308 Stainless Steel: Properties and Key Applications

308 Stainless Steel is classified as an austenitic stainless steel, known for its excellent corrosion resistance and high-temperature strength. This grade is primarily alloyed with chromium (20-22%) and nickel (10-12%), with the addition of molybdenum (up to 2%) to enhance its resistance to pitting and crevice corrosion. The presence of these elements contributes to its non-magnetic properties and good weldability, making it suitable for various applications in harsh environments.

Comprehensive Overview

308 Stainless Steel is particularly valued in the welding industry due to its ability to maintain strength and corrosion resistance at elevated temperatures. It is often used as a filler metal for welding other stainless steels, especially in applications where high strength and resistance to oxidation are required. The steel's high chromium and nickel content provides a robust protective layer against corrosion, while its austenitic structure ensures excellent toughness and ductility.

Advantages (Pros):
- Corrosion Resistance: Exceptional resistance to a wide range of corrosive environments, including acidic and alkaline conditions.
- Weldability: Excellent for welding applications, particularly as a filler metal for other stainless steels.
- High-Temperature Strength: Retains strength and stability at elevated temperatures, making it suitable for high-heat applications.

Limitations (Cons):
- Cost: Generally more expensive than lower-grade steels due to alloying elements.
- Work Hardening: Can become hard and brittle if not properly handled during machining or forming processes.
- Susceptibility to Stress Corrosion Cracking: In certain environments, particularly chloride-rich conditions, it can be prone to stress corrosion cracking.

Historically, 308 Stainless Steel has played a significant role in the development of welding technologies, particularly in the fabrication of structures requiring high strength and durability. Its market position remains strong, especially in industries such as construction, automotive, and aerospace.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S30800	USA	Closest equivalent to AISI 308
AISI/SAE	308	USA	Commonly used filler metal
ASTM	A240	USA	Standard specification for stainless steel plates
EN	1.4301	Europe	Equivalent to AISI 304, with higher nickel content
JIS	SUS308	Japan	Similar properties to AISI 308
ISO	308	International	Standard designation for austenitic stainless steel

The differences between equivalent grades can significantly affect performance in specific applications. For instance, while 1.4301 (AISI 304) has a lower nickel content, it may not perform as well in high-temperature applications compared to 308.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Cr (Chromium)	20.0 - 22.0
Ni (Nickel)	10.0 - 12.0
Mo (Molybdenum)	0.0 - 2.0
C (Carbon)	≤ 0.08
Mn (Manganese)	≤ 2.0
Si (Silicon)	≤ 1.0
P (Phosphorus)	≤ 0.045
S (Sulfur)	≤ 0.03

The primary role of chromium is to enhance corrosion resistance, while nickel contributes to the steel's toughness and ductility. Molybdenum improves resistance to pitting and crevice corrosion, particularly in chloride environments. Carbon, although present in low amounts, can affect the steel's hardness and strength.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	515 - 690 MPa	75 - 100 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	205 - 310 MPa	30 - 45 ksi	ASTM E8
Elongation	Annealed	40 - 50%	40 - 50%	ASTM E8
Reduction of Area	Annealed	60 - 70%	60 - 70%	ASTM E8
Hardness (Rockwell B)	Annealed	70 - 90 HRB	70 - 90 HRB	ASTM E18
Impact Strength (Charpy)	-40°C	40 J	30 ft-lbf	ASTM E23

The combination of these mechanical properties makes 308 Stainless Steel suitable for applications requiring high strength and ductility, particularly in structural and welding applications. Its ability to withstand significant deformation without fracturing is crucial in dynamic loading conditions.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temperature	8.0 g/cm³	0.289 lb/in³
Melting Point/Range	-	1400 - 1450 °C	2552 - 2642 °F
Thermal Conductivity	Room Temperature	16 W/m·K	92 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	Room Temperature	500 J/kg·K	0.119 BTU/lb·°F
Electrical Resistivity	Room Temperature	0.72 µΩ·m	0.0000143 Ω·in
Coefficient of Thermal Expansion	Room Temperature	16.0 x 10⁻⁶ /K	8.9 x 10⁻⁶ /°F

The density of 308 Stainless Steel makes it suitable for applications where weight is a consideration, while its thermal conductivity and specific heat capacity are critical in high-temperature applications, such as in heat exchangers and furnace components.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Chlorides	3-10	20-60 / 68-140	Good	Risk of pitting
Sulfuric Acid	10-30	20-60 / 68-140	Fair	Susceptible to stress corrosion
Acetic Acid	5-20	20-60 / 68-140	Excellent	Good resistance
Sea Water	-	20-60 / 68-140	Good	Risk of crevice corrosion
Ammonia	-	20-60 / 68-140	Excellent	Resistant to stress corrosion

