Because both ferrite and retained austenite are resistant to corrosion, they appear white under a microscope, making them often confusing without careful observation.

Ferrite and retained austenite coexist in the microstructure, typically in hypoeutectoid steel quenched parts. Ferrite in hypoeutectoid steel quenched parts generally occurs in three forms: polygonal undissolved ferrite, massive proeutectoid ferrite, and reticular or semi-reticular proeutectoid ferrite, all of which are bright white. Polygonal and massive ferrite have distinct boundaries and often occur in the blank areas at the angles of martensite needles. Fine-tuning the focus reveals that the white phase is coplanar with the martensite phase. Reticular or semi-reticular ferrite is distributed along the grain boundaries of the prior austenite and is relatively fine.

1. Differentiation by Microstructure

Retained austenite has no distinct boundaries; its shape varies with the distribution of the martensite needles. The retained austenite in the quenched microstructure of hypoeutectoid steel generally does not exist independently, but is organically integrated with the acicular martensite after quenching. Therefore, its color is slightly darker than that of ferrite, and the acicular martensite is often faintly visible as raised.

2. Inference from the Heat Treatment Process

The material and heat treatment method determine the microstructure. If the quenching heating and holding time of hypoeutectoid steel is insufficient or the temperature is too low, white polygonal undissolved ferrite will appear in the quenched microstructure. If there are too many workpieces in the furnace, the workpiece is taken out of the furnace for too long, the cooling rate in the furnace is greater than the furnace cooling rate of annealing but less than the air cooling rate of normalizing, or the workpiece remains in the air for too long after being taken out of the furnace, blocky proeutectoid ferrite will appear in the quenched microstructure.

If the quenching cooling rate is insufficient, the proeutectoid ferrite in the steel is generally distributed in a reticular or semi-reticular pattern along the grain boundaries of the original austenite.

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

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