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B381 F4


B381 F4 is a pure titanium forging material (non-alloyed titanium, reinforced by interstitial elements), which comes from ASTM B381 “Standard Specification for Titanium and Titanium Alloy Forgings” (current version ASTM B381-21). With the characteristics of “corrosion resistance × biocompatibility× medium strength”, it replaces stainless steel in low-stress corrosion scenarios. The following system analysis:

I. Standards and brand system

1. Core standards

  • International specification: ASTM B381 (covering 39 brands of titanium and titanium alloy forgings, F4 belongs to non-alloy titanium series, and belongs to pure titanium with F1/F2/F3, and is classified by interstitial element content);
  • Version iteration: the old version of ASTM B381-06a has been replaced by ASTM B381-21 (updated impurity detection and performance requirements);
  • Domestic adaptation: there is no completely equivalent brand, and the composition and properties of pure titanium of TA3 in GB/T 25137 (titanium and titanium alloy forgings) are close to F4 (TA3 has higher interstitial element content and slightly higher strength).

2. Brand code

  • Commercial name: no exclusive alias, collectively referred to as “ASTM B381 F4 pure titanium forgings”;
  • Classification logic: F4 is a brand with high content of interstitial elements (O/N/C) in pure titanium (strength is better than F1/F2, and plasticity is slightly lower), and there is no UNS independent number (belonging to pure titanium system, which together with F1/F2/F3 constitutes the strength gradient of pure titanium).

II. Chemical composition (mass fraction%, ASTM B381 standard)

F4 is high-purity pure titanium+interstitial elements, and impurities are strictly controlled;
element Content range Core role
Ti margin Matrix, providing low density (4.5 g/cm³) and basic corrosion resistance.
O 0.18~0.25 Gap strengthening core (O content ↑→ α phase hardness ↑, strength exceeding 600 MPa)
N ≤0.05 Strictly control the precipitation of titanium nitride to avoid embrittlement
C ≤0.10 Restrain titanium carbide and ensure weldability and formability.
H ≤0.015 Prevention of hydrogen embrittlement (hydrogen absorption is forbidden during forging processing/service)
Fe ≤0.30 Impurity elements, exceeding the limit to reduce corrosion resistance (strictly controlled)

III. Mechanical properties (annealed state, forgings)

F4 is better than low-gap pure titanium (such as F1) but weaker than alloy titanium (such as F2/Ti-6Al-4V) due to the strengthening of interstitial elements;
Performance index Typical value (ASTM requirements) Comparison F1 (low-gap pure titanium)
tensile strength 500~600 MPa  25% higher （F1≈400 MPa）
yield strength 300~400 MPa 30% higher （F1≈275 MPa）
extensibility（δ₅） 15~25% 25% lower（F1≈20~30%）
hardness 100~140 HB 25% higher （F1≈80~120 HB）
heat resistance The corrosion rate in 3.5% NaCl is less than 0.01 mm/year. Equivalent to F1 (far more than stainless steel)

IV. Requirements for Heat Treatment and Processing

  1. Annealing treatment (necessary, balance performance):
    • Temperature: 700~800℃ (heat preservation for 1~2 hours, α phase region, eliminating forging stress);
    • Cooling: air cooling (keeping α single-phase structure, giving consideration to strength and plasticity);
    • Function: restore cutting and welding performance (convenient for subsequent machining).
  2. Machining and welding:
    • Machining: the cutting performance is better than that of titanium alloy, so cemented carbide tool and sufficient cooling are recommended (to avoid hydrogen embrittlement caused by cutting heat);
    • Welding: TIG welding (ERTi-1 welding wire) is adopted, and the whole process is protected by high-purity argon gas (preventing O, N and H pollution, and pure titanium is easy to absorb impurities at high temperature).

V. Main application fields (medium and low stress+corrosion resistance scene)

F4 relies on “corrosion resistance of pure titanium × medium strength × biocompatibility”, covering the following fields (the performance is between F1 and alloy titanium, and the cost is lower than alloy titanium):
  1. Ocean engineering:
    • Seawater pipelines and fasteners (seawater erosion+biofouling resistance, 3 times longer service life than brass and lower cost than alloy titanium);
    • Shallow sea cage frame and ship accessories (medium strength requirements, 30% cost reduction instead of stainless steel).
  2. Chemical industry and energy:
    • Dilute sulfuric acid, hydrochloric acid (non-oxidizing acid) pipeline, reactor lining (corrosion resistance is better than 316L, and the cost is equivalent);
    • Food processing equipment (such as brewing barrel and dairy pipeline, sanitary grade+organic acid corrosion resistance).
  3. Medical health:
    • Low stress implants: dental implant abutment, cosmetic implant (biocompatible+medium strength, lower cost than alloy titanium);
    • Medical instruments: disinfection containers, surgical tools (corrosion resistance of body fluids, no metal ion precipitation).
  4. General machinery:
    • Corrosion-resistant pump valves and pressure vessels (medium and low pressure, replacing stainless steel to improve corrosion resistance);
    • Precision instrument parts (such as watch movements, optical equipment, easy to process complex shapes).

Key summary

  • Core advantages:

    • Cost-performance ratio: the strength is better than that of low-gap pure titanium (F1), and the cost is lower than that of alloy titanium (F2), which is suitable for low-stress corrosion-resistant scenes;

    • Corrosion resistance: consistent with pure titanium, the corrosion rate in seawater and dilute acid is less than 0.01 mm/year;

    • Friendly processing: the hot/cold workability is better than that of alloy titanium, and the manufacturing cost is 15-20% lower.

  • Limitations:

    • The strength is only 60% of alloy titanium (alloy titanium needs to be replaced in high stress scenes);

    • The interstitial elements are sensitive (excessive O/N will reduce plasticity, so the composition should be strictly controlled).

B381 F4 is “the benchmark of medium-strength pure titanium forgings”, which fills the application gap between F1 (low strength and high plasticity) and alloy titanium (high strength and high cost) through the classification of interstitial elements, and becomes “cost-effective optimization” in the fields of marine anticorrosion, chemical corrosion resistance and medical general use, redefining the strength adaptation boundary of pure titanium.
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