Our discussion has dealt with the cause of retained austenite (parts 1 and 2). Here, we will address the reasons why retained austenite can be a problem, and we will conclude our discussion with the control and elimination of retained austenite.
Problems of Retained Austenite
This article will discuss the problems of retained austenite as a cause of concern among companies that are conducting the process of carburizing. When the steel is raised up to an elevated temperature into the austenite region, the austenite is a stable phase and the phase has reoriented itself into the face-centered cubic (FCC) lattice. Austenite will allow carbon diffusion from a hydrocarbon-based atmosphere.
Once the appropriate amount of carbon has diffused into the steel through the surface and into the sub-surface, the steel is cooled down. Other products/phases will begin to form. The final phase formation will depend on the rate of cooling from the austenitizing/carburizing temperature.
If the steel is cooled rapidly enough, the newly formed austenite becomes unstable and decomposes into the other phases. The hope is that the austenite will transform the carburized surface into the phase of martensite (BCT lattice).
The objective is for the carburized surface to transform into martensite. However, the steel surface will cool down at the rate that the chosen cooling medium will allow. Quenching options are typically water, oil, polyalkaline glycol in water or blended gases (only good for quenching on alloy carburizing steels).
If it is cooled too quickly, there is a strong likelihood of surface cracks being initiated. If it is cooled too slowly, however, some of the now unstable austenite can remain. This is known as retained austenite.
The next factor of concern will be the final cooled temperature as seen on the iron-carbon equilibrium (ICE) diagram. The steel has to cool down to below what is known as the martensite-start line (Ms line on the ICE diagram). This is the horizontal temperature line at which martensite will BEGIN to form.
Please note that the transformation from austenite to martensite is not instantaneous. One can assume full transformation from austenite to martensite when the quenched steel passes down to what is known as the Mf line (martensite-finish line). This line can be near room temperature or somewhat below or above ambient temperature, dependent on the surface carbon content of the carburized case.