Stainless Steel Screws
| Stainless Steel: When to use it,
and what do the numbers mean? |
Stainless Steel was developed about 90 years ago to solve the rusting problems of standard grades of steel. It is best used where the eventual corrosion of plated products would create a real problem – salt-water boats or docks (above water), and redwood siding quickly come to mind.
There are 5 different classes of stainless steel. All of the classes are magnetic with the exception of “Austenitic”. Austenitic includes grades 201, 301, 302, 303, 304 and 316. The austenitic class has a higher content of chromium which increases the corrosion resistance.
Only a few of the many engineered grades of Stainless Steel are suitable for fasteners – those that can be “cold headed” easily, are corrosion resistant, and have adequate torsional and shear strengths. Of these, the 300 series of stainless steels is most popular. These alloys contain about 18% chromium and 8% nickel for good corrosion resistance, but make them non-magnetic and non-heat-treatable. Differences between 304 and 305 series stainless steel are slight, but 316 stainless adds molybdenum, which increases corrosion resistance, strength, and cost.
When a bit of corrosion resistance can be sacrificed for strength and reduced cost, 410 stainless steel is used. It contains about 12% chromium, no nickel, and sufficient carbon to allow heat treatment, making it mildly magnetic. Because it can be heat-treated, it is used extensively for self-drilling fasteners. It has a slightly “coated” surface finish appearance.
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