Music wire vs stainless steel springs: cost, corrosion and life

The short answer

Music wire is a high carbon spring steel with high strength and a low cost. It is the default for small springs in a dry, indoor environment where the part is protected by a housing or a coating. Stainless steel is chosen when the spring will see moisture, a washdown, a marine atmosphere or a product where rust is not acceptable.

The choice is not usually about load. A stainless spring can be designed to carry the same load as a music wire spring, but it may need a larger wire or a different coil geometry because the allowable stress is lower. The trade is corrosion resistance and a higher material cost against strength and price.

If the environment is dry and the spring is plated or housed, music wire is usually the better commercial choice. If the environment is wet, the plating on music wire becomes a maintenance item, and stainless becomes the simpler answer.

Music wire: strength at low cost

Music wire is cold drawn, high carbon steel with a fine surface and high tensile strength. It is available in small diameters, it takes a good finish and it is widely stocked. For a small spring with a high load in a confined space, it is often the only economical way to get the stress high enough.

The weakness is corrosion. Music wire will rust in a damp environment, and a zinc or nickel plating only delays the start. A plated spring also carries a hydrogen embrittlement risk if the wire is high strength, so the plating route needs a bake and a process control.

Music wire is magnetic. That matters for a sensor, a relay or a motor application where a magnetic spring can disturb a field. Stainless 302 is also magnetic in the cold worked condition, so the substitution is not automatic. A non-magnetic grade or a copper alloy may be needed.

Stainless steel: corrosion and temperature

Stainless spring wire is usually 302, 304 or 316, with 17-7PH for higher strength. The chromium oxide layer gives the corrosion resistance. The layer needs oxygen to repair itself, so a stainless spring in a stagnant, chloride-rich crevice can still corrode, especially at a cut end or a scratch.

Grade 302 is the general purpose choice for springs. It has good strength in the cold worked condition and reasonable corrosion resistance in indoor and mildly damp environments. Grade 316 adds molybdenum for better resistance to chlorides and marine air, at a higher cost and usually a lower attainable strength.

Stainless steel has a lower shear modulus than carbon steel, so the same geometry gives a lower rate. That is one reason a direct substitution changes the load. The rate has to be recalculated with the stainless modulus, and the stress check has to use the stainless allowable stress.

Which stainless grade

For a general enclosure or an indoor appliance, 302 is usually enough. For a coastal site, a washdown or a food environment, 316 is the safer starting point. For a spring that needs high strength and good corrosion resistance, 17-7PH can be age hardened after forming, but the process is more complex and the cost is higher.

Do not assume that 316 is better in every case. It is softer in the annealed condition, it can be more difficult to form, and it may not reach the same spring temper as 302. A 316 spring that is substituted for a 302 spring without a redesign may not meet the load.

The finish also matters. Passivation removes free iron from the surface and helps the oxide layer form. A pickled or passivated stainless spring is more corrosion resistant than a bare one, especially after cutting or grinding. Specify passivation if the environment is aggressive.

Fatigue and life

Music wire has a high tensile strength and good fatigue resistance in a clean, dry environment. Its weakness is that a corrosion pit becomes a stress raiser. A plated music wire spring that starts to corrode at a scratch can fail sooner than an unplated one in a dry test.

Stainless 302 has good fatigue resistance for a general purpose spring, but its allowable stress is lower than music wire. A spring that is redesigned from music wire to stainless may need more coils or a larger wire to keep the stress in the safe range, which changes the space and the rate.

Shot peening and stress relief can improve the fatigue life of either material, but the process window differs. A peened stainless spring still needs passivation if the surface is exposed. The post-forming sequence should be written for the material, not copied from a carbon steel part.

Cost beyond the wire price

The wire price is only the first line. Music wire is cheaper per kilogram, but a plated music wire spring adds a plating step, a bake, a handling leg and a corrosion inspection. A stainless spring avoids the plating but pays more for the wire and may need passivation.

The volume matters. At a high volume, the material cost difference can dominate. At a low volume, the setup and the finishing route can dominate, and a stainless part that avoids a plating subcontract may be simpler to manage even if the wire costs more.

Add the cost of a corrosion failure: a return, a repair, a line stop or a safety recall. A spring that is hidden inside a sealed product is expensive to replace. In that case, the higher material cost of stainless is cheap insurance.

• Wire cost per kilogram and the scrap rate
• Plating, bake and passivation costs
• Handling and lead time for a subcontracted finish
• Corrosion inspection and salt spray testing, if required
• Cost of a field failure and the access needed to replace the part

How to choose

Start with the environment, then the load, then the cost. If the spring is dry and protected, music wire is the default. If it sees moisture, a washdown or a marine atmosphere, move to stainless and recalculate the rate and the stress for the lower modulus and the lower allowable stress.

If the spring is magnetic-sensitive, check the actual grade rather than assuming that stainless is non-magnetic. Cold worked 302 and 316 are magnetic. A non-magnetic requirement may need a different alloy or a different mechanism.

Finally, write the grade and the temper on the drawing, not just the word stainless. The temper decides the strength and the forming behaviour, and the grade decides the corrosion resistance. A supplier cannot choose correctly from a generic note.