304 vs 347 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Type 304 and Type 347 are two of the most widely used austenitic stainless steels in industry. Engineers, procurement managers, and manufacturing planners commonly weigh trade-offs between initial material cost, resistance to corrosion (especially after welding), weldability, and in-service strength when selecting between them. Typical decision contexts include food and beverage equipment, chemical processing lines, architectural applications, and welded assemblies exposed to elevated temperatures.
The principal metallurgical distinction is that Type 347 is deliberately alloyed with a stabilizing element that preferentially combines with carbon, preventing chromium carbide formation during slow cooling or welding. That stabilization reduces the risk of intergranular chromium-depletion and associated corrosion, which is the chief reason designers compare 304 and 347 when welding and temperature exposure are concerns.
1. Standards and Designations
Major standards and common international designations:
- ASTM/ASME: Type 304 (UNS S30400), Type 347 (UNS S34700). Common product specification: ASTM A240 (plate, sheet).
- EN: 1.4301 (304), 1.4550 / 1.4552 often referenced for stabilized grades (347 variants).
- JIS: SUS304 corresponds to 304; SUS347 corresponds to 347.
- GB (China): 0Cr18Ni9 (approx. 304), 0Cr18Ni10Nb (approx. 347).
Classification: both are stainless steels (austenitic). They are not carbon, alloy, tool steels, or HSLA steels.
2. Chemical Composition and Alloying Strategy
The following table shows typical composition ranges (wt%) for annealed commercial grades. Ranges reflect common ASTM/EN specifications and commercial practice; exact limits depend on the specification and product form.
| Element | 304 (typical range, wt%) | 347 (typical range, wt%) |
|---|---|---|
| C | ≤ 0.08 | ≤ 0.08 |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 0.75 | ≤ 1.0 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | 18–20 | 17–19 |
| Ni | 8–10.5 | 9–13 |
| Mo | ~0 | ~0 |
| V | — trace | — trace |
| Nb (niobium) | — negligible | ~0.10–1.0 |
| Ti (titanium) | — negligible | sometimes present in small amounts in special variants |
| B | — trace | — trace |
| N | ≤ ~0.10 | ≤ ~0.10 |
How alloying affects behavior: - Chromium (Cr) provides the stainless character by forming a passive oxide film. - Nickel (Ni) stabilizes the austenitic structure and improves toughness and formability. - Carbon (C) increases strength but can combine with Cr to form chromium carbides at grain boundaries when cooled slowly, causing local Cr depletion. - Niobium (Nb) in 347 ties up carbon as niobium carbides (NbC or (Nb,Ti)C), preventing chromium carbide precipitation—this is the primary stabilization strategy in 347. - Low or absent molybdenum means both grades are less resistant to localized chloride pitting than Mo-bearing austenitics.
3. Microstructure and Heat Treatment Response
Microstructure: - Both grades are fully austenitic (face-centered cubic) in the annealed condition. The matrix is ductile with high toughness at ambient and sub-ambient temperatures. - In 304, slow cooling through the sensitization range (approximately 450–850 °C) can permit formation of chromium-rich carbides (Cr23C6) at grain boundaries, producing chromium-depleted zones susceptible to intergranular corrosion. - In 347, niobium forms stable niobium carbide or carbonitride particles that preferentially consume carbon and prevent significant Cr23C6 formation at grain boundaries. This preserves chromium continuity and mitigates intergranular attack after welding or slow cooling.
Heat treatment response: - Austenitic stainless steels are not hardened by quench-and-temper in the way ferritic or martensitic steels are. Solution annealing (typically around 1,000–1,100 °C followed by rapid cooling) dissolves precipitates and restores corrosion resistance. - For 304: solution anneal if the material has been exposed to sensitization temperatures to re-dissolve Cr carbides and then rapidly quench to avoid re-precipitation. - For 347: stabilization reduces the need for solution annealing to prevent sensitization after welding or slow cooling, although solution annealing is still used for cleaning up fabrication-induced precipitates or for specific property requirements. - Cold working increases dislocation density and can produce significant work hardening; in some circumstances, strain-induced martensite can form in 304 during heavy cold deformation, increasing strength but lowering ductility. Stabilized grades can show slightly different work-hardening behavior but remain austenitic.
4. Mechanical Properties
Typical annealed mechanical properties vary with product form (sheet, plate, bar) and manufacturer. The table gives representative ranges for commonly supplied annealed conditions; users should specify required mechanical property tests for procurement.
| Property (annealed, typical) | 304 | 347 |
|---|---|---|
| Tensile Strength (UTS) | ~480–700 MPa | ~480–700 MPa |
| Yield Strength (0.2% offset) | ~205–310 MPa | ~205–310 MPa |
| Elongation (in 50 mm) | ~40–60% | ~40–60% |
| Impact Toughness (Charpy) | Excellent, retains toughness at low T | Excellent, retains toughness at low T |
| Hardness (Brinell/Rockwell B) | Moderate (soft in annealed state) | Similar to 304 |
Interpretation: - In the annealed condition both grades have very similar mechanical properties because the base matrix is austenitic stainless steel. Differences in strength or toughness are usually small and driven by cold work, fabrication history, or specific alloying levels (e.g., slightly higher Ni in some 347 variants). - Work hardening can raise strength markedly during forming operations; selection should consider forming schedules and post-forming treatment.
5. Weldability
Weldability of austenitic stainless steels is generally excellent compared to high-carbon steels; however, susceptibility to post-weld sensitization differs.
