316Ti vs 321H – Composition, Heat Treatment, Properties, and Applications
แบ่งปัน
Table Of Content
Table Of Content
Introduction
316Ti and 321H are both austenitic stainless steels used where combination of corrosion resistance and elevated-temperature performance is required. Choosing between them is a recurring dilemma for engineers and procurement teams balancing corrosion resistance, high-temperature strength, weldability, and life‑cycle cost: 316Ti is a molybdenum‑bearing grade stabilized with titanium for improved resistance to sensitization, while 321H is a titanium‑stabilized chromium‑nickel grade offered with higher carbon for improved creep and strength at elevated temperature. These differences make the two grades attractive for overlapping but distinct service envelopes—316Ti where pitting resistance and general corrosion resistance are prioritized, and 321H where long‑term stability in high‑temperature oxidizing environments and creep resistance are important.
1. Standards and Designations
Common international standards and designations where these grades are found:
- ASTM/ASME: A240 (plate, sheet, and strip), A312 (pipe), A403 (fittings) — 316Ti and 321/321H variants are specified.
- EN: EN 1.4571 (316Ti), EN 1.4878 (321H) equivalents are used in European standards.
- JIS: JIS G4303/G4313 family includes stabilized austenitics with local designations.
- GB (China): GB/T standards list stainless grades comparable to 316Ti and 321H.
Classification: - Both 316Ti and 321H are stainless steels (austenitic). They are not carbon steels, alloy steels, tool steels, or HSLA.
2. Chemical Composition and Alloying Strategy
Table: typical composition ranges (expressed as weight percent). Values vary by specification and product form; the table shows representative ranges commonly cited in standards. Always consult the relevant standard or mill certificate for exact composition.
| Element | 316Ti (typical range) | 321H (typical range) |
|---|---|---|
| C | ≤ 0.08 (control to low carbon) | ~0.04–0.10 (higher‑C variant of 321) |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 1.0 | ≤ 1.0 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | ~16–18 | ~17–19 |
| Ni | ~10–14 | ~9–12 |
| Mo | ~2.0–3.0 | ≤ 0.5 (typically none) |
| V | trace or not specified | trace or not specified |
| Nb | — (not a primary stabilizer) | — (Ti is the stabilizer; Nb sometimes used in related grades) |
| Ti | controlled addition (≥ 5 × C, up to ~0.7) | controlled addition (≥ 5 × C, up to ~0.7) |
| B | trace | trace |
| N | trace to small additions possible | trace |
Notes on alloying strategy: - 316Ti: built on the 316 family—chromium, nickel, and molybdenum provide superior general and pitting corrosion resistance. Titanium is added to stabilize carbon, forming carbon‑titanium precipitates to avoid chromium carbide precipitation during exposure in the sensitization range (roughly 450–850 °C). - 321H: derived from 321 (Cr–Ni with Ti stabilization) but furnished with higher carbon to improve creep strength and sustained high‑temperature properties. Titanium in 321H ties carbon and reduces sensitization while retaining higher C for creep strength.
Alloying effects: - Chromium provides passivity and general corrosion resistance. - Nickel stabilizes the austenitic structure and improves toughness. - Molybdenum in 316Ti increases resistance to localized corrosion (pitting/crevice). - Titanium ties up free carbon to prevent intergranular corrosion after welding or exposure to sensitizing temperatures; in 321H the higher carbon increases strength at elevated temperatures but requires correct titanium content to prevent sensitization.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both grades are fully austenitic in the annealed condition with dispersed carbides or intermetallics depending on thermal history. - Titanium forms TiC or Ti(C,N) precipitates preferentially ahead of chromium carbide formation, maintaining chromium in solution at grain boundaries.
