347 Stainless Steel Excels in Hightemperature Applications

In chemical plants with high-temperature pipelines, food processing facilities with steam sterilization equipment, or even within jet engine combustion chambers, there exists a critical need for materials that can withstand extreme heat and corrosive environments. Material selection in these applications is crucial—poor choices may reduce efficiency at best, or cause safety hazards at worst. Is there a stainless steel that maintains stability under high pressure and temperature while offering exceptional corrosion resistance? The answer is yes: 347 stainless steel.

Known as 1.4550 or X6CrNiNb18-10 (EN standard) and UNS S34700 (UNS standard), 347 stainless steel is an austenitic stainless steel specifically engineered for high-temperature environments. Its most notable characteristics include superior heat resistance and corrosion protection, particularly within the 400°C to 800°C temperature range. The material maintains excellent mechanical properties and chemical stability at elevated temperatures, making it invaluable across various industrial sectors requiring heat and corrosion resistance.

While similar to 304 and 321 stainless steels, 347 stainless steel distinguishes itself through the addition of niobium (Nb) and tantalum (Ta). These elements serve as crucial stabilizers by forming stable carbides with carbon in the steel, preventing chromium precipitation at grain boundaries and subsequent intergranular corrosion. Essentially, niobium and tantalum act as structural stabilizers, enhancing the steel's durability under high temperatures.

During welding or other high-temperature processes, carbon in stainless steel tends to bond with chromium, forming chromium carbides that precipitate at grain boundaries. This "sensitization" phenomenon reduces chromium content near these boundaries, compromising corrosion resistance—particularly in corrosive media. Intergranular corrosion can then propagate rapidly along grain boundaries, weakening the material and potentially causing fractures. The addition of niobium and tantalum effectively prevents sensitization, improving overall material performance.

• High-Temperature Resistance: The most prominent feature of 347 stainless steel is its ability to operate long-term at 400°C to 800°C without significant performance degradation, making it ideal for high-temperature equipment and components.
• Corrosion Resistance: Comparable to 304 and 321 stainless steels, 347 resists various corrosive media including acids, alkalis, and salts. However, performance may be compromised in strongly oxidizing (e.g., nitric acid) or strongly reducing (e.g., hydrochloric acid) environments.
• Weldability: 347 stainless steel exhibits excellent welding characteristics, accommodating common methods like TIG and shielded metal arc welding. Post-weld heat treatment is typically unnecessary, though optimal results require proper techniques and materials.
• Non-Heat-Treatable: Unlike some stainless steels that gain strength through heat treatment, 347 derives its strength from chemical composition and cold working processes.
• Mechanical Properties: The steel offers favorable mechanical properties including high tensile strength, yield strength, and elongation, ensuring reliable performance under various loads and deformations.

Given its exceptional properties, 347 stainless steel serves critical roles in multiple industries:

• Chemical Processing: Reactors, piping systems, and heat exchangers handling corrosive substances.
• Petrochemical: Refinery components like furnace tubes and cracking pipes, plus corrosion-resistant transfer lines.
• Food Processing: Equipment requiring non-toxic, corrosion-resistant, and easily cleanable materials.
• Energy: Boilers, steam turbines, and nuclear reactor components demanding high-temperature and high-pressure resistance.
• Aerospace: Jet engine parts including combustion chambers and turbine blades requiring extreme heat resistance and oxidation protection.
• Waste Management: Incinerators and other equipment exposed to high-temperature gases and corrosive byproducts.

• Corrosion Resistance: Good resistance to multiple corrosive media, though caution is advised with strongly oxidizing/reducing acids.
• Mechanical Properties: High tensile strength, yield strength, and elongation—specific values depend on material specifications and processing.
• Forging Characteristics: Excellent hot workability for forming operations.
• Machinability: Moderate difficulty due to high strength and toughness, requiring appropriate tooling and cutting parameters.

Both 347 and 321 stainless steels are popular high-temperature alloys with many similarities. The key difference lies in their stabilization methods: 321 uses titanium (Ti), while 347 employs niobium and tantalum. While often interchangeable, their corrosion resistance may differ in specific environments. Generally, 347 demonstrates superior high-temperature creep resistance compared to 321.

When specifying 347 stainless steel:

• Verify compliance with relevant standards (ASTM, EN, JIS, etc.).
• Review material certifications documenting chemical composition, mechanical properties, and heat treatment status.
• Source from reputable suppliers with proven quality control.
• Select appropriate specifications and dimensions based on design requirements to prevent waste or safety issues.

347 stainless steel plays a vital role across industries where extreme temperatures and corrosive conditions demand reliable material performance. Through stabilization with niobium and tantalum, it maintains structural integrity at high temperatures while resisting intergranular corrosion. When selecting high-temperature stainless steels, 347 remains a proven choice—though final decisions should account for specific operating conditions and environmental factors.