308 Stainless Steel exhibits excellent resistance to a variety of corrosive environments, particularly in acidic and alkaline conditions. However, it is susceptible to stress corrosion cracking in chloride-rich environments, which can be a critical consideration in marine applications. Compared to 304 Stainless Steel, 308 offers improved resistance to pitting and crevice corrosion, making it a better choice for applications in harsh environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	870	1600	Suitable for high-temperature applications
Max Intermittent Service Temp	925	1700	Can withstand short-term exposure
Scaling Temperature	900	1650	Risk of oxidation beyond this temperature
Creep Strength considerations	600	1112	Begins to lose strength at elevated temps

At elevated temperatures, 308 Stainless Steel maintains its mechanical properties and exhibits good oxidation resistance. However, prolonged exposure to temperatures above 900 °C (1650 °F) can lead to scaling and degradation of the protective oxide layer, necessitating careful consideration in high-temperature applications.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER308L	Argon	Low carbon content for reduced carbide precipitation
MIG	ER308L	Argon/CO2	Good for thicker sections
SMAW	E308L	-	Suitable for all positions

308 Stainless Steel is highly weldable, making it a preferred choice for welding applications. Preheating is generally not required, but post-weld heat treatment may be beneficial to relieve stresses and improve corrosion resistance. Common defects include porosity and cracking, which can be mitigated through proper welding techniques.

Machinability

Machining Parameter	[308 Stainless Steel]	AISI 1212	Notes/Tips
Relative Machinability Index	30%	100%	Requires sharp tools and slower speeds
Typical Cutting Speed	30 m/min	60 m/min	Use of coolant recommended

308 Stainless Steel has lower machinability compared to carbon steels, requiring careful selection of tooling and cutting parameters. High-speed steel or carbide tools are recommended, and the use of cutting fluids can enhance tool life and surface finish.

Formability

308 Stainless Steel exhibits good formability, allowing for cold and hot forming processes. However, it is important to note that excessive work hardening can occur, necessitating careful control of bending radii and forming techniques to avoid cracking.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	1010 - 1120 / 1850 - 2050	30 minutes	Air	Relieve stresses, improve ductility
Solution Treatment	1000 - 1100 / 1830 - 2010	30 minutes	Water	Enhance corrosion resistance

Heat treatment processes such as annealing and solution treatment are crucial for optimizing the microstructure and properties of 308 Stainless Steel. These treatments help to relieve internal stresses and enhance the material's corrosion resistance, making it suitable for demanding applications.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Construction	Structural components	High strength, corrosion resistance	Durability in harsh environments
Automotive	Exhaust systems	High-temperature strength, weldability	Performance under thermal stress
Aerospace	Engine components	High strength, oxidation resistance	Safety and reliability at high altitudes
Food Processing	Equipment and piping	Corrosion resistance, ease of cleaning	Hygiene and durability

308 Stainless Steel is widely used in various industries due to its excellent mechanical properties and corrosion resistance. In the construction sector, it is favored for structural components exposed to harsh environments. In the automotive and aerospace industries, its high-temperature strength and weldability make it ideal for critical applications.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	308 Stainless Steel	304 Stainless Steel	316 Stainless Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Moderate strength	High strength	308 offers better high-temp performance than 304
Key Corrosion Aspect	Good in acidic environments	Good in general environments	Excellent in chloride environments	316 is superior for marine applications
Weldability	Excellent	Excellent	Good	308 is preferred for welding applications
Machinability	Moderate	Good	Moderate	304 is easier to machine than 308
Formability	Good	Excellent	Good	304 has better formability than 308
Approx. Relative Cost	Moderate	Low	High	308 is more cost-effective than 316
Typical Availability	Common	Very Common	Common	304 is the most widely available

When selecting 308 Stainless Steel, considerations include its cost-effectiveness, availability, and specific application requirements. While it offers excellent performance in high-temperature and corrosive environments, it may not be the best choice for every application, particularly where chloride exposure is a concern. The choice between 308, 304, and 316 grades will depend on the specific environmental conditions and mechanical requirements of the application.

In summary, 308 Stainless Steel is a versatile material with a strong balance of properties that make it suitable for a wide range of applications, particularly in welding and high-temperature environments. Its unique combination of corrosion resistance, strength, and weldability ensures its continued relevance in modern engineering and manufacturing.