Performance of Titanium Pipe Fittings, Hydrogen Content Control and Key Points of Heat Treatment

  Corrosion resistance: Although titanium is a thermodynamically active metal with a low equilibrium potential and strong corrosion tendency, it possesses excellent stability in oxidizing, neutral, and weakly reducing media, and has outstanding corrosion resistance.

 Heat resistance: It can be used for long-term operation at temperatures up to 600°C or even higher.

Non-magnetic and non-toxic: It will not be magnetized in a strong magnetic field and is non-toxic.

Low elastic modulus: Its elastic modulus is approximately 57% of that of steel.

Gas absorption performance: At high temperatures, it readily reacts with various elements and compounds, and has the characteristic of absorbing gases.

Titanium pipe fittings have a high risk of hydrogen embrittlement. The key is to control the hydrogen content to be ≤ 0.015wt% (150ppm). For aviation/high-end equipment, it is often required to be ≤ 0.010wt% (100ppm), as excessive hydrogen content will reduce the impact toughness and notch tensile strength of the titanium pipe fittings, leading to an increase in brittleness. The control of hydrogen content runs through the entire process from raw materials, processing, heat treatment, welding, cleaning and inspection.

 The raw materials use vacuum self-dissolution arc melting (VAR) or electron beam cold bed melting (EBCHM) titanium ingots with low hydrogen content, with the hydrogen content of the raw materials being ≤ 0.008%.

 Pay attention to surface cleanliness. Before processing, thoroughly remove oil, fingerprints, and coolant residues, and when contacting it, the operator must wear dust-free gloves. During heat treatment, it must at a vacuum environment or inert atmosphere, and there should be no moisture in the furnace.

 The welding environment should have a humidity of less than 60%, use argon arc welding, and dehydrogenate after welding.

 The acid washing solution requires a low hydrogen formula, and after acid washing, it must be thoroughly rinsed with water and dried.

 Testing uses inert gas fusion - thermal conductivity method, and PT/UT checks for hydrogen-induced cracks.

When the heat treatment temperature does not exceed 540℃, the thickness of the oxide film on the surface of the titanium pipe parts increases slowly; after exceeding this temperature, the oxidation rate significantly accelerates, and the formed oxygen contamination internal diffusion layer is relatively brittle, which is prone to cause cracks on the surface of the parts and even lead to failure. The methods for removing the oxygen contamination layer include mechanical processing, acid washing, and chemical grinding, etc. To reduce the oxidation contamination, the heating time should be shortened as much as possible under the premise of meeting the process requirements, and vacuum furnaces or inert gas-protected furnaces should be preferred to avoid or reduce direct heating in air furnaces.