The Iron-Carbon Diagram (ICD)
The map of steels: what really happens during heating and cooling
AI generated The iron- diagram is the most important map in materials science. It shows you which microstructure a steel takes on at which temperature and carbon content — and thus why the weld hardens when . Whoever reads the ICD understands hardening, crack susceptibility, and heat treatment instead of just memorizing them.
The (ICD) is the map on which every steel is located. The horizontal axis shows the content, the vertical axis the temperature. The diagram tells you: At this temperature and this C content, your steel has exactly this microstructure.
Pure iron undergoes two lattice transformations when heated. Below 911 °C it is — a soft, easily deformable microstructure with a body-centered cubic lattice that can barely dissolve (max. 0.02 %). Between 911 °C and 1392 °C it becomes , a face-centered cubic lattice that absorbs significantly more (up to 2.06 %). That is exactly the crux: is readily soluble in , practically not in .
If you cool a steel slowly, the wants to leave the again. At 723 °C — the pearlite temperature — the decomposes into a fine mixture of and (iron carbide Fe₃C). This lamellar mixture is called . At 0.8 % lies the eutectoid point: all the turns into at once. Below that (0.02–0.8 % C), forms first, the rest becomes ; above that (0.8–2.06 % C), forms first at the grain boundaries, the rest becomes .
At 2.06 % the steel region ends and cast iron begins. Cast iron has so much that graphite or ledeburite forms during solidification — that is why it is brittle and castable, but not forgeable.
For you as a welder, the ICD is the explanation for everything that happens in the weld. When , the heats the steel above 900 °C — everything becomes . When the then cools quickly, the has no time to precipitate as . Instead, the lattice suddenly collapses and forms — glass-hard, brittle, crack-prone. The more , the harder and more dangerous the .
That is why two parameters are so important: The carbon content determines the maximum , and the cooling rate determines how much of it actually forms. Together, both explain why you preheat thick plates and weld high-carbon steels only with caution.