Classification and use of titanium pipe fittings

The titanium tube is light in weight, high in strength and superior in mechanical properties. It is widely used in heat exchange equipment such as tubular heat exchangers, coil heat exchangers, serpentine heat exchangers, condensers, evaporators and pipelines. Many nuclear power industries use titanium tubes as their standard units.

classification

Titanium is an isomer, with a melting point of 1720 ° C. It is a close-packed hexagonal lattice structure below 882 ° C, called α titanium; above 882 ° C is a body-centered cubic character structure called beta titanium. Titanium alloys with different microstructures are obtained by adding appropriate alloying elements and gradually changing the phase transition temperature and phase content by using the different characteristics of the above two structures of titanium. At room temperature, titanium alloys have three kinds of matrix structures, and titanium alloys are classified into the following three types: α alloys, (α + β) alloys and β alloys. China is represented by TA, TC, and TB respectively.

Alpha titanium alloy

It is a single-phase alloy composed of α-phase solid solution. It is α phase at normal temperature or at a higher practical application temperature. It is structurally stable and has higher wear resistance than pure titanium and has strong antioxidant capacity. The strength and creep resistance are maintained at a temperature of 500 ° C to 600 ° C, but the heat treatment is not strengthened, and the room temperature strength is not high.

Beta titanium alloy

It is a single-phase alloy composed of β-phase solid solution. It has high strength without heat treatment. After quenching and aging, the alloy is further strengthened. The room temperature strength can reach 1372～1666MPa. However, the thermal stability is poor and it is not suitable for use at high temperature.

++β titanium alloy

It is a two-phase alloy with good comprehensive properties, good structural stability, good toughness, plasticity and high temperature deformation properties. It can perform hot pressure processing well, and can be quenched and aged to strengthen the alloy. The strength after heat treatment is about 50% to 100% higher than the annealing state; the high temperature strength is high, and it can work for a long time at a temperature of 400 ° C to 500 ° C, and its thermal stability is inferior to that of the α titanium alloy.

The most commonly used of the three titanium alloys are α-titanium alloy and α+β-titanium alloy; the α-titanium alloy has the best machinability, the α+β-titanium alloy is the second, and the β-titanium alloy is the worst. The alpha titanium alloy is coded TA, the beta titanium alloy code is TB, and the alpha + beta titanium alloy code is TC.

Titanium alloys can be classified into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (titanium-molybdenum, titanium-palladium alloys, etc.), low-temperature alloys, and special functional alloys (titanium-iron hydrogen storage materials and titanium-nickel memory alloys). . The composition and properties of typical alloys are shown in the table.

The heat treated titanium alloy can obtain different phase compositions and microstructures by adjusting the heat treatment process. It is generally believed that the fine equiaxed structure has good plasticity, thermal stability and fatigue strength; the needle-like structure has high durability, creep strength and fracture toughness; the equiaxed and needle-like mixed structure has better comprehensive performance.

use

Titanium alloy has high strength and low density, good mechanical properties, and good toughness and corrosion resistance. In addition, the titanium alloy has poor processability and is difficult to cut, and it is very easy to absorb impurities such as hydrogen, nitrogen, nitrogen and carbon during hot working. There is also poor wear resistance and complicated production processes. The industrial production of titanium began in 1948. The development of the aviation industry has enabled the titanium industry to grow at an average annual growth rate of about 8%. At present, the annual output of titanium alloy processing materials in the world has reached more than 40,000 tons, and nearly 30 titanium alloy grades. The most widely used titanium alloys are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).

Titanium alloys are mainly used to make aircraft engine compressor components, followed by structural components for rockets, missiles and high-speed aircraft. In the mid-1960s, titanium and its alloys were used in the general industry for the production of electrodes for the electrolysis industry, condensers for power stations, heaters for petroleum refining and seawater desalination, and environmental pollution control devices. Titanium and its alloys have become a corrosion resistant structural material. It is also used to produce hydrogen storage materials and shape memory alloys.

China began research on titanium and titanium alloys in 1956; industrial production of titanium materials began in the mid-1960s and developed into TB2 alloys.

Titanium alloy is a new important structural material used in the aerospace industry. Its specific gravity, strength and service temperature are between aluminum and steel, but its specific strength is high and it has excellent seawater corrosion resistance and ultra-low temperature performance. In the United States in 1950, it was used for the first time on the F-84 fighter-bomber as a non-bearing member such as a rear fuselage insulation panel, an air hood, and a tail hood. In the 1960s, the use of titanium alloys moved from the rear fuselage to the middle fuselage, partially replacing structural steel to make important load-bearing components such as bulkheads, beams, and flaps. The amount of titanium alloy used in military aircraft has increased rapidly, reaching 20% to 25% of the structural weight of the aircraft. Since the 1970s, civilian machines have begun to use titanium alloys in large quantities. For example, Boeing 747 passenger aircraft used more than 3,640 kilograms of titanium. Titanium for aircraft with Mach number less than 2.5 is mainly used to replace steel to reduce the structural weight. Another example is the US SR-71 high-altitude high-speed reconnaissance aircraft (with a flying Mach number of 3 and a flying height of 26,212 m). Titanium accounts for 93% of the aircraft's structural weight, and is known as the “all-titanium” aircraft. When the thrust-to-weight ratio of the aero-engine is increased from 4 to 6 to 8 to 10, and the compressor outlet temperature is increased from 200 to 300 ° C to 500 to 600 ° C, the low-pressure compressor disk and blades originally made of aluminum must be Use titanium alloys or titanium alloys instead of stainless steel to make high-pressure compressor discs and blades to reduce structural weight. In the 1970s, titanium alloys used in aircraft engines generally accounted for 20% to 30% of the total weight of the structure. They are mainly used in the manufacture of compressor components, such as forged titanium fans, compressor discs and blades, cast titanium compressors, and intermediates. Machine casing, bearing housing, etc. The spacecraft mainly utilizes the high specific strength, corrosion resistance and low temperature resistance of titanium alloys to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frames and rocket casings. Titanium alloy plate welded parts are also used in artificial earth satellites, lunar modules, manned spacecraft and space shuttles.