Flat High-Carbon Spring Steel Strip

§1Two steels cover 95 % of flat springs

The blue-tempered ribbon inside a clock is one of the finest carbon steels made — and it is one of only two compositions doing nearly all the flat-spring work in industry.

SAE 1074 is the volume grade — thicknesses 0.050–2.362 mm (annealed to about 6.35 mm), stocked in warehouses, fed to four-slide machines and presses for clips, flat and spiral springs. SAE 1095 is the clock-and-motor-spring steel, carrying higher stress, usually bought tempered, polished and coloured in 0.050–1.58 mm; the ends of coiled mainsprings are locally annealed before bending and piercing. The remaining few per cent split between SAE 1064, the most ductile of the family for deep-drawn and severely formed parts, and barcoid at 1.25–1.32 % carbon for band saws, cutlery and scraper blades — wear articles first, springs second.

The strip route matters to quality: forging-grade rod is pickled, baked and spheroidise-annealed so the cementite balls up for cold rolling, with process anneals between passes and roll finish transferring directly to strip finish. For allowable stresses, hardened 1095 at HRC 46 and above designs to the oil-tempered A229 torsion-spring curves; softer tempers and 1074 design to the hard-drawn A679 curves.

§2Cold-rolled tempers No. 5 to No. 1

Without any heat treatment at all, rolling reduction alone spans dead-soft to blanking-hard. Each temper number is really a bending contract — all bends quoted over a radius equal to the thickness.

• No. 5 — dead soft. Annealed after rolling; bends flat on itself with or across the grain and takes deep drawing. Hardness ceilings HRB 83 (1074) and HRB 86 (1095); commercial stock runs HRB 75–85. No. 4 is the same steel given a light skin pass purely for finish — some mills use the two numbers interchangeably.
• No. 3 — quarter hard. One firm pass (as little as 0.025 mm of reduction lifts hardness at least three HRB points): bends flat on itself across the grain, to a right angle with it. The blanking-plus-severe-forming temper. HRB 87–93 for 1074; 89–95 for 1095.
• No. 2 — half hard. Bends to a right angle across the grain only; along the grain it demands a radius of several thicknesses. HRB 91–97, all compositions.
• No. 1 — full hard. Minimum HRB 98 (95–105 accepted): blank it, form it barely, bend it rarely. The top of what rolling alone can do — warehouse full-hard runs about HRC 28 / 889 MPa for 1074 and HRC 33 / 1020 MPa for 1095.

Parts made from tempers No. 5 through No. 2 are commonly hardened and tempered after forming (§7); No. 3 parts sometimes serve as-rolled where only modest springiness is needed.

§3Hardened-and-tempered strip — T1, T2, T3

The mill can deliver strip already at full spring temper, hardened and tempered as a continuous ribbon with a uniformity no batch furnace matches.

Spheroidise-annealed strip is uncoiled and pulled through a hardening oven at 788–838 °C, quenched in oil or lead near 327 °C, air-cooled, then drawn through a tempering oven near 410 °C — furnaces around 21 m long running four to twelve strands abreast, with temperature, bath and line speed balanced against composition, thickness and target hardness. Very thin gauges may be quenched between water-cooled plates; some alloys air-blast quench.

§4Strength by thickness

Tempered strip is stronger the thinner it is — and even the hardness scale used to measure it shifts as the gauge drops.

Intermediate gauges interpolate cleanly; T2 runs a step lower and T3 two steps lower again. Note the instrumentation detail hiding in column two: below about 0.4 mm the superficial 15N and 30N scales replace HRC because a standard Rockwell C indenter would punch through the sample — quote hardness on the scale appropriate to the gauge, or acceptance testing turns into an argument.

§5Edge conditions No. 1 to No. 6

A slit edge is a row of ready-made crack starters. Whether that matters depends entirely on what happens to the strip next.

Equipment sets hard limits: filing works to about 1.78 mm thickness, slitting to about 3.3 mm, and edge rolling to 63.5 mm width — with thin sections excluded from edge rolling because they buckle. Heavy bars such as 9.5 × 4 mm are cold-drawn to shape instead. Width and thickness tolerances occupy nine full tables in ASTM A682; cite them, don't copy them.

§6Finish and colour

The blue of clock steel is chemistry's most famous cosmetic — and it is only cosmetic.

Black tempered strip carries the dark oxide of muffle hardening, normal on heavy gauges of roughly 0.78–6.35 mm, some scale expected. Blue tempered strip, 0.127–2.3 mm, is the scaleless favourite: the ribbon is shielded from air through the hardening oven and lead-quenched, leaving a blue oxide of admittedly uneven shade. Either can be polished bright between high-speed brushes or abrasive buffs, and polished strip can be coloured — the uniform straw or blue of clock spring — by a controlled pass through a lead bath and timed cooling. The colour signals nothing about hardness or temper and offers only mild corrosion resistance; specify the finish and the hardness separately, always.

§7Heat treatment of formed parts

Parts formed from annealed or rolled-temper strip get their spring properties in a short, sharp cycle — thin sections leave no margin for a lazy furnace.

Harden at 788–816 °C with a soak of only one to three minutes — enough to austenitise, not enough to decarburise a 0.3 mm section — and quench in warm agitated oil. Temper at 316–371 °C, trading temperature against time to hit the target hardness. Springs made from mill-tempered strip skip all that and take only a stress-equalising bake at 204–316 °C for 15–30 minutes to relax forming stresses.

Contents

§8Quick reference