1. Preface
I would like to share with you the identification of “Ferrite” and “Retained Austenite”, as part of a series of articles on identifying similar structures in steel.
2. Identification of retained ferrite and austenite
Because Ferrite and Retained Austenite do not corrode, they both appear white when observed under a microscope. However, they can be easily confused if not observed properly.
Fortunately, it is relatively easy to distinguish between the two structures by mastering certain methods. Two common methods include:
1. Distinguish from tissue morphology
Ferrite and retained austenite often coexist in the microstructure of hypoeutectoid steel that has been subjected to quenching. There are typically three forms of ferrite in these tempered parts: undissolved polygonal ferrite, massive proeutectoid ferrite, and reticular or semi-reticular proeutectoid ferrite. All of these forms of ferrite are white and shiny in appearance.
Polygonal and massive ferrites have well-defined boundaries and are often found in the blank areas between the martensite needles. Upon closer inspection, the white phase and martensite phase can be seen in the same plane.
Reticular or semi-reticular ferrite is finely distributed along the original grain boundary of austenite.
Retained Austenite, on the other hand, lacks well-defined boundaries and its shape changes with the shape of the martensite needle distribution. It generally does not exist alone, but is organically combined with needle-shaped martensite after quenching. As a result, its color is slightly darker than that of ferrite, and the phenomenon of needle-shaped martensite is often faintly visible.
2. Inferred from heat treatment process
If the quenching heat preservation time for hypoeutectoid steel is insufficient or the temperature is too low, undissolved white polygonal ferrite will appear in the resulting microstructure.
Fig. 1 Undissolved white polygonal ferrite
As illustrated in Figure 1, the microstructure of 45 steel that was subjected to water quenching at 760℃ for 25 minutes consists of undissolved white polygonal ferrite, medium carbon quenched black martensite, light gray martensite, and a residual austenite matrix.
If there are many parts in the furnace and the casting time is excessive, the cooling rate of the parts will be greater than the cooling rate in an annealing furnace but less than the cooling rate of the normalizing air. Alternatively, if parts are left in the air for a long time after threading, the resulting microstructure will contain massive proeutectoid ferrite.
Fig. 2 White massive proeutectoid ferrite
As represented in Figure 2, the microstructure of steel 45 was obtained after heating at 840°C for 25 minutes, followed by quenching in water and tempering at 600°C for 60 minutes. The white massive structure is eutectoid ferrite, while the remaining structure is tempered sorbite.
This result was due to multiple parts present in the heating furnace during testing, and the furnace door was not kept closed during quenching as required. Instead, the oven door was kept open after the first sample was tempered and until the last sample was tempered.
As a result, in the later stages of quenching, approximately half of the quenched samples showed massive proeutectoid ferrite. This amount increased from less to more with the prolongation of the quenching time, with the massive proeutectoid ferrite content in the last quenched sample reaching up to 40% (volume fraction).
Due to the open door of the furnace, when the temperature of the parts in the furnace was below AC3, the cooling rate of the parts was greater than that of cooling (equivalent to annealing) but less than that of air cooling (equivalent to normalization) . This resulted in the precipitation of massive proeutectoid ferrite.
If the quench cooling rate was not sufficient, the proeutectoid ferrite in the steel was generally distributed along the original grain boundary of the austenite in the form of a lattice or semi-lattice.
Fig. 3 White reticular proeutectoid ferrite
As shown in Fig. 3, the microstructure of steel 45 after heating at 900°C for 25 minutes and quenching in oil consists of white fine-mesh pre-eutectoid ferrite, quenched black troostite, feathered upper bainite, light gray martensite and an austenite residual. headquarters.
Residual austenite, which is not in the same plane as martensite, is only visible in the quenched structure when the quenching heat is severely superheated. In normal hardening, residual austenite is not prominently present.
Fig. 4 White retained austenite
As shown in Figure 4, the microstructure of steel 45 after heating at 900°C for 25 minutes and quenching in water consists of quenched medium-carbon black martensite and white residual austenite.
The shape of the residual austenite changes depending on the angle at which it intersects the martensite.
3. Conclusion
In this post we present the methods for identifying retained ferrite and austenite. We hope you find this information useful.
It should also be noted that a thorough understanding of the iron-carbon phase diagram, combined with the perspectives discussed in the article, will make the identification process much easier.
