Stainless Steel (Austenitic) (Fe+Cr+Ni)
Austenitic stainless steels are the most widely used stainless steel family, accounting for over 70% of all stainless steel production. Characterized by their face-centered cubic (FCC) crystal structure, they offer excellent corrosion resistance, formability, and cryogenic toughness. The 300-series (304, 316) are the industry workhorses.
Properties
- Density
- 7,930–8,000 kg/m³
- Melting Point
- 1,400–1,450 °C
- Boiling Point
- 2,750 °C
- Thermal Conductivity
- 15–16 W/(m·K)
- Electrical Resistivity
- 7.2 × 10⁻⁷ Ω·m
- Tensile Strength
- 515–690 MPa
- Yield Strength
- 205–310 MPa
- Hardness
- 150–220 HB
- Elongation
- 40–60%
- Crystal Structure
- FCC (Austenite)
Common Grades
- 304 (1.4301): The most common grade — 18% Cr, 8% Ni. General purpose, food-safe, excellent formability.
- 316 (1.4401): Molybdenum-bearing grade with superior corrosion resistance. Marine and chemical industry standard.
- 316L (1.4404): Low-carbon version of 316, preventing sensitization during welding. Preferred for welded structures.
- 321 (1.4541): Titanium-stabilized grade for high-temperature service (up to 900°C).
- 304L (1.4307): Low-carbon 304 for welded applications where intergranular corrosion is a concern.
- 310S (1.4845): High-chromium (25% Cr, 20% Ni) heat-resistant grade for furnace parts, kiln linings. Service up to 1,050°C.
- 303 (1.4305): Free-machining austenitic with added sulfur. Screw-machine parts, fittings, shafts.
Applications
- Food & Beverage: Processing tanks, piping, conveyor systems, dairy equipment, breweries
- Chemical Processing: Reactors, heat exchangers, storage tanks, piping systems
- Architecture: Facades, handrails, elevator panels, kitchen equipment, signage
- Medical: Surgical instruments, implants, hospital equipment, pharmaceutical vessels
- Marine: Boat fittings, desalination plants, offshore platforms (316 grade)
FAQ
What is the difference between 304 and 316 stainless steel?
The primary difference is molybdenum content: 316 contains 2–3% Mo, giving it significantly better resistance to chlorides and pitting corrosion. 304 is sufficient for indoor and mild outdoor use, while 316 is required for marine, chemical, and salt-exposed environments.
Why does stainless steel sometimes rust?
Stainless steel can develop surface rust through contamination with carbon steel particles, exposure to chlorides beyond its grade's capability, damage to the passive chromium oxide layer, or use of inappropriate grades. Proper grade selection and passivation prevent these issues.
Is austenitic stainless steel magnetic?
Austenitic stainless steels (304, 316) are generally non-magnetic in the annealed condition. However, cold working can induce partial martensite transformation, creating slight magnetism. This is normal and does not affect corrosion resistance.
What is sensitization in stainless steel?
Sensitization occurs when austenitic stainless steel is held at 450–850°C (e.g., during welding), causing chromium carbides to precipitate at grain boundaries. This depletes chromium from adjacent areas, creating zones vulnerable to intergranular corrosion. Prevention: use low-carbon (L) grades like 316L or stabilized grades like 321.
Can stainless steel be used at cryogenic temperatures?
Yes — austenitic grades maintain excellent toughness down to -269°C (liquid helium). Unlike carbon steel and ferritic grades, austenitic stainless steel does not undergo ductile-to-brittle transition. Grade 304L is commonly used for LNG tanks, cryogenic vessels, and laboratory dewars.