321 vs 347 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Type 321 and Type 347 are two commonly specified stabilized austenitic stainless steels used where the combination of corrosion resistance, weldability, and elevated-temperature stability is required. Engineers, procurement managers, and manufacturing planners frequently decide between them when balancing corrosion performance, fabrication behavior, and lifecycle cost — for example, choosing between better resistance to intergranular attack after welding versus marginally lower material cost or availability.
The core metallurgical distinction is that 321 is stabilized primarily by titanium additions while 347 is stabilized by niobium (columbium) — each forms stable carbonitride precipitates that reduce chromium carbide formation during thermal exposure. Because both are 18–8 style austenitic stainless steels, they are often compared for applications such as exhaust components, furnace hardware, and chemical process equipment where sensitization and elevated-temperature service are concerns.
1. Standards and Designations
Common standards and designations where 321 and 347 appear:
- ASTM/ASME: ASTM A240 / ASME SA-240 (stainless steel plate, sheet, and strip)
- EN: EN 10088 series (European stainless steel standards)
- JIS: JIS G4303 / G4311 (Japanese stainless steels) — equivalents exist but check specific grade mapping
- GB: Chinese GB/T standards (comparable grades exist; verify chemistry and designation)
- UNS: UNS S32100 (Type 321), UNS S34700 (Type 347)
Classification: both 321 and 347 are austenitic stainless steels (stainless), not carbon steels, tool steels, or HSLA. They are stabilized austenitic stainless grades intended to limit sensitization and intergranular corrosion.
2. Chemical Composition and Alloying Strategy
The following table shows typical compositional elements and common control ranges for Types 321 and 347 in their standard commercial variants. Values shown are representative ranges found in common standards (e.g., ASTM A240, EN specifications); exact compositional limits should be checked on the mill certificate for a given heat.
| Element | Type 321 (typical ranges) | Type 347 (typical ranges) |
|---|---|---|
| C (wt%) | ≤ 0.08 | ≤ 0.08 |
| Mn (wt%) | ≤ 2.0 | ≤ 2.0 |
| Si (wt%) | ≤ 1.0 | ≤ 1.0 |
| P (wt%) | ≤ 0.045 | ≤ 0.045 |
| S (wt%) | ≤ 0.03 | ≤ 0.03 |
| Cr (wt%) | 17–19 | 17–19 |
| Ni (wt%) | 9–13 | 9–13 |
| Mo (wt%) | — / traces | — / traces |
| V (wt%) | — / traces | — / traces |
| Nb (wt%) | typically ≤ 0.10 (may contain trace) | typically 0.8–1.25 |
| Ti (wt%) | typically 0.5–1.0 (at least ~5×C) | typically ≤ 0.10 (may contain trace) |
| B (wt%) | trace if present | trace if present |
| N (wt%) | small amounts (≤ 0.1) | small amounts (≤ 0.1) |
How the alloying strategy works: - Cr and Ni produce the base austenitic stainless matrix, giving corrosion resistance and ductility. - Titanium or niobium preferentially combines with carbon and nitrogen to form stable carbide/nitride particles (TiC/TiN or NbC/Nb(C,N)), preventing chromium carbide precipitation at grain boundaries during thermal cycles (sensitization). - Low carbon limits also reduce the amount of chromium carbide that could form; the stabilizers act as a safety margin, particularly important during welding or prolonged exposure in the sensitization range (~450–850°C).
3. Microstructure and Heat Treatment Response
Microstructure: - Both grades are fully austenitic (face-centered cubic) in the solution-annealed condition. - Stabilizing elements form finely dispersed titanium or niobium carbonitrides. Their distribution and size depend on melt practice, hot working, and thermal history. - If stabilizer content is insufficient relative to carbon, chromium carbides can precipitate at grain boundaries during exposure to sensitizing temperatures, reducing intergranular corrosion resistance.
Heat treatment and processing response: - Solution annealing: typical solution anneal temperatures for austenitic stainless steels range from about 1010°C to 1120°C followed by rapid cooling (water or air) to retain a homogeneous austenitic structure. Both 321 and 347 are normally supplied in the annealed state. - Tempering/quenching: unlike ferritic or martensitic steels, traditional quench-and-temper cycles are not applicable to these austenitic grades; they do not transform martensitically in a way that benefits from tempering. - Normalizing: not commonly used for austenitic stainless steels. - Thermo-mechanical processing: cold work (rolling, drawing) increases strength via work hardening and influences grain deformation; subsequent anneal is used to restore ductility. - Stabilization effectiveness: niobium carbonitrides generally form very stable precipitates across a broad temperature range and can provide excellent stabilization for higher-temperature or longer-duration exposures. Titanium stabilization is effective for many common fabrication/welding cycles but requires control of Ti/C ratio to avoid coarse precipitate formation.
4. Mechanical Properties
Both grades exhibit mechanical properties typical of 18–8 austenitic stainless steels in the annealed condition. Because they are closely related alloys, their mechanical-property ranges overlap substantially.
| Property (annealed, typical ranges) | Type 321 | Type 347 |
|---|---|---|
| Tensile strength (MPa) | 500–700 (typical) | 500–700 (typical) |
| Yield strength, 0.2% (MPa) | 190–310 (typical) | 190–310 (typical) |
| Elongation (%) | 40–60% | 40–60% |
| Impact toughness | High toughness at room temp; no specific universal Charpy requirement | High toughness at room temp; similar to 321 |
| Hardness (annealed) | ~70–95 HRB (approx.) | ~70–95 HRB (approx.) |
Interpretation: - Mechanical strengths are essentially similar in the annealed condition because base Ni–Cr levels are comparable. - Work hardening during forming will raise strength and reduce ductility for both grades; the propensity for rapid work hardening is similar. - Any small differences in high-temperature creep or long-term strength can stem from differing stability and distribution of carbide/nitride precipitates (niobium-stabilized material can show better creep resistance in some long-term high-temperature applications).
