State diagram - the foundation of understanding steels
The study of the microstructure of all steels usually begins by considering iron-carbon metastable phase diagram (Fe-C) (picture). It is also called iron-cementite state diagram (iron-iron carbide). This diagram provides a foundation for understanding the structure and phase composition of both carbonaceous, and alloy steels, as well as various heat treatments, to which they are exposed. Cm., eg, “work” status diagrams or, may be, more correct phase diagram, during crystallization of steel in the article “Cast steel structure“.
Solubility of carbon in austenite and ferrite
In the low carbon left edge of the metastable iron-carbon diagram, we see ferrite (alpha iron), which has maximum solubility 0,022 % at a temperature 727 ° C and austenite (gamma iron), which dissolves 2,11 % carbon at temperature 1148 °C. It is seen, that the austenite region (gamma iron) much larger compared to the ferrite area (alpha iron). This clearly indicates a significantly higher solubility of carbon in austenite with a maximum 2,11 % at a temperature 1148 °C.
Strengthening of carbon steels, like many other steels, based precisely on this difference in the solubility of carbon in alpha iron (ferrets) and gamma iron (austenites).
In the carbon-rich side of the metastable Fe-C diagram, we find cementite (Fe3C). Less interest, excluding high alloy steels, represents delta ferrite at the highest temperatures.
Single-phase diagram areas
The vast majority of steels have only two allotropic forms of iron:
- alpha iron with body-centered cubic (OCK) atomic lattice and
- gamma iron with face-centered cubic (Fcc) atomic lattice.
At room temperature, bcc ferrite is temperature stable 912 °C (point A3), at which it turns into fcc austenite. Austenite converts back to ferrite at a temperature 1394 °C (point A4).
This high temperature ferrite is called delta ferrite, despite, that its crystal structure is identical to that, what is alpha ferrite. Delta ferrite remains stable until, how it melts at temperature 1538 °C.
Biphasic Chart Areas
Areas with a mixture of two phases (such as "ferrite + cementite ", «Austenite + cementite "and" ferrite + austenite "are located between single-phase regions. At the highest temperatures are the region of the liquid phase, and below it there are two-phase regions - "liquid + austenite », "liquid + cementite "and liquid + delta ferrite ". During heat treatment of steels, the formation of a liquid phase is always avoided.
Steel plot diagram
The "steel" section of the iron-carbon phase diagram is located in the carbon content range from 0 to 2,08 %, and the "cast iron" section - from 2,08 to 6,67 % carbon.
It is convenient to divide the "steel" section of the iron-carbon diagram into three parts according to the carbon content:
- hypoeutectoid – from 0 to 0,77 % FROM;
- eutectoid – 0,77 % With and
- zaevtektoidnoe - from 0,77 to 2,11 % FROM.
A very important phase transformation of the metastable iron-carbon phase diagram occurs at 0,77 % carbon. This transformation is eutectoid, and its product is called perlite (ferrite + cementite):
Gamma iron (austenite) —> alpha iron (ferrite) + Fe3C (cementite)
Critical steel temperatures
The most important boundaries of single-phase regions have special names., These include (in international designations):
- A1 - eutectoid temperature, which is the minimum temperature for austenite;
- A3 - low-temperature austenite boundary at low carbon content (the boundary between the regions "gamma iron" and "gamma iron" + ferrite");
- Aсm - the opposite boundary of the austenitic region at high carbon content (the boundary between the regions "gamma iron" and "gamma iron" + cementite).
Sometimes the letters c are added to the designations of these temperatures., e and r, eg, As1, As3 and Accm. The letter c stands for, that phase transformations occur during heating, letter e - in phase equilibrium, and the letter r - when cooling.
Effect of doping on critical temperatures
When added to iron-carbon alloy (steel) alloying elements position of boundaries А1, A3 and acm, and also the eutectoid content of carbon changes. In general, the elements, which stabilize austenite (nickel, manganese, nitrogen, copper) lower the temperature A1, whereas elements, which stabilize ferrite (chromium, silicon, aluminum, titanium, niobium, molybdenum, tungsten) increase the temperature A1.
Carbon content, at which austenite has a minimum temperature, called eutectoid content (0,77 % carbon by mass for the case of a metastable phase diagram). A mixture of ferrite and cementite phases with this carbon composition, which is formed upon slow cooling has a characteristic lamellar structure, which is called perlite. Pearlite is a collection of alternating ferrite and cementite plates. These plates, after holding at a temperature close to A1 coarse ("Spheroidized") into cementite particles, distributed in a ferrite matrix.