Nickel-Base Spring Alloys

§1The nickel-base family

Nickel buys three things steel cannot always give at once: corrosion resistance across a wide chemical range, useful strength at elevated temperature, and — in several grades — no magnetism at all.

Nickel is a strong, tough, white metal, the third-most magnetic element after iron and cobalt, yet its spring alloys are chosen precisely where those steel-like properties must be traded away. All the nickel-base spring alloys resist a broad span of corroding media and work both well below freezing and well above the reach of steel. Several are non-magnetic, which matters for gyroscopes and indicating instruments. They are drawn and rolled to the American Wire Gauge, in wire and strip, with the available range depending on the alloy.

They split cleanly by hardening mechanism. Monel 400 and Inconel 600 reach high tensile strength by cold drawing or rolling and cannot be hardened by heat. Monel K-500 and Inconel X-750 add small amounts of aluminium and titanium (and, in the Inconel, columbium) that make them precipitation-hardenable: they are formed soft or nearly hard, then hardened by a long age. That single difference decides how each is bought, formed and heat-treated.

§2Monel 400 — the affordable one

A solid-solution nickel-copper alloy, nearly non-magnetic, the least costly of the four — and cold-formed only.

Monel 400 is roughly two-thirds nickel and one-third copper, a good combination of reasonable strength and strong corrosion resistance, particularly in sea water. It is the cheapest nickel-base spring alloy and the lowest in tensile strength, but it can be stressed a little above phosphor-bronze and nearly as high as beryllium-copper, and it is at home in equipment handling food and beverages. It is supplied full hard for spring use up to about 6 mm diameter, with larger sizes at reduced hardness; rods are reduced 33–75 %, the larger reductions on the smaller wire. Because it cannot be hardened by heat, all its strength is cold-worked in. Its working range is −73 to 220 °C at average design stress.

Composition runs nickel 63–70 %, copper the remainder, with iron to 2.5 %, manganese to 2.0 % (1.25 % in strip), silicon to 0.5 %, carbon to 0.30 % and sulphur to 0.024 %. Modulus is 179 265 MPa in tension and 65 500 MPa in torsion; the elastic limit is 65–70 % of tensile in tension, 38–42 % in torsion. Conductivity is only 3.6 % of copper — it is not a conductor material — and density is 8.83 g/cm³. Strip is graded ½, full and spring hard at Rockwell 90B, 95B and 100B, reaching 910 MPa spring hard.

§3Monel K-500 — age-hardenable Monel

Monel 400 with a little aluminium and titanium added — which moves it into the precipitation-hardening group and lets it reach strengths nearly those of stainless steel.

K-500 is the same nickel-copper base as Monel 400, but 2–4 % aluminium (and optional titanium) makes it hardenable by ageing. It is non-magnetic and obtainable to about 13 mm diameter for cold-wound springs, larger for hot-wound. The attraction is that it can be formed soft — taking sharp bends, punching and piercing — and then hardened to tensile strengths and hardnesses above plain Monel. Its range is −73 to 232 °C at average design stresses, usually kept below 345 MPa. The heat treatment is a long one and worth noting for scheduling: precipitation-harden at 538 °C for eight hours, then drop slowly to 480 °C over an hour before furnace- or air-cooling — a cycle that runs slightly longer than a working day.

Composition holds nickel 63–70 %, copper the remainder, aluminium 2.0–4.0 %, with iron, silicon and manganese limited; modulus after hardening is 179 265 MPa in tension and 65 500 MPa in torsion — the same as Monel 400 — and density is 8.47 g/cm³.

§4Inconel 600 — nickel-chromium for heat

A non-magnetic nickel-chromium-iron alloy, cold-drawn to high strength, whose value is holding load hot where steel would relax.