5. Weldability
General weldability: - Both grades weld well with standard austenitic stainless welding procedures (TIG, MIG, resistance welding) because of low carbon and austenitic structure which resists cracking. - Stabilization reduces the risk of intergranular corrosion after welding by binding carbon, eliminating the need for some post-weld solution annealing operations in many cases.
Relevant weldability indices: - Carbon-equivalent formula commonly used to assess hardenability and cracking tendency: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - More detailed phosphorus–manganese–chromium formula: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both grades generally have low $CE_{IIW}$ and $P_{cm}$ values relative to martensitic steels, indicating good weldability. - The presence of stabilizers (Ti or Nb) reduces post-weld sensitization risk; however, coarse stabilizer precipitates or improper Ti/Nb ratios can create local heterogeneities. Welding procedures should still follow best practices: controlled heat input, appropriate filler selection, and, if necessary, post-weld solution anneal for critical severe service. - 347 may perform better than 321 in some long-duration or high-temperature weld exposures because niobium forms more stable precipitates at certain temperatures; nevertheless, both are considered weldable and are frequently used in welded assemblies.
6. Corrosion and Surface Protection
Corrosion behavior: - Both grades are corrosion-resistant in a wide range of environments typical for 18–8 austenitic stainless steels. Their stabilization strategy specifically targets resistance to intergranular corrosion following thermal exposure (welding or prolonged heating in the sensitization range). - Neither grade is significantly more resistant to uniform corrosion than the other in common aqueous or atmospheric environments; differences show up in specialized high-temperature or sensitization-sensitive contexts.
When to use corrosion indices: - PREN (Pitting Resistance Equivalent Number) is applicable for comparing pitting resistance (primarily relevant when Mo and N are significant): $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - For 321 and 347, Mo is typically absent or present only in trace amounts, and N is low, so PREN is not a discriminating index between these two grades.
Surface protection for non-stainless contexts: - Not applicable here — both are stainless. For non-stainless steels the choices would be galvanizing, painting, or coatings.
7. Fabrication, Machinability, and Formability
Fabrication notes: - Machinability: austenitic stainless steels, including 321 and 347, are more difficult to machine than carbon steels due to high work hardening and low thermal conductivity. Carbide tooling, rigid setups, and controlled feeds/speeds are recommended. - Formability: both grades have good ductility and can be formed, deep-drawn, and spun; however, they work harden rapidly — frequent annealing may be needed for heavy deformation sequences. - Surface finish: susceptibility to galling and tool wear requires attention; electropolishing or passivation improves corrosion resistance post-fabrication. - Welding consumables: match or slightly higher nickel content filler alloys are commonly used; filler choice should consider service temperature and corrosion environment.
8. Typical Applications
| Type 321 (common uses) | Type 347 (common uses) |
|---|---|
| Aircraft and automotive exhaust systems | Chemical process equipment exposed to higher continuous temperatures |
| Furnace hardware, bake ovens, and heat exchangers where weld stability is needed | Pressure vessels and piping in high-temperature service where prolonged exposure at elevated temperatures may occur |
| Petrochemical components, fasteners, and springs where stabilization against sensitization is desired | Boiler and superheater tubing, where niobium stabilization can aid long-term creep resistance |
| Food processing equipment where thermal cycles and welding are common | High-temperature furnace components and petrochemical fittings with long dwell at elevated T |
Selection rationale: - Choose based on the dominant service factor: if welding and moderate thermal cycling are the primary concern, both perform well; if long-term creep or sustained exposure at higher temperatures is expected, niobium-stabilized 347 can offer an advantage. Availability, form (tube, plate, coil), and local supplier inventories also influence selection.
9. Cost and Availability
- Cost: both grades are broadly comparable in price because their base Ni and Cr contents are similar. Type 347 can be slightly more expensive in some regions due to niobium content and market variations of that alloying element.
- Availability: both are widely available in sheet, plate, tube, and bar forms from major stainless mills and distributors. Specific product forms (e.g., heavy gauge plate or specialty tube sizes) should be confirmed with suppliers; lead times can vary by region and market cycle.
10. Summary and Recommendation
| Criteria | Type 321 | Type 347 |
|---|---|---|
| Weldability | Excellent; Ti stabilization reduces sensitization risk | Excellent; Nb stabilization reduces sensitization risk (often preferred for higher-T weld exposures) |
| Strength–Toughness | Typical austenitic strength and high toughness; similar to 347 | Comparable to 321; potential edge in long-term high-temperature stability |
| Cost | Commonly slightly lower or similar | Comparable; can be marginally higher due to Nb |
Recommendation: - Choose Type 321 if: you need a well-proven stabilized austenitic stainless for welded assemblies and thermal cycling where titanium stabilization is effective; if material forms and supplier inventory favor 321; or if cost sensitivity and conventional elevated-temperature service (moderate durations) are primary concerns. - Choose Type 347 if: the component will see prolonged exposure at higher temperatures, where niobium-stabilized carbides provide superior stability and potential creep resistance; if the welding or service history indicates long dwell times in the sensitization range; or if specifications call for Nb-stabilized alloying for performance reasons.
Concluding note: Both 321 and 347 are excellent choices where stabilization against sensitization is required. The decision typically hinges on the specific thermal profile (duration and peak temperature), long-term high-temperature creep expectations, and logistics (product form availability and cost). For mission-critical or long-duration high-temperature applications, consult material test data and supplier certificates for the specific lot, and consider engineering evaluation (creep tests, corrosion exposure trials, or weld procedure qualifications) to validate the chosen grade for the intended service.