High carbon steels
In high-carbon steels, the amount of carbon, dissolved in austenite, increases, and the iron atoms move apart in relation to each other. It stretches chemical bonds, that hold iron atoms together, which leads to the appearance of deformation energy. There is a limit to that, how much of this deformation austenite can withstand. Carbon amount, which dissolves in austenite, before reaching this limit, called the limit of solubility of carbon in steel.
Cementite - carbon phase in steel
Solubility limit of carbon in austenite, eg, at a temperature 820 ºС is 1 %. If high carbon steel, which contains 1,5 % carbon, heat up to 820 ºS, then only 1 % of these 1,5 % dissolve in austenite. Where does the rest of the carbon go? This extra carbon enters a new phase. This new phase - cementite - has an important difference from austenite and ferrite. It's a chemical compound, which only exists in one composition.
Chemical formula of cementite - Fe3C, that is, for every carbon atom in this compound there are three iron atoms. The weight fraction of carbon in cementite is 6,7 %.
Cementite, like ferrite and austenite, has a regular crystal structure. This crystal structure is more complex, than body-centered cubic ferrite structure or face-centered cubic austenite, but she is well known. Cementite is a separate phase and is present in steel as discrete grains. therefore, those "extra" 0,5 % carbon in steel at 820 ° С will enter into individual cementite grains, mixed with austenite grains. In this way, the microstructure of the steel will look like a two-phase mixture of austenite and cementite.
Phase diagram for hypereutectoid steels
On the picture 1 iron-carbon phase diagram, including part with a high carbon content - for hypereutectoid - high carbon - steels. Cementite plays an important role here.
The region of existence of austenite is shown as a dark central region. Since cementite only exists in one chemical composition, it is shown in the diagram as a vertical line at the carbon content 6,7 %.
Solubility limit of carbon in austenite
Line in the diagram, designated Acm denotes the solubility limit of carbon in austenite. When 820 ° C this line gives a point with the carbon content 1 % – only so much carbon can dissolve in austenite at 820 ° C. High carbon alloys with carbon content to the right of line Acm are in the slightly shaded biphasic area on the right. This area is designated “γ + Acm» – it consists of a mixture of austenite and cementite grains.
Formation of cementite during heating of high-carbon steel
As an example, consider a high-carbon steel containing 0,95 % carbon. If this steel is heated to 760 ºS, then its point on the phase diagram of the figure 1 there will be a white circle with horizontal arrows passing through it. Since this point lies in the two-phase region γ + Acm, then the steel should consist of a mixture of austenite of composition O (0,85 % carbon) and cementite (0,67 % carbon). The diagram does not give a view of the microstructure, but experiments show, what does she look like, as shown at the bottom of the picture 1. All cementite is in the form of small spherical grains, which are randomly distributed over the austenite grains. The grain size of austenite is much larger, than cementite grains. Austenite grains have clear boundaries.
Formation of cementite when cooling high-carbon steel
Let us apply a phase diagram to, to understand, how the microstructure of high-carbon steel changes during heat treatment. Let's take the same steel with carbon content 0,95 %, heat it up to temperature 850 ºС and hold at this temperature for minutes 20. As shown in the phase diagram of the figure 2, this state of steel corresponds to a point in the single-phase austenitic region.
If we had a microscope, in which you can see the microstructure of the heated steel, then we would see the microstructure, similar to that, what's on the right in the picture 2. When the steel temperature drops to 760 ºС its point on the phase diagram falls into the two-phase region austenite + cementite. It means, that when the steel cools, cementite grains should be released. Really, according to experiments, cementite falls out along the boundaries of austenite grains - just like ferrite in hypoeutectoid steels.
As seen on the right in the picture 2, microstructure of steel at temperature 760 ºС contains the same primary austenite grains, which were at temperature 850 ºS, with borders, which are filled with thin, lamellar cementite grains.
The purpose of heat treatment of steel is to change its microstructure
One cannot fail to see a significant difference in the microstructure in the figure. 2 from him, which is shown in the figure 1 - after heating the steel from room temperature to 760 ºS. Note, that both microstructures contain the same amount of cementite and austenite, but the distribution of cementite is completely different.
Unlike austenite and ferrite, cementite is very brittle.. Therefore, microstructured steel, what's in the picture 2, with interconnected cementite plates will not be as tough and ductile as steel with the microstructure in the figure 1 with small, insulated cementite grains. These cementite grains give the steel good cutting properties.. These steels are used to make, eg, straight razors.
This is a good example of, as heat treatment of steel – not only high-carbon – can change its microstructure and, hence, its mechanical properties.
Note, microstructures, which are shown in the figures 1 and 2 exist only at high temperatures 760 ºS, not at room temperature. Phases and microstructures at room temperature - in a separate article.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007