Inconel 600 is a nickel-chromium-iron alloy — nickel 72 % minimum, chromium 14–17 %, iron 6–10 % — cold-drawn to high tensile strength in wire to about 6 mm, and not hardenable by heat. Though dearer than stainless steel, it earns its place in heat: compression springs run to 650 °C stressed as high as 450 MPa with about a 5 % load loss in 24 hours, while 345 MPa buys much longer life. In the annealed condition it is used for heat-treating baskets and furnace conveyor belts to 1200 °C. As a spring it appears in steam and regulating valves, boilers, compressors, turbines and jet engines. Because modulus falls with temperature, the reduced value at the working temperature must be used in design — the hero curve above shows the fall from about 214 GPa at room temperature to 194 GPa at 370 °C.

The elastic limit is 65–70 % of tensile in tension and 40–45 % in torsion; conductivity is very low and density is 8.415 g/cm³. Like Monel, it cannot be hardened by heat, but it is stress-equalised after coiling to shed residual stress.

§5Inconel X-750 — the high-temperature spring

Inconel with aluminium, columbium and titanium added — precipitation-hardenable, non-magnetic, and the alloy of choice when the spring must survive real heat.

X-750 is the age-hardenable member of the Inconel family: nickel-plus-cobalt 70 % minimum, chromium 14–17 %, iron 5–9 %, with titanium 2.25–2.75 %, aluminium 0.40–1.00 % and columbium-plus-tantalum 0.70–1.20 %. It comes as bar, rod, wire and strip, is coiled or formed hot or cold, then precipitation-hardened to higher strength than plain Inconel. It withstands 455 °C readily and works below freezing, appearing in gas-turbine rotor blades, steam valves and rocket thrust chambers. Spring-temper wire heated at 650 °C for four hours reaches 1517 MPa at small diameters, tapering to 1310 MPa above 13 mm. As with Inconel 600, its modulus — the upper curve in the hero chart — falls from about 214 GPa at room temperature to 176 GPa at 650 °C, and the reduced value must be used at the working temperature.

Springs may be furnace- or air-cooled. Above 455 °C, lower the stress to no more than 380 MPa and expect about a 5 % load loss in 24 hours. Some makers, unwilling to keep furnaces hot overnight, run the single cycle above for all tempers with excellent results; the SAE lists alternative cycles for specific temperature and stress bands.

§6Heat treatment and stress equalising

The cold-worked pair are stress-equalised; the age-hardenable pair are precipitation-hardened. Both operations depend on wire size and severity of service.

Monel 400 and Inconel 600 cannot be hardened, but a stress-equalising heat after coiling removes residual stress and steadies the load. Monel is equalised at 232–260 °C for average service, 288–316 °C for severe, and 343–371 °C for high-temperature use; Inconel 600 at 370–400 °C, 425–450 °C and 455–480 °C respectively. Time at heat scales with size and severity — from around 30–45 minutes for the smallest wire in average service up to two or three hours for larger wire at high temperature.

The service-temperature ranges below place the four alloys against one another: Monel 400 and K-500 cover the low-to-moderate band with the corrosion and non-magnetic advantages, while the two Inconels reach into genuine heat.

§7The minor nickel alloys

Beyond the four mainstays sit a handful of nickel alloys used occasionally, each filling a narrow need.

"A" nickel is commercially pure, cold-worked nickel — 99 % minimum — silvery, magnetic, with corrosion resistance like Monel and good spring properties; tensile runs 862–1034 MPa, it resists caustic soda even above 315 °C, and it serves food and chemical work. Duranickel is a precipitation-hardened nickel-aluminium-titanium alloy usable to 260 °C at 345 MPa, in larger sizes than "A" nickel, reaching 1200–1380 MPa after a heat treatment identical to Monel K-500. Both carry a modulus of 206 840 MPa in tension and 75 840 MPa in torsion.

Incoloy — about 30 % nickel, 22 % chromium, 45 % iron — is rarely used for springs but reaches properties nearly Inconel's, cold-formed and not heat-hardenable, and tolerates higher temperatures than Inconel. Permanickel, a high-nickel titanium-magnesium alloy similar to Duranickel but with better electrical and thermal conductivity, is used for thermostat tension springs.

§8Quick reference