347H Stainless Steel: Properties and Key Applications

347H stainless steel is a high-performance austenitic stainless steel grade, primarily known for its excellent resistance to oxidation and corrosion at elevated temperatures. It is classified under the 300 series of stainless steels, which are characterized by their high chromium and nickel content. The primary alloying elements in 347H include chromium (Cr), nickel (Ni), and niobium (Nb), with a low carbon content that enhances its weldability and resistance to intergranular corrosion.

Comprehensive Overview

347H stainless steel is an austenitic stainless steel that is particularly suited for high-temperature applications. Its composition typically includes around 18% chromium, 11% nickel, and 5% niobium, which contribute to its unique properties. The addition of niobium stabilizes the structure and helps prevent carbide precipitation during welding, making it an ideal choice for applications that require welding in high-temperature environments.

Key Characteristics:
- High Temperature Resistance: 347H can withstand temperatures up to 900°C (1650°F) without significant loss of mechanical properties.
- Corrosion Resistance: It exhibits excellent resistance to a variety of corrosive environments, including sulfuric and phosphoric acids.
- Weldability: The low carbon content and niobium addition enhance its weldability, making it suitable for fabrication processes.

Advantages:
- Excellent oxidation resistance at high temperatures.
- Good mechanical properties at elevated temperatures.
- Enhanced weldability compared to other stainless steels.

Limitations:
- Higher cost compared to standard stainless steels.
- Not suitable for applications involving strong reducing environments.

347H stainless steel has a strong market presence, particularly in industries such as petrochemical, power generation, and aerospace, where high-temperature performance is critical. Its historical significance lies in its development to address the challenges of intergranular corrosion in high-temperature applications.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S34709	USA	Closest equivalent to AISI 347 with higher carbon content.
AISI/SAE	347H	USA	High carbon variant of 347 for improved high-temperature strength.
ASTM	A240	USA	Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip.
EN	1.4961	Europe	Equivalent designation in European standards.
JIS	SUS347H	Japan	Japanese Industrial Standard equivalent.

The differences between 347 and 347H primarily lie in the carbon content, with 347H having a higher carbon percentage, which enhances its strength at elevated temperatures. This distinction is crucial when selecting materials for applications where thermal stability is paramount.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
Cr (Chromium)	17.0 - 19.0
Ni (Nickel)	9.0 - 12.0
Nb (Niobium)	5.0 - 7.0
C (Carbon)	0.04 - 0.10
Mn (Manganese)	2.0 max
Si (Silicon)	1.0 max
P (Phosphorus)	0.045 max
S (Sulfur)	0.030 max

The primary role of chromium is to enhance corrosion resistance, while nickel contributes to the steel's toughness and ductility. Niobium stabilizes the austenitic structure and prevents carbide precipitation, which is critical during welding processes. The low carbon content minimizes the risk of intergranular corrosion, making 347H particularly suitable for 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
Hardness (Rockwell B)	Annealed	85 max	85 max	ASTM E18
Impact Strength	-20°C	40 J	30 ft-lbf	ASTM E23

The combination of high tensile and yield strength, along with good elongation, makes 347H stainless steel suitable for applications that require structural integrity under mechanical loading. Its impact strength at low temperatures ensures performance in cryogenic applications.

Physical Properties

Property	Condition/Temperature	Value (Metric - SI Units)	Value (Imperial Units)
Density	Room Temperature	7.93 g/cm³	0.286 lb/in³
Melting Point	-	1400 - 1450 °C	2552 - 2642 °F
Thermal Conductivity	Room Temperature	16.2 W/m·K	112 BTU·in/(hr·ft²·°F)
Specific Heat Capacity	Room Temperature	500 J/kg·K	0.12 BTU/lb·°F
Electrical Resistivity	Room Temperature	0.72 µΩ·m	0.0000143 Ω·in

The density and melting point of 347H indicate its robustness, while its thermal conductivity and specific heat capacity suggest good thermal management properties in high-temperature applications. These physical properties are essential for industries where thermal stability is critical.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C/°F)	Resistance Rating	Notes
Sulfuric Acid	10%	25°C / 77°F	Good	Risk of pitting at higher temperatures.
Phosphoric Acid	20%	25°C / 77°F	Excellent	Very good resistance.
Chlorides	3%	60°C / 140°F	Fair	Susceptible to pitting corrosion.
Sea Water	-	25°C / 77°F	Good	Generally resistant, but caution advised.