Heat‑treatment and processing effects: - Annealing (solution treatment) at typical austenitizing temperatures followed by rapid cooling returns both materials to a ductile, single‑phase austenitic microstructure. - For 316Ti, standard solution anneal eliminates previous carbide precipitation; the Ti–C precipitates remain stable if Ti is sufficient relative to carbon. - 321H is normally supplied in the solution‑annealed condition; the higher carbon provides greater precipitation strengthening on longer exposures at elevated temperature, enhancing creep strength. - Normalizing, quenching & tempering are not applicable to austenitic stainless steels in the same sense as for ferritic/pearlitic steels; mechanical properties are achieved mainly by cold work, solution anneal, and aging/precipitation effects at service temperature. - Thermo‑mechanical processing (cold work + anneal) can increase strength via strain hardening; long service exposure between about 500–800 °C can cause complex carbide and intermetallic precipitation affecting toughness and corrosion resistance if titanium is inadequate.
4. Mechanical Properties
Mechanical properties depend on product form (sheet, plate, pipe), heat treatment, and testing standard. The table below provides qualitative comparative descriptors rather than absolute numeric values—consult mill certificates for precise figures.
| Property | 316Ti (annealed, typical behavior) | 321H (annealed or stabilized) |
|---|---|---|
| Tensile strength | Moderate — consistent with austenitic 316 family | Similar or slightly higher at elevated temperatures due to C strengthening |
| Yield strength | Moderate — good ductility | Slightly higher yield at high temperatures; room‑temperature yield similar to 316Ti |
| Elongation | High (good ductility and formability) | Good, but may be modestly reduced if higher C or cold‑worked |
| Impact toughness | High at room temperature; good low‑temperature toughness | Good at room temp; retains toughness at elevated temp, but long exposures may affect toughness if precipitation occurs |
| Hardness | Low to moderate (soft, ductile, easily cold‑worked) | Comparable; higher C can marginally increase hardness |
Interpretation: - At room temperature both grades exhibit the characteristic ductility and toughness of austenitic stainless steels. - 321H’s higher carbon and stabilization strategy give it an edge for high‑temperature creep and strength retention over extended service, while 316Ti offers slightly better resistance to localized corrosion because of molybdenum.
5. Weldability
Weldability of austenitic stainless steels is generally good; two aspects are important here: susceptibility to sensitization/intergranular corrosion and cold‑work/hardenability effects near welds.
Key weldability indices: - Carbon equivalent (IIW formula) is useful for assessing hardening tendency in welding: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The chromium‑equivalent or Pcm formula is also used for assessing weld cracking susceptibility: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 316Ti: Generally excellent weldability. Low carbon and titanium stabilization reduce risk of intergranular corrosion after welding. Molybdenum does not seriously impair weldability but increases the alloy’s tendency to form low‑melting phases in rare circumstances; standard filler metallurgy and control of heat input avoid problems. - 321H: Also weldable but higher carbon elevates the theoretical carbon equivalent measures, increasing the need for controlled heat input and potential post‑weld treatments in thick sections. Titanium stabilization mitigates chromium carbide formation, but when carbon content is deliberately higher (as in 321H) control of Ti:C ratio is critical. Preheating is typically unnecessary for thin sections; for heavy sections and cyclic high‑temperature service, welding procedure qualification is recommended.
Overall: both grades are considered weldable with standard procedures; 316Ti is often considered easier with less requirement for additional controls when corrosion resistance is the primary concern, while 321H requires attention when used in thick sections or in applications where post‑weld heat treatment and creep performance matter.
6. Corrosion and Surface Protection
- For stainless grades, corrosion performance is driven primarily by chromium content and presence of molybdenum and nitrogen.
- PREN (pitting resistance equivalent number) is helpful for comparing pitting resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Application of PREN: 316Ti benefits from its molybdenum content, yielding a higher PREN than 321H in typical compositions; 321H’s lack of Mo means lower pitting resistance in chloride environments.
Non‑stainless steels: - Not applicable here since both grades are stainless. If using alternative carbon steels, coatings (galvanizing, painting, linings) would be necessary.
