Why metal moves at all

Every weld starts with a small pool of molten steel at somewhere between 1,500 and 2,000°C. The parent metal a few millimetres either side climbs to 200–400°C, which is enough to make it expand significantly — mild steel grows by roughly 12 microns per metre per degree. That hot metal wants to swell, but it's surrounded by cold plate that won't let it. So it yields, plastically, in compression. Then it cools, contracts, and pulls on the cold metal around it, which refuses to give. The result is distortion.

None of this is a fault in your technique. It's physics, and every fabricator fights it. What separates a tidy assembly from a banana-shaped one is understanding which factors you can actually influence: how much heat you put in, how the part is restrained, and the order in which you run your beads.

The main shapes distortion takes

It helps to name what you're looking at, because each type responds to different remedies.

• Transverse shrinkage — the weld pulls the two plates towards each other across the joint. On a single-V butt in 10mm plate you might see 2–3mm of closing up over a metre. This is what makes a long butt joint slowly shorten as you weld it.
• Longitudinal shrinkage — contraction along the length of the weld. Smaller than transverse shrinkage, but it's the one that bows a beam or pulls a fabricated rail out of straight.
• Angular distortion — the classic "hinge" effect, where a fillet or single-V weld lifts the plate on one side. More heat on one face means more contraction on that face.
• Buckling and bowing — the cumulative result on thin sheet or long slender assemblies, where there simply isn't enough stiffness to resist the pull.

Thin material, high heat input, and few passes all make these worse. Thick material, low heat input and balanced welding make them better.

Restraint is a double-edged tool

Clamps, strongbacks and jigs feel like the answer, and up to a point they are. Hold a frame square in a jig and weld it, and it will usually come out square — until you release the clamps, at which point the stored stress redistributes and the frame springs out of shape. That's the trap: restraint doesn't remove the stress, it just hides it until the jig lets go.

Heavy restraint also raises the risk of cracking, particularly in thicker sections and higher-carbon steels, because the shrinking weld is fighting something that won't move. For most small workshop work, moderate restraint plus good sequencing beats brute force every time. Think of the jig as a guide, not a prison.

Sequencing: the cheapest fix you have

Before you alter anything about the design, look at the order of your welds. A few habits are worth building in:

• Balance the heat. Weld alternate sides of a joint, or opposite sides of a fabrication, so the contraction pulls cancel out. On a T-section, weld one side, then the other, rather than running both beads on the same face.
• Backstep. Instead of running a bead from one end to the other, start 200mm in and weld back towards the start, then repeat. Each new bead's shrinkage pulls against the previous one and the net movement drops noticeably.
• Weld from the centre outwards. On a long seam or a plate with several stiffeners, work outwards so the free ends can move as they want to.
• Skip and stagger. On long runs, weld short 100–150mm sections in a staggered pattern, letting each cool before you return.
• Save the strong tacks for last. Leaving final welds until after the assembly is tacked and squared means the tacks do some of the restraint work for you.

These cost nothing but a bit of thought, and they'll handle most of the distortion you'd otherwise be straightening later.

Heat input, tacks and copper

Amps and travel speed matter more than most people credit. Running hotter than you need, or travelling slower than you need, puts more heat into the part and produces more contraction. For distortion-sensitive work in a small shop, a slightly undersized electrode with a brisk travel speed and a short arc will usually leave less movement than a big comfortable bead.

Other practical measures worth keeping to hand:

• Tack spacing. On 3mm sheet, tacks every 50–75mm is a sensible starting point; on 6mm plate, 100–150mm is usually fine. Closer tacks on thin material.
• Copper backing bars and heat sinks. A copper bar clamped behind the joint draws heat away quickly and holds a clean root profile, which cuts both distortion and post-weld cleaning.
• Presetting. Deliberately set the parts slightly out of true in the opposite direction before welding — a few millimetres over a 2m gate frame — so the contraction brings them back to straight. Experience tells you how much; start small and keep notes.
• Peening. Light hammer or needle-gun peening of a hot bead stretches the cooling metal and reduces the pull. Keep it gentle — heavy peening can crack or over-work the weld.

When to correct, and when to live with it

Some movement is inevitable and not worth chasing. If a 1,200mm fabricated rail arrives 2mm out over its length in a non-critical application, it's probably fine. If a mating face is out by 3mm, it needs sorting before it leaves the bench.

Localised heat is your friend for correction: a hot spot on the convex side followed by rapid cooling pulls the metal back towards the heat. Oxy-acetylene with a small nozzle and a wet rag gives you fine control. For anything larger, mechanical straightening with a press or a bit of controlled persuasion is quicker and kinder than trying to chase it with heat. Whatever the method, check with a straight edge or a laser before you commit, and remember that straightening adds its own residual stress — it's better to get the assembly close on the way in than to fight it on the way out.