347H stainless steel exhibits excellent resistance to a variety of corrosive agents, particularly in acidic environments. However, it is susceptible to pitting corrosion in chloride-rich environments, which is a critical consideration for marine applications. Compared to 316L stainless steel, which also offers good corrosion resistance, 347H provides superior performance in high-temperature applications but may be less effective in reducing environments.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	900°C	1650°F	-
Max Intermittent Service Temp	1000°C	1832°F	-
Scaling Temperature	1150°C	2102°F	-
Creep Strength Considerations	600°C	1112°F	Begins to lose strength.

347H stainless steel maintains its mechanical properties at elevated temperatures, making it suitable for applications in power plants and chemical processing. Its oxidation resistance allows it to perform well in environments where thermal cycling occurs. However, care must be taken to avoid prolonged exposure to temperatures above 900°C, where creep may become a concern.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER347H	Argon	Excellent results with proper technique.
MIG	ER347H	Argon + 2% O2	Good for thin sections.
Stick	E347	-	Suitable for field applications.

347H stainless steel is highly weldable, and the use of appropriate filler metals ensures strong welds with minimal risk of defects. Preheating may be required for thicker sections to avoid cracking. Post-weld heat treatment can further enhance the properties of the weld.

Machinability

Machining Parameter	347H Stainless Steel	AISI 1212	Notes/Tips
Relative Machinability Index	50	100	Moderate machinability; requires carbide tooling.
Typical Cutting Speed	30 m/min	60 m/min	Adjust speeds based on tooling and operation.

347H stainless steel has moderate machinability, which can be improved with the use of high-speed steel or carbide tools. Proper cutting speeds and feeds are essential to avoid work hardening.

Formability

347H stainless steel can be formed using standard techniques, both hot and cold. However, it exhibits work hardening, which may require additional force during forming operations. The minimum bend radius should be considered to avoid cracking.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Solution Annealing	1050 - 1100 °C / 1922 - 2012 °F	30 min	Air or Water	Dissolve carbides, enhance ductility.
Stress Relief	400 - 600 °C / 752 - 1112 °F	1 - 2 hours	Air	Reduce residual stresses.

Heat treatment processes such as solution annealing enhance the ductility and toughness of 347H stainless steel. The metallurgical transformations during these treatments significantly impact the microstructure, improving its performance in high-temperature applications.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection (Brief)
Petrochemical	Reactor Vessels	High temperature and corrosion resistance	Essential for process integrity.
Power Generation	Boiler Tubes	Oxidation resistance at high temperatures	Ensures long service life.
Aerospace	Exhaust Systems	High strength and thermal stability	Critical for performance.

347H stainless steel is commonly used in applications where high-temperature strength and corrosion resistance are paramount. Its selection is often driven by the need for reliability and durability in harsh environments.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	347H Stainless Steel	316L Stainless Steel	Inconel 625	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High strength	Good ductility	Excellent strength	347H offers better high-temp performance.
Key Corrosion Aspect	Good in acids	Excellent in chlorides	Excellent in high temps	316L better for chloride environments.
Weldability	Excellent	Good	Moderate	347H is easier to weld.
Machinability	Moderate	Good	Poor	347H requires careful machining.
Approx. Relative Cost	Moderate	Lower	Higher	Cost considerations vary by application.
Typical Availability	Moderate	High	Low	347H may be less available than 316L.

When selecting 347H stainless steel, considerations include its cost-effectiveness, availability, and specific application requirements. Its high-temperature performance makes it a preferred choice in industries where reliability is critical. However, the potential for pitting in chloride environments may necessitate careful evaluation against alternatives like 316L or Inconel 625, depending on the specific application and environmental conditions.

In summary, 347H stainless steel is a versatile and robust material that excels in high-temperature and corrosive environments, making it a valuable choice for various industrial applications.