature. This is particularly true of carburized alloys, since the case carbon content is roughly 0.9 percent carbon, and the martensite finish temperature is approximately minus-100°C. The amount of retained austenite is dependent of the carbon content, alloy content (nickel and manganese are strong austenite stabilizers), quenchant temperature, and subsequent thermal processing . Large amounts of retained austenite can be estimated by metallography  but for smaller amounts of retained austenite (less than 15 percent), other methods such as x-ray diffraction is required .
The remaining retained austenite can be transformed to martensite by either sub-zero or cryogenic treatment or by tempering.
Stabilization of Retained Austenite
After hardening, if the steel has been held at room temperature or the martempering temperature for a prolonged period, the retained austenite becomes stabilized. This means that it is more difficult to transform to martensite after a sub-zero treatment. Figure 2 shows the amount of retained austenite that is present after stabilizing at temperature for different periods of time.
Figure 2 shows that, initially, there is 18 percent retained austenite. If the sub-zero treatment to minus-180°C is carried out within five minutes attaining 20°C, nearly 70 percent of the retained austenite is transformed. If the part is held for 50 hours (3,000 minutes), only 30 percent of the retained austenite will be transformed to martensite.
If hardening is interrupted around the Ms temperature, a similar effect occurs. If the part is held at approximately the Ms temperature, then cooled to room temperature, there will be additional retained austenite transformation to martensite. However, the amount of retained austenite after sub-zero treatment would be increased. The amount of retained austenite transformed would be less than if a direct quench to room temperature had occurred.
The amount of retained austenite is dependent on the austenitizing temperature. As austenization temperature is increased, greater quantities of austenite are retained upon quenching. This is shown in Figure 3.
Stabilization of retained austenite is thought to be due to a pinning mechanism. During the stabilization period, the carbon is redistributed to the matrix by diffusion. Interstitial carbon atoms pin the austenite/martensite interface . As the length of time increases after quenching, more pinning occurs. This requires a greater energy to get over the activation energy hump and restart the austenite to martensitic transformation. Sub-zero treatments have the necessary driving force to drive the retained austenite to martensite reaction to completion.
In this short article, the causes of retained austenite were examined. Alloy content, especially nickel and manganese, has a strong influence on the stability of retained austenite and the martensite finish temperature, Mf . It was shown that for most common steels, the martensite finish temperature is below room temperature, and often at sub-zero temperatures. Stabilization of retained austenite occurs by pinning of the retained austenite and martensite interface by the diffusion of carbon to interstitial sites. Increasing holding times at above the martensite finish temperature increases the driving force necessary to drive the austenite transformation to completion.
Should there be any comments on this article, or suggestions for further columns, please contact the author, or the editor.
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