Constant-Modulus Alloys

§1What "constant modulus" means

A spring's rate is set by its modulus of elasticity. If the modulus changes with temperature, so does the rate — and a weighing scale or an instrument that drifts with the weather is useless.

Most metals soften as they warm: the elastic modulus falls, and a spring made from them exerts slightly less load at the same deflection. For a machine spring that hardly matters, but for a food-weighing scale, a gyroscope, a measuring device or a computing scale it is a defect. A small family of special nickel alloys is built to have a low or zero thermoelastic coefficient — their modulus barely moves across the working range, so spring stiffness stays constant. They also carry low hysteresis and low creep, and good corrosion resistance. The range that matters is narrow, about −46 to +66 °C, which is exactly where scales and instruments live.

The trade is cost and availability. These alloys are expensive, none is regularly stocked in a wide variety of sizes, and all are used only in small wire diameters and thin strip. Every one is covered by patents, and several suppliers will not fabricate springs from them at all because of the special processing they demand — so they should never be specified without first talking to the spring maker.

§2Elinvar, Iso-Elastic and the classics

The line begins in a French watch factory and runs through a family of nickel-iron-chromium alloys, each tuned for a slightly better constant modulus.

Elinvar — a nickel-iron-chromium alloy developed in France — was the first constant-modulus material used for watch hairsprings. It is austenitic and hardens only by cold drawing and rolling. Adding titanium, tungsten and molybdenum gave later alloys precipitation-hardening ability and better characteristics, spawning trade names such as Elinvar Extra, Durinval, Modulvar and Nivarox; the original is still used, but the newer alloys are more useful.

Iso-Elastic, a nickel-iron-chromium-cobalt alloy, is popular because it is easier to fabricate than Ni-Span C, and its temperature-compensating behaviour meets the requirements of weights-and-measures boards. It is used in dynamometers, computing and food-weighing scales and precision instruments. Its modulus is 179 260 MPa in tension and 63 430 MPa in torsion, with tensile strength at least 1170 MPa and hardness Rc 32–36. Elinvar Extra, a nickel-chromium-titanium-iron alloy with small cobalt, aluminium and silicon, is precipitation-hardened: cold-worked 50 % and aged two hours at 675 °C, it reaches 1380 MPa, a modulus of 193 000 MPa and hardness Rc 42, for hairsprings and instrument springs.

§3Ni-Span C alloy 902 — the popular one

The most widely used constant-modulus alloy, and unusually flexible: two heat treatments give two very different materials from the same wire.

Ni-Span C alloy 902, a nickel-iron-chromium-titanium alloy developed by the International Nickel Company, is the most popular of the group. It is usually formed or coiled in the 50 % cold-worked condition and then precipitation-hardened. The standard heat is 480 °C for eight hours; a higher heat of 675 °C for three hours gives greater hardness; and for the greatest precision and uniform performance, a two-step cycle — 400 °C for two hours, then the oven raised to 650 °C for two more, then air cool — is preferred. After the standard treatment the alloy shows a modulus of 189 600 MPa in tension and 68 950 MPa in torsion, tensile strength at least 1380 MPa, and recommended design stresses of 415–480 MPa in compression, 345–380 MPa in extension and 825–895 MPa in torsion.

Driven harder, the same alloy transforms. After the high-temperature treatments its modulus rises to 203 400 MPa in tension and 77 220 MPa in torsion, and tensile strength reaches 2480 MPa in strip and 2345 MPa in wire, at hardnesses of Rc 56–59 (strip) and Rc 51–55 (wire) — with design stresses then taken the same as stainless steel type 302. It is this range, from a precise low-drift spring to a very high-strength one, that makes alloy 902 the default constant-modulus choice.

§4Elgiloy and Havar — cobalt-base strength

Two cobalt-chromium-nickel-iron alloys grouped here for their precision-spring pedigree, both non-magnetic and both very strong after ageing.

Elgiloy — a cobalt-chromium-nickel-iron alloy once called 8-J Alloy, Durapower and Cobenium — was developed by the Elgin National Watch Company with Battelle and three wire producers. It is non-magnetic, suits below-freezing and elevated temperatures to 400 °C provided torsional stress stays under 515 MPa, and reaches its properties by a combination of heavy cold work and a long heat. Strip is cold-rolled 85 % and then heated at 480 °C for five hours after forming; wire is cold-drawn 45 % and heated at 525 °C for five hours after coiling. It runs watch mainsprings, indicating instruments, compasses and stepper motors.

Havar, a cobalt-chromium-nickel-iron alloy with small tungsten and molybdenum from Hamilton Technology, is likewise non-magnetic and good from subzero to 400 °C at torsional stresses to 515 MPa. Hardened after forming at 510 °C for three hours, it reaches tensile strengths over 2070 MPa, hardness Rc 56–60 and a modulus of 206 800 MPa, in watch mainsprings and motor, spiral and flat springs.

§5Heat treatment and cold work

Every alloy here is set by a deliberate pairing of cold work and a precipitation heat, and the numbers are specific enough that they must be followed to the letter.

The pattern is consistent: cold-work to a defined reduction — commonly 50 % for the nickel-iron-chromium alloys, 45–85 % for the cobalt alloys — form or coil, then age. The ageing temperature is the lever that sets the final properties. Ni-Span C is the clearest illustration: 480 °C for eight hours gives the low-drift precision material; the two-step 400 °C-then-650 °C cycle gives the most uniform performance; the higher single heats push it to very high strength. Because these alloys are bought unhardened and hardened after forming, distortion during ageing must be anticipated and, where it matters, fixtured out — exactly as for beryllium-copper and the age-hardenable nickel alloys.

§6Other materials occasionally used

Beyond the constant-modulus alloys proper, the handbook gathers a set of specialty materials used for springs under particular circumstances.

Rocket wire has a chemistry like music wire but is specially selected for uniformity — carbon 0.80–1.00 %, manganese 0.25–1.00 % — and hard-drawn 10–20 % above music-wire tensile (25–28 % higher in the lightest sizes), for small springs failing at high operating stress. Hastelloy alloy B, a nickel-molybdenum-iron alloy, resists hydrochloric acid at all concentrations, with spring-temper tensile of 1550–1800 MPa, liftable a further 345 MPa by ageing at 790 °C for 16 hours; alloy C is similar but slightly lower, and resists wet chlorine and strong oxidisers.

Titanium alloys weigh about a third of steel, resist corrosion and serve at elevated temperature; age-hardened at 425 °C they give high tensile strengths for space-vehicle springs. Stainless type 18-2 (ASTM A 313, type XM-28), a low-nickel austenitic developed as a 302/304 substitute, is non-magnetic with good corrosion resistance and, in heavier sizes to 13 mm, higher tensile strength than 302; after a 370 °C heat it shows a modulus of 200 000 MPa in tension and 67 570 MPa in torsion. Wood, glass, plastic, aluminium and fibreglass have all served as springs under special circumstances.

§7Specifying and sourcing

The engineering is only half the problem; these alloys demand a conversation with the maker before the drawing is released.

When a design genuinely needs constant stiffness across temperature — a scale, a precision instrument, a hairspring — one of these alloys is the answer, and Ni-Span C alloy 902 is the usual first choice for its combination of availability and tunable properties. Iso-Elastic is easier to fabricate; Elinvar Extra, Elgiloy and Havar serve the smallest, most demanding precision springs. But every one is expensive, patented, stocked only in limited sizes, and dependent on a heat treatment that is part of the specification. The single most useful rule on this page is procedural: agree the alloy, the size and the heat treatment with the spring manufacturer before committing, because some will not make the part at all.

§8Quick reference