Austenitic stainless steels are a class of stainless steels that have a face-centered cubic crystal structure. The austenitic structure is achieved by alloying steel with nickel and/or manganese and nitrogen. Nickel is the main alloying element, typically comprising 8-20% of the steel’s composition. Common austenitic stainless steels include the 300 series such as 304 and 316. 304 is the most common stainless steel, used for things like kitchen sinks and utensils. 316 has added molybdenum for increased corrosion resistance. Austenitic stainless steels are characterized by having excellent corrosion resistance, good formability and weldability, and high strength and ductility when compared to other classes of stainless steel.
What Is Austenitic Stainless Steel Made Of
The balance of these alloying elements stabilizes the austenitic structure, enhances properties like strength and corrosion resistance, and provides the versatility that makes austenitic stainless steels useful for many applications. Austenitic stainless steels are made up of several elements added to iron:
- – Iron (Fe) – Makes up the base metal. Iron content ranges from 50-80%.
- – Chromium (Cr) – Added at 16-26%. Chromium forms a protective oxide layer that gives stainless steel its corrosion resistance.
- – Nickel (Ni) – The main alloying element, typically 8-20% nickel. Nickel stabilizes the austenitic structure at room temperature.
- – Manganese (Mn) – Up to 10% manganese. It enhances corrosion resistance and stabilizes the austenitic structure.
- – Nitrogen (N) – Small amounts (0.1-0.4%) are added to enhance strength and corrosion resistance.
- – Carbon (C) – Kept low, usually below 0.08%, to prevent carbide precipitation which can reduce corrosion resistance.
- – Molybdenum (Mo) – Added in some 300 series steels like 316(2-3% Mo). Further improves corrosion resistance.
- – Copper (Cu) – Added to some austenitic steels in the 1-4% range. Enhances corrosion resistance and strength.
- – Other minor alloying elements like silicon, phosphorus, and sulfur may also be present in small amounts.
Types of Austenitic Stainless Steel Grades
Here are some of the most common austenitic stainless steel grades and their typical properties:
- 304 – One of the most widely used austenitic stainless steel. Also referred to as 18/8 or A2 stainless.
– Composition: 18-20% Cr, 8-10.5% Ni
– Yield Strength: 290 MPa
– Tensile Strength: 515 MPa
– Elongation: 40%
– Excellent corrosion resistance and good formability and weldability. Used for kitchenware, furniture, architecture, etc.
- 316 – Austenitic stainless steel with added molybdenum for increased corrosion resistance. Also known as marine-grade stainless steel.
– Composition: 16-18% Cr, 10-14% Ni, 2-3% Mo
– Yield Strength: 290 MPa
– Tensile Strength: 515 MPa
– Elongation: 40%
– Superior corrosion resistance in harsh environments like marine applications. Used for chemical processing equipment.
- 301 – An austenitic stainless steel that can be hard rolled/cold worked to high mechanical properties.
– Composition: 16-18% Cr, 6-8% Ni, 1.5% Mn
– Yield Strength: 450 MPa
– Tensile Strength: 860 MPa
– Elongation: 40%
– Used for springs, fasteners, and structural applications needing high strength.
- 321- Titanium stabilized austenitic stainless steel resistant to intergranular corrosion at high temps.
– Composition: 17-19% Cr, 9-12% Ni, 5xC min, Ti
– Excellent resistance to corrosion and oxidation. Used for exhaust systems and heat exchangers.
Applications of Austenitic Stainless Steel
Austenitic stainless steels, especially 304 and 316, are used across many industries from small consumer goods to large industrial applications. Their excellent corrosion resistance and good mechanical strength make them a versatile material choice. Brands leverage the material properties to convey qualities like luxury, durability, and safety.
- Kitchenware and Appliances:
– Sinks, pots, pans, utensils – Companies like Moen, LG, and Whirlpool use 304 and 316 stainless.
- Architecture and Construction:
– Cladding, handrails, facades – The Gateway Arch in St. Louis has a 304 stainless steel exterior.
- Chemical and Food Processing:
– Tanks, process equipment – 316L stainless steel is often specified due to its corrosion resistance.
- Marine Applications:
– Boat propellers, offshore rigs – 316 stainless resists saltwater corrosion. Rolls-Royce uses 316 for marine propulsion systems.
- Medical Devices and Implants:
– Surgical tools, implants – Johnson & Johnson uses 316L stainless for orthopedic implants.
- Automotive Parts:
– Exhaust components, trim parts – 304 stainless is used for chrome trim on iconic cars like Chevrolet Corvettes.
- Consumer Goods:
– Watch cases, jewelry, utensils – Rolex uses 904L stainless steel for luxury watch cases. Primanti Brothers’ iconic sandwiches are served on 304 stainless trays.
FAQ
- Is Austenitic Stainless Steel Better Than Stainless Steel?
In applications where corrosion resistance, toughness, and weldability are priorities, and the higher cost is justified, austenitic stainless steels would be considered superior to other stainless steels. But no one stainless steel is “best” for all situations.
2. Can Austenitic Stainless Steel Get Rusted?
Yes, while austenitic stainless steels have excellent corrosion resistance thanks to the chromium and nickel content, they are not fully rust-proof. Certain environments and damage conditions can lead to rust formation on the surface over time. Proper material selection, handling, and maintenance are required.
3. Will a Magnet Stick to Austenitic Stainless Steel?
No, a magnet will not stick to austenitic stainless steel. This is because austenitic stainless steels have a non-magnetic austenitic crystal structure.
4. Is Austenitic Stainless Steel an Alloy or Metal?
Austenitic stainless steel is an alloy, not a pure metal. Austenitic stainless steel gets its ‘stainless’ and non-magnetic properties through intentional alloying. The alloying makes it superior for many uses compared to pure iron metal.
5. What Are Other Types of Stainless Steel Beside Austenitic Stainless Steel?
The main types of stainless steel besides austenitic are:
- 1. Ferritic Stainless Steel:
Ferritic stainless steels contain chromium (10.5-27%) as the main alloying element. They have a ferritic body-centered cubic crystal structure which makes them magnetic. Common grades are 409 and 430. Used for automotive exhaust systems and appliances. More corrosion resistant than martensitic grades.
- 2. Martensitic Stainless Steel:
Martensitic stainless steels are alloyed with chromium (12-18%) and carbon. They can be heat-treated to high strength and hardness. Grades include 420 and 440. Used for knives, surgical tools, and cutlery. Moderate corrosion resistance.
- 3. Duplex Stainless Steel:
Duplex stainless combines a ferritic and austenitic microstructure. Common grades are 2205 and 2507 which contain chromium, nickel, molybdenum, and nitrogen. Excellent strength and corrosion resistance used for marine applications.
- 4. Precipitation Hardening Stainless Steel:
Grades like 17-4PH stainless are alloyed with copper and niobium/titanium/aluminum. They can be precipitation-hardened to high strength similar to heat-treatable alloys. Used for aerospace components.
- 5. Nitrogen Alloyed Stainless Steel:
Nitrogen is added to high-alloy austenitic and duplex stainless steels for increased strength. Grade 316LN uses nitrogen for improved corrosion resistance.