The iron-carbon phase diagram shows the dependence of the phase composition of steel on temperature: austenite, ferrite or mixtures thereof. In pure iron, austenite converts to ferrite when cooled to 912 ºS. Temperature, at which austenite begins to turn into ferrite, traditionally called temperature A3.
Eutectoid point of steels
The addition of carbon to iron leads to a decrease in temperature A3. The maximum decrease in this temperature is 727 ºС - achieved with carbon content 0,77 %. This point on the iron-carbon phase diagram is called eutectoid point or, sometimes, pearlite point. At this point in the phase diagram, a eutectoid reaction occurs, that is the reaction, in which one solid phase transforms into two solid phases. Become, which have a carbon content of less 0,77 % are called hypoeutectoid steels or low carbon steels. Eutectoid temperature 727 ºС is traditionally designated A1.
Solubility of carbon in ferrite and austenite
Become, which are 100% austenite occupy the central dark area of the phase diagram in the figure 1. Become, which are ferrite, should have in the coordinates "temperature - carbon content" in a narrow dark area on the left edge of the phase diagram. Maximum carbon content, which can dissolve in ferritic iron, is only 0,02 % and this happens at the eutectoid temperature 727 ºS. It means, that ferrite is essentially pure iron, since it is pure in relation to carbon by 99,98 % and cleaner.
Austenite can dissolve much more carbon, than ferrite. At the eutectoid temperature, austenite dissolves 0,77 % carbon, what in 38 times more, what ferrite can contain at the same temperature. Austenite contains more carbon, than ferrite, because the face-centered crystal structure of austenite has more space between the iron atoms, than the body-centered crystal structure of ferrite.
Hypoeutectoid steels – steel 40
Consider the shaded area of the phase diagram in the figure 1, which is denoted γ + α. This area, is a set of coordinates "temperature - carbon content", in which the steel is a mixture of ferrite and austenite.
Imagine a high temperature microscope, in which we are considering a polished steel surface with carbon content 0,4 % – low carbon steel 40 – at a temperature 760 ºS. Since a point with such a chemical composition and at such a temperature lies in the region γ + α, then the steel will be a mixture of ferrite and austenite. An example of such a structure is shown at the bottom of the figure. 1. The phase diagram also provides information on the composition of these two phases.. Austenite grains should have a composition, which is indicated in the phase diagram of the figure 2 letter N, and ferrite - composition, which is denoted by the letter M.
Cooling steel 40 - ferrite along the boundaries of austenite grains
As an illustration of the usefulness of the phase diagram, consider the following simple experiment. Carbon steel 0,4 % – low carbon steel 40 – first heated to temperature 850 ºС and withstand about 10 minutes. After this short exposure, all grains in this steel will be pure face-centered austenite grains with a carbon content. 0,4 %. The view of this structure in an imaginary high-temperature microscope is shown on the left in the figure. 2. Further, slowly lower the temperature of the steel to 760 ºS. What will happen to the microstructure? According to the phase diagram, the steel after cooling must be two-phase – austenite-ferrite mixture. Experiments show, what ferrite, which is formed from pure austenite during its cooling is always located at the boundaries of austenite grains. This is shown in the right microstructure of the figure. 2. Ferrite forms as multiple α-grains along the primary austenite grain boundaries.
Heating steel 40 - ferrite in the form of bulk grains
Let's compare the microstructure on the right in the figure. 2 and the microstructure below in the figure 1. Both of them are a mixture of austenitic and ferritic grains with the same volume fraction of both.. However, these two microstructures are very different in the distribution of these ferrite grains.. Microstructure of the pattern 1 usually formed by heating mild steel from room temperature to temperature 760 ºS.
Heat treatment and microstructure of steel
This example demonstrates the remarkable property of steels: microstructure of steel – not just low carbon – controlled by heat treatment. And the microstructure of steels, in its turn, controls their mechanical properties.
The microstructure of steel is usually referred to as the features of the grains: their forms, size, distribution and phase type. Note, what phase, which are shown in the figures 1 and 2 exist only at high temperatures 760 ºS, not at room temperature. Phases at room temperature - a separate conversation.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007