321 vs 347 – Composition, Heat Treatment, Properties, and Applications

Introduction

Type 321 and Type 347 are both austenitic, chromium-nickel stainless steels widely used in engineered systems where corrosion resistance, formability, and elevated-temperature stability are required. Engineers, procurement managers, and manufacturing planners frequently decide between them when balancing corrosion performance, fabrication behavior, long-term stability at temperature, and cost.

The primary technical distinction between the two grades is the choice of the carbide-stabilizing element: Type 321 is stabilized with titanium (Ti), while Type 347 is stabilized with niobium (columbium, Nb). That difference controls how each grade resists chromium carbide precipitation (sensitization) during welding or service at 450–850 °C, and it influences long-term stability, particularly for high-temperature or cyclic applications.

1. Standards and Designations

• ASTM/ASME: A240 / SA-240 (common for plate and sheet).
• UNS: 321 = UNS S32100; 347 = UNS S34700.
• EN: 321 / 347 equivalents exist but consult EN numbers (e.g., EN 1.4541 for 321 sometimes, check current cross-references).
• JIS / GB: Japanese and Chinese standards have similar stabilised austenitics; check local cross-reference tables for the precise designation.

Classification: Both 321 and 347 are stainless steels (austenitic, non-magnetic in the annealed condition). They are not carbon steels, alloy-carbon tool steels, or HSLA steels.

2. Chemical Composition and Alloying Strategy

Table — Typical nominal composition ranges (weight percent). Values are indicative for annealed, commercially-specified material; consult the specific standard or supplier mill certificate for exact limits.

Notes: - 321 uses titanium additions sized relative to carbon to tie up C as TiC/Ti(C,N) to prevent Cr23C6 formation. Standards typically require Ti ≥ 5 × C up to a practical maximum. - 347 uses niobium (often with a small amount of tantalum as natural impurity) to form NbC/Nb(C,N) for the same purpose. Specification limits and typical Nb contents vary by standard and product form. - Neither grade normally contains significant molybdenum; they are not Mo-bearing duplex or super-austenitic families.

How alloying affects properties: - Chromium provides general passive-film corrosion resistance. - Nickel stabilizes the austenitic phase and improves toughness and formability. - Titanium or niobium prevent sensitization by forming stable carbides and carbonitrides, protecting chromium from tying up as Cr-carbides at grain boundaries during exposure to sensitizing temperatures. - Small N additions increase strength through interstitial strengthening.

3. Microstructure and Heat Treatment Response

• Microstructure (annealed): Both grades are fully austenitic with a face-centered cubic (FCC) matrix. Stabilizing carbides/nitrides (TiC/TiN in 321, NbC/Nb(C,N) in 347) are present, generally as fine precipitates distributed at grain boundaries and within grains.
• Sensitization resistance: Stabilizers preferentially form carbides; this prevents chromium-depleted zones at grain boundaries and protects against intergranular corrosion following exposure to 450–850 °C.
• Heat-treatment response:
• Anneal (typical): Solution anneal at ~1010–1150 °C followed by rapid cooling to retain the austenitic structure and dissolve undesirable precipitates.
• Normalizing/Quenching & Tempering: These are not standard routes for austenitic stainless steels — they do not harden by quench and temper like martensitic steels. Thermomechanical processing influences grain size and texture, but chemical stabilization primarily governs high-temperature behaviour.
• High-temperature service: Over long exposures at elevated temperature, Ti-stabilized 321 can form complex Ti-rich precipitates and, if Ti/C ratio is inadequate or if long exposure occurs, may develop secondary chromium carbide precipitation. Nb-stabilized 347 tends to maintain strength and resist grain boundary chromium depletion better during extended high-temperature service, which is why 347 (and the 347H variant with higher C) is often specified for prolonged elevated-temperature operation.

4. Mechanical Properties

Table — Typical mechanical property ranges for annealed material at ambient temperature (indicative; product form and specification determine guaranteed values).

Interpretation: - In the annealed condition at room temperature, mechanical properties of 321 and 347 are very similar. The stabilizing element has only a modest effect on static tensile/yield strength and ductility under ambient conditions. - At elevated temperature and over long exposure times, 347 (niobium-stabilized) can show superior retention of ductility and creep resistance because niobium carbides are more stable and less likely to coarsen in certain service regimes compared with titanium precipitates — this is particularly relevant for long-duration high-temperature service and cyclic thermal exposure.

