316Ti vs 904L – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners frequently weigh stainless steel choices against an array of competing priorities: corrosion resistance in aggressive media, weldability and fabrication cost, mechanical performance at service temperature, and supply-chain availability. Two austenitic stainless steels that commonly surface in those trade-offs are 316Ti (a titanium-stabilized variant of 316) and 904L (a high‑alloy, low‑carbon austenitic grade with elevated nickel, molybdenum, and copper).
The principal metallurgical distinction between these grades is their alloying strategy: one uses titanium to stabilize carbon and avoid intergranular carbide precipitation, while the other relies on higher nickel and molybdenum (and added copper) to achieve enhanced general and localized corrosion resistance. Because of that, 316Ti and 904L occupy overlapping but distinct application spaces where corrosion regime, temperature stability, weldability, and cost drive selection.
1. Standards and Designations
- 316Ti
- Common designations: UNS S31635, EN 1.4571, ASTM A240 (as part of 316 family with Ti-stabilization in some specifications).
- Classification: Austenitic stainless steel / stainless alloy.
- 904L
- Common designations: UNS N08904, EN 1.4539.
- Classification: High-alloy austenitic stainless steel (often used for highly corrosive environments).
Other regional standards (JIS, GB) may refer to equivalent or similar chemistry under different identifiers; specify the exact standard and material certificate required when ordering.
2. Chemical Composition and Alloying Strategy
The following table lists elements commonly specified for these grades. Values shown are typical composition ranges used in industry specifications; actual values must be confirmed per material certification from suppliers.
| Element | 316Ti (typical range, wt%) | 904L (typical range, wt%) |
|---|---|---|
| C | ≤ 0.08 (stabilized by Ti) | ≤ 0.02 (low C) |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 1.0 | ≤ 1.0 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | ~16–18 | ~19–23 |
| Ni | ~10–14 | ~23–28 |
| Mo | ~2–3 | ~4–5.5 |
| V | trace/none | trace/none |
| Nb | none | none (not normally added) |
| Ti | ~0.5–0.7 (or a stoichiometric amount to tie up C) | none |
| B | trace/none | trace/none |
| N | low (trace) | ≤ 0.1 (often small amounts) |
| Cu | none | ~1–2 |
How alloying affects properties: - Titanium in 316Ti preferentially forms stable carbonitrides (TiC/TiN) that prevent chromium carbide precipitation at grain boundaries during exposure to intermediate temperatures (sensitization), preserving intergranular corrosion resistance after thermal cycles or welding. - 904L achieves corrosion resistance primarily through a higher nickel content (stabilizes austenite and increases resistance to chloride stress corrosion cracking) and elevated molybdenum for improved pitting and crevice corrosion resistance. Copper addition improves resistance to reducing acids (notably sulfuric acid).
3. Microstructure and Heat Treatment Response
Both grades are essentially fully austenitic in the normalized/solution-annealed condition.
- 316Ti
- Microstructure: Austenitic matrix with Ti-stabilized carbides/nitrides distributed at high-temperature processing stages. In properly solution-annealed material, carbide precipitation is suppressed and grain-boundary sensitization is minimized.
- Heat treatment response: Typical practice is solution annealing (e.g., 1040–1150 °C range for austenitic stainless steels) followed by rapid quench to maintain a single-phase austenite. 316Ti is not hardenable by quench-and-temper in the way ferritic/martensitic steels are; strength is primarily modified by cold work.
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Sensitization resistance: Improved over 316/316L because Ti ties up carbon, preventing Cr-carbide formation.
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904L
- Microstructure: Fully austenitic, generally free of intentionally added stabilizers. Low carbon eliminates significant carbide precipitation risk; small nitrogen additions (if present) further strengthen austenite.
- Heat treatment response: Solution annealing and rapid quenching are used to dissolve any undesirable precipitates. Like other austenitic stainless steels, mechanical properties are adjusted by cold work rather than hardening heat treatments.
- Sensitization resistance: Inherently low-chromium carbide formation due to low C content; no Ti or Nb required.
Note: Conventional normalizing, quenching & tempering cycles used for carbon or martensitic steels are not applicable for these austenitic grades.
4. Mechanical Properties
Mechanical properties depend on product form (sheet, plate, bar), cold work, and heat treatment. The table below provides representative annealed-condition ranges commonly cited for these types of austenitic stainless steels; use supplier mill certificates for design calculations.
| Property (annealed) | 316Ti (representative) | 904L (representative) |
|---|---|---|
| Tensile strength (UTS) | ~480–620 MPa | ~500–700 MPa |
| Yield strength (0.2% proof) | ~170–300 MPa | ~200–350 MPa |
| Elongation (A, %) | ~40% (good ductility) | ~35–45% (good ductility) |
| Impact toughness | Good, retains toughness at low temp | Comparable; generally good toughness |
| Hardness (HRB/HRC) | Annealed: low (~70–95 HRB) | Annealed: similar range |
Interpretation: - Both are ductile, tough austenitic grades. 904L’s higher nickel and molybdenum content typically result in comparable or slightly higher strength in some product forms versus stabilized 316Ti, but differences are often modest compared to the effects of cold work. - Toughness in both grades is generally high; neither displays a sharp ductile-to-brittle transition as ferritic/martensitic steels do.
5. Weldability
Austenitic stainless steels are among the most weldable metallic alloys, but composition influences weld behavior.
