Carbon in steels – near equilibrium structures.

Alloying of iron with carbon can produce a wide variety of properties. In general steels become stronger and less ductile with increasing carbon content. However a wide variety of combinations of properties can be obtained by altering the both the composition along with thermal and mechanical processing. Steel containing containing 0.4 per cent (by mass) of carbon is twice as strong as pure iron. Steels with 1 per cent carbon are nearly 3 times as strong. Iron-carbon alloys are classified by carbon content in the table below.

Classifcation Carbon content (mass or weight percent)
Intersitial free steel ?
Extra low carbon steel ?
Mild steel 0.1-0.25
Medium carbon steel 0.25-0.45
High carbon steel 0.45-1.50
Rarely used 1.50-2.5
Cast Iron 2.5-4.0

Examples of different carbon contents in different applications are shown in this figure.
Applications of steels with different carbon contents (weight or mass percent)

Slowly cooling from the high temperature crystal structure (austenite) in carbon steels will develop microstructures as shown in this figure. At room temperature less than 0.01 per cent carbon exists in equilibrium in solid solution in the low temperature crystal structure of iron (ferrite). Carbon is instead found to exist in an intermetallic compound, iron carbide, of three iron atoms and one carbon atom. Greater proportions of this compound form at higher carbon contents. b = 0.3, c = 0.6, d = 0.8 in the figure below. The cementite forms as an intimate mixture with lamalae of ferrite and cementite. Above the eutectic composition of 0.8 per cent the carbide forms first at the previously existing austenite grain boundaries as shown in case d (1.1 per cent carbon) below.
Microstructure on slow cooling from austenite
Eutectic area of equilibrium phase diagram.

Faster cooling will produce structures further away from the equilibrium structure. Fast cooling can produce ferrite supersaturated with carbon, which has a tetragonal closed packed structure called martensite. Intermediate cooling rates or isothermal transformation can form bainite.

Key to steel explaination

4 Responses

  1. It’s reported that significant improvements can be made over pearlitic rail steels by using low carbon, carbide free, bainitic steels.

  2. Hypereutectoid steels are not useful due to the formation of brittle proeutectoid cementite networks that form at the prior austenite grain boundaries. However, adding high levels of silicon can suppress the formation of these networks. How does silicon do this, i.e., what is the mechanism? Is there a carbon- silicon ratio that can tell me how much silicon to add to the steel to minimize these continuous networks?

  3. Dear Bruce, I wouldn’t say that this makes them non-useful. However if we want to avoid the low toughness we need to avoid forming these networks. I believe I read that this can be done by deformation at high temperature, if this can be done to cause the cementite as to be present as spheroids. High deformation will cause large nucleation rate of cementite and if there is enough sites this will prevent the formation of the cementite films on the grain boundaries.

    Unfortunately I can’t remember the reference for the above. I think there may be by a paper with David Edmonds of the University of Leeds as one of the authors.

    Alternatives which might work in theory would be to try to refine as much as possible the austenite grain size or to try to allow the cementite to globalise, i.e. by show heating up to the austenitisation temperature, which would not be sufficient to completely dissolve the cementite.

    I don’t believe that silicon will suppress the formation of cementite at high temperatures, it may in fact promote the formation of cementite (at high temperatures).

    This can be shown by thermodynamic calculations.

    Fast cooling from fully austenite may be one way to avoid cementite formation (or at least growth of large particles). However formation on g.b.s will be very fast.

    Another way is very short austenitising heat treatments with rapid heating. This can be seen in the flash bainite process ( you might find a paper by Suersh Babu that deals with this.

    Sorry I don’t know the proper references about this I can only give a clue for the time being.

    If you have luxury of changing the composition you could try shifting the eutectoid to higher carbon contents. Of other alloying to lower the cementite stability (this usually means removing alloying elements).

  4. hello
    thank you for this websit
    i want t-t-t diagram for all low alloy steel used for oilpipe line .
    thank you

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