I am a mechanical engineer, and just like everyone else I had classes on the crystal structure of metals, phase diagrams, and the various heat treatments.

However, even after diving back into that recently and researching online, I can't make sense of how everything fits together. I know the high level, the low level, but can't link the two together.

For example:

  1. How can the phase diagram of steel be linked to its physical properties? Say, given a certain carbon weight fraction, is it possible to tell from the phase diagram if the steel is going to be hard, resilient, ductile? E.g. cementite is hard and brittle, austenite is ductile and soft?
  2. How do heat treatments work? The way I see it, the phase diagram is "steady state", and heat treatments are about spending a specific amount of time in a phase region to add "a little bit of that" etc. (or quickly cooling it to stop the reactions), but I can't link it back to the phase diagram (there is a 3rd dimension, time).

Apologies if this is not a very clear question, but this is exactly what I hope to find: clarification on a quite complex topic.


1 Answer 1


Steel - effect of carbon content

(Source: Roy Beardmore, http://www.roymech.co.uk/ (defunct, via the Internet Archive)

As carbon content increases, the ability to resist a sudden impact decreases, as measured by the Charpy Impact Test.

As carbon content increases, ultimate tensile strength and Brinell Hardness increases.

As far as a phase diagram:

enter image description here

increasing $\ce{}$ (cementite) content makes the steel more brittle and hard.

Near 0.76% carbon, the steel will be pearlite which is ductile (good for wires). Below 0.76% carbon, there will be increased alpha-iron (ferrite). Above 0.76% there can be Ledeburite. Austenite only exists at high temperature. Depending upon the cooling rate of Austenite, Bainite (slow cooling) or Martensite (rapid cooling) can form.

  • $\begingroup$ Thanks for this good answer (and just in time for the bounty!) . So is my intuition of 2] correct? Are durations to spend in certain phase regions extracted from experience, or is there some kind of law that dictates how much of the steel transforms in function of time? $\endgroup$ Commented May 19, 2015 at 16:34
  • $\begingroup$ Nevermind, it's in the wikipedia links you've given. $\endgroup$ Commented May 19, 2015 at 16:36
  • $\begingroup$ @MisterMystère When a material undergoes some phase transformation with time we call it Ageing. This cannot be figured out from a phase diagram. Its observed from Microscopic techniques after some hours. Phase Diagram just tells us at some T temperature and some X composition what is(are) the phase(are) present in our sample. Heat Treatments- we usually want to either soften(anneal) or harden(quenching) the material for our purpose. $\endgroup$
    – Gowtham
    Commented May 20, 2015 at 6:33
  • $\begingroup$ Say we want martensite, we heat material above $727 C$ and then rapidly cool it below the $M_{s} $ and then $M_{f}$. But if we just want pearlite/combination of ferrite and cementite we cool slowly(furnace cooling) to room temperature. Note that sometimes cooling rates wont be enough to be considered "equilibrium cooling" and some austenite may be retained. $\endgroup$
    – Gowtham
    Commented May 20, 2015 at 6:39
  • 1
    $\begingroup$ Please don't just link to the pictures you use, make sure to attribute them correctly. Here we have a typical case of link-rot. I'll try to fix it as well as I can. $\endgroup$ Commented Dec 17, 2019 at 14:19

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