Overview
We examine again, the solid-state iron–carbon ($\text{Fe}\text{--}\text{C}$) phase diagram, emphasising its role as the fundamental map for understanding steel metallurgy. Historical milestones in metallography are explored while addressing modern uncertainties regarding the stability of various iron carbides at low temperatures. It is argued that cementite ($\text{Fe}_3\text{C}$) remains the stable phase under these conditions, contrary to some models suggesting transition carbides. To improve the diagram’s utility, a case is made for the routine inclusion of the $T_0$ curve, which defines where austenite ($\gamma$) and ferrite ($\alpha$) share identical free energies. Additionally, the reason why excess carbon is retained in bainitic ferrite is described, suggesting that equilibrium between austenite and tetragonal ferrite provides a better explanation than traditional models.
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Key metallurgical themes
- Low-temperature carbide stability: Evidence confirms that cementite remains the stable carbide phase at low temperatures. There aren't stable phase fields of other carbides at low temperatures.
- Routine inclusion of the $T_0$ curve: Incorporating the $T_0$ boundary directly onto the equilibrium diagram provides an indispensable tool for interpreting diffusionless and displacive transformations, setting the upper thermodynamic limit for carbon supersaturation in ferrite.
- Carbon retention in bainitic ferrite: The persistence of carbon in bainitic ferrite beyond classical paraequilibrium limits is rationalised through equilibrium of austenite with tetragonal ferrite.
