What Makes Steel “Stainless”?
Add at least 10.5% chromium to steel and it forms a passive oxide layer that resists corrosion. That’s stainless steel. The specific grade depends on what else is added and how it’s processed.
Austenitic (300 Series) — Most Common
Non-magnetic (mostly), non-heat-treatable, excellent corrosion resistance. Strengthened only by cold working.
304 Stainless (18/8)
18% chromium, 8% nickel. The “default” stainless steel — roughly 50% of all stainless produced worldwide.
- Tensile strength: 75 ksi (annealed)
- Use for: Kitchen equipment, food processing, architectural, chemical tanks, medical
- Machining: Fair — work hardens. Use sharp tools, positive rake, constant feed. Don’t dwell.
- Welding: Excellent with TIG or MIG. Use 308L filler.
316 Stainless
Adds 2–3% molybdenum for improved corrosion resistance, especially against chlorides (salt water, chemicals).
- Tensile strength: 75 ksi
- Use for: Marine hardware, chemical processing, pharmaceutical, coastal architecture
- Cost: ~20% more than 304
- Rule of thumb: If it sees salt, chemicals, or body fluids → 316. Everything else → 304.
303 Stainless (Free-Machining)
304 with added sulfur for dramatically improved machinability. Machines like butter compared to 304.
- Use for: Screw machine parts, fittings, shafts, bushings
- Warning: Sulfur kills weldability and slightly reduces corrosion resistance. Don’t weld 303.
Martensitic (400 Series) — Hardenable
410 Stainless
Heat-treatable to 40 HRC. Moderate corrosion resistance. Magnetic.
- Use for: Valves, pumps, cutlery, turbine blades
440C Stainless
The hardest common stainless — heat treats to 58-60 HRC. Excellent wear resistance, fair corrosion.
- Use for: Bearings, knife blades, surgical instruments, nozzles
Precipitation Hardening — Best of Both Worlds
17-4 PH (630)
Can be heat-treated to high strength (up to 190 ksi) while maintaining good corrosion resistance. Machine it in Condition A (soft), then age-harden.
- Use for: Aerospace, nuclear, oil & gas, high-strength fasteners, medical implants
- Conditions: H900 (strongest, 190 ksi), H1025, H1075, H1150 (toughest, 135 ksi)
Comparison Table
| Grade | Corrosion | Strength (ksi) | Machinability | Weldability | Magnetic |
|---|---|---|---|---|---|
| 303 | Good | 75 | Excellent | Poor | Slight |
| 304 | Excellent | 75 | Fair | Excellent | No* |
| 316 | Superior | 75 | Fair | Excellent | No* |
| 410 | Moderate | 65–180† | Good | Fair | Yes |
| 440C | Moderate | 285† | Poor (hardened) | Poor | Yes |
| 17-4 PH | Good | 135–190† | Good (Cond A) | Fair | Yes |
*Can become slightly magnetic after cold working. †Depends on heat treatment.
Passivation
All stainless steel should be passivated after machining. The process (nitric or citric acid bath) removes free iron from the surface and restores the protective chromium oxide layer. Required for medical, food, and chemical applications. Spec: ASTM A967 or AMS 2700.
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