Stainless Steel Grades

What Makes Steel 'Stainless'

Stainless steel contains a minimum of 10.5% chromium, which forms a passive oxide layer that protects the steel from corrosion. Different grades add other elements — nickel, molybdenum, titanium — to enhance specific properties like corrosion resistance, strength, or weldability.

The two most common grades — 304 and 316 — are austenitic (non-magnetic, non-hardenable by heat treatment). They account for over 70% of all stainless steel used in industry.

Common Grades Comparison

304 vs 316 — The Key Decision

The most common question in stainless selection is 304 or 316. The difference is molybdenum — 316 contains 2% Mo, 304 does not. Molybdenum dramatically improves resistance to pitting and crevice corrosion, especially in chloride environments.

Rule of thumb: if the environment contains chlorides (seawater, brackish water, coastal atmosphere, chemical processes with chlorides), use 316. For general indoor or non-chloride service, 304 is sufficient and cheaper.

L Grades (Low Carbon)

The 'L' designation (304L, 316L) means low carbon (max 0.03% vs 0.08% for standard). Low carbon prevents carbide precipitation during welding — a phenomenon where chromium carbides form at grain boundaries when the metal passes through the 425–900°C range, depleting the adjacent area of chromium and causing intergranular corrosion.

Always specify L grades for welded applications. The slight cost premium is far cheaper than dealing with weld decay later.

Duplex Stainless Steel

Duplex stainless (e.g., 2205) has a mixed microstructure — half austenite, half ferrite. This gives it roughly twice the yield strength of standard 316, plus superior chloride stress corrosion cracking resistance. It is increasingly used in oil and gas, desalination, and chemical processing.

Super duplex (2507) takes this further with even higher chromium, nickel, and molybdenum for the most aggressive environments.

Advantages of Stainless Steel

• Excellent corrosion resistance: The passive chromium oxide layer self-heals if scratched, providing long-term protection.
• Hygienic surface: Smooth, non-porous surface is ideal for food, pharmaceutical, and sanitary applications.
• High and low temperature strength: Performs well from cryogenic (-200°C) to high temperatures (up to 870°C).
• Low maintenance: Does not need painting or coating — the surface naturally resists corrosion.
• Aesthetic appeal: Bright, clean appearance is valued in architectural and food applications.

Limitations

• Higher cost: 2–4× the cost of carbon steel — justified only where corrosion resistance is needed.
• Galling and seizing: Stainless threads tend to gall and seize — use anti-seize compound and avoid over-tightening.
• Lower thermal conductivity: At 16 W/m·K vs 50 for carbon steel — heat transfer is 3× slower, relevant for heat exchanger tubes.
• Work hardening: Hardens rapidly during machining — requires sharp tools and slower speeds.
• Sensitisation: Standard grades lose corrosion resistance after welding — use L grades.
• Stress corrosion cracking: In hot chloride environments (>60°C), austenitic grades can crack — use duplex.

Chemical Compatibility