Stainless Steel Fasteners: A2 and A4 Chemical Composition and How to Stop Galling

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓A2 covers alloys 302, 304, 304L, 321 and 347; A4 covers 316 and 316L.
• ✓Both have 0.08 max carbon, 1.0 max silicon and 2.0 max manganese. A2 has 17.0 to 20.0 chromium and 8.0 to 13.0 nickel; A4 has 16.0 to 18.5 chromium, 10.0 to 14.0 nickel and 2.0 to 3.0 molybdenum.
• ✓Galling is cold welding between the male and female threads. Stainless, aluminium and titanium fasteners are most prone, because tightening breaks their protective oxide film.
• ✓In the extreme the threads seize. Keep tightening and the fastener can twist off or strip its threads.
• ✓To stop galling: slow down the installation rpm, and lubricate the threads with a lubricant containing a substantial amount of molybdenum disulfide. Use silicone grease where stainless bolts go into aluminium.
• ✓Use different stainless grades for the bolt and nut, as they work harden at different rates: 400 series nuts work well on 316 bolts, and 304 with 316 has also shown useful results.
• ✓Rough threads gall more. The internal (cut) thread is usually the rougher one; most stainless bolts have rolled threads.

Who it helps: Anyone fitting stainless bolts and nuts who wants to avoid seized or stripped threads.