347 Stainless Steel: Properties and Key Applications

347 stainless steel is an austenitic stainless steel grade known for its excellent corrosion resistance and high-temperature strength. It is primarily alloyed with chromium (Cr) and nickel (Ni), with the addition of niobium (Nb) to stabilize the structure against carbide precipitation during welding and high-temperature service. This stabilization is crucial for maintaining the steel's integrity in environments where it may be exposed to elevated temperatures.

Comprehensive Overview

347 stainless steel is classified as an austenitic stainless steel, which is characterized by its face-centered cubic (FCC) crystal structure. This structure provides excellent ductility and toughness, making it suitable for various applications. The primary alloying elements in 347 stainless steel include:

• Chromium (Cr): Typically 17-19%, which enhances corrosion resistance and contributes to the steel's overall strength.
• Nickel (Ni): Usually 9-12%, which improves ductility and toughness, especially at low temperatures.
• Niobium (Nb): Added to stabilize the steel against carbide precipitation, particularly during welding.

Key Characteristics

347 stainless steel exhibits several significant properties:
- Corrosion Resistance: Excellent resistance to oxidation and corrosion in a variety of environments, including acidic and high-temperature conditions.
- High-Temperature Strength: Retains strength and toughness at elevated temperatures, making it suitable for applications in heat exchangers and furnace components.
- Weldability: Good weldability due to the presence of niobium, which minimizes the risk of carbide precipitation during welding.

Advantages and Limitations

Advantages (Pros)	Limitations (Cons)
Excellent corrosion resistance	Higher cost compared to carbon steels
Good high-temperature strength	Lower strength compared to some alloy steels
Good weldability	Susceptible to stress corrosion cracking in certain environments

347 stainless steel is commonly used in industries such as chemical processing, aerospace, and power generation due to its unique combination of properties. Its historical significance lies in its development as a solution for applications requiring both corrosion resistance and high-temperature performance.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S34700	USA	Closest equivalent to AISI 347
AISI/SAE	347	USA	Commonly used designation
ASTM	A240	USA	Standard specification for stainless steel plates
EN	1.4550	Europe	Equivalent designation in European standards
DIN	X6CrNiNb18-10	Germany	Minor compositional differences to be aware of
JIS	SUS347	Japan	Japanese Industrial Standard equivalent

The differences between equivalent grades can affect selection based on specific application requirements. For example, while both 347 and 321 stainless steels are stabilized grades, 321 contains titanium, which may be preferable in certain environments where titanium's properties are beneficial.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Cr (Chromium)	17.0 - 19.0
Ni (Nickel)	9.0 - 12.0
Nb (Niobium)	0.5 - 1.0
C (Carbon)	≤ 0.08
Mn (Manganese)	≤ 2.0
Si (Silicon)	≤ 1.0
P (Phosphorus)	≤ 0.045
S (Sulfur)	≤ 0.03

The primary role of the key alloying elements in 347 stainless steel includes:
- Chromium: Enhances corrosion resistance and forms a protective oxide layer.
- Nickel: Improves toughness and ductility, especially at low temperatures.
- Niobium: Stabilizes the steel against carbide precipitation, particularly during welding and high-temperature applications.

Mechanical Properties

Property	Condition/Temper	Typical Value/Range (Metric - SI Units)	Typical Value/Range (Imperial Units)	Reference Standard for Test Method
Tensile Strength	Annealed	520 - 750 MPa	75 - 109 ksi	ASTM E8
Yield Strength (0.2% offset)	Annealed	205 - 310 MPa	30 - 45 ksi	ASTM E8
Elongation	Annealed	40%	40%	ASTM E8
Reduction of Area	Annealed	60%	60%	ASTM E8
Hardness (Rockwell B)	Annealed	85 - 95 HRB	85 - 95 HRB	ASTM E18
Impact Strength (Charpy V-notch)	-20°C	40 J	29.5 ft-lbf	ASTM E23

The combination of these mechanical properties makes 347 stainless steel suitable for applications that require high strength and ductility, particularly under mechanical loading conditions. Its ability to maintain strength at elevated temperatures also makes it ideal for structural integrity in high-temperature environments.

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	16.2 W/(m·K)	112 BTU/(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	0.0000013 Ω·in
Coefficient of Thermal Expansion	20 - 100 °C	16.0 x 10⁻⁶ /K	8.9 x 10⁻⁶ /°F
Magnetic Permeability	Room Temperature	Non-magnetic	Non-magnetic

The practical significance of key physical properties includes:
- Density: Affects weight considerations in structural applications.
- Thermal Conductivity: Important for heat exchanger applications where efficient heat transfer is required.
- Coefficient of Thermal Expansion: Critical for applications involving temperature fluctuations, as it affects dimensional stability.

