A2 Stainless Steel (ISO 3506 group A2 (304 / 1.4301), commonly supplied A2-70) is the most common stainless group for indoor and general outdoor fasteners. When cup finishing washers are specified in this grade, the dimensions remain per Trade series (cup finishing washer); the material determines corrosion behavior (good general corrosion resistance) and the service temperature range (approx. -200 °C to +400 °C, subject to the property class and the joint). Both are covered below with the size table.
Why Choose A2 Stainless Steel for Cup Finishing Washers
The most common stainless group for indoor and general outdoor fasteners. For cup finishing washers, this matters when good general corrosion resistance is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 3506 group A2 (304 / 1.4301), commonly supplied A2-70 |
|---|---|
| Magnetic response | Largely non-magnetic |
| Service temperature | approx. -200 °C to +400 °C, subject to the property class and the joint |
| Corrosion characteristic | Good general corrosion resistance |
Galvanic & Galling Considerations
For stainless cup finishing washers the compatibility question is galvanic rather than galling: joined to aluminum or galvanized steel in wet service, stainless drives corrosion of the less-noble metal. Isolate with a coating, sleeve or non-conductive washer where the joint sees moisture. Note also that stainless is softer than hardened carbon steel, so where the part carries a wear or shear duty, check the load against the grade before substituting.
Comparable Grades
For Cup Finishing Washers, the alternative stainless grade is A4 (316). A2 (304) covers general indoor and outdoor service. A4 (316) adds molybdenum, which is what buys the chloride resistance for marine and de-icing-salt exposure. Where strength matters more than corrosion, plated alloy steel in class 8.8 or 10.9 is the swap to make.
Using Stainless Fasteners in Aluminum
Stainless steel is more noble than aluminum, so with an electrolyte present — rain, condensation, salt spray — the aluminum becomes the anode and corrodes at the joint. Area ratio decides how bad it gets: a small stainless fastener in a large aluminum section is normally acceptable, a large stainless fastener in thin aluminum is not. Where the joint gets wet, isolate the metals with a coated or nylon washer and a sealant, or specify an aluminum or coated fastener instead.
Sizes of Cup Finishing Washers Available in A2 Stainless Steel
| For screw size | Inner diameter d1 (mm) | Outer diameter d2 (mm) | Overall height (mm) | Sheet thickness (mm) | Material | Finish | Typical use |
|---|---|---|---|---|---|---|---|
| No.6 / M3.5 | 4.0 | 11.0 | 3.0 | 0.6 | Stainless A2 | Polished | Trim, panelling |
| No.8 / M4 | 4.6 | 12.7 | 3.5 | 0.6 | Stainless A2 | Polished | Trim, panelling |
| No.10 / M5 | 5.4 | 14.3 | 4.0 | 0.7 | Stainless A2 | Polished | Cabinetry |
| M6 | 6.5 | 17.5 | 4.5 | 0.8 | Steel | Nickel | Cabinetry |
| M8 | 8.5 | 22.0 | 5.0 | 0.9 | Steel | Nickel | Furniture |
