ASTM A370

What the test does

A370 is an umbrella rather than a single procedure, and tension is its largest part. A coupon taken from the steel product — a machined round, a flat cut from sheet or plate, or an untouched full cross-section of bar, tube or reinforcing bar — is pulled along its axis until it necks and breaks, force and extension being recorded throughout. The bend clauses force a second coupon around a mandrel of stated radius to a required angle, and the stretched outer surface is examined for cracking. Hardness and notched-bar impact are covered by reference and run on separate testers.

What it measures, and why it matters

Tension reports yield point or yield strength, tensile strength, elongation over a stated gauge length and, where required, reduction of area. Yield governs the load a section carries before permanent set and is what most product specifications are written around; tensile strength sets the margin before rupture. Elongation and reduction of area describe ductility — whether the steel redistributes stress at a weld toe or bolt hole rather than cracking there — and a low value on an otherwise acceptable heat is often the first sign of trouble at the mill. The bend test checks surface soundness and formability, and Charpy impact whether the grade stays tough at the temperature the specification names.

Specimen

Tension coupons may be machined rounds, sheet-type flats, sub-size flats for thin or narrow product, or full cross-sections tested as rolled. Gauge length follows the coupon: 200 mm (8 in.) for full cross-section pieces unless the product specification says otherwise, 50 mm (2 in.) for the standard sheet-type coupon and the 12.5 mm round, and shorter lengths for sub-size flats and the proportionally smaller rounds, whose dimensions are tabulated in the standard. Orientation, the number tested per lot and any Charpy temperature come from the product specification, not from A370. Steel needs no humidity conditioning.

What the machine must be capable of

A 12.5 mm round of ordinary structural steel parts at roughly 60 kN, but a full cross-section bar, tube or reinforcing-bar coupon routinely demands 600 kN to 1 000 kN or more, so steel-product laboratories standardise on 600 kN frames and larger. Force measurement is verified to Practice E4.

Speed is controlled only around the yield. Any convenient rate may be used up to half the specified yield; from there the crosshead-separation rate is capped through the yield and a higher cap applies for the tensile-strength portion, with a floor beneath each. On a machine with a loading-rate indicator, a stress rate of 70 to 690 MPa/min (10 000 to 100 000 psi/min) is offered as an alternative through the yield. The per-inch limits should be read from the current edition. Bend speed is not an important variable.

Offset yield work needs an extensometer classified to Practice E83, a Class B-2 device covering the usual 0.2 % offset and tighter classes being called for as the offset gets smaller. The requirement relaxes as strain grows, so elongation to fracture can be followed with a lower class; the class applicable to a given offset and strain range should be taken from the current edition.

Machined coupons run in wedge grips; full-section coupons need jaw faces matched to the product — deeply serrated inserts for deformed bar, threaded adapters where the coupon allows — because a smooth face on a ribbed or scaled surface slips or bites unevenly and pulls the specimen off axis. Bend work needs its own guided jig. Hardness and impact leave the frame entirely, for a Brinell or Rockwell tester and a pendulum machine. Temperature is left open: no ambient band is set for tension, and the bend clause says only room temperature.

What goes wrong in practice

Slip in the wedges is the commonest fault on full-section product, and rarely obvious: mill scale breaks down under the jaw faces, the coupon creeps out a fraction at a time, and the flattened early curve depresses the apparent yield. Over-correcting is the opposite failure: jaws set too hard on a heat-treated round start a fracture at the grip line, the coupon breaks outside the reduced section, and the test is void.

A full-section angle, channel or tube gripped without an adapter that centres it bends as well as stretches, giving a low, scattered yield with nothing in the data to show why. Extensometer slip on a scaled surface is quieter still: the curve looks clean but reads short, and the error lands entirely on modulus and offset yield.

Related and equivalent standards

ISO 6892-1 is the nearest international counterpart for the tension part, close in intent but not interchangeable: strain-rate control is central to the ISO method, whereas A370 permits either crosshead-separation limits or a stress-rate window. ASTM A1058 covers the same ground in SI units only.

A370 mostly directs work elsewhere while adding steel-specific exceptions: tension follows E8/E8M, bend defers to E190 and E290, Brinell to E10, Rockwell to E18, impact to E23. It is also confused with the product specifications that invoke it, which supply the acceptance limits A370 does not.