Steel bar has a grain, like wood, running along its length. A head machined from larger bar cuts across that grain at the head-to-shank fillet; a forged head bends it up into the head. Forging that develops the grain flow improves impact toughness, fracture toughness and fatigue strength.
The specifications check the result. Upset or forged headed bolts up to 1-1/2 in., three diameters long or more, get a full-size wedge test under ASTM A193 and must not break or crack where the head meets the shank. For nuts, ASTM A194 allows hot or cold forging or machining from bar, then quenching and tempering, and the marking says which: 2H for a forged or punched nut, 2HB for one machined from bar.
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model dataBolt drawn as a 1 in. heavy hex bolt to ASME B18.2.1 (1-5/8 in. across flats, 43/64 in. head), 6 in. under the head as an example; the socket head cap screw as a 1 in. screw in ASME B18.3 proportions, 4 in. under the head; nut as a 1 in. heavy hex nut (1.625 in. across flats, 0.984 in. thick), tapped 1-8 UNC. The stock gathered for the bolt head, about 2 in. of 1 in. bar, is the head's volume divided by the bar's area; the upset shapes are volume-conserving sketches, not simulations, and the socket blow keeps the head's volume, so the head grows taller as the socket forms. The one-blow limit of about three bar diameters, the 1700–2300 °F forging range and grain flow: Forging Industry Association design guide; flow lines revealed by macroetching: ASTM E340. Heat treatment after forging and the wedge test: ASTM A193. Nut routes and the 2H and 2HB marks: ASTM A194. Heat treatment is done by an outside Houston processor. The two-station press is drawn generic after a photo of the shop's own press. For the hex head the shop's set-up is a hex recess in the bottom die and a hex-shaped punch sized to it; the 30° relief drawn in that punch's face to form the head's chamfer, and the other inserts and punches, are a plausible set-up for each job; the induction coil is generic; glow colours are qualitative; the forging blows are shown in slow motion and the heating and cooling in time-lapse; how a forged nut's hole is made is not shown. The finished parts are marked INF (the maker's mark), the grade and the heat number, in three lines on a bolt head and in three places round a nut face; H-24A117 is an example heat.
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