Cup Finishing Washers manufactured in A4 Stainless Steel (ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80) are dimensioned to Trade series (cup finishing washer). The sections below give the mechanical and physical properties of the grade, its behavior in corrosive and elevated-temperature service, and the full size table.
Why Choose A4 Stainless Steel for Cup Finishing Washers
Specified for marine, coastal and chemical applications. For cup finishing washers, this matters when superior, including marine and chloride environments is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80 |
|---|---|
| Magnetic response | Largely non-magnetic |
| Service temperature | approx. -200 °C to +400 °C, subject to the property class and the joint |
| Corrosion characteristic | Superior, including marine and chloride environments |
Corrosion Behavior in Service
A4 (316) adds 2–3% molybdenum to the A2 (304) composition, which dramatically improves resistance to chloride-pitting corrosion. For Cup Finishing Washers in marine, coastal, swimming-pool or food-processing environments, A4 is the default selection.
Comparable Grades
For Cup Finishing Washers, the alternative stainless grade is A2 (304). 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
The galvanic couple here runs the wrong way: aluminum is the less noble metal, so it corrodes and the fastener does not. What governs the rate is the ratio of areas — a large anode and a small cathode is a tolerable combination, the reverse concentrates the attack into thin material around the hole. An isolating washer plus sealant breaks the circuit; where that is impractical, match the fastener to the substrate.
Sizes of Cup Finishing Washers Available in A4 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 |
