Why Alloy Knowledge Matters at the Forge

Every blacksmith quickly learns that not all metal behaves the same under the hammer. A length of mild steel will move predictably, whilst a piece of tool steel might fight back or crumble at the edges if you get it too hot. Understanding the common alloys available to the UK smith — mild steel, wrought iron and tool steels — is not academic. It determines how you heat, hit, quench and finish your work. It also keeps you safe: some alloys contain elements that can spit or fume if mishandled. This guide covers the practical properties of the metals you are most likely to find at the scrapyard, the stockholder or the farm auction.

Mild Steel: The Reliable Workhorse

Mild steel, also called low-carbon steel, is the default stock for most modern blacksmithing. Its carbon content is typically below 0.25 per cent, which makes it soft, ductile and forgiving. You can forge it at a bright orange to yellow heat, and it will bend, taper and draw out without cracking. It forge-welds reasonably well with practice, though it is less forgiving than wrought iron.

• Working range: Forge between 900°C and 1,250°C. Below 800°C it becomes stiff and prone to cracking.
• Hardening: Mild steel will not harden appreciably by quenching. It is tough but not wear-resistant.
• Common uses: Gates, railings, fire tools, brackets, decorative scrollwork and general forge work.

Mild steel is cheap and widely available in square bar, round bar, flat stock and plate. Its main drawback is that it rusts quickly if left unfinished, so a wire brush and wax, oil or paint is usually needed.

Wrought Iron: The Historic Material with a Difference

True wrought iron is almost pure iron with a small amount of slag — iron silicate — rolled into the structure. It has a fibrous, woody grain that makes it extraordinarily resistant to corrosion and fatigue. It forges at a lower temperature than mild steel and has a distinctive, almost buttery feel under the hammer. When you bend it, you can sometimes see the grain open up at the edges.

• Working range: Forge at a yellow heat, but avoid overheating. It can burn if you push it too far.
• Forge welding: Wrought iron welds easily and cleanly, often at a lower heat than mild steel.
• Availability: Genuine wrought iron is scarce and expensive. Most "wrought iron" sold today is actually mild steel.

If you are restoring historic ironwork or want maximum corrosion resistance, wrought iron is worth seeking out. But for everyday projects, mild steel is the practical substitute.

Tool Steels: Hardness and Edge Retention

Tool steels contain higher carbon and often alloying elements such as chromium, vanadium or tungsten. They are designed to be hardened and tempered, which makes them ideal for chisels, punches, drifts, axes, knives and hammers. However, they are much less forgiving to forge. They have a narrower working range, and they can crack if quenched incorrectly.

• Working range: Usually 800°C to 1,100°C. Overheating can cause grain growth and brittleness.
• Hardening: Requires a proper quench — often oil or water — followed by tempering to relieve stress.
• Common types: Old files, leaf springs, coil springs, and purpose-made bar stock such as O1 or 5160.

When forging tool steel, work at a steady heat and avoid prolonged soaking. Normalise the piece before hardening if you have done a lot of hammering. And always temper after quenching: an untempered tool is a shattering hazard.

Reading the Metal Before You Strike

You cannot always trust a label, especially with scrap. A spark test on a grinding wheel is your best first check. Mild steel gives a few short, straight sparks. Wrought iron gives almost no sparks — just a dull glow. Tool steel gives a bright, bushy spark stream with many forks. A file test can also help: a file bites into mild steel but skates over hardened tool steel. Finally, a simple bend test on a thin offcut tells you about ductility. These tests take seconds and save hours of frustration.

Choosing Stock for Your Project

Match the alloy to the job. For decorative work, gates, and structures that need to be tough but not hard, mild steel is the sensible choice. It is affordable, available and easy to forge. For restoration or marine environments where corrosion is a concern, seek out genuine wrought iron if your budget allows. For cutting tools and striking tools, tool steel is the only sensible option — but be prepared to invest in careful heat treatment.

Remember that your forge, anvil and hammer technique also influence the outcome. A well-forged piece of mild steel will outperform a badly forged piece of tool steel every time. Start with mild steel, learn its language, then move on to the more demanding alloys when you are ready. The metal will tell you what it needs — you just have to listen.