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Spring flat bars are hot-rolled flats of silicon-manganese, chromium or chromium-vanadium spring steel, made to close tolerances for cutting, forming and hardening into leaf springs. They carry the suspension of trucks, trailers, buses, rail wagons and farm machinery.
A leaf spring is a stack of curved steel leaves, clamped at the centre and fixed to the chassis at the ends, which bends under load and returns. To do that millions of times without breaking, the steel must be hardened and tempered to high strength — typically 1,300–1,700 MPa tensile — and must be free of surface defects and decarburisation, because a fatigue crack starts at the surface. Spring flat bars are the hot-rolled material from which the leaves are cut.
The spring maker cuts the flat to length, punches the centre hole, rolls the eye ends, tapers the leaf if it is parabolic, then heats, bends to camber and quenches in oil, and tempers. Most springs are then shot-peened, which puts the surface into compression and greatly improves fatigue life, and painted. Parabolic springs, with one to four tapered leaves instead of many flat ones, are lighter and are now standard on many trucks and trailers; they need thicker flats and higher-hardenability grades.
Grade is chosen by thickness, because a thick leaf must harden right through. Silicon-manganese steels (60Si2Mn, 56Si7) and plain chromium steels (55Cr3 / SUP9, 60Cr3 / SUP9A) suit thinner leaves; chromium-vanadium 51CrV4 (SUP10, SAE 6150) and chromium-molybdenum-vanadium 52CrMoV4 are used for thick and parabolic leaves. Ribbed and grooved flats (EN 10092-2) have a profile that locates the leaves in the stack.
In the Middle East the market is the replacement and trailer trade: the region's large truck and trailer fleets wear out springs in heavy, hot service, and spring workshops and trailer builders in Saudi Arabia, the UAE and elsewhere make and repair them. Spring flats are imported, mostly from China, India, Turkey and Europe.
Spring flats are ordered by width × thickness, edge shape, grade and length. Thickness tolerance matters more than for an ordinary flat, because leaf stiffness rises with the cube of the thickness.
| Region | Standard | What it covers |
|---|---|---|
| Europe | EN 10092-1 CEN-CENELEC |
Hot-rolled spring steel flat bars — dimensions and tolerances on shape and dimensions |
| Europe | EN 10092-2 CEN-CENELEC |
Hot-rolled spring steel ribbed and grooved flat bars — dimensions and tolerances |
| Europe | EN 10089 CEN-CENELEC |
Hot-rolled steels for quenched and tempered springs — technical delivery conditions (55Cr3, 51CrV4, 52CrMoV4…) |
| Japan | JIS G4801 JSA |
Spring steels (SUP6, SUP9, SUP9A, SUP10, SUP11A…) |
| China | GB/T 1222 SAC |
Spring steels (60Si2Mn, 55CrMnA, 60CrMnA, 50CrVA…) |
| USA | ASTM A689 ASTM |
Carbon and alloy steel bars for springs |
| USA | SAE J1268 | Hardenability bands for carbon and alloy H steels (5160H, 6150H…) |
| India | IS 3431 BIS |
Steel for the manufacture of volute, helical and laminated springs for automotive suspension |
| Germany | DIN 17221 DIN replaced by EN 10089 |
Hot-rolled steels for quenched and tempered springs |
Grade equivalents and the standards bodies: Steel grades & standards
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