Key weldability equations used for qualitative assessment: - Typical chromium equivalent for assessing hardenability and weld cracking tendency: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Combined parameter to estimate cold 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: The titanium stabilization markedly reduces the risk of intergranular corrosion after welding by preventing chromium carbide precipitation. However, Ti increases the tendency for hard-to-machine inclusions and may require proper filler selection; filler metals commonly used are 316L/316 filler variants to avoid depletion of stabilizer in the weld zone. Preheat/post-weld heat treatment is typically not required, though control of heat input is used to avoid excessive precipitation of titanium-rich phases. - 904L: Excellent weldability in the sense of ductility and fusion; low carbon reduces carbide precipitation risk. The high nickel and molybdenum content mean filler selection should match chemical and mechanical requirements (matched high-alloy filler to maintain corrosion resistance); high-alloy weld consumables are more expensive. Hot cracking risk is not typically higher than other austenitics, but welding parameters must account for higher thermal expansion and contraction tendencies.
In both grades, welding consumables, joint design, and heat input control are important for maintaining corrosion resistance in the heat-affected zone.
6. Corrosion and Surface Protection
- For non-stainless steels, protective systems include galvanizing, painting, epoxy linings, or cathodic protection; such measures are not primary considerations for these stainless grades.
- For stainless grades, pitting resistance is commonly quantified by the Pitting Resistance Equivalent Number (PREN): $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ This index gives a rough indication of resistance to localized (pitting) corrosion in chloride-containing environments.
Corrosion behavior: - 316Ti: Offers good general corrosion resistance typical of 316 family due to chromium content and molybdenum for pitting resistance. The titanium stabilization primarily preserves corrosion resistance after exposure to sensitizing thermal cycles or welding by preventing chromium carbide precipitation. - 904L: Designed for superior resistance to both general and localized corrosion. Higher Ni and Mo raise the PREN relative to 316Ti, improving resistance to pitting and crevice corrosion in chloride-bearing environments. Copper improves resistance to reducing acids (e.g., sulfuric acid). 904L is often chosen where chloride pitting or aggressive acidic environments are present and where minimizing maintenance is critical.
Note: PREN is an index—not a substitute for application-specific corrosion testing—because actual performance depends on microstructure, surface finish, temperature, flow conditions, and chemical environment.
7. Fabrication, Machinability, and Formability
- Machinability
- 316Ti: Work-hardens; titanium stabilization can make tooling wear somewhat higher than for 316L. Standard austenitic stainless machining practices (sharp tooling, rigid setups, high coolant flow, controlled speeds) apply.
- 904L: Generally more difficult to machine than 316-class alloys due to higher nickel content and toughness; cutter life is shorter and cutting parameters must be conservative.
- Formability
- Both grades are readily formed in the annealed condition. 316Ti retains formability similar to other 316 variants; 904L can be cold-formed but springback and work-hardening should be accounted for.
- Surface finishing
- Both can be polished, passivated, and electropolished. 904L may be more challenging to electropolish uniformly because of alloying differences.
8. Typical Applications
| 316Ti — Typical Uses | 904L — Typical Uses |
|---|---|
| High-temperature components where sensitization is a concern (e.g., furnace components, heat exchangers, boiler and superheater tubes) | Chemical processing equipment in highly corrosive environments (e.g., sulfuric acid production, pickling lines) |
| Process piping and vessels that see thermal cycles or welding where stabilization is needed | Seawater handling and offshore equipment where pitting and crevice corrosion risk is high |
| Exhaust systems, turbocharger components, and furnace fixtures | Flue gas desulfurization systems, acid handling, and environments with reducing acids due to Cu addition |
| General-purpose corrosion-resistant components where 316 family properties are sufficient but stability against sensitization is required | High-integrity applications where extended service life in harsh chemical media outweighs material cost |
Selection rationale: - Choose 316Ti where thermal cycles or fabrication could cause sensitization and where standard 316 corrosion resistance is adequate. - Choose 904L where higher pitting/crevice corrosion resistance and resistance to specific acids (especially sulfuric acid) are required and where lifecycle cost justifies higher material cost.
9. Cost and Availability
- Cost: 904L is significantly more expensive than 316Ti on a per-kilogram basis because of the much higher nickel and molybdenum content and the addition of copper. Material cost can materially affect project budgets for large equipment.
- Availability: 316Ti is broadly available in plate, sheet, tube, and bar from many mills and distributors. 904L is available but less ubiquitous; longer lead times or special-order production may be required for large sections or custom forms. Filler metals for welding 904L are also higher cost and may have limited availability in some regions.
10. Summary and Recommendation
| Attribute | 316Ti | 904L |
|---|---|---|
| Weldability | Very good; Ti reduces sensitization risk in HAZ | Very good; low C helps, but filler selection important |
| Strength–Toughness | Good ductility and toughness; typical austenitic behavior | Comparable or slightly higher strength in some forms; tough and ductile |
| Corrosion performance | Good general and localized corrosion resistance; stabilized for thermal exposure | Superior localized corrosion resistance (pitting/crevice) and resistance to reducing acids |
| Relative cost | Lower | Higher |
Conclusion and selection guidance: - Choose 316Ti if: - The dominant concern is preventing sensitization after welding or exposure to intermediate temperatures (e.g., welded pressure parts, heat-affected components). - Standard 316-class corrosion resistance (with Mo for pitting) is adequate for the service environment. - You require a more economical option with broad availability and predictable fabrication behavior.
- Choose 904L if:
- Service includes highly aggressive chloride environments, crevice or pitting-prone conditions, or reducing acids (e.g., sulfuric acid), and superior corrosion resistance is required.
- Long service life and reduced maintenance in aggressive chemical environments justify higher material and fabrication costs.
- Procurement can accommodate higher-cost high-alloy consumables and possible lead times.
Always verify material selection with environment-specific corrosion data, mechanical requirements, welding procedure qualification, and life-cycle cost analysis. For critical systems, conduct application-specific corrosion testing or consult metallurgy specialists and mill certifications before final specification.