Circlip materials and finishes

Carbon spring steel is the default

The great majority of DIN 471, DIN 472 and DIN 6799 rings are produced in carbon spring steel, commonly to C67S or an equivalent grade, hardened and tempered to give the elastic behaviour the part depends on. It holds its free diameter through fitting, carries thrust well, and is inexpensive enough that replacing rather than reusing a ring is never a difficult decision.

Specified for a dry, enclosed, indoor assembly, it is the correct answer and there is nothing to gain from paying more.

Where carbon steel stops being the right answer

Carbon spring steel will corrode. In a sealed gearbox running in oil that is academic; in equipment washed down, exposed to weather, run in humid air or in contact with water, it is not. Corrosion in a groove is worse than corrosion on an open surface, because rust products pack into the groove and prevent the ring from seating fully — the ring loses its shoulder long before it loses meaningful section.

Temperature is the other limit. The spring temper the ring depends on is a heat treatment, and sustained service at elevated temperature will draw it. Where an assembly runs hot continuously, the ring needs specifying against that temperature rather than assumed to be fine.

When to specify stainless

Stainless retaining rings are worth the cost where the assembly is washed down, exposed to the weather, used in food, marine or medical equipment, or where a corroded ring would be expensive to reach. Martensitic grades such as 1.4122 are common where spring temper and moderate corrosion resistance are both needed; austenitic grades such as 1.4310 are used where corrosion resistance matters more.

Stainless rings are not a straight upgrade in every respect — some grades carry lower thrust ratings than the carbon equivalent for the same nominal size. Where a joint is close to its load limit, check the rating for the specific material rather than assuming parity.

Finishes on carbon rings

Plain or phosphated and lightly oiled is the usual supplied condition for carbon rings. Phosphating gives modest corrosion protection in storage and holds oil, which is enough for an assembly that will be closed up and lubricated.

Zinc plating is available and gives better protection in service, with one caveat worth knowing: electroplating hardened spring steel introduces a risk of hydrogen embrittlement, where absorbed hydrogen causes delayed brittle fracture. Reputable platers bake plated spring components to drive the hydrogen out. On a safety-critical joint, confirm that baking has been carried out rather than assuming it.

Magnetism

Carbon spring steel rings are magnetic, as are martensitic stainless grades. Austenitic stainless is essentially non-magnetic in the annealed condition, though the cold work involved in forming a spring ring raises its magnetic response. Where a genuinely non-magnetic part is required — near sensors, in some medical and instrument work — it needs specifying explicitly and confirming on the certificate, not inferred from the word stainless.