How to Distinguish Ferrite and Retained Austenite?

1. Preface

How to Distinguish Ferrite and Retained Austenite? 1

I’d like to share with you the identification of “ferrite” and “retained austenite”, one of the series of articles on the identification of similar structures of steels.

2. Identification of ferrite and retained austenite

Since ferrite and retained austenite are not corroded, they are all white when observed under the microscope.

If they are not carefully observed, they are easily confused.

However, it is very easy to distinguish these two kinds of differences by mastering the methods.

Two common methods are as follows:

1. Distinguish from tissue morphology

Ferrite and retained austenite co-exist in the microstructure, and are generally hypoeutectoid steel quenched parts.

There are roughly three forms of ferrite in the quenched parts of hypoeutectoid steel: polygonal undissolved ferrite, massive proeutectoid ferrite and reticular or semi reticular proeutectoid ferrite, all of which are white and bright.

Polygonal and massive ferrites have obvious boundaries and often exist in the blank area of the angle between the needles of martensite.

Fine tuning and focusing will find that the white phase and the martensite phase are on the same plane.

The reticular or semi reticular ferrite is distributed along the original austenite grain boundary, which is fine.

The retained austenite has no obvious boundary line, and its shape changes with the shape of martensite needle distribution.

The residual austenite in the quenched structure of hypoeutectoid steel generally does not exist alone, but is organically combined with the needle like martensite after quenching.

Therefore, the color is slightly darker than that of ferrite, and the phenomenon of needle like martensite is often dimly seen.

2. Inferred from heat treatment process

If the quenching heat preservation time of hypoeutectoid steel is insufficient or the temperature is low, white polygonal undissolved ferrite will appear in the quenched structure.

How to Distinguish Ferrite and Retained Austenite? 2

Fig. 1 white polygonal undissolved ferrite

As shown in Fig. 1, the microstructure of 45 steel after water quenching at 760 ℃ for 25 min is white polygonal undissolved ferrite + Black Medium Carbon quenched martensite + light gray martensite + residual austenite matrix.

If there are many workpieces in the furnace and the tapping time is too long, the cooling rate of workpieces in the furnace is greater than the annealing furnace cooling rate but less than the normalizing air cooling rate, or the workpieces stay in the air for too long after tapping, the massive proeutectoid ferrite will appear in the quenched structure.

How to Distinguish Ferrite and Retained Austenite? 3

Fig. 2 white massive proeutectoid ferrite

As shown in Fig. 2, the microstructure of 45 steel after heating at 840 ℃ for 25min and water quenching and then tempering at 600 ℃ for 60min.

The white massive structure is eutectoid ferrite, and the rest is tempered sorbite.

This is because there are many workpieces in the heating furnace during the test, and the furnace door is not closed at any time during quenching as required;

Instead, the furnace door is always open after the first sample is quenched and until the last sample is quenched.

Therefore, in the late quenching period, about half of the quenched samples appeared massive proeutectoid ferrite.

The amount of massive proeutectoid ferrite increases from less to more with the extension of quenching time.

The content of massive proeutectoid ferrite in the last quenched sample is as high as about 40% (volume fraction).

Since the furnace door is not closed, when the temperature of workpieces in the furnace is lower than AC3, the cooling rate of workpieces in the furnace is greater than that with cooling (equivalent to annealing) and less than that with air cooling (equivalent to normalizing), so massive proeutectoid ferrite precipitates.

If the quenching cooling rate is not enough, the proeutectoid ferrite in the steel is generally distributed along the original austenite grain boundary in the form of network or semi network.

How to Distinguish Ferrite and Retained Austenite? 4

Fig. 3 white reticular proeutectoid ferrite

As shown in Fig. 3, the microstructure of 45 steel after heating at 900 ℃ for 25 min and oil quenching is as follows: white fine mesh pre eutectoid ferrite + Black quenched troostite + feathery upper bainite + light gray martensite + residual austenite matrix.

Only when the quenching heating is severely overheated, the residual austenite which is not on the same plane as the martensite can be observed in the quenched structure, and the residual austenite is not obvious in the normal quenched structure.

How to Distinguish Ferrite and Retained Austenite? 5

Fig. 4 white retained austenite

As shown in Fig. 4, the microstructure of 45 steel after heating at 900 ℃ for 25 min and water quenching is black quenched medium carbon martensite + white residual austenite.

The morphology of residual austenite varies with the angle of intersection of martensite.

3. Conclusion

In this post, we share the identification methods of ferrite and retained austenite. I believe you will have some harvest after reading it.

Of course, it can also be clearly felt from here that if you thoroughly understand the iron carbon phase diagram, and then understand some of the viewpoints described in the article, it is actually very easy.

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