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What heat does to steel — and why the metal fabricator needs to know it
A farrier does not just shape steel — he transforms it. Every time a piece is heated in the forge and cooled, the steel's internal grain structure changes, and with it its hardness, toughness and strength. The apprenticeship subject of Basic Blacksmithing requires, according to the learning objectives, that the apprentice gains understanding of how heating affects steel and can apply normalizing, tempering and hardening of individual steel types and achieves skill in forge welding. It sounds theoretical, but it is deeply practical: incorrectly heat-treated tools break, and incorrectly treated shoes wear unevenly.
Steel is iron with a small content of carbon, and it is precisely the carbon content that determines what you can do with the material. Soft steel with low carbon content is tough and easy to forge — it is shoe material. Steel with higher carbon content can be hardened and is therefore used for tools such as shoe knives, pliers and chisels. As a smith you should know what type of steel you are dealing with before you choose heat treatment: trying to harden a steel with too low a carbon content has no effect, and overheating a tool steel can destroy it.
During forging the steel's grain structure becomes uneven: some areas are hammered heavily others slightly and internal stresses develop. Through normalization the component is heated evenly above the steel's transformation temperature and then cooled slowly in still air. The result is a fine-grained uniform structure without internal stresses—the steel is normalized back to a known state. This is typically done after heavy forging before further processing.
With hardening the steel is heated again above the transformation temperature but instead of slow cooling it is rapidly cooled in water or oil. The rapid cooling freezes a very hard structure in the steel. The price is brittleness: a newly hardened workpiece is as hard as glass and can shatter if struck. Therefore hardening never stands alone.
After hardening, the part is tempered: it is reheated to a significantly lower temperature and held there, thereby trading some of the hardness for toughness. Traditionally the metal fabricator reads the tempering temperature on the tempering colors that draw across the polished surface — from light straw yellow through brown and violet to blue — where the yellow tones correspond to lower temperature and greater retained hardness than the blue. The balance between hardness and toughness is chosen based on the tool's use: a chisel should be hard, a striking tool should be tough.
Forge welding is the oldest welding method: two items are heated to welding temperature — where the steel is almost white-hot and the surface begins to become sticky — and then hammered together so they join into one piece. The method requires a clean surface, the right temperature and quick, secure hammer blows. In the horseshoe smith trade forge welding is used among other things in the production of certain hand-forged shoes and modifications and the skill is an independent objective in Blacksmithing.
The learning objectives explicitly mention work with both gas and coal forges. The gas forge is quick to light, gives even heat and is standard in many mobile workshops. The coal forge allows very high temperatures concentrated on a small area — an advantage for forge welding — but requires more attention and experience in controlling the fire. Both place demands on extraction and fire safety: the Working Environment Authority's rules on work with open flame and flue gases apply in the smithy as in any other workshop.
Metallurgy is the invisible half of blacksmithing. Two horseshoes can look the same and behave completely differently because one is forged and cooled correctly and the other is overheated and full of stress. The metal worker who understands their steel makes tools that last, shoes that wear evenly — and avoids breaks that always come at the most inconvenient time.