ASTM 347 Stainless Steel: Stabilized Austenitic Grade for High-Temperature & Corrosive Environments
ASTM 347 stainless steel (UNS S34700) is a niobium-stabilized austenitic grade designed to resist intergranular corrosion in welded structures and maintain mechanical integrity at elevated temperatures (up to 800°C/1472°F). By adding niobium (Nb) as a stabilizer, this alloy mitigates chromium carbide precipitation during welding or high-temperature exposure, making it ideal for chemical processing, aerospace, and power generation applications. This article explores its chemical composition, mechanical properties, manufacturing nuances, and comparative advantages over standard 304/321 grades.
1. Chemical Composition (ASTM A240/A480 Compliance)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | ≤ 0.08% | Minimized to reduce carbide formation; Nb stabilization compensates for higher C tolerance |
| Chromium (Cr) | 17.00 – 19.00% | Forms passive Cr₂O₃ layer; provides oxidation resistance up to 870°C (1600°F) |
| Nickel (Ni) | 9.00 – 13.00% | Stabilizes austenitic microstructure; enhances toughness at cryogenic and elevated temperatures |
| Niobium (Nb) | 10×C min (typically 0.80-1.00%) | Binds carbon into NbC precipitates, preventing chromium depletion at grain boundaries |
| Manganese (Mn) | ≤ 2.00% | Improves hot workability; partial substitute for nickel in cost-sensitive applications |
| Silicon (Si) | ≤ 1.00% | Enhances oxidation resistance; aids in deoxidation during smelting |
| Phosphorus (P) | ≤ 0.045% | Controlled impurity; excessive levels reduce ductility and corrosion resistance |
| Sulfur (S) | ≤ 0.030% | Minimized to prevent hot cracking during welding and reduce inclusion formation |
2. Mechanical Properties at Room Temperature
- Tensile Strength (σb): ≥ 515 MPa (75 ksi) per ASTM A240
- Yield Strength (σ0.2): ≥ 205 MPa (30 ksi); retains 60-70% of RT strength at 600°C (1112°F)
- Elongation (δ): ≥ 40% in 50mm (2in); superior formability for deep drawing and spinning
- Hardness (HB): ≤ 217 Brinell; solution-annealed condition for optimal machinability
- Impact Toughness (CVN): ≥ 100 J at -196°C (-320°F); suitable for LNG and cryogenic applications
- Creep Resistance: 100,000-hour rupture strength of 103 MPa (15 ksi) at 650°C (1202°F)
3. Manufacturing Process & Heat Treatment
- Melting: Electric arc furnace (EAF) + argon oxygen decarburization (AOD) with vacuum degassing to achieve ultra-low carbon and high Nb homogeneity. Niobium added as ferro-niobium (FeNb) during ladle refinement.
- Hot Working: Hot rolled at 1150-1250°C (2102-2282°F) with controlled cooling to prevent NbC dissolution; water quenched to retain austenitic structure.
- Cold Working: Cold rolled for thin gauges (≤ 5mm) with intermediate annealing at 1050-1120°C (1922-2048°F) to relieve stresses and restore corrosion resistance.
- Solution Annealing: Heated to 1040-1120°C (1904-2048°F) followed by rapid water quenching to dissolve NbC and redistribute chromium uniformly.
- Stabilization Treatment: Optional 850-900°C (1562-1652°F) soak for 2-4 hours to precipitate NbC at grain boundaries, enhancing resistance to knife-line attack in HAZ (heat-affected zones).
- Surface Finishing: Pickled in nitric-hydrofluoric acid blend (10-20% HNO₃ + 2-5% HF) to remove scale; passivated in 20-30% HNO₃ at 50-70°C (122-158°F) for 30-60 minutes.
4. Key Application Sectors
Petrochemical & Refining
Hydrocarbon processing equipment (reactors, distillation columns), catalytic reformer tubing, and sulfuric acid coolers where resistance to polythionic acid stress corrosion cracking (PASCC) is critical.
Aerospace & Aviation
Jet engine exhaust systems, afterburner components, and aircraft hydraulic lines operating at 500-800°C (932-1472°F) with exposure to sulfur-bearing fuels.
Power Generation
Boiler superheater tubes, heat recovery steam generators (HRSG), and nuclear fuel reprocessing vessels requiring ASME Section III NCA-3800 compliance.
Pharmaceutical & Biotech
High-purity pipelines, autoclaves, and fermentation tanks for GMP/ISO 9001 facilities where weld integrity and cleanability are paramount.
Exhaust Systems
Automotive and industrial exhaust manifolds, turbocharger housings, and emission control systems resistant to thermal cycling and condensate corrosion.
Welded Fabrications
Pressure vessels (ASME BPVC Sec VIII Div 1), storage tanks (API 650), and structural components where post-weld heat treatment (PWHT) is impractical.
5. Comparison with Competing Grades
| Grade | Stabilizer | Carbon Content | Key Strengths | Limitations |
|---|---|---|---|---|
| ASTM 347 | Nb (10×C min) | ≤ 0.08% | Superior high-temperature strength; resistant to knife-line attack; weldable without PWHT | Higher cost than 321; Nb precipitates may reduce toughness if over-stabilized |
| ASTM 321 | Ti (5×C min) | ≤ 0.08% | Lower cost than 347; effective for temperatures below 600°C (1112°F) | Prone to knife-line attack in HAZ; TiC dissolves above 700°C (1292°F) |
| ASTM 304 | None | ≤ 0.08% | Lower cost; excellent formability and weldability in non-critical applications | Susceptible to intergranular corrosion when welded; limited to < 425°C (797°F) |
| ASTM 304L | None | ≤ 0.03% | Resists weld decay via low carbon; cost-effective for thin sections | Reduced high-temperature strength; not stabilized for prolonged exposure |
6. Technical Considerations & Best Practices
- Welding Guidelines: Use ER347 filler metal (AWS A5.9); maintain interpass temperature below 150°C (302°F) to avoid NbC dissolution. Post-weld pickling/passivation recommended for critical applications.
- Heat Treatment: Avoid prolonged exposure at 425-850°C (797-1562°F) to prevent sigma phase formation. Stabilization treatment (850-900°C) required if sensitized during service.
- Corrosion Resistance: Not recommended for halides (e.g., NaCl, HCl) or reducing acids (e.g., hydrochloric, hydrofluoric). For chloride environments, consider ASTM 316L or 904L.
- Machining: Use carbide tools with positive rake angles; sulfurized or chlorinated cutting oils improve chip control. Work-hardening rate is ~40% higher than carbon steel.
- Inspection Standards: Verify compliance via ASTM A262 Practice E (Strauss test) for intergranular corrosion susceptibility; PMI testing for Nb content confirmation.
- Surface Finishes: #4 brushed finish for architectural applications; electropolished (Ra ≤ 0.5 µm) for pharmaceutical/food contact surfaces to reduce bacterial adhesion.
7. Request a Custom ASTM 347 Stainless Steel Quote
Require ASTM 347 stainless steel in custom forms—sheets, plates, bars, or welded pipes? Our metallurgical team provides tailored solutions for high-temperature and corrosive service conditions. Submit your specifications (dimensions, tolerances, surface finish) for a competitive quote and lead time estimate.