The stainless family
Stainless steels contain enough chromium to support a passive surface film; the commonly used classification threshold is 10.5% chromium. The passive film belongs to the alloy surface, unlike zinc or paint applied over another steel. BSSA’s family reference describes the principal groups.
Family, grade example and selection question
The grade names below are examples within families, not cross-standard equivalents or promises of one property value across a row.
| Family | Typical grade examples | Selection question |
|---|---|---|
| Austenitic | 304; 316 | Does the exposure call for the molybdenum-bearing option, and what forming/finish is required? |
| Ferritic | 430 | Does this grade meet the corrosion and joining needs of the actual component? |
| Martensitic | 410 | What hardened-and-tempered condition balances hardness and toughness? |
| Duplex | 2205 | How will fabrication retain the intended ferrite/austenite balance? |
| Precipitation-hardening | 17-4PH | Which solution and aging condition is required for service? |
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Producer data and the selected product standard supply the numerical requirements.
304, 316 and the low-carbon L variants
304 and 316 are austenitic examples; 316 contains molybdenum, which changes resistance to localized corrosion in relevant environments. It is not an assurance against chloride attack. In 304L and 316L, L identifies a low-carbon variant, used to reduce sensitization and associated intergranular-corrosion risk from welding/thermal exposure. It does not mean low strength, a polished finish or immunity to every corrosion mechanism. Read the exact product standard and service condition. BSSA explains the composition and welding context .
For the mechanisms and exposure factors, see why stainless steel can still rust .
Stainless does not mean corrosion-proof. Stainless does not mean always non-magnetic. Ferritic and martensitic families are magnetic; austenitic grades can develop a magnetic response after cold work. A magnet is therefore not a substitute for grade documentation.
For the family comparison and the effects of forming and weld ferrite, see why stainless steel can be magnetic .
Surface finish is a separate choice
A sheet can be supplied with a mill finish, mechanically finished surface or a specified decorative texture. Record visible faces, directional appearance, roughness where functional, and an approved sample where visual matching matters. A bright surface alone gives no alloy identification; bright bar can also be carbon or alloy steel.
Contamination from carbon-steel particles or unsuitable fabrication tools can compromise the intended surface. Cleaning and post-fabrication treatment should fit the stainless grade and the finished component.
| Finish | How it is described | Order detail |
|---|---|---|
| 2B | Cold rolled, heat treated and descaled, with a light finishing pass. | Identify applicable standard, visible face and surface acceptance. |
| BA | Bright annealed in a protective atmosphere; a bright mill finish. | Confirm the specified finish designation and reference sample. |
| No.4 | A mechanically polished/brushed directional finish in the applicable system. | Agree direction, sample and functional roughness; do not infer one universal grit or Ra. |
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Similar finish labels across standards require their own definitions; none replaces the alloy grade.
Corrosion, forming and joining
Temperature, chlorides, deposits, crevices and cleaning conditions distinguish service environments. The familiar 316 designation is not a corrosion-proof label. Compare grade-specific data for the actual exposure rather than choosing only by the word stainless.
Austenitic sheet, ferritic sheet, hardened martensitic stock and duplex plate have different fabrication responses. For duplex welding, heat input and cooling influence phase balance. For a precipitation-hardening part, the aging state belongs in the material description. Annealing likewise has a family-specific objective.
A drawing-ready material identity
For a folded enclosure, identify the exact stainless grade/product standard, sheet thickness, surface finish on the visible face, bend requirements and cleaning after fabrication. For a shaft, include bar condition, machining allowance and final heat treatment. For tubing, specify the tubular product rather than borrowing a sheet designation.
A magnet can help observe magnetic response, but it cannot fully identify stainless steel: families differ and cold work can change the response. Use the material record and appropriate identification testing when identity matters. Adjacent references cover sheet and plate and pipe and tube .