Stainless Steel Wire and Strip

§1The film, and how to keep it

Stainless steel is not inert — it is self-repairing. An invisible chromium-oxide film forms on the surface, and everything in stainless practice comes down to protecting or restoring that film.

With 12–20 % chromium (nickel to about 10 % assisting), the film resists town and country atmospheres, steam, sea water, fruit juices, fuels, soaps and a long list of process chemistry; marine chloride and sulphurous industrial air are the classic spoilers. Strip the film — by machining, pickling or heat treatment — and the metal is briefly vulnerable while it regrows over some hours: fingerprints etch in permanently, and a speck of ordinary steel resting on the surface will rust and masquerade as "rusting stainless".

Passivation accelerates the regrowth while dissolving embedded foreign particles: 20 % nitric acid by volume in water, ideally at 60 °C, 10 minutes immersion, rinse. A five-minute 50-50 nitric dip suits the straight-chromium 400 grades. Round-wire springs of the 300 series are often shipped unpassivated with no penalty — except after shot-peening, when passivation is mandatory to dissolve the specks of carbon-steel shot that would otherwise rust and pit the surface. Coiler wire also arrives thinly coated in lead or copper as die lubricant; the coating dulls the finish harmlessly, and where food or sensitive chemistry is involved a five-minute room-temperature pass through the 20 % nitric tank strips it (hot, stronger acid at about 82 °C strips everything).

§2Austenitic 300 series — 302, 304, 316

The 18-8 wires cannot be hardened by heat at all. Like music wire, their strength is cold work — which sets both their size range and their weaknesses.

Spring-temper (full-hard) wire runs from about 0.13 to 4.75 mm; larger diameters come only in ¾- or ½-hard tempers with proportionally lower tensile strength, which is the cue to switch to 17-7 PH or a martensitic grade. Fully annealed, these alloys are non-magnetic; drawing and rolling realign the structure enough to make finished springs faintly magnetic — the low-temperature stress-relieving bake walks some of that back. They serve happily below freezing.

§3Martensitic 400 series — 414, 420, 431

Straight-chromium and heat-treatable: formed soft, then hardened and tempered like carbon steel. Magnetic, strong in big sections — and brittle in the cold.

These grades carry little or no nickel and behave in the quench like the spring steels of earlier sheets. Their territory is wire above 4.75 mm and any smaller spring wound to an index D/d below about 4, where cold-worked austenitic wire would crack in coiling. The hard limits: they are unsatisfactory below freezing — shock resistance collapses — and they are prone to grain growth and embrittlement in careless heat treatment. Corrosion resistance is real but a class below the 300s, and best on clean, polished, scale-free surfaces.

• Type 414. Hard-drawn to 4.75 mm it gives away about 15 % of 302's tensile — so it is normally formed annealed and hardened and tempered to 1207–1551 MPa. Process-anneal for severe forming at 677 °C for 4 hours (it will not fully anneal by slow cooling); harden at 996–1010 °C. A regular choice for hard-rolled strip, stampings and flat springs.
• Type 420. The standard for large diameters — and famously the 1.45 mm wire of the Garand rifle's recoil spring. It has no stainless character until hardened. Treat it with respect: charge into a furnace near 538 °C, raise slowly to 788 °C, then to the 996–1010 °C hardening range; quench in warm oil (≤120 °C) and temper immediately — as-quenched springs at HRC 50–54 will crack if left standing. Draw at 232–371 °C to a final HRC 46–51. Process anneal 771 °C, 3 hours; full anneal 899 °C then cool at 17–28 °C per hour to 593 °C.
• Type 431. Modern processing — mill-hardened and tempered, then cold-drawn — yields bright wire with tensile approaching music wire, in 1.25–8 mm plus bar and strip. Corrosion resistance sits just below 302; flexible watchbands wound from tight rectangular coils made its reputation.

Heat-treatment housekeeping for the whole 400 group: soak sluggish sections at least 20 minutes per 25 mm at temperature; never use cyanide salts (they carburise away the stainless); avoid manufactured furnace atmospheres that glue scale to the surface — dry hydrogen or dry cracked ammonia are safe.

§417-7 PH — strength after coiling

One alloy inverts the usual order of operations: coil the spring first, then let a modest oven make it strong — with no quench and no distortion.

17-7 PH — 17 % chromium, 7 % nickel, aluminium 0.75–1.50 % as the precipitation-hardening agent — combines spring properties with heat and corrosion resistance, in wire from 0.13 to 11 mm and in strip. Three supply conditions cover practice:

• Annealed (HRB 78–92): for deep drawing and severe forming, followed by the full two-stage treatment — solution treat 760 °C for 1½ hours, air-cool or water-quench, then age 454–510 °C for 30–90 minutes to HRC 40–45.
• Condition C, cold-worked (HRC 40–44): mill drawing or rolling has already transformed the structure, so springs coil like 18-8 wire and then take only the ageing step — 482 °C for one hour is the preferred schedule — finishing at HRC 47–50. Hooks and legs do not move during ageing; no distortion allowance is needed. This is the standard spring route.
• Extra-hard temper: mill-aged, for straightened-and-cut rods and parts needing only trivial forming.

Where a 300-series wire runs out of size, temperature or strength, 17-7 PH is usually the first alternative on the table — before the martensitics, unless magnetism or sub-4 indexes force the issue.

§5Properties and compositions

One table carries most stainless spring design. Note how E and G climb across the families — and how the densities differ enough to matter in mass budgets.

¹ For 300-series wire not at full spring temper, design with E down around 186 GPa. Elastic limits here are quoted, as throughout this series, as a fraction of ultimate tensile — a convenience figure standing in for the confusion of proportional limits, proof stresses and yield definitions: what the designer actually wants is the stress that must not be exceeded before objectionable permanent set.

Manganese runs to 2 % maximum in the austenitics and 1 % in the martensitics; silicon 1 % maximum throughout; phosphorus and sulphur at the usual residual ceilings. Wrap-test acceptance matches carbon wire: hardened wire to 4 mm winds on itself, larger sizes on a two-diameter arbor, without cracking.

§6Strip tempers

Austenitic strip is graded by rolling reduction, exactly like its wire is graded by drawing — and every step of hardness costs a step of bendability.

Type 420 strip follows the martensitic logic instead: fabricate soft, then harden and temper to roughly HRC 42–52 and 1517 MPa. Elastic limit in tension for stainless strip runs 65–75 % of tensile across the board.

§7Working, joining and cleaning

Most stainless "corrosion failures" are process failures. The rules below close the usual gaps.

Shot-peening deserves its own line: shot around 0.40 mm hammered onto the surface leaves a compressive pre-stress that subtracts directly from working tension — fatigue life gains of three- to ten-fold are ordinary. On stainless, follow it with cleaning and passivation (or use stainless shot) so embedded particles cannot pit the film. Carburising and nitriding, by contrast, trade away the very chromium chemistry that makes the steel stainless — leave them out of spring practice.

Contents

§8Quick reference