Quality Control And Performance Characteristics Of Titanium Pipe Fittings

For its excellent corrosion resistance and heat resistance, titanium pipe fittings are widely used in chemical, aerospace and marine engineering fields. However, titanium is an active metal and has a strong affinity for gap elements such as hydrogen and oxygen. If improperly controlled during processing and heat treatment, the mechanical properties will be degraded due to hydrogen embrittlement or oxygen pollution, which seriously affects the service safety and service life of pipe fittings. Therefore, it is necessary to strengthen the control of hydrogen content and oxidation behavior from the source of the process, and at the same time fully understand the inherent characteristics of titanium pipe fittings in order to rationally select materials and optimize the process.

1. Control and treatment of hydrogen content exceeding the standard

Hydrogen is one of the most harmful impurities in titanium and titanium alloys. When the hydrogen content in titanium pipe fittings is too high, brittle hydrides (TIH₂) will precipitate, which significantly reduces the impact toughness and notch tensile strength of the material, resulting in an increased risk of brittle fracture. In order to ensure the reliability of use, it is generally stipulated that the hydrogen content in titanium pipe fittings shall not exceed 0.015% (mass fraction).

In order to reduce the absorption of hydrogen, surface cleaning before welding and heat treatment is essential. Fingerprints, wear marks, grease and other residual organic matter must be completely removed, because these pollutants will decompose at high temperatures to produce hydrogen sources. At the same time, the atmosphere in the heat treatment furnace should be strictly controlled to avoid the presence of water vapor, because the reaction of water vapor with titanium at high temperatures will release hydrogen. If the hydrogen content of titanium pipe fittings is found to exceed the standard, dehydrogenation can be carried out by vacuum annealing treatment to reduce the hydrogen content to within the allowable range.

2. Formation and prevention and control of oxidative pollution

The oxidation behavior of titanium in the air varies significantly with temperature. When the temperature does not exceed 540℃, the thickening of the surface oxide film is relatively slow, but after this temperature is exceeded, the oxidation rate accelerates sharply. At high temperatures, oxygen not only forms a surface oxide layer, but also diffuses inward to form an oxygen-rich brittle pollution layer. The contaminated layer has high hardness and extremely low plasticity, which can easily induce surface cracks during force or cooling, which seriously damages the quality of pipe fittings.

Methods such as machining, pickling or chemical grinding can be used to remove the oxidized contaminated layer, but it is more critical to prevent it from the heat treatment process. The heating time should be shortened as much as possible, and priority should be given to vacuum furnaces or inert gas protection furnaces for heat treatment to avoid long-term heating in air furnaces.If the conditions are limited and must be treated in an air furnace, appropriate protective measures (such as coating protective coatings, filling with inert gases, etc.) should be taken to minimize the infiltration of oxygen.

3. The main performance characteristics of titanium pipe fittings

In addition to the need to strictly control hydrogen and oxygen pollution, titanium pipe fittings themselves have the following outstanding performance advantages:

1) Excellent corrosion resistance

Although titanium is a thermodynamically unstable metal, it is easy to form a dense and stable passivated oxide film on its surface. It exhibits excellent corrosion resistance in many corrosive media such as oxidizing, neutral and weakly reducing properties, especially resistant to chloride ion corrosion, which is better than most stainless steels.

2) Good heat resistance

Titanium pipe fittings can be used for a long time at temperatures up to 600℃ or even higher, maintaining sufficient mechanical strength and creep resistance, and are suitable for high-temperature heat transfer and transportation pipelines.

3) Non-magnetic

Titanium is a paramagnetic metal and will not be magnetized in a large magnetic field, so it is suitable for magnetism-sensitive instrumentation and electronic equipment environments.

4) Low elastic modulus

The elastic modulus of titanium is about 57% that of steel. This characteristic makes titanium pipe fittings more flexible when subjected to alternating loads, which is conducive to absorbing vibration and thermal expansion, but at the same time, it is necessary to pay attention to the deformation caused by insufficient rigidity. Problems.

5) Strong suction characteristics

Titanium is an active metal, which can easily react with various elements and compounds such as hydrogen, oxygen, nitrogen, carbon, etc. at high temperatures to form a gap solid solution or compound. This characteristic is not only the basis of its corrosion resistance, but also determines that it must adopt a strict protective atmosphere during high-temperature processing and heat treatment, otherwise it will lead to deterioration of performance.

The application value of titanium pipe fittings is based on the effective control of its hydrogen and oxygen pollution. Through strict surface cleaning, reasonable annealing process and suitable protective atmosphere, titanium can give full play to the excellent properties of corrosion resistance, heat resistance, non-magnetic and low modulus, so as to ensure the long-term safe operation of pipe fittings under harsh working conditions.