How Do You Join Steel Without Welding? Hammer a Hot Pin Flat

A pin with a head at one end is passed through holes drilled through the parts to be joined, and the projecting end is deformed to form a second head, which traps the parts between them. In the hot method the pin is heated until red, driven into place and hammered over, and as it cools it contracts along its length, which pulls the two heads together and squeezes the parts with considerable force. That clamping is what makes the joint strong, since the friction between the clamped surfaces carries much of the load rather than the pin being loaded sideways. A cooled joint is therefore under permanent tension and is extremely difficult to work loose.

Hot riveting was a gang operation with defined roles:

• •A heater working a portable forge, bringing pins to temperature
• •A catcher receiving the hot pin, frequently thrown, in a metal cone
• •A holder pressing a heavy bar against the existing head from behind
• •A riveter hammering the projecting end over, originally by hand and later pneumatically
• •Pace set by how fast a pin cools, which allowed only seconds
• •Noise, heat and danger that defined the working conditions of shipyards

Welding displaced the method across most of heavy industry from the 1930s and the advantages were substantial. A welded structure needs no holes, and every hole removes material and concentrates stress. It is lighter, since no overlapping plates and no pins are needed. It is faster and needs fewer workers. It is watertight without additional sealing. Against that, welding introduces its own problems, since heat distorts the parts, the weld metal may be brittle, and a crack in a welded structure can run continuously across the whole thing, which cannot happen where a joint interrupts it. Several catastrophic ship failures in the 1940s were attributed to exactly that.

Not everything is riveted hot, and the cold methods are what most people actually encounter. A small pin in soft metal is simply hammered over cold, which is how leather work, saucepan handles and traditional armour are assembled. A blind fastener carries a mandrel through its centre with a bulge at the end, and pulling that mandrel with a tool draws the bulge back into the body, expanding it against the far side, after which the mandrel snaps at a designed weak point and is discarded, which allows a joint to be made from one side only. Self-piercing versions punch through the sheets and flare inside them without any hole being drilled, and are used in enormous numbers in car bodies.

The method survives wherever its particular advantages matter. Aircraft structures are riveted extensively, since aluminium alloys used in airframes lose strength when welded and since a riveted joint arrests a crack rather than allowing it to propagate, which is fundamental to designing structures that fail safely. Blind rivets, set from one side by pulling a mandrel that snaps off, are used throughout manufacturing where only one side is accessible. Joining dissimilar materials that cannot be welded together uses them. Repair of historic structures uses hot riveting to match the original, which requires reviving a nearly extinct skill. And the joints themselves are visible on every preserved bridge and ship from the period.

A pin driven hot and hammered over contracts as it cools, pulling the heads together and clamping the parts so that friction carries much of the load. Hot work needed a gang of four and had to be completed within seconds before the pin cooled. Welding displaced it by removing holes and weight, at the cost of allowing cracks to run continuously. Aircraft and blind fastening keep the method in use.