Classification of Stainless Steel
Both ferrite and residual austenite are resistant to corrosion, appearing white under a microscope. This can often lead to confusion if not observed carefully.
However, there are straightforward methods for distinguishing between the two. The two commonly used methods are as follows:
Differentiation Based on Microstructure
Ferrite and residual austenite coexist in the microstructure, typically in sub-critical quenched steel.
There are approximately three types of ferrite present in sub-critical quenched steel: polyangular undissolved ferrite, blocky pro-eutectoid ferrite and networked or semi-networked pro-eutectoid ferrite, all of which are relatively white and bright.
Polyangular and blocky ferrite have distinct boundaries and often exist in the blank areas of martensite lath corners. Fine focus adjustment reveals that the white phase and martensite phase are on the same plane.
Networked or semi-networked ferrite is distributed along the original austenite grain boundary and is comparatively thin.
Residual austenite doesn’t have a clear boundary line. Its shape changes according to the distribution shape of the martensite lath.
Residual austenite in sub-critical quenched steel microstructure doesn’t usually exist independently, but is organically combined with the needle-like martensite after quenching.
Therefore, its color is slightly darker than ferrite and the bulging phenomenon of needle-like martensite can often be faintly seen.
Inference from Heat Treatment Process
If the heating and insulation time during sub-critical quenching of steel is insufficient or the temperature is too low, white polyangular undissolved ferrite may appear in the quenched microstructure.
Figure 1 shows the microstructure of 45 steel heated at 760°C for 25 minutes and quenched in water. It consists of white polyangular undissolved ferrite, black medium-carbon quenched martensite, light-gray martensite, and residual austenite matrix.
If there are many workpieces in the furnace, the cooling time is too long, the cooling rate of the workpiece in the furnace is greater than the annealing furnace cooling rate but less than the normal fire air cooling rate, or the workpiece stays in the air for too long after removal from the furnace, blocky pro-eutectoid ferrite may appear in the quenched microstructure.
Figure 2 shows the microstructure of 45 steel heated at 840°C for 25 minutes, quenched in water, and then tempered at 600°C for 60 minutes. The white blocky structure is the pro-eutectoid ferrite, while the remaining structure is tempered sorbite.
This was due to the fact that the furnace was loaded with many workpieces during the test.
Quenching was not performed one by one as required and the furnace door was not closed immediately.
As a result, blocky pro-eutectoid ferrite appeared in about half of the quenched samples during the later stage of quenching. The amount of blocky pro-eutectoid ferrite increased with the extension of the quenching time.
The last quenched sample contained up to about 40% (volume fraction) of blocky pro-eutectoid ferrite.
Because the furnace door was not closed, when the temperature of the workpiece in the furnace dropped below Ac3, the cooling rate of the workpiece in the furnace was greater than the slow cooling (equivalent to annealing) but less than air cooling (equivalent to normal fire), so blocky pro-eutectoid ferrite precipitated out.
If the quenching cooling rate is not high enough, the pro-eutectoid ferrite in the steel generally appears as networked or semi-networked along the original austenite grain boundary.
Figure 3 shows the microstructure of 45 steel heated at 900°C for 25 minutes and quenched in oil. It consists of white fine-networked pro-eutectoid ferrite, black quenched troostite, feather-like upper bainite, light-gray martensite, and residual austenite matrix.
Only when the quenching heating is severely overheated, residual austenite, which is not on the same plane as martensite, can be observed in the quenched microstructure. The residual austenite is not obvious in the normal quenched microstructure.
Figure 4 shows the microstructure of 45 steel heated at 900°C for 25 minutes and quenched in water. It consists of black quenched medium-carbon martensite and white residual austenite, the morphology of which varies with the different intersections of martensite.