metal - What chemical characteristics give types of steel their properties?

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.

Answer

(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:

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.