View martensite under a light microscope with a change in carbon content also changes. On the picture 1 on the right is schematically shown, that the hardening of pure austenite with a carbon content below or above the eutectoid content 0,77 % can give a hardened structure with 100 %-nym martensite. As shown on the left in the picture 1, it is customary to divide martensite into two types - rack and plate – depending on its carbon content.
Rack and lamellar types of martensite
With a carbon content of 0 to 0,6 % martensite is called lath, and if the content is more 1 % – lamellar. At an intermediate carbon content from 0,6 to 1,0 % the structure of martensite is a mixture of these two types of martensite. The view of martensite under a light microscope is a bit disappointing: lath martensite looks indistinct and cloudy, as seen in the picture 2 for steel with carbon content 0,18 %.
In lamellar martensite, it is sometimes possible to see individual plates, but only then does the carbon content increase 1 %. This is shown in the figure 3 for martensite with 1,4 % carbon.
The reason, why the plates become visible is, that this structure is not on 100 % martensitic. White areas, which surround the martensite plates, are austenite grains, which did not turn during quenching. This austenite is called residual. This austenite is soft compared to martensite - it is because of this austenite hardness in hardened steel decreases with increasing carbon content above 0,9 % as it shown on the picture 4.
If lamellar martensite contains little or no residual martensite, individual plates cannot be distinguished (picture 5), although they can be clearly seen under a transmission electron microscope.
Comparison of photos in drawings 3 and 5 shows, which is very difficult (Besides, sure, experienced metallurgists) distinguish between pure lath martensite and mixed lath-lamellar structure only by optical microstructure.
When martensite is removed from the quench bath, it is called freshly hardened martensite. Hardness data in the picture 4 just refer to freshly hardened martensite. The big problem with this "fresh" martensite is, what, if the carbon content is more than 0,2-0,3 %, then the steel in this state is very fragile. This fragility can be removed by some loss of hardness., if hardened steel is slightly heated. This process is called vacation..
Therefore, hardened steels are almost always tempered to increase the toughness of the steel.. The resulting martensite is called tempered martensite. The increased tempering temperature allows the carbon atoms, which are "captured" in the GCT structure, move a little. This movement of atoms has two effects:
– enables the bcc structure to change into bcc structure;
– enables the formation of very small carbide particles.
Martensite under a light microscope
These small carbides are too small, to see them through an optical microscope. However, alkaline etching of the steel surface, which contains these carbides, makes it rough - so the tempered martensite looks dark under an optical microscope. Unlike tempered martensite, etching untempered martensite gives it a smooth surface - it looks white under a microscope. Release temperature for, to darken martensite under a microscope, very low - about 150 ° C. All types of martensite in the figures 2, 3 and 5 were slightly released.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007