B381 F6
B381 F6 is a Fe microalloyed strengthened pure titanium forging (belonging to the category of low-alloyed pure titanium), which comes from ASTM B381 Standard Specification for Titanium and Titanium Alloy Forgings (current version ASTM B381-21). Through “Fe microalloying+interstitial element (O) synergistic strengthening”, the strength bottleneck of pure titanium is broken through, and the cost-performance balance of “corrosion resistance of pure titanium+strength of near-alloyed titanium” is realized in the middle and high stress corrosion resistance scene. The following system analysis:
I. Standards and brand system
1. Core standards
- International specification: ASTM B381 (covering pure titanium and titanium alloy forgings, F6 is the only microalloyed brand in pure titanium series, which refines the grain and strengthens the matrix through Fe, which is different from “pure titanium+gap strengthening” of F1-F5);
- Domestic adaptation: there is no completely equivalent brand, and the pure titanium composition of TA5 in GB/T 25137 is similar (TA5 contains 0.3-0.8% Fe and 0.2-0.3% O, but F6 controls the Fe content more strictly, focusing on the forging properties).
2. Brand characteristics
- Commercial name: no exclusive alias, collectively referred to as “ASTM B381 F6 titanium forgings”;
- Positioning: the “transitional brand” from pure titanium to titanium alloy (Fe, as a cheap β stabilizing element, achieves 80% strength of alloy titanium at 1/10 cost, and its corrosion resistance is still close to pure titanium).
II. Chemical composition (mass fraction%, ASTM B381 standard)
F6 enhances the strength through the dual mechanism of “Fe microalloying (β phase strengthening)+O gap strengthening”, and the elements have precise functions:
| element | Content range | Core role |
|---|---|---|
| Ti | margin | Matrix, providing low density (4.5 g/cm³) and basic corrosion resistance. |
| Fe | 0.30~0.80 | Key alloying elements! Form β phase, refine grains, and strengthen by solid solution/precipitation (Fe atoms pin dislocations to improve strength). |
| O | 0.20~0.30 | Gap strengthens α phase, balancing strength and plasticity (O content should be matched with Fe to avoid embrittlement) |
| N | ≤0.05 | Strictly control titanium nitride (TiN) to avoid intergranular embrittlement. |
| C | ≤0.10 | Inhibit titanium carbide (TiC) and ensure weldability. |
| H | ≤0.015 | Prevent hydride (TiH₂) embrittlement (hydrogen absorption is forbidden for forgings) |
III. Mechanical properties (annealed state, forgings)
Because of Fe microalloying and O-gap strengthening, the performance of F6 is far superior to that of high-gap pure titanium (such as F5) and close to that of low-end titanium alloy (such as Ti-3Al-2.5V);
| Performance index | Typical value (ASTM requirements) | Contrast F5 (high-gap pure titanium) | Comparative Ti-3Al-2.5V (low-end alloy titanium) |
|---|---|---|---|
| tensile strength | 600~750 MPa | 15% higher (F5 ≈ 550 ~ 650 MPa) | 20% lower (alloy titanium ≈750~900 MPa) |
| yield strength | 400~550 MPa | 15% higher F5≈350~450 MPa） | 25% lower (alloy titanium ≈500~650 MPa) |
| extensibility（δ₅） | 10~18% | 15% lower（F5≈12~22%） | 20% higher (alloy titanium ≈8~15%) |
| hardness | 140~180 HB | 15% higher （F5≈120~160 HB） | 15% lower (alloy titanium ≈160~200 HB) |
| heat resistance | The corrosion rate in 3.5% NaCl is less than 0.01 mm/year. | Equivalent to F5. | Excellent 30% (alloy titanium contains Al/V, and its corrosion resistance is slightly lower) |
IV. Requirements for Heat Treatment and Processing
-
Annealing treatment (core technology, balance performance):
- Temperature: 750~850℃ (heat preservation for 1~2 hours, α+β phase region, Fe promotes recrystallization and refines grains);
- Cooling: air cooling (retaining α+β biphase structure, Fe stabilizing β phase, improving plasticity and hot workability);
- Advantages: Fe lowers the recrystallization temperature of pure titanium, has better hot workability than F5 (high-gap pure titanium), and can forge more complex shapes (such as curved flanges and special-shaped forgings).
