How to upset metal

How to upset metal comes down to one fact: hammering moves steel, it does not remove it. Drive a hot bar along its length and it gets shorter and fatter right where it is hot. Smiths call that upsetting steel, and it is the opposite of drawing a bar out. I use it for bolt heads, shoulders, collars, and any time a piece of stock is thinner than the finished part needs to be. If you are working through the basics, the getting started hub collects the other beginner guides on this site.

This is a reference explainer, not a shop-class substitute. It describes how the process works and how smiths commonly do it. Your stock, your fuel, and your anvil will change the details, so start small on scrap.

What upsetting does to steel

Hot steel keeps its volume as you work it. If you squeeze a bar along its length, the metal has nowhere to go but outward, so the section gets thicker and the bar gets shorter. Drawing out does the reverse: you hit across the bar, it gets longer and thinner. If drawing out is new to you, read how to draw out first, because the two moves are halves of the same skill.

Upsetting shows up in a lot of everyday forging:

• Heads and shoulders. Bolt heads, rivet heads, nail heads, and the shoulder on a tenon all start as an upset on the end of a bar.
• Extra mass. If a part needs a thick spot, such as a bulb for a finial or a collar for a hinge pin, upsetting puts steel where you need it instead of forging a big bar down.
• Forge welding prep. Smiths often upset the ends before scarfing, because scarfing and welding both take away material and the joint ends up thinner than the bar. The how to forge weld guide covers the weld itself.
• Recovering a short piece. If you drew a tang or a tenon too thin, upsetting can bring some of the lost thickness back, at the cost of length.

Everything below applies whether you do it at an anvil, in a vise, or on a press. I work on Bessie, my 148 lb Trenton, so the examples assume a hand hammer and a solid anvil.

Why the heat has to be in the right place

Steel moves where it is hottest and softest. Cold steel behind the hot zone is stiff and resists the blow. That is exactly what you want, because the blow compresses the hot zone while the cold bar acts as a backstop.

The trouble is a hot zone that is too long. A long hot section of thin bar acts like a column under load, and it bends sideways instead of getting thicker. Smiths call that buckling. A rule of thumb you will hear at any club forge is to keep the hot zone short, a few bar diameters at most, and to upset in several quick heats rather than one long one. That is owner consensus, not a law, but it matches how a bar behaves. A fat, short hot zone upsets cleanly. A long thin one folds.

So heat control is the real skill:

• Heat only what you want to move. In a propane forge, the whole chamber is hot, so slide in only the end you plan to upset and let the rest of the bar stay out the door. In a coal forge, bury the end in the fire and keep the rest above it.
• Heat it through. A bar that is bright on the surface but dark in the center squashes unevenly and can crack. Give it time in the fire so the color is even into the core.
• Work at forging heat, not below it. Steel hammered too cool resists the blow and can crack, especially higher-carbon steels. Flame and steel color shift with the light in your shop, so learn what a good forging heat looks like on scrap in your own space. If the color fades toward dull red, stop hammering and reheat.

Upsetting the end of a bar

Upsetting the end of a bar is the version most beginners meet first. Start with a bar of mild steel, something forgiving, rather than a high-carbon or alloy bar. Square the end first with a cut or a few light blows, because a ragged or slanted end gives you a crooked upset. Then:

• Heat the last short section of the bar to a bright, even forging heat.
• Grip the cold part of the bar firmly in tongs that fit the stock.
• Stand the bar on its cold end on the anvil face, hot end up, and strike the top square down the axis of the bar.
• Keep the blows straight. A blow that lands off to one side bends the bar instead of upsetting it.
• Reheat when the color drops, and repeat.

That is the standard method for a bar you can hold upright. Variations you will see are the same idea in a different grip:

Ramming the bar down

For a lighter bar, you can hold it vertical with the hot end down and bring it onto the anvil face, letting the weight of the bar and your arm do the compression. Smiths call this jump or ram upsetting. It works for short, light pieces and gets awkward fast as the bar gets longer or heavier.

Clamping in the vise

For short pieces, a solid post vise lets you clamp the bar with the hot end sticking up and hammer it down. Only a stout vise built for hammering belongs in this job, and never hit the vise body or jaws themselves. The jaws will also pull heat out of the bar at the grip, which helps keep the cold zone stiff.

Whichever method you pick, you will notice the end mushrooms: the top flares out under the hammer while the bar below stays narrow. Turn the bar and work the sides with the hammer to dress it back toward a clean shape, then upset again. Alternate between compressing and shaping and you end up with a head instead of a mushroom.

Upsetting in the middle of a bar

Sometimes the thick spot goes in the middle of the bar, such as a collar or a swelling you plan to draw out into something else. The principle is the same, but now you need two cold backstops, one on each side of the hot zone.

Heat only the short section in the middle. Then cool the bar on both sides of it, either by letting the ends sit out of the fire or by wetting them (more on that in the cooling section below). Stand the bar on end on the anvil and strike the top, so the compression piles up in the hot middle section. If the bar is too long to stand up, you can hold one end in the vise and hammer the other, but the same rule applies: the force has to run straight along the bar, and the hot zone has to be short.

Mid-bar upsets are harder to keep straight than end upsets, because there is steel on both sides trying to bend. Expect to straighten as you go.

Fixing a bend before it becomes a fold

If the bar starts to bend, stop upsetting and fix it right away. While the steel is still hot, lay it on its side on the anvil and strike the outside of the bend until the bar is straight again. Then go back to upsetting.

