321 vs 347H – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Choosing between stainless steels 321 and 347H is a common decision point for engineers, procurement managers, and manufacturing planners working with high-temperature or corrosive environments. The trade-offs typically center on corrosion resistance under thermal exposure, weldability and fabrication ease, long-term high-temperature strength, and lifecycle cost.
The principal distinction between these two austenitic, stabilized stainless steels lies in their stabilization strategy against carbide precipitation at elevated temperatures: one grade is titanium-stabilized while the other is niobium-stabilized and offered in a higher-carbon variant for improved high-temperature strength. This difference governs their resistance to intergranular attack after thermal cycling, their creep and rupture behavior, and influences selection for furnace hardware, boiler and superheater tubes, and chemical-plant components.
1. Standards and Designations
- Common standards and designations:
- ASTM/ASME: 321 (often given as ASTM A240 / ASME SA240), 347H (ASTM A240 / ASME SA240 high-carbon variant of 347)
- EN: equivalents appear as X6CrNiTi17-12 or similar for 321; 347/347H variants mapped to EN grades with columbium/niobium stabilization
- JIS/GB: national standards provide corresponding designations and composition bands
- Classification:
- Both 321 and 347H are austenitic stainless steels (stainless family).
- They are not carbon steels, tool steels, or HSLA — they are stainless (corrosion-resistant) alloys intended for elevated temperatures.
2. Chemical Composition and Alloying Strategy
The two grades share the same austenitic matrix chemistry (austenite-stabilizing nickel and chromium-based composition) but differ in stabilizing elements and carbon control.
Table: presence/role of elements (qualitative)
| Element | 321 | 347H | Role / Notes |
|---|---|---|---|
| C (carbon) | Low carbon austenitic | Higher carbon variant (H) | Carbon influences creep strength and precipitation behavior |
| Mn (manganese) | Present (minor) | Present (minor) | Austenite stabilizer, affects hot-working |
| Si (silicon) | Present (trace) | Present (trace) | Deoxidizer, minor effect on properties |
| P (phosphorus) | Trace control | Trace control | Impurity control for toughness |
| S (sulfur) | Trace control | Trace control | Affects machinability; kept low |
| Cr (chromium) | Major alloying element | Major alloying element | Primary corrosion resistance contributor |
| Ni (nickel) | Major alloying element | Major alloying element | Stabilizes austenite, improves ductility and toughness |
| Mo (molybdenum) | Typically minimal/absent | Typically minimal/absent | Not a design feature for these grades |
| V (vanadium) | Not a stabilizer here | Not a stabilizer here | Generally not used in these grades |
| Nb (niobium / columbium) | Not used as primary stabilizer | Present as stabilizer | Forms Nb-carbonitrides that pin carbides and grain boundaries |
| Ti (titanium) | Present as stabilizer | May be present only in small amounts | Forms Ti-carbonitrides to prevent chromium carbide precipitation |
| B (boron) | Trace if any | Trace if any | Not a design driver |
| N (nitrogen) | Low levels | Low levels | Affects strength and pitting resistance slightly |
Explanation - Both alloys are chromium-nickel austenitic stainless steels. Chromium gives the passive film for general corrosion resistance; nickel stabilizes the austenitic phase and improves toughness. - 321 uses titanium as a stabilizer: titanium preferentially forms titanium carbides/nitrides, which tie up carbon and prevent chromium carbide precipitation at grain boundaries during long thermal exposures. - 347H uses niobium (columbium) as the stabilizer and is supplied in a higher-carbon variant (the "H") to enhance high-temperature strength and creep resistance. Niobium has similar stabilizing action as titanium but is especially effective when combined with higher carbon for long-term elevated-temperature strength.
3. Microstructure and Heat Treatment Response
Microstructure - At room temperature both grades are single-phase austenite (face-centered cubic), with alloying additions and stabilizers present as fine carbides/nitrides. - Stabilizing precipitates: 321 shows titanium carbonitrides; 347H shows niobium carbonitrides. These precipitates are typically fine and distributed at grain boundaries and within grains.
