Stainless steel is the most common CNC material, but SUS304, SUS316, and SUS440C differ significantly. The wrong choice means premature corrosion, insufficient strength, or unnecessary cost.
Quick comparison
- SUS304: General-purpose, the price baseline. Good for general machinery, fails in saltwater/strong chlorides.
- SUS316: Adds 2-3% molybdenum, resists saltwater and chlorides. ~1.5× the cost of SUS304. Choice for marine, chemical, semiconductor, medical.
- SUS440C: Martensitic, high-carbon, heat-treatable to HRC 58+. Used for bearings, cutting tools, mold guides. Weaker corrosion resistance — not for prolonged liquid contact.
Decision flow
- Saltwater / chlorides? → SUS316 (use SUS316L for welded parts)
- Need hardness HRC 50+? → SUS440C
- Otherwise → SUS304
Common mistakes
"Stainless steel never rusts" — wrong, every grade rusts in the wrong environment. "SUS316 is always better than SUS304" — only when chlorides matter; otherwise it's just 50% more expensive. "SUS440C is the strongest stainless" — it's a hard-but-rust-prone tool steel, not a structural material.
