SS 347/ 347H

SS 347 and SS 347H are both stainless steel grades that belong to the austenitic stainless steel family, primarily designed for high-temperature applications where resistance to oxidation and corrosion is critical. These alloys are particularly favored for their stability at high temperatures, due to the presence of niobium, which helps to avoid sensitization during welding and exposure to extreme environments.

1. SS 347

SS 347 is a stabilized version of stainless steel 304, alloyed with niobium (or tantalum), which provides enhanced resistance to intergranular corrosion. The alloy’s primary benefit is its ability to resist carbide precipitation (sensitization), which occurs during welding or high-temperature exposure, making it suitable for high-temperature services.

Chemical Composition of SS 347:

Typical composition of SS 347 includes the following elements:

        Chromium (Cr) 17.0–19.0%
        Nickel (Ni) 9.0–13.0%
        Niobium (Nb) 0.5–1.0%
        Carbon (C) 0.08% max
        Manganese (Mn) 2.0% max
        Silicon (Si) 0.75% max
        Phosphorus (P) 0.045% max
        Sulfur (S) 0.03% max
    

The addition of niobium allows SS 347 to perform well at temperatures between 800°F and 1650°F (427°C and 899°C) without experiencing sensitization.

Mechanical Properties of SS 347:

• Tensile Strength: 75,000–95,000 psi (515–655 MPa)
• Yield Strength: 30,000–45,000 psi (205–310 MPa)
• Elongation: 40% in 8 inches (200 mm)
• Hardness: 170–200 Brinell
• Modulus of Elasticity: 28,000 ksi (193 GPa)

Key Properties of SS 347:

• Corrosion Resistance: Excellent in a variety of aggressive environments, especially where sulfuric and nitric acids are present. It is also resistant to oxidation in high temperatures.
• Temperature Resistance: Excellent stability in service temperatures up to 1650°F (899°C).
• Weldability: SS 347 has good weldability and is resistant to weld decay (sensitization), making it suitable for high-temperature welded applications.
• Machinability: SS 347 exhibits reasonable machinability but requires more effort than SS 304 due to its high strength.

Common Applications of SS 347:

• Chemical Processing Equipment: Reactors, heat exchangers, and distillation columns.
• Power Generation: Furnace parts, gas turbine components, and superheater tubes.
• Petrochemical Industry: Components exposed to high temperatures and aggressive chemicals, such as valves, pumps, and piping.
• Aerospace: Aircraft engine components.
• Food Processing: Certain food-grade equipment requiring resistance to high temperatures.

2. SS 347H

SS 347H is a high-carbon variant of SS 347. The main difference between SS 347 and SS 347H is the higher carbon content in SS 347H, which improves its high-temperature strength and creep resistance. SS 347H is a better option when enhanced strength is required at elevated temperatures.

Chemical Composition of SS 347H:

The typical composition for SS 347H is as follows:

        Chromium (Cr) 17.0–19.0%
        Nickel (Ni) 9.0–13.0%
        Niobium (Nb) 0.5–1.0%
        Carbon (C) 0.04–0.10%
        Manganese (Mn) 2.0% max
        Silicon (Si) 0.75% max
        Phosphorus (P) 0.045% max
        Sulfur (S) 0.03% max
    

The higher carbon content improves SS 347H’s strength and makes it suitable for applications requiring increased stress resistance at high temperatures.

Mechanical Properties of SS 347H:

• Tensile Strength: 90,000–105,000 psi (620–725 MPa)
• Yield Strength: 40,000–55,000 psi (275–380 MPa)
• Elongation: 30% in 8 inches (200 mm)
• Hardness: 200–220 Brinell
• Modulus of Elasticity: 28,000 ksi (193 GPa)

Key Properties of SS 347H:

• High Strength: Due to the higher carbon content, SS 347H offers superior strength at elevated temperatures, making it ideal for high-stress applications.
• Corrosion Resistance: Similar to SS 347, SS 347H offers excellent resistance to high-temperature corrosion and oxidation.
• Temperature Resistance: SS 347H can withstand continuous exposure to temperatures up to 1650°F (899°C) and can perform well at higher temperatures.
• Weldability: SS 347H maintains good weldability, though post-weld heat treatment may be required to maintain its properties.

Common Applications of SS 347H:

• Power Generation: Boiler tubes, heat exchangers, superheater components, and gas turbines.
• Chemical Processing: Pressure vessels, reactor components, and distillation columns operating under high temperatures.
• Petrochemical: Equipment exposed to high temperatures, such as reactor vessels, piping, and flanges.
• Furnace and Furnace Components: Parts in continuous or batch furnaces requiring excellent strength and corrosion resistance at high temperatures.

Comparison Table

Property	SS 347	SS 347H
Carbon Content	0.08% max	0.04–0.10%
Strength	Good	Higher, enhanced strength at high temperatures
Temperature Resistance	800°F (427°C) to 1650°F (899°C)	Up to 1650°F (899°C) and higher
Corrosion Resistance	Excellent	Excellent
Weldability	Good, with limited post-weld heat treatment needed	Good, post-weld heat treatment may be necessary
Applications	General high-temperature environments	High-stress applications, power generation, chemical processing