318 Stainless Steel (Duplex): Properties and Key Applications

318 Stainless Steel, also known as Duplex Stainless Steel, is a unique alloy that combines the beneficial properties of both austenitic and ferritic stainless steels. Classified as a duplex stainless steel, it typically contains a balanced microstructure of approximately 50% austenite and 50% ferrite. The primary alloying elements include chromium (Cr), nickel (Ni), and molybdenum (Mo), which significantly influence its corrosion resistance, mechanical strength, and overall performance.

Comprehensive Overview

318 Stainless Steel is particularly known for its high strength and excellent resistance to corrosion, making it suitable for various demanding applications. The alloy typically contains around 24% chromium, 6% nickel, and 3% molybdenum, which contribute to its robust performance in harsh environments. The dual-phase microstructure provides enhanced toughness and ductility compared to traditional austenitic stainless steels.

Advantages:
- Corrosion Resistance: Exceptional resistance to pitting and crevice corrosion, especially in chloride environments.
- Mechanical Strength: Higher yield strength compared to austenitic grades, allowing for thinner sections in structural applications.
- Weldability: Good weldability with appropriate filler materials, making it suitable for fabrication.

Limitations:
- Cost: Generally more expensive than standard austenitic stainless steels due to alloying elements.
- Brittleness at Low Temperatures: Potential for reduced toughness in cryogenic applications.
- Sensitivity to Sigma Phase Formation: At elevated temperatures, prolonged exposure can lead to the formation of sigma phase, which can embrittle the alloy.

Historically, duplex stainless steels like 318 were developed to overcome the limitations of austenitic and ferritic grades, particularly in terms of strength and corrosion resistance. Today, they occupy a significant position in the market, especially in industries such as oil and gas, chemical processing, and marine applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S31803	USA	Closest equivalent to 318L
AISI/SAE	318	USA	Minor compositional differences to be aware of
ASTM	A240/A240M	USA	Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and general applications
EN	1.4462	Europe	European equivalent with similar properties
JIS	SUS318	Japan	Japanese standard designation

The differences between equivalent grades can be subtle but significant. For instance, while S31803 and 1.4462 have similar compositions, their mechanical properties and corrosion resistance can vary slightly due to differences in manufacturing processes and heat treatment.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Chromium (Cr)	24.0 - 26.0
Nickel (Ni)	4.5 - 6.5
Molybdenum (Mo)	2.5 - 3.5
Manganese (Mn)	0.5 - 1.5
Nitrogen (N)	0.08 - 0.20
Carbon (C)	≤ 0.03
Phosphorus (P)	≤ 0.03
Sulfur (S)	≤ 0.02

The primary role of chromium is to enhance corrosion resistance, while nickel contributes to toughness and ductility. Molybdenum further improves resistance to pitting and crevice corrosion, particularly in chloride environments. Nitrogen is added to increase strength and improve resistance to stress corrosion cracking.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	620 - 850 MPa	90 - 123 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	450 - 650 MPa	65 - 94 ksi	ASTM E8
Elongation	Annealed	25 - 40%	25 - 40%	ASTM E8
Hardness (Rockwell B)	Annealed	85 - 95 HRB	85 - 95 HRB	ASTM E18
Impact Strength (Charpy)	-20°C	40 J	29.5 ft-lbf	ASTM E23

The combination of high tensile and yield strength makes 318 Stainless Steel suitable for applications requiring structural integrity under mechanical loading. Its elongation and impact strength indicate good ductility and toughness, essential for dynamic loading conditions.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	-	7.8 g/cm³	0.283 lb/in³
Melting Point	-	1400 - 1450 °C	2552 - 2642 °F
Thermal Conductivity	20 °C	15 W/m·K	86 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	20 °C	500 J/kg·K	0.12 BTU/lb·°F
Electrical Resistivity	20 °C	0.72 µΩ·m	0.00000072 Ω·m
Coefficient of Thermal Expansion	20 - 100 °C	16.5 x 10⁻⁶ /K	9.2 x 10⁻⁶ /°F

The density of 318 Stainless Steel indicates a robust material, while its thermal conductivity and specific heat capacity suggest it can handle thermal stresses effectively. The coefficient of thermal expansion is crucial for applications involving temperature fluctuations, ensuring dimensional stability.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3-10	20-60 °C / 68-140 °F	Excellent	Risk of pitting in high concentrations
Sulfuric Acid	10-30	20-40 °C / 68-104 °F	Good	Limited resistance at high temperatures
Hydrochloric Acid	1-5	20-25 °C / 68-77 °F	Fair	Not recommended for high concentrations
Sea Water	-	Ambient	Excellent	Highly resistant to seawater corrosion

318 Stainless Steel exhibits excellent resistance to a variety of corrosive agents, particularly in marine environments and chemical processing applications. Its performance against chlorides is noteworthy, making it suitable for offshore and coastal applications. However, caution is advised in environments with high concentrations of sulfuric and hydrochloric acids, where alternative materials may be more appropriate.

