Titanium Flanges: Engineering Guide for ASTM B381 Forgings
Technical handbook for Titanium Grade 2, Grade 5, Grade 7, and Grade 12 flanges. The ultimate solution for wet chlorine, seawater, and oxidizing acid environments where stainless steel fails.
- ASTM B381 Specification: Forging properties vs. plate/bar.
- Safety Critical: Wet Chlorine vs. Dry Chlorine ignition risks.
- Cost/Performance: When to upgrade from Gr 2 to Gr 12 or Gr 7.
1. Titanium Flanges: The Ultimate Corrosion Barrier
Titanium flanges are not just "premium" components; they are often the only material capable of surviving in wet chlorine, hypochlorites, and hot seawater. Unlike stainless steel, which relies on a passive film that can break down in chlorides (pitting), titanium's TiO₂ oxide film is self-healing and stable in high-chloride environments.
1.1 Understanding ASTM B381
When ordering flanges, specification is key. ASTM B381 is the specific standard for Titanium and Titanium Alloy Forgings. Do not confuse it with ASTM B348 (Bars) or B265 (Plates), although the chemistry is similar. B381 ensures the grain structure produced by forging is suitable for the multi-axial stresses flanges endure.
2. The "Big Four" Titanium Grades for Flanges
While there are nearly 40 grades, Sunhyings focuses on the four most critical for industrial piping.
Grade 2 (UNS R50400)
Commercially Pure (CP). The workhorse. Moderate strength (Yield ~40 ksi), excellent ductility. Used in 90% of chemical plants for wet chlorine and seawater cooling.
Grade 5 (UNS R56400)
Ti-6Al-4V. The high-strength alloy. Yield >120 ksi. Used in offshore risers and aerospace where weight and strength are critical. Note: Lower corrosion resistance in reducing acids than CP grades.
Grade 7 (UNS R52400)
Ti-0.15Pd. Grade 2 + Palladium. The "Gold Standard" for corrosion. The Pd addition prevents crevice corrosion in hot brine and reducing acids (pH < 1). Expensive but necessary for extreme conditions.
Grade 12 (UNS R53400)
Ti-0.3Mo-0.8Ni. The smart alternative. Improved heat resistance and crevice corrosion resistance over Gr 2, at a much lower cost than Gr 7. Ideal for hot brine applications.
3. ASTM B381 Chemical & Mechanical Properties
Comparison of the forging specifications (simplified for reference).
| Grade (Forging) | Alloying Elements | Tensile (min) | Yield (min) | Elongation |
|---|---|---|---|---|
| F-2 (Gr 2) | Unalloyed | 50 ksi (345 MPa) | 40 ksi (275 MPa) | 20% |
| F-5 (Gr 5) | 6% Al, 4% V | 130 ksi (895 MPa) | 120 ksi (828 MPa) | 10% |
| F-7 (Gr 7) | 0.12-0.25% Pd | 50 ksi (345 MPa) | 40 ksi (275 MPa) | 20% |
| F-12 (Gr 12) | 0.3% Mo, 0.8% Ni | 70 ksi (483 MPa) | 50 ksi (345 MPa) | 18% |
*Grade F-2H is available for pressure vessels requiring higher minimum UTS (58 ksi). Always specify if "H" grade is needed for ASME code compliance.
4. Critical Safety: Corrosion & Ignition Hazards
4.1 Wet vs. Dry Chlorine: A Matter of Life and Death
WARNING: Titanium reacts violently (ignites) in DRY Chlorine gas. Titanium flanges must NEVER be used in dry chlorine service. However, in WET Chlorine (water content >0.5% - 1.5%), titanium is one of the only materials that is virtually immune to corrosion. Ensure your process maintains sufficient moisture content.
4.2 Seawater & Crevice Corrosion
Titanium is immune to general corrosion in seawater. The limiting factor is Crevice Corrosion (under gaskets).
- Grade 2: Safe up to ~80°C in seawater. Above this, crevice corrosion may occur.
- Grade 12: Resistant up to ~260°C.
- Grade 7: Resistant >260°C and in low pH (acidic) brines.
5. Manufacturing: Forging & Welding Challenges
Processing titanium requires strict adherence to metallurgical controls to avoid embrittlement.
5.1 The "Beta Transus" Risk
Forging must be carefully controlled relative to the Beta Transus temperature (approx 880-915°C for Gr 2, 995°C for Gr 5). If forged entirely in the Beta field without proper working, the grain structure becomes coarse (Widmanstätten), reducing ductility. Sunhyings ensures controlled Alpha-Beta forging for optimal toughness.
5.2 Welding & Gas Shielding
Welding titanium flanges (e.g., Slip-On or Socket Weld) requires inert gas shielding (Argon/Helium) not just at the arc, but for the cooling trail.
Visual Quality Check:
- Silver / Light Straw: Good shield, ductile weld.
- Blue / Purple: Oxygen contamination, brittle. (Reject).
- White / Flaky powder: Total failure. (Reject).
5.3 Iron Contamination (Ferroxyl Test)
Iron particles embedded from machining tools can ruin titanium's oxide film. We perform Ferroxyl Tests (blue spot test) to ensure surface purity. Any contamination is removed via pickling (HNO₃-HF).
6. Design Strategy: Managing Cost
Titanium is expensive. For large diameters (e.g., > 6 inches), solid titanium flanges can be cost-prohibitive.
Value Engineering Solution: Use a Lap Joint Flange (Stub End).
- Stub End: Made of Titanium Gr 2 (wetted part).
- Backing Flange: Made of Carbon Steel (galvanized or painted).
This design maintains titanium corrosion resistance on the process side while utilizing steel's strength and lower cost for the bolting load.
7. Key Industry Applications
- Chlor-Alkali Plants: Anode/Cathode circulation, wet chlorine gas coolers (Gr 2).
- Desalination (MSF/RO): Heat exchangers, ejectors, and high-velocity seawater piping (Gr 2 or Gr 12).
- Offshore Oil & Gas: Fire water systems (immune to stagnant seawater), Risers (Gr 5 for high strength-to-weight ratio).
- Pulp & Paper: Bleaching stages (Chlorine Dioxide - ClO₂) where stainless steel corrodes rapidly.
8. FAQ – Purchasing Titanium Flanges
No. Direct arc welding of Titanium to Steel forms brittle intermetallic compounds (TiC, TiFe) that will crack immediately. You must use a mechanical connection (flange) or an explosion-bonded transition joint.
Grade 12 is much cheaper than Grade 7 (no Palladium). If your process is hot brine or mild acid, Grade 12 is usually sufficient. Grade 7 is reserved for extreme reducing acids or very low pH, high-temp environments where Grade 12 might fail.
Titanium is paramagnetic (effectively non-magnetic). This makes it ideal for specific naval or electronic applications where magnetic interference must be minimized.
9. Request a Technical Quote
Sunhyings supplies ASTM B381 flanges in solid titanium and lap-joint configurations. Send us your specs (Grade, Class, Schedule) for a rapid quote.
Resources
- Titanium Grades Comparison Chart (PDF)
- Corrosion Data: Titanium in Chlorine & Seawater
- ASTM B381 Forging Tolerances