SS 321/ 321H

SS 321 vs SS 321H

SS 321 and SS 321H are both austenitic stainless steels primarily designed for high-temperature applications. They are alloyed with titanium, which helps stabilize the material and prevent sensitization (intergranular corrosion) during welding and high-temperature exposure.

1. SS 321

SS 321 is a stabilized stainless steel that contains titanium, which prevents carbide precipitation at grain boundaries, thus resisting intergranular corrosion during high-temperature service. It offers excellent oxidation and corrosion resistance in high-temperature environments.

Chemical Composition of SS 321:

        Chromium (Cr)       17.0–19.0%
        Nickel (Ni)         9.0–12.0%
        Titanium (Ti)       5×(C + N) min
        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
        Iron (Fe)           Balance
    

Mechanical Properties of SS 321:

• 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 321:

• Corrosion Resistance: Excellent resistance to oxidation and corrosion in high-temperature environments.
• Temperature Resistance: Suitable for continuous service at temperatures up to 900°F (482°C) and intermittent service up to 1600°F (871°C).
• Weldability: Excellent weldability, with minimal concerns of sensitization. Post-weld heat treatment is not required.
• High-Temperature Strength: Retains strength and resistance to oxidation at high temperatures.

Common Applications of SS 321:

• Aerospace components (e.g., jet engine parts)
• Heat exchangers
• Chemical processing equipment (e.g., reactors, pipelines)
• Pressure vessels
• Welded structures in high-temperature environments

2. SS 321H

SS 321H is a high-carbon variant of SS 321. The key difference is the higher carbon content, which increases its strength at high temperatures. This makes SS 321H ideal for high-stress, high-temperature applications where better creep resistance is required.

Chemical Composition of SS 321H:

        Chromium (Cr)       17.0–19.0%
        Nickel (Ni)         9.0–12.0%
        Titanium (Ti)       5×(C + N) min
        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
        Iron (Fe)           Balance
    

Mechanical Properties of SS 321H:

• 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 321H:

• High-Temperature Strength: SS 321H offers better strength at elevated temperatures due to the higher carbon content.
• Corrosion Resistance: Excellent oxidation resistance and corrosion resistance at elevated temperatures.
• Temperature Resistance: Suitable for continuous service up to 1050°F (566°C) and intermittent service up to 1600°F (871°C).
• Weldability: Good weldability, but post-weld heat treatment may be necessary to preserve the material's properties.

Common Applications of SS 321H:

• Furnace parts
• Power generation equipment (e.g., superheater tubes, boiler components)
• Chemical reactors and other high-temperature process equipment
• Petrochemical applications (e.g., pumps, valves)
• Marine environments

Comparison of SS 321 and SS 321H

Property	SS 321	SS 321H
Carbon Content	0.08% max	0.04–0.10%
Tensile Strength	75,000–95,000 psi (515–655 MPa)	90,000–105,000 psi (620–725 MPa)
Yield Strength	30,000–45,000 psi (205–310 MPa)	40,000–55,000 psi (275–380 MPa)
Elongation	40% in 8 inches (200 mm)	30% in 8 inches (200 mm)
Hardness	170–200 Brinell	200–220 Brinell
Temperature Resistance	900°F (482°C) continuous, 1600°F (871°C) intermittent	1050°F (566°C) continuous, 1600°F (871°C) intermittent
Creep Strength	Moderate	High due to increased carbon content
Weldability	Good, no post-weld heat treatment required	Good, post-weld heat treatment may be required
Applications	General high-temperature environments	High-stress, high-temperature applications