How to read a binary phase diagram (liquidus, solidus, and the two-phase region between them), the lever rule for calculating phase fractions, and the iron-carbon phase diagram that underlies all ferrous heat treatment.
A phase diagram is a map of which phase — or mixture of phases — is thermodynamically stable at a given composition and temperature, and reading one correctly is a prerequisite for nearly everything that comes after it in ferrous metallurgy. This module builds that reading skill from the ground up: the liquidus and solidus lines, tie lines, the lever rule for calculating exactly how much of each phase is present, and the specific eutectoid reaction on the iron-carbon diagram that turns austenite into pearlite.
By the end of this module you should be able to explain why martensite doesn't appear anywhere on the equilibrium iron-carbon diagram even though it's the entire point of quenching steel — a non-equilibrium result this module sets up directly and Module 9's heat treatment content resolves with TTT and CCT diagrams.