-
Machining and welding:
- Machining: Fe refines the grain, and its cutting performance is better than that of alloy titanium (the surface roughness is lower, and the tool loss is reduced by 30%). It is recommended that cemented carbide tools+medium speed cutting;
- Welding: TIG welding (ERTi-1 or ERTi-Fe welding wire) is adopted, and Fe can improve the toughness of the weld, which can avoid preheating welding (unlike the strict argon protection of high-gap pure titanium, F6 has a slightly higher tolerance to pollution).
V. Main application fields (medium and high stress+corrosion resistance+low cost scenario)
F6 relies on “Fe strengthening cost performance+corrosion resistance of pure titanium”, covering the following fields (replacing F5 pure titanium and low-end alloy titanium, reducing cost by 20-30%):
-
Ocean engineering:
- High-stress components: valve body of deep-sea Christmas tree and impeller of high-pressure seawater pump (resistant to 4000m water depth and corrosion, with strength superior to F5 and cost lower than alloy titanium);
- Wear-resistant structure: fasteners for ship propeller edge and offshore platform (Fe improves wear resistance, and its service life is 5 times longer than that of brass).
-
Chemical industry and energy:
- Medium and high pressure equipment: electrode bracket of chlor-alkali electrolyzer and flange of medium pressure reaction kettle (resistant to wet chlorine/dilute acid corrosion and supported by Fe under medium and high pressure conditions);
- Geothermal development: 250~300℃ geothermal well pipeline (corrosion resistance+medium and high strength, 40% cost reduction instead of stainless steel).
-
Medical health:
- High-stress implants: bone plates and joint fixation nails (biocompatible+medium-high strength, the fixation effect is better than F5 pure titanium, and the cost is lower than alloy titanium implants);
- Surgical instruments: orthopedic saw blade, high-stress fixture (body fluid corrosion resistance+wear resistance, life exceeding 3 times of stainless steel instruments).
-
Military industry and high-end equipment:
- Light weapon parts: rifle barrel liner, missile tail (lightweight+medium and high strength, specific strength is better than steel, weight loss is 30%);
- Precision machinery: suspension joint of racing car, high-stress bracket of unmanned aerial vehicle (Fe strengthening to improve fatigue strength, no fracture after 10 cycles).
Key summary
-
Core advantages:
-
Breakthrough in performance: the strength of Fe microalloying is 15% higher than that of F5, which is close to the low-end alloy titanium, and the corrosion resistance still maintains the level of pure titanium;
-
Cost revolution: Fe is a cheap alloy element (only 1/10 of Al/V), and its manufacturing cost is 30% lower than that of titanium alloy;
-
Friendly processing: Fe improves hot workability and weldability, and the production efficiency is increased by 20% (preheating welding and complex forging can be avoided).
-
-
Limited challenges:
-
High-temperature short board: the strength decays quickly above 300℃ (Fe strengthening fails at high temperature, which is lower than the high-temperature stability of alloy titanium);
-
Composition sensitivity: the Fe/O ratio needs to be strictly controlled (imbalance will easily lead to β phase coarsening, reduce plasticity and increase quality inspection cost by 15%).
-
B381 F6 is the benchmark of Fe microalloyed pure titanium. Through “cheap elements+precise formula”, the double breakthrough of “pure titanium performance upgrade” and “alloy titanium cost degradation” is achieved in the field of medium and high stress corrosion resistance, and the “critical point of cost performance” for the transition from pure titanium to titanium alloy is redefined.