Practical notes: - 316Ti: preferred for chloride‑bearing environments (sea water, chemical process streams) because of Mo‑enhanced pitting resistance and Ti stabilization to prevent sensitization. - 321H: better suited for high‑temperature oxidizing environments (exhaust systems, heaters, boilers) where creep resistance and resistance to high‑temperature corrosion/oxidation are priorities; not optimal for aggressive chloride environments unless protected.
7. Fabrication, Machinability, and Formability
- Machinability: Austenitic stainless steels are generally more adhesive and work‑hardening than ferritic steels. 316Ti and 321H machine similarly, though increased carbon in 321H can slightly improve tool interaction but may also increase work hardening locally.
- Formability: Both grades form well in the annealed condition; 316Ti typically has slightly better formability due to lower carbon and the presence of molybdenum does not impede forming. 321H is formable but processes should account for potential springback and strain‑hardening behavior.
- Surface finishing: Both accept standard grinding, polishing, and passivation treatments. Passivation chemicals and parameters are the same as for other austenitic grades but verifying post‑processing corrosion resistance is recommended, especially after welding.
- Cold work: Cold deformation increases strength but reduces ductility; final anneal may be used to restore formability and corrosion resistance.
8. Typical Applications
| 316Ti — Typical Uses | 321H — Typical Uses |
|---|---|
| Chemical process equipment exposed to chlorides, heat exchangers, tanks and piping in corrosive environments, marine fittings, pharmaceutical equipment where pitting resistance matters | Exhaust stacks, furnace and boiler components, high‑temperature process piping, heat‑resistant fittings, aircraft and engine exhausts where sustained high temperature exposure and creep resistance are required |
| Food and beverage equipment where corrosion resistance and cleanability are needed | High‑temperature structural components and expansion joints in petrochemical/industrial heaters |
Selection rationale: - Choose 316Ti for environments where pitting and crevice corrosion from chlorides or aggressive process fluids are primary concerns and where welded assemblies must avoid intergranular corrosion. - Choose 321H for sustained high‑temperature service where creep strength, resistance to oxidation, and stability after prolonged exposure are more critical than maximum pitting resistance.
9. Cost and Availability
- Cost: 316Ti typically commands a premium relative to unstabilized 316 and some 321 variants due to molybdenum content and titanium addition. 321H’s cost is influenced by heat‑treatment, higher carbon content, and market availability; since it lacks Mo it can be less costly than 316Ti in raw alloy content terms but specialty supply and product forms may affect price.
- Availability: Both are widely available in common product forms (sheet, plate, pipe, tube, bar, and fittings) from major stainless steel producers. 316Ti is ubiquitous in process industries; 321H is commonly available where high‑temperature alloys are stocked. Long lead times are possible for large diameters, heavy sections, or special finish/traceability requirements.
10. Summary and Recommendation
| Criterion | 316Ti | 321H |
|---|---|---|
| Weldability | Very good — titanium stabilization reduces sensitization risk | Good — higher C requires attention to heat input and Ti:C control |
| Strength–Toughness | Good combination at room temp; moderate high‑temp strength | Better sustained high‑temp strength/creep for long exposures |
| Cost | Higher alloy cost (Mo) but widely stocked | Comparable or lower alloy cost; specialty demand for high‑temp forms may vary availability |
Recommendation: - Choose 316Ti if your primary requirement is corrosion resistance in chloride‑bearing or aggressive chemical environments, combined with the need to maintain corrosion resistance after welding and good general mechanical properties. - Choose 321H if your application exposes components to prolonged elevated temperatures where creep resistance, oxidation resistance, and long‑term dimensional stability are the priority, and where pitting in chloride environments is not the dominant failure mode.
Final note: both materials serve important but different niches. Specify the exact alloy, product form, heat treatment, and acceptance tests in procurement documentation and request mill certificates. For critical welded assemblies or long‑term elevated temperature service, perform application‑specific assessments (welding procedure qualification, corrosion testing, and creep life estimation) rather than relying on generic grade selection.