Hobson technical article
What Is Stainless Steel? A2, A4 and BUMAX Fastener Grades Explained
A one-page technical extract from BUMAX (Bulten Stainless), distributed by Hobson, explaining what stainless steel is, the difference between stainless and acid-resistant steel, and the common steel types used for fasteners.
What this covers
- What stainless steel is
- Stainless or acid-resistant
- Common types of steel
- A2, A4 and BUMAX
Applies to Threaded Rod (Allthread), Hex Bolts & Set Screws, Machine Screw, Hex Flange Nut, Hex Nut, All-Metal Prevailing Torque Nut, Socket Head Cap Screw, Socket Countersunk Screw, Flat Washer and Mudguard / Penny Washer.
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- Stainless steel is steel with at least 12% chromium, often with nickel and molybdenum added.
- Four main types by structure: ferritic, martensitic (12 to 18% chromium, can be hardened), austenitic (16.5 to 26% chromium, 7 to 25% nickel) and ferritic-austenitic.
- In everyday use "stainless" means 18/8 austenitic steel without molybdenum (A2, 304), and "acid-resistant" means the molybdenum grades such as A4 (316). The acid-resistant types generally resist corrosion better.
- A2 (304) suits moderate exposure: air away from the coast, fresh water, and many alkali and salt solutions. It should not be used in non-oxidising acids such as hydrochloric acid, or in brackish water or seawater.
- A4 (316) is the most common steel for A4 fasteners, intended for normal exposure in a maritime climate.
- BUMAX 88 and BUMAX 109 use 316L high-molybdenum steel with better corrosion resistance than normal A4, for somewhat more aggressive environments such as water containing chlorides and non-oxidising acids.
Who it helps: Anyone choosing between 304, 316 and BUMAX fasteners.
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