Vacuum annealing of titanium and its alloys
A method of removing the saturated layer of a gas
Removing scale and gas saturated layers is difficult because scale adheres firmly to base metals and is stable in many chemically active media. Titanium dioxide - refractory, insoluble in water, in dilute solution of acid and alkali. It only reacts with hydrofluoric acid and slowly dissolves in concentrated sulfuric acid.
Mechanical, chemical and electrochemical methods are used to clean the surface of titanium semi-finished products from oxide and gas saturated layers.
The main purpose of vacuum annealing of titanium alloys is to reduce their hydrogen content to a safe concentration at which hydrogen embrittlement can be excluded. In this case, a number of additional tasks are addressed: a) the gas saturation of the surface layer is reduced, thus frequently eliminating the need for etching titanium alloys after heat treatment; B) remove residual stress and reduce deformation of semi-finished products and belts in subsequent process operations (welding, machining, etc.); C) mechanical properties that meet the requirements of technical conditions.
Based on the corresponding evaluation, similar to the above VIAM recommended for semi-finished products, components and structural components in a thin section (< 6 mm) vacuum annealing temperature below: titanium BT - 0 and VT1-00-550 + 10 0 ℃; For PTV3 alloy OT4-1, OT4 VT5, VT5L, VT16-660 + 10 0 ℃; For VT6 alloy VT6L VT20 VT22-750 plus or minus 10 0 ℃; Annealing time 2 hours
The quality control of annealed parts (structural components) is carried out visually. There should be no tonal colors on the surface that deviate significantly from the given geometry.
Titanium and its alloys have low wear resistance, high adhesion, and large friction coefficients paired with almost all materials. These disadvantages of titanium alloys limit their use in the manufacture of friction parts. Therefore, if bolts and nuts are made of any titanium alloy, the bolt joint is one piece. When you try to remove the nuts from the bolts, they are broken by threaded connections. So for now, the bolts are made of titanium and the nuts are made of stainless steel.
Doping and heat treatment did not significantly improve the friction resistance of titanium alloys. This defect in titanium alloys is tried to be eliminated by chemical heat treatment. The greatest success is achieved through nitriding and oxidation, and these processes, though limited, are used in industry. Practical acceptable carburizing and boriding methods were found.
Chemical heat treatment of titanium and its alloys, commonly used in the processing of steel, is not acceptable, especially in the presence of hydrogen containing gases and their mixtures, because the significant hydrogen concentration of the metal is sufficient to produce hydrogen brittleness. Therefore, instead of being nitrided in ammonia, nitridation takes place in pure nitrogen, carefully purified from oxygen and water.
Under the temperature below 882 ℃ in the process of titanium nitride, formed on the surface of titanium nitride thin layer d, is the nitrogen-rich layer under it. The nitride layer is golden yellow with thickness of 4-20 m and microhardness of 12-16gpa. With the distance from the surface to the metal depth, the microhardness of the nitrogen-rich white layer gradually decreases until the microhardness characteristics of the base metal are reached. For nitrification depth, use the thickness of the layer with high microhardness.
When nitrifying + b- alloy under nitride d layer, there is a changed layer, which is replaced by the structure represented by a- and b- phases. The amount of a phase decreases from 100% to the characteristic value of the alloy with the distance from the coating. These structural changes are due to the -stabilization of nitrogen.
Nitriding increased the wear resistance and heat resistance of titanium products by dozens of times. At the same time, the ductility of elongation, especially transverse narrowing, is reduced several times. The durability limit of the 10-7 cycle base is reduced by 10-25%.
In addition, the nitriding layer is very thin, so adjusting the nitriding part to the required size will be very difficult. As a result, although this is the most common type of chemical-heat treatment, nitriding of titanium and its alloys is used only in limited amounts.
Oxidation is also used for surface hardening of titanium and its alloys. The properties of the gaseous saturated layer formed on the surface of the semi-finished product when heated in oxygen and air and the products made of titanium and its alloys are discussed above. At more than 850-900 ℃ temperature oxidation of thick oxide layer on titanium and its alloy machinery and the use of an adverse effect on performance. However, when the oxidation temperature is not too high the thickness of the oxide layer formed by small, can significantly improve the wear resistance of parts made of titanium alloy, but not significantly reduce the mechanical and service properties of titanium and its alloys.