5. Weldability

• Both 321 and 347 have good weldability typical of austenitic stainless steels: low carbon content and the presence of stabilizers reduce risk of intergranular attack after welding.
• Key welding considerations:
• Proper filler selection and welding procedure remain important to avoid hot cracking and to control delta-ferrite when necessary.
• Post-weld annealing is usually not required solely to avoid intergranular corrosion, provided the stabilizer-to-carbon ratio and process control are correct.
• Important weldability indices (examples — use them qualitatively):
• Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• Chromium equivalent (Pcm) — a welding-cracking susceptibility estimator: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Qualitative interpretation:
• Both grades score well for general fusion welding because of low C and stabilization. Niobium in 347 appears in the $P_{cm}$ term; while contributing to resistance to sensitization, it can slightly influence welding solidification behavior. In practice, weldability differences are small; selecting appropriate filler metal (often matching or using 308/309 family fillers as specified) and controlling heat input is more impactful than the Ti vs Nb choice.
• For repair welding or fabrication where repeated thermal cycling occurs, 347 can be preferred when long-term stability of the stabilizing carbide is critical.

6. Corrosion and Surface Protection

• General corrosion: Both grades form a chromium-rich passive film and show corrosion resistance similar to 304 in many environments. Neither contains Mo, so pitting resistance in chloride environments is not as high as Mo-bearing grades.
• Intergranular corrosion: Both are stabilized against sensitization by their respective stabilizers; however, correct stabilizer levels relative to carbon content and controlled processing are required.
• Use of PREN: The Pitting Resistance Equivalent Number is commonly used where Mo and N provide pitting resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• For 321 and 347 (Mo ~ 0), PREN is mainly driven by Cr and N and is therefore modest; PREN is of limited value for differentiating these two grades because both lack Mo.
• Surface protection for non-stainless steels: Not applicable here — both are stainless. However, where enhanced protection is required (chloride service, sea-water), consider Mo-bearing or duplex stainless steels or coatings.

7. Fabrication, Machinability, and Formability

• Machinability: Austenitic stainless steels work-harden rapidly; 321 and 347 are similar to 304 in this regard. Machining strategies (rigid set-up, sharp tooling, high positive rake, heavy coolant) apply equally.
• 347 may be marginally more difficult to machine if higher Nb-carbide content increases tool wear in some feeds, but differences are small in practice.
• Formability: Both exhibit excellent cold formability and deep-drawing characteristics in the annealed condition. Springback and work hardening behaviour are comparable.
• Surface finish and polishing: Both polish well and accept most surface treatments; welded areas should be passivated if elevated corrosion resistance is required.

8. Typical Applications

Table — Typical uses of each grade and selection rationale.

Selection rationale: - Choose 321 when typical exposure involves occasional or short high-temperature excursions and when titanium stabilization is effective for expected thermal cycles. - Choose 347 when long-term exposure at elevated temperature or prolonged service in the sensitization-temperature range demands the stability of niobium carbides (347H variant may be specified for higher creep/strength at temperature due to higher carbon content).

9. Cost and Availability

• Cost: 347 is frequently modestly more expensive than 321 because niobium is a higher-cost alloying addition than titanium. Market pricing fluctuates with niobium raw material costs.
• Availability: Both grades are widely available in sheet, plate, tube, and bar forms from major mills. 321 historically has very broad availability as it has long been a common alloy in aerospace and industrial applications. 347 and 347H are well supplied but availability in certain product forms or special tempers can be more limited and lead times slightly longer.
• Procurement advice: Specify exact UNS/ASTM grade and product form on purchase orders; if lead time or cost is critical, confirm mill stock or consider substitution with engineering approval.

10. Summary and Recommendation

Table — Quick comparison (qualitative).

Conclusions: - Choose Type 321 if you need a stabilized austenitic stainless with excellent general corrosion resistance, good weldability, and cost sensitivity where the service includes thermal cycles or short-duration high-temperature exposure. 321 is a common choice for furnace parts, expansion joints, and applications where titanium stabilization performs adequately. - Choose Type 347 if the application involves prolonged exposure at elevated temperatures, extended service near the sensitization range, or where long-term creep and grain-boundary stability are critical. 347 (or 347H for higher-temperature strength) is preferred when niobium carbide stability gives measurable lifecycle advantages despite a modest premium in cost.

Final practical note: Always review the specific ASTM/UNS/EN limits and request mill certificates for critical projects. For critical high-temperature or corrosive environments, perform application-specific corrosion testing and consult metallurgists to validate grade selection and welding/fabrication procedures.