Weldability indices often used: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (a more welding-sensitive carbon equivalent for steels with many alloying elements): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both 304 and 347 have low carbon contents and moderate nickel, giving good general weldability with most common processes (GTAW/TIG, GMAW/MIG, SMAW). - 304 can be vulnerable to intergranular corrosion if welding produces slow cooling through the sensitization range; post-weld solution annealing or low-carbon variants (304L) are common remedies. - 347’s stabilizing element lowers the effective risk of sensitization after welding because carbon is preferentially bound in Nb-bearing precipitates rather than chromium carbides. Thus, in applications requiring heavy welding, prolonged exposure to the sensitization range, or where post-weld heat treatment is impractical, 347 is often preferred. - Care with filler selection: to maintain corrosion performance, matching or low-carbon filler metals are recommended for both grades when corrosion resistance is a primary requirement.
6. Corrosion and Surface Protection
- As stainless steels, both rely on a chromium-rich oxide passive film for corrosion resistance. Neither contains significant molybdenum, so localized pitting resistance in chloride environments is limited compared to Mo-bearing grades (e.g., 316).
- PREN (Pitting Resistance Equivalent Number) is typically used for assessing resistance to chloride pitting: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Because Mo ≈ 0 for both grades and N is low, PREN values for 304 and 347 are similar and modest, meaning both should be used cautiously in aggressive chloride environments.
Intergranular corrosion: - 304: Susceptible to intergranular attack if sensitized by slow cooling or welding; mitigation strategies include using 304L (low carbon), solution annealing, or post-weld passivation. - 347: Stabilization by niobium prevents significant chromium carbide precipitation, so the risk of intergranular corrosion after welding or slow cooling is much reduced.
Surface protection for non-stainless steels is not applicable here; however, stainless surfaces can be passivated (chemical treatments) to enhance oxide film uniformity and minimize corrosion initiation.
7. Fabrication, Machinability, and Formability
- Formability: Both grades are highly formable in the annealed condition and are used for deep drawing, bending, and complex shapes. 304 is widely used for forming; 347 forms similarly though slightly higher alloy content can marginally affect formability.
- Work hardening: Austenitic stainless steels work-harden rapidly; tooling and forming sequences must account for springback and increasing forces.
- Machinability: Both grades are more difficult to machine than carbon steels. Typical machinability is on the order of 40–60% of free-cutting steels; 304 can be slightly less machinable due to work hardening. Use of rigid tooling, carbide inserts, low cutting speeds and positive rake geometry is recommended.
- Finishing: Both take good surface finishes and can be polished to high aesthetics. Electropolishing and passivation improve corrosion resistance and appearance.
- Welding and forming sequences should be planned to minimize repeated thermal cycles that could cause sensitization (a concern for 304 but mitigated in 347).
8. Typical Applications
| Type 304 – Typical Uses | Type 347 – Typical Uses |
|---|---|
| Food processing equipment, kitchenware, sinks, architectural trim | Aircraft and exhaust components, furnace parts, high-temperature welded assemblies |
| Chemical and pharmaceutical equipment not exposed to high chloride conditions | Chemical process equipment where welding and slow cooling occur; heat exchangers under cyclic temperatures |
| Decorative and architectural facades | Boiler tubes, superheater tubing, and applications requiring stabilization against sensitization |
| Fasteners, bolts, and general fabrication | Automotive exhaust manifolds and other elevated-temperature welded components |
Selection rationale: - Choose 304 for general-purpose corrosion resistance, lower cost, and widespread availability where chloride pitting and post-weld sensitization are not primary concerns or where low-carbon variants (304L) or post-weld solution annealing can be used. - Choose 347 when welded components will experience slow cooling, elevated service temperatures, or when intergranular corrosion associated with sensitization must be avoided without post-weld heat treatment.
9. Cost and Availability
- Cost: 347 is typically more expensive than 304 due to the addition of niobium and often higher nickel content. Pricing varies with market nickel and niobium supply, product form, and processing.
- Availability: 304 is one of the most widely available stainless steels in many product forms (sheet, plate, tube, bar, wire). 347 is commonly available but less ubiquitous than 304; lead times may be slightly longer for some product forms or tight-tolerance products.
- For procurement: specify required grade, product form, surface finish, and any heat-treatment or testing requirements (e.g., PMI, corrosion testing) to avoid sourcing delays.
10. Summary and Recommendation
Summary table (qualitative):
| Criterion | 304 | 347 |
|---|---|---|
| Weldability (general) | Excellent; susceptible to sensitization unless low-C or post-weld treated | Excellent; improved resistance to sensitization due to stabilization |
| Strength–Toughness (annealed) | Good, typical austenitic properties | Similar to 304 in annealed condition |
| Resistance to weld-induced intergranular corrosion | Moderate without mitigation | Superior for welded/slow-cooled components |
| Cost | Lower (more economical) | Higher (due to stabilizer and sometimes higher Ni) |
| Availability | Very high | High, but less ubiquitous than 304 |
Conclusions — practical guidance: - Choose 304 if: - You need a versatile, cost-effective austenitic stainless for general corrosion resistance, food processing, architectural components, or applications where welding is limited or where 304L/solution anneal options are acceptable. - Localized chloride pitting is not severe and post-weld heat treatment is feasible when required.
- Choose 347 if:
- The component will be heavily welded, will see slow cooling through sensitization temperatures, or will operate at elevated temperatures where carbide precipitation is a concern.
- You require good intergranular corrosion resistance in welded structures without the need for post-weld solution annealing.
Final note: both materials are robust engineering choices. For critical applications, specify exact material standards, request manufacturer mill certificates showing composition and mechanical test results, and consider laboratory testing (e.g., corrosion testing, weld trials) under representative conditions before final selection.