Do not keep hammering a bent bar end-on. The bend gets sharper and the metal laps over, leaving a cold shut: a seam that never welded back together. A cold shut can hide inside a part that looks fine on the outside and then open up later under load or when you bend or file the piece. If you see a fold forming, the safe move is to cut the bar back past it and start again, not to bury it with more hammering.

The best prevention is a short hot zone, square blows, and not trying to gain too much thickness in a single heat. Many small upsets beat one heroic one.

Cooling the rest of the bar

Smiths often cool the bar behind the hot zone to keep the backstop stiff. Some do this by holding the bar so the cold part stays out of the fire. Others dip or splash the cool side with water so the heated area stays short.

Treat water near hot steel as a real hazard:

• Water on hot steel flashes to steam and can spit. Keep your face back and your eye protection on.
• Keep the water bucket stable and out of your swing path. Spilled water on the floor around a forge and anvil is a slip risk.
• Keep the water on the cool side of the bar, away from the hot zone. Dunking the part you are about to hammer can crack the steel.
• In medium or high-carbon steels, steel cooled quickly from a high heat can harden. Hardened steel is brittle and can snap, so do not flex or bend that section cold. Plan to normalize or anneal it afterward; the annealing steel guide explains the basics.

For mild steel this is a much smaller worry. If you are upsetting something that matters, like a tool steel or a knife blank, send the real metallurgy questions to real testing sources. Knife Steel Nerds is the place I would read first, rather than guessing from forum threads.

Tools and anvil setup

You do not need special gear to upset a bar, but a few basics help:

• Tongs that fit. Tongs that grip the bar on its full diameter hold it straight against the shock of a blow. Loose tongs let the bar twist and bend, and the shock goes straight into your hand and wrist. The blacksmith tongs guide covers what to look for.
• A hammer you can swing straight. A moderate hand hammer that you can bring down flat and square beats a heavy one you cannot control. A square, flat hammer face matters because the bar end is flat and you want a flat contact.
• A solid anvil face. Upsetting sends force straight down into the anvil, so the anvil needs to sit firmly on a stable stand. A wobbling stand loses energy and makes the bar bounce. Never strike the bare anvil face with the hammer, which chips both.
• A heading tool, if you make a lot of heads. Smiths who make nails and rivets use a header, a plate with a hole that holds the shank while the head is upset above it. A heading tool controls the shape of the head so you are not trimming a mushroom.

If you use a bigger machine, a hydraulic press or power hammer will upset much faster than a hand hammer, but the same rules about heat, axis, and bar length still apply. Machines will fold a badly set up bar even faster than you can.

Safety at the anvil

Upsetting is gentler than some forging, but it has hazards of its own.

• Eye protection. Wear safety glasses rated to ANSI Z87.1 at the anvil. Scale flies off hot steel under the hammer, and an end-on blow throws it in unpredictable directions. The blacksmith safety gear guide covers the full kit.
• Hearing. Steel on steel is loud, and a long session of upsetting adds up. See hearing protection for blacksmithing.
• Clothing. Natural fibers such as cotton or wool, not synthetics, which can melt and stick to skin. Closed leather shoes, and no loose cuffs for hot scale to land in.
• Shock to the hand. An off-axis blow or a loose tong grip sends the shock into your wrist. If your hand stings after a blow, your grip, your hammer, or your aim is wrong. Fix that rather than pushing through.
• Hot ends. The end of a bar you just upset stays hot for a long time and looks like any other piece of steel once it dulls. Set hot work down in the same spot every time, and never leave it where someone can grab it.
• A bar that jumps. A bar standing on its end can slip or kick when you strike it. Stand so the bar cannot swing into your leg, and let it fall away from you if it slips.

Do not hold hot work with bare hands, and do not rely on gloves as a substitute for tongs. Gloves help with radiant heat and a stray spark, not a hot bar.

Which steels upset easily

Mild steel upsets most easily. It forges soft, tolerates a wide range of heats, and forgives mistakes. That is why it is the right choice for learning.

Higher-carbon and alloy steels are less forgiving. They need to be worked within a narrower heat range, they can crack if you upset them too cool, and some will harden in still air or from a splash of water. If you plan to upset a tool steel or a knife steel, check the manufacturer's data sheet for forging guidance, and read the steel's page on Knife Steel Nerds before you plan the work. The steel selector tool can help you narrow down which steel suits a project, but it will not replace the data sheet.

Salvaged steel is a gamble. Railroad spikes, old bolts, and scrap of unknown alloy do not always behave like mild steel, and a bad upset can open up as a crack on the first heat or the fifth. Practice on known stock before you commit unknown steel to a project that matters. The mystery steel article covers how to think about unidentified stock.

When upsetting is the wrong move

Upsetting is slow and tiring by hand. If a part needs a lot more thickness than the bar has, ask whether a thicker bar would be easier. Buying the right size stock almost always beats forging mass into the wrong size.

It is also the wrong tool when the final part depends on grain flow and strength in a way you cannot judge. A cold shut, a crack, or a folded seam inside a hook, a hammer, or any load-bearing piece is a failure waiting to happen. I do not make safety-critical parts at home, and this article is not engineering advice. If a failed part could hurt someone, buy a rated part from a source that can stand behind it.

For everything else, upsetting is a core move that makes you a better smith. Learn it on mild steel and scrap, and bolt heads, collars, and welding prep all grow out of the same motion.