When compared to other stainless steels, such as 316L and 2205, 318 offers superior resistance to pitting and stress corrosion cracking, particularly in chloride-rich environments. However, 316L may perform better in highly acidic conditions, while 2205 provides enhanced strength.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	300 °C	572 °F	Suitable for continuous use at this temperature
Max Intermittent Service Temp	350 °C	662 °F	Short-term exposure can be tolerated
Scaling Temperature	600 °C	1112 °F	Risk of oxidation beyond this temperature
Creep Strength considerations	500 °C	932 °F	Creep resistance begins to diminish at this temperature

At elevated temperatures, 318 Stainless Steel maintains good mechanical properties, although prolonged exposure can lead to oxidation and the formation of sigma phase, which can embrittle the alloy. It is essential to consider the service environment and temperature fluctuations when selecting this material for high-temperature applications.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER318L	Argon	Good results with proper technique
MIG	ER318L	Argon/CO2 mix	Suitable for thicker sections
SMAW	E318-16	-	Requires preheat for thicker sections

318 Stainless Steel is generally considered to have good weldability, particularly with the appropriate filler metals. Preheating may be necessary for thicker sections to avoid cracking. Post-weld heat treatment can enhance the mechanical properties and relieve residual stresses.

Machinability

Machining Parameter	318 Stainless Steel	AISI 1212	Notes/Tips
Relative Machinability Index	40%	100%	Requires slower cutting speeds
Typical Cutting Speed	20-30 m/min	60-80 m/min	Use carbide tools for best results

Machining 318 Stainless Steel can be challenging due to its strength and toughness. It is advisable to use high-speed steel or carbide tools and to maintain lower cutting speeds to achieve optimal results.

Formability

318 Stainless Steel exhibits good formability, allowing for cold and hot forming processes. However, due to its strength, it may require higher forces compared to austenitic grades. The material can be bent and shaped with appropriate tooling, but care must be taken to avoid work hardening.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Solution Annealing	1020 - 1100 °C / 1868 - 2012 °F	30 minutes	Air or water	Dissolve carbides and improve ductility
Stress Relieving	300 - 600 °C / 572 - 1112 °F	1 hour	Air	Reduce residual stresses

Heat treatment processes such as solution annealing are crucial for optimizing the microstructure and properties of 318 Stainless Steel. This treatment helps dissolve carbides and enhances ductility, making the material more suitable for fabrication.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Oil and Gas	Offshore platforms	High strength, corrosion resistance	Essential for harsh marine environments
Chemical Processing	Storage tanks	Excellent resistance to aggressive chemicals	Ensures longevity and safety
Marine	Shipbuilding	Resistance to seawater corrosion	Critical for structural integrity
Power Generation	Heat exchangers	Good thermal conductivity and corrosion resistance	Efficient heat transfer in harsh conditions

Other applications include:
- Pulp and Paper Industry: Used in bleaching and chemical recovery processes.
- Food Processing: Equipment requiring high hygiene standards and corrosion resistance.
- Pharmaceuticals: Equipment and piping systems where cleanliness and corrosion resistance are paramount.

The selection of 318 Stainless Steel for these applications is primarily due to its superior mechanical properties and corrosion resistance, which are essential for maintaining performance and safety in challenging environments.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	318 Stainless Steel	316L Stainless Steel	2205 Duplex Stainless Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Good ductility	Higher strength	318 offers a balance of strength and ductility
Key Corrosion Aspect	Excellent in chlorides	Good in acids	Excellent in chlorides	318 is superior in chloride environments
Weldability	Good	Excellent	Good	316L is easier to weld
Machinability	Moderate	Good	Moderate	316L is easier to machine
Formability	Good	Excellent	Moderate	316L offers better formability
Approx. Relative Cost	Higher	Moderate	Higher	Cost varies with market demand
Typical Availability	Moderate	High	Moderate	316L is more commonly stocked

When selecting 318 Stainless Steel, considerations such as cost-effectiveness, availability, and specific application requirements are crucial. While it may be more expensive than standard austenitic grades, its superior performance in corrosive environments often justifies the investment. Additionally, its unique properties make it suitable for niche applications where other materials may fail.

In summary, 318 Stainless Steel is a versatile and robust material that excels in various demanding applications, particularly in environments where corrosion resistance and mechanical strength are paramount. Its unique properties and advantages make it a preferred choice in many industries, ensuring safety and longevity in critical applications.