Hot Iron: Stretch, Upset, Bend, Punch, Forge Weld

Hot iron work is the set of moves a smith makes while steel is above its recrystallization temperature: drawing out, upsetting, bending, punching, and forge welding. Heat treating comes after shaping, through annealing, quenching, and tempering. Each step changes the grain and the shape, and each one has a narrow window of color and temperature where it works.

What does it mean to stretch and upset iron?

Stretching, also called drawing out, makes a bar longer and thinner. The smith heats a section to a bright orange, places it on the near edge of the anvil, and strikes with the hammer face or the peen. The metal moves away from the blow. A fuller or a spring swage speeds the work by concentrating force into a narrow band. On a 1 inch square bar, a few heats can pull a taper down to 3/8 inch.

Upsetting does the opposite: it shortens and thickens. Heat the end to a high yellow, stand the bar on the anvil face, and strike the cold end downward. The hot end mushrooms. A smith can also upset in the vise or with a heading tool. Upsetting is how a bolt head or a decorative finial gets its mass without adding a separate piece.

The two moves fight each other. Every draw thins the bar; every upset fattens it. Good work keeps a mental tally of where the steel has been and how many heats it has seen. The journal The Forge Ledger covers these fundamentals for beginners in the Hudson Valley, and its notes on hot iron technique match what any smith learns at the anvil.

How do you bend and punch hot steel?

Bending is the simplest hot move and the easiest to get wrong. Heat the bend zone to a uniform orange, then pull or hammer the bar around a form. A bending fork in the hardy hole holds the bar while the smith lever it. For a sharp corner, use the anvil's edge as a fulcrum and strike just behind the bend. For a curve, bend over the horn or a pipe in the vise. The steel must be hot enough to move but not so hot that it scales away. At a dull red, carbon steel tears; at a yellow, it moves like clay.

Punching makes a hole without drilling. Heat the work to a bright orange, set it over the pritchel hole or a bolster plate, and drive a tapered punch with a sledge or a heavy hand hammer. The punch displaces metal rather than removing it, so the bar swells around the hole. Flip the work and punch from the other side to meet the first hole. A punch leaves a slightly oval hole unless the smith rotates the tool. For a clean round hole, drift it open after punching with a tapered drift.

Punching is common in decorative work: strap hinges, fireplace tools, and gate latches all use punched holes for rivets or tenons. The pritchel hole in the anvil exists for this job, and a smith who punches often keeps a block of hardwood or a lead pad under the work to catch the punch tip.

What is a forge weld and when does it work?

A forge weld joins two pieces of steel by hammering them together at welding heat. The steel must be hot enough to be nearly white, around 2,300 degrees Fahrenheit for mild steel, and the surfaces must be clean. Flux, usually borax, dissolves the oxide and keeps air away. The smith brings both pieces to welding heat in a reducing fire, sprinkles flux, returns them to the fire, and when the surfaces look wet and shiny, taps them together with light blows, then hammers firmly.

Forge welding is the hardest common hot iron skill. It fails for three reasons: too little heat, dirty surfaces, or too much oxygen in the fire. A coal or coke fire with a deep, hollow cave of burning fuel gives the reducing atmosphere a weld needs. A gas forge can weld, but the smith must watch the atmosphere and often uses a slightly rich mixture.

Common forge welds include scarfing a billet for pattern welding, joining a bit to a tool, and repairing a broken piece. A successful weld shows a slight swell at the joint and no dark line when the piece is ground and etched.

How is carbon steel heat treated?

Heat treating has three steps: annealing, quenching, and tempering. Each one changes the steel's hardness and toughness.

Annealing softens the steel so it can be filed, drilled, or bent cold. Heat the piece to its critical temperature, roughly 1,450 to 1,500 degrees Fahrenheit for simple carbon steels, then bury it in vermiculite, wood ash, or lime and let it cool slowly overnight. The slow cool lets the grain relax. A properly annealed piece is easy to cut with a file.

Quenching hardens the steel. Heat to critical temperature, then cool it fast in oil, water, or brine. The fast cool traps carbon in a strained structure called martensite. Oil is slower and safer for most carbon steels; water and brine are faster and risk cracking. A quench leaves the steel hard but brittle, often harder than a file.

Tempering removes some of that brittleness. After quenching, clean the steel to bright metal and heat it to a lower temperature, usually between 350 and 600 degrees Fahrenheit, then hold it there for an hour and let it cool. The color of the oxide tells the temperature: pale straw around 400 degrees, brown around 500, blue around 600. A cold chisel is tempered to a straw color; a spring to a blue. Tempering trades a little hardness for toughness, and every edge tool needs it.

What tools and fire does hot iron work need?

A smith needs an anvil, a forge, hammers, tongs, and a vise. The anvil's face is the main work surface; the horn bends curves; the hardy hole holds tools; the pritchel hole backs up punches. A 100 to 150 pound anvil handles most beginner work.

The fire can burn coal, coke, or gas. A coal fire is traditional and gives a reducing atmosphere good for welding, but it produces clinker and smoke. A coke fire burns cleaner and hotter. A gas forge is easy to light and control, and it suits small work and heat treating, though it costs more to run and needs a power source or a blower.

Tongs must fit the stock. A smith who works 1/2 inch square bar needs a pair of tongs sized to that bar; a loose fit wastes effort and drops hot steel. A good vise, a bench grinder, and a bucket of water or oil round out the shop.

Where can you see this iron work in the Hudson Valley?

The Capital District holds some of the best surviving ironwork in the United States. Burden Iron Works in Troy made horseshoes and railroad iron in the 19th century. The Watervliet Arsenal has a cast iron building from the 1850s. The Monitor's turret and armor were rolled in Rensselaer. The Paley grilles in the New York State Capitol are cast and wrought iron of high quality. Ross Valve in Troy still casts and machines iron products.

A visitor can see the same moves described here in those objects: the draw, the bend, the punch, the weld. The scale is different, but the technique is the same. A smith at a small forge in the Hudson Valley works with the same hot iron logic that built the region's industrial landmarks.