Heat treatment and processing response - Austenitic stainless steels are generally non-hardenable by quenching; strength adjustments are via cold work or solution annealing. - Solution annealing followed by rapid cooling dissolves precipitates and restores corrosion resistance if done correctly. - For stabilized grades, the stabilizer ties up carbon during welding or slow cooling, reducing the risk of chromium carbide precipitation (sensitization). - 347H, with higher carbon and niobium, is designed to retain better creep resistance and maintain grain boundary stability under prolonged high-temperature exposure; however, welding procedures should still control thermal cycles to avoid undesirable precipitates.
Process effects - Normalizing is not a standard practice for these austenitic grades; annealing (solution treatment) is the usual thermal process to restore structure after fabrication. - Thermo-mechanical processing (controlled rolling for tubes or forgings) primarily affects grain size and creep strength; both grades respond similarly in terms of recrystallization and grain growth, but 347H's precipitation behavior improves creep resistance at higher temperatures.
4. Mechanical Properties
Table: qualitative comparison at ambient and elevated service considerations
| Property | 321 | 347H | Notes |
|---|---|---|---|
| Tensile strength (room temp) | Similar | Similar | Both have comparable room-temperature tensile properties typical of austenitic stainless steels |
| Yield strength | Comparable | Slightly higher at elevated temperatures | 347H's higher carbon and Nb precipitation improve high-temperature strength |
| Elongation / ductility | Good, higher ductility | Good, slightly reduced ductility vs 321 | Higher carbon modestly reduces ductility in 347H |
| Impact toughness | Excellent at room temp | Excellent at room temp | Both maintain good toughness; careful control required after cold work |
| Hardness | Similar in annealed condition | Similar (may be slightly higher if cold worked) | Hardness increases with cold work for both grades |
Interpretation - At room temperature the mechanical properties are broadly similar, and both offer good toughness and ductility characteristic of austenitic stainless steels. - In long-term elevated-temperature service, 347H typically offers better tensile/yield retention and creep resistance due to higher carbon and niobium-stabilized precipitates that strengthen the matrix and slow boundary degradation.
5. Weldability
Weldability is critical for fabrication and service reliability.
Factors - Both 321 and 347H are generally considered weldable by standard austenitic stainless welding practices, but stabilization strategy and carbon content affect susceptibility to sensitization and secondary phases. - Lower carbon in 321 reduces the tendency to form chromium carbides, and titanium stabilization helps prevent sensitization. 347H's niobium stabilization and higher carbon require care with weld thermal cycles to ensure niobium effectively ties up carbon and avoids local chromium-depleted zones.
Common weldability indices (for interpretation) - Carbon equivalent (IIW form) often used qualitatively to assess hardenability/weld-cracking risk: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - A more detailed parameter for stainless steels: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation (qualitative) - Both grades give moderate values in these indices relative to ferritic/quenched steels; they are not prone to hydrogen-assisted cold cracking but can exhibit solidification cracking and formation of deleterious intermetallic phases if improper filler or heat input is used. - Preheat is generally not required for austenitic stainless steels, but post-weld heat treatment and filler selection should be chosen to preserve stabilization effectiveness: for 321 ensure titanium-to-carbon ratio is adequate in weld metal; for 347H choose filler and procedure that accomodate niobium stabilization and prevent local depletion.
6. Corrosion and Surface Protection
General corrosion - Both 321 and 347H rely on chromium to form a passive oxide film; they provide good general corrosion resistance in many atmospheres and mild chemical environments.
Intergranular corrosion and high-temperature sensitization - The stabilizers (Ti in 321 and Nb in 347H) are specifically included to prevent chromium carbide precipitation at grain boundaries during exposure to sensitizing temperature ranges, thereby reducing susceptibility to intergranular corrosion. - 347H's niobium stabilization combined with its higher carbon content improves resistance to intergranular attack during prolonged high-temperature exposure and thermal cycling common in boiler and superheater applications.
