upset
To thicken and shorten a soft or heated piece of metal, by forging or hammering on the end, to shape, for example, rivets or internal combustion engine valves.
upset: hammer-forging that thickens metal by compression
Upsetting is a forging operation that compresses a heated metal workpiece along its length, causing it to thicken and shorten. A punch or die strikes the end face of the material, driving metal outward and downward to increase cross-sectional area while reducing length. The process works only on soft metals or heated stock, since cold metal resists plastic deformation and risks cracking instead.
The most common industrial application is rivet manufacture. A length of wire or rod is heated to cherry red or orange (typically 1100 to 1200 degrees Celsius for steel), inserted into a die cavity, and struck on the end with a ram. The confined die forces the metal to bulge outward, forming the characteristic rounded head. Valve stems for internal combustion engines are similarly upset to create the wider head that seats in the cylinder.
Upset forging machines range from simple manual hammers to fully automated header machines that can produce thousands of fasteners per hour. Horizontal upset headers grip the rod in a die, advance it to the right length, then strike the protruding end with a horizontal ram. Vertical machines use gravity or hydraulic pressure. The critical variable is heat: metal that cools below the recrystallization temperature becomes too hard to move without fracturing, so production lines include induction heaters or furnaces to maintain stock temperature as it feeds through the die.
Cold upsetting is possible for small sections of very ductile metals like copper or aluminum, but requires far greater force. Hot upsetting is standard practice for steel and iron. The process creates characteristic forging surfaces: the die marks remain visible on the upset head, and flash (thin material squeezed out between die halves) must be trimmed afterward. Incomplete upsetting leaves insufficient head diameter; excessive upsetting under too much force causes the workpiece to buckle or shear before the head forms properly.
The term comes from the vernacular use of "upset" to mean displaced or disturbed, referring to the metal being pushed or disturbed from its original form. In heavy forging, "upsetting" can also describe the heating and preparation of a billet before it enters the main press, though that usage is less common in modern trade language.