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	20-50 / 68-122	Fair	Susceptible to SCC
Nitric Acid	10-30	20-60 / 68-140	Excellent	Resistant to localized attack
Sea Water	-	Ambient	Good	Suitable for marine applications
Acetic Acid	5-20	20-60 / 68-140	Fair	Risk of localized corrosion

347 stainless steel exhibits excellent resistance to a variety of corrosive environments, particularly in acidic and high-temperature conditions. Its resistance to pitting and stress corrosion cracking (SCC) makes it suitable for applications in chemical processing and marine environments. Compared to other stainless steel grades, such as 316 and 321, 347 offers superior performance in high-temperature applications due to its niobium stabilization.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	925	1700	Suitable for continuous service
Max Intermittent Service Temp	870	1600	Can withstand short-term exposure
Scaling Temperature	1000	1832	Begins to lose strength above this temp
Creep Strength considerations begin	600	1112	Important for long-term applications

347 stainless steel performs well at elevated temperatures, maintaining its strength and oxidation resistance. However, care must be taken to avoid prolonged exposure to temperatures above 925 °C (1700 °F), as this can lead to scaling and loss of mechanical properties. Its oxidation resistance is particularly beneficial in applications such as furnace components and heat exchangers.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER347	Argon	Good for thin sections
MIG	ER347	Argon + 2% O₂	Suitable for thicker sections
SMAW	E347	-	Requires preheat

347 stainless steel is highly weldable, thanks to its niobium content, which minimizes the risk of carbide precipitation during welding. Preheating may be required for thicker sections to avoid cracking. Post-weld heat treatment can enhance the mechanical properties and relieve residual stresses.

Machinability

Machining Parameter	[347 Stainless Steel]	Benchmark Steel (AISI 1212)	Notes/Tips
Relative Machinability Index	40%	100%	Requires slower cutting speeds
Typical Cutting Speed (Turning)	30 m/min	60 m/min	Use carbide tooling

347 stainless steel has moderate machinability. It is recommended to use slower cutting speeds and appropriate tooling to achieve optimal results. The presence of niobium can lead to work hardening, so careful attention to cutting conditions is necessary.

Formability

347 stainless steel can be formed using both cold and hot working processes. Cold forming is feasible but may require additional force due to work hardening. Hot forming is preferred for complex shapes, as it reduces the risk of cracking and improves ductility.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	1010 - 1120 / 1850 - 2050	1-2 hours	Air or Water	Relieve stresses, improve ductility
Solution Treatment	1050 - 1100 / 1920 - 2010	1 hour	Water	Dissolve carbides, enhance corrosion resistance

During heat treatment, 347 stainless steel undergoes metallurgical transformations that enhance its microstructure and properties. Annealing relieves internal stresses and improves ductility, while solution treatment dissolves carbides, enhancing corrosion resistance.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Chemical Processing	Heat exchangers	High-temperature strength, corrosion resistance	Suitable for aggressive environments
Aerospace	Exhaust systems	High-temperature strength, weldability	Required for high-performance applications
Power Generation	Boiler components	Corrosion resistance, high-temperature strength	Essential for durability and safety
Marine	Shipbuilding	Corrosion resistance, weldability	Ideal for marine environments

Other applications include:
- Food processing equipment
- Pharmaceutical manufacturing
- Oil and gas pipelines

347 stainless steel is chosen for these applications due to its unique combination of high-temperature strength and excellent corrosion resistance, ensuring reliability and longevity in demanding environments.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	347 Stainless Steel	316 Stainless Steel	321 Stainless Steel	Brief Pro/Con or Trade-off Note
Key Mechanical Property	Moderate strength	High strength	High strength	316 offers better corrosion resistance in chloride environments
Key Corrosion Aspect	Good in acidic environments	Excellent in chlorides	Good in high-temperature applications	321 is better for high-temperature applications
Weldability	Good	Excellent	Good	316 is preferred for critical welds
Machinability	Moderate	Good	Moderate	316 is easier to machine due to lower work hardening
Formability	Good	Good	Good	All grades are suitable for forming
Approx. Relative Cost	Moderate	Higher	Moderate	316 is generally more expensive
Typical Availability	Common	Very Common	Common	316 is widely available due to its popularity

When selecting 347 stainless steel, considerations include cost-effectiveness, availability, and specific application requirements. Its unique properties make it suitable for a range of applications, particularly where high-temperature performance and corrosion resistance are critical. Additionally, safety considerations in high-stress environments should guide material selection, ensuring that the chosen grade meets all operational demands.

In summary, 347 stainless steel is a versatile and reliable material, particularly valued in industries where performance under extreme conditions is paramount.