Technical Library
Most stainless fastener problems come from one of three things: picking a free-machining grade for a corrosive environment, assuming stainless is stronger than it is, or galling a thread on assembly. All three are avoidable at the specification stage.
| Grade | ISO class | Corrosion resistance | Machinability | Typical use |
|---|---|---|---|---|
| 303 | - | Moderate. Lowest of the austenitics. | Excellent | Turned and machined precision parts where sulphur-aided machining matters more than maximum corrosion resistance. |
| 304 / 18-8 | A2 | Good. General atmospheric and indoor. | Fair | The default general-purpose stainless fastener. |
| 316 | A4 | Very good. Chlorides, marine, de-icing salt. | Fair | Coastal, marine, chemical, road-salt exposure. |
| 410 / 416 | C1 / C4 | Limited. Needs a coating outdoors. | Good | Martensitic, hardenable, magnetic. Self-drilling screw points and higher-hardness parts. |
303 is a machining grade, not a marine grade. The sulphur added to make it machine cleanly forms manganese sulphide inclusions, and those inclusions are where pitting corrosion starts. It is an excellent choice for precision turned parts in a benign environment and a poor one anywhere near chlorides. If a part is going outdoors near salt, specify 316 / A4.
A2-70 yields at about 450 MPa against 640 MPa for a plain 8.8 steel bolt. Substituting stainless into a joint designed at 8.8 removes roughly a third of the clamp capacity. Re-check the joint, or specify A4-80 and confirm it still closes the gap.
Austenitic stainless has a tenacious oxide film and poor natural lubricity. Under the pressure and heat of tightening the threads can cold-weld, seizing the fastener solid before it is home. Mitigate with an anti-seize or wax, slower driving speed, rolled rather than cut threads, and by pairing dissimilar grades (for example an A4 bolt with an A2 nut) rather than identical ones.
Only approximately, and only when annealed. Cold forming a 304 screw induces martensite and the finished part will often respond to a magnet. That is normal and is not evidence of the wrong grade being supplied. If a genuinely non-magnetic part is required it needs to be specified and tested as such.
Passivation is not a coating. It is an acid treatment that dissolves free iron left on the surface by forming, machining and handling, so the chromium oxide layer can re-form uniformly. Without it, embedded iron particles rust and the part appears to be failing when the base material is fine.
It is worth specifying on machined stainless parts, and it is not a substitute for choosing the right grade in the first place. Ask for it explicitly - it is not always included by default.
How to use these tables. The values here are standard reference figures published to help you specify and compare parts. They are guidance, not a substitute for your own engineering standard, and several of them (torque above all) depend on assumptions about friction, lubrication and joint material that will differ in your application. Where a number matters, confirm it against the governing specification and against the part drawing. We will supply the plant's actual test data with any quotation.