- The three main constituents of austenitic stainless steel are:
- Iron: As the base metal, iron makes up the majority of the composition, typically ranging from 60% to 70%.
- Chromium: This is the key element that gives austenitic stainless steel its corrosion resistance. It usually constitutes between 18% and 20% of the composition.
- Nickel: This element contributes to the austenitic structure and enhances properties like ductility, toughness, and strength at low temperatures. The nickel content in austenitic stainless steel typically ranges from 8% to 10%.
While these three elements form the core of austenitic stainless steel, other elements can be added in smaller amounts to further enhance specific properties. These include:
- Carbon: Increases strength and hardness but decreases ductility and toughness.
- Manganese: Improves strength, workability, and hardenability.
- Nitrogen: Provides additional strength and enhances corrosion resistance.
- Molybdenum: Increases high-temperature strength and resistance to certain types of corrosion.
b) Both 304 and 316 are popular grades of austenitic stainless steel, but they do have some key differences:
Molybdenum content:
- 304: Does not contain molybdenum.
- 316: Contains 2-3% molybdenum, which significantly improves its corrosion resistance, especially in chloride-rich environments like saltwater.
Corrosion resistance:
- 304: Offers good corrosion resistance to mild environments, food acids, and freshwater.
- 316: Offers superior corrosion resistance to chlorides, acids, and saltwater, making it ideal for marine applications, chemical processing, and food processing equipment exposed to harsh cleaning solutions.
Applications:
- 304: Widely used in kitchen equipment, architectural trim, automotive parts, and other general applications where good corrosion resistance is needed but saltwater exposure is minimal.
- 316: Widely used in marine hardware, chemical processing equipment, medical implants, high-temperature applications, and food processing equipment requiring extra corrosion resistance.
Cost:
- 304: Less expensive due to the absence of molybdenum.
- 316: More expensive due to the addition of molybdenum.
Other differences:
- Formability: Both are highly formable and weldable, but 304 may be slightly easier to work with.
- Strength: 316 may have slightly higher strength due to the molybdenum content.
Here’s a table summarising the key differences:
| Feature | 304 | 316 |
| Molybdenum content | No | 2-3% |
| Corrosion resistance | Good | Superior, especially to chlorides and salt |
| Applications | Kitchen equipment, architecture, automotive | Marine, chemical processing, medical, food processing with harsh cleaning |
| Cost | Less expensive | More expensive |
| Formability | Highly formable | Highly formable |
| Strength | Good | Slightly higher |
Application 1: Food processing equipment (tanks, pipes, utensils)
- Possible grades: Austenitic stainless steels like AISI 304 (18/8) or 316 (18/10 Mo).
- Key features:
- Corrosion resistance: Resists corrosion from food acids, cleaning chemicals, and saltwater.
- Formability and weldability: Easy to shape and weld for complex equipment designs.
- Hygiene: Smooth, non-porous surface prevents bacteria growth and is easy to clean.
- Austenitic: Non-magnetic and has good low-temperature toughness.
Application 2: Chemical piping and tanks
- Possible grades: Depending on the specific chemicals, AISI 316L (16/10 Mo low carbon), 904L, or super austenitic grades like 254SMO.
- Key features:
- High corrosion resistance: Resists strong acids, alkalis, and oxidising environments.
- Pitting resistance: Resists localised corrosion from chlorides and other aggressive ions.
- Crevice corrosion resistance: Resists corrosion in tight spaces like under gaskets.
- High strength and durability: Handles pressure and temperature extremes.
Application 3: Marine hardware (fasteners, fittings, shafts)
- Possible grades: AISI 316L (16/10 Mo low carbon), 17-4 PH, or duplex stainless steels like 2205.
- Key features:
- Excellent saltwater corrosion resistance: Resists pitting and crevice corrosion from seawater.
- High strength and toughness: Withstands mechanical loads and stresses.
- Good machinability and weldability: Easy to fabricate and join for complex hardware.
- 17-4 PH and duplex grades: Offer higher strength and hardness for demanding applications.
Application 4: Medical implants (surgical instruments, prosthetics)
- Possible grades: AISI 316LVM (16/10 Mo very low carbon) or 316L with electropolishing.
- Key features:
- Biocompatibility: Non-toxic and does not reject human tissue.
- High corrosion resistance: Resists body fluids and sterilisation chemicals.
- High strength and fatigue resistance: Withstands repeated stresses without breaking.
- Smooth surface finish: Minimises risk of infection and tissue irritation.
Application 5: Cryogenic tanks and vessels (storing extremely cold liquids)
- Possible grades: AISI 304L (18/8 low carbon) or 904L.
- Key features:
- Excellent low-temperature toughness: Maintains strength and ductility at very cold temperatures.
- Good weldability: Can be welded reliably for strong and leak-proof tanks.
- Corrosion resistance: Resists contamination from cryogenic liquids and cleaning chemicals.
- 304L: Offers a good balance of properties and affordability.