Use of corrosion indices - The Pitting Resistance Equivalent Number (PREN) is relevant for assessing pitting resistance in chloride environments: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - For 321 and 347H, PREN is of limited utility because these grades are not designed primarily for high pitting resistance (Mo poor); PREN is more meaningful for duplex or high-Mo austenitics/ferritics.
Surface protection for non-stainless alternatives - Not applicable here (both are stainless). For non-stainless steels, protection would include galvanizing, painting, or plating.
7. Fabrication, Machinability, and Formability
- Machinability: Austenitic stainless steels work-harden rapidly; both 321 and 347H require sharp tooling, rigid setups, and proper cutting parameters. 347H (higher carbon) may machine slightly less easily than 321, but difference is modest.
- Formability and bending: Both are highly formable in annealed condition. 321 may show marginally better formability due to lower carbon content, while 347H's higher-carbon and precipitate structure can stiffen the material.
- Surface finish: Both take similar finishing and polishing processes; pickling and passivation treatments are standard after welding to restore chromium-rich passive film.
8. Typical Applications
| 321 – Typical Uses | 347H – Typical Uses |
|---|---|
| Exhaust components and heat exchangers in aircraft and automotive systems | Boiler tubes, superheater and reheater tubing in fossil and nuclear plants |
| Chemical process equipment exposed to moderate high temperatures and corrosive atmospheres | High-temperature furnace hardware and piping requiring long-term creep resistance |
| Food processing equipment and heat-treatment fixtures where stabilization is desired | Petrochemical high-temperature piping and vessel components where sensitization risk is high |
| Aerospace and engine components where titanium stabilization is well understood | Components exposed to long-duration thermal cycles where niobium stabilization preserves grain boundary integrity |
Selection rationale - Choose 321 when general high-temperature corrosion resistance and good weldability are required, and cost or forming ease is a priority. - Choose 347H when service involves prolonged exposures at elevated temperatures where intergranular corrosion (sensitization) and creep resistance are primary concerns, and slightly higher material cost is acceptable.
9. Cost and Availability
- Cost: 321 is commonly available and typically less expensive than 347H because niobium-alloyed, high-carbon stabilized grades are specialty items and use more costly alloying and processing controls.
- Availability: 321 is widely stocked in plates, sheets, bars, and welded/seamless tubing. 347H is available in standard product forms but may be less common in some market regions and in large-diameter seamless products — lead times and minimum orders can be longer.
- Procurement note: specify exact grade and stabilization requirement (Ti vs Nb, carbon range) on purchase orders to avoid receiving the non-H variant of 347 or an unstabilized grade.
10. Summary and Recommendation
Table: qualitative summary
| Criterion | 321 | 347H |
|---|---|---|
| Weldability | Good — easier to control with Ti stabilization | Good — requires attention to Nb stabilization and filler choice |
| Strength–Toughness (high-temp) | Good at moderate temps | Better high-temperature strength and creep resistance |
| Cost | Generally lower / widely available | Generally higher / speciality grade |
Conclusion (recommendations) - Choose 321 if: - The application involves moderate high temperatures with occasional thermal cycling, where good general corrosion resistance and fabrication ease are required. - Cost, formability, and readily available product forms are important. - Choose 347H if: - Service involves prolonged elevated-temperature exposure, long-term creep stress, or repeated thermal cycles that risk sensitization and intergranular corrosion. - Higher-temperature mechanical property retention and grain-boundary stability are critical and you can accept higher material cost and slightly more demanding welding/fabrication controls.
Final practical note - For critical components exposed to long-duration high-temperature service, specify the stabilized grade, required post-fabrication heat treatment (if any), weld filler composition, and inspection criteria. Early engagement of metallurgy and welding engineering during design and procurement avoids costly field failures or remanufacture.