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Vacuum heat treatment process of titanium alloy
Advantages of titanium and titanium alloys: low density, high specific strength, higher heat resistance than aluminum, and good corrosion resistance.
Disadvantages of titanium and titanium alloys: poor thermal conductivity, poor wear resistance, low elastic modulus, high chemical activity.
The characteristics of vacuum heat treatment of titanium alloy:
(1) The martensitic transformation does not cause significant strengthening of the alloy
(2) The formation of ω phase should be avoided
(3) Allotropic transformation is difficult to refine grains
(4) Poor thermal conductivity
(5) Chemically active
(6) The β phase transition point has a large difference
(7) When heating in the β phase region, the β grain growth tendency is large
Main types of vacuum heat treatment for titanium alloys:
1. Vacuum annealing
Objective: To reduce the hydrogen content in the surface layer of titanium alloy to a safe concentration and eliminate the possibility of hydrogen embrittlement. In addition, reduce the residual stress and ensure the mechanical properties and performance of the alloy.
Note: reduce the hydrogen concentration of the titanium alloy surface layer to a level that will not cause hydrogen embrittlement (chronic fracture) during later use; reduce the residual stress (especially welding stress) to a level that cannot affect the performance of titanium alloy components The level of negative impact; it is not allowed to guarantee the mechanical properties and performance of titanium alloy components, and the unfavorable factors such as the evaporation of alloying elements from the surface layer must be reduced to the lowest level; the vacuum annealed components maintain the original size; An oxide film is formed on the surface of the part to prevent the metal from interacting with water vapor and other hydrogen-containing gases.
Influencing factors: vacuum annealing temperature and holding time.
2. Enhanced vacuum heat treatment
Quenching aging is the main way of vacuum heat treatment strengthening of titanium alloy, so it is called enhanced vacuum heat treatment. Strengthening vacuum heat treatment is an important way to improve the strength index of titanium alloy. Similarities and differences between titanium alloy enhanced vacuum heat treatment and steel and aluminum:
(1) Martensite can be obtained by vacuum quenching both steel and titanium, but the martensite strength of steel is high and the strengthening effect is strong, and vacuum tempering softens the steel; while the hardness of titanium martensite is not high, the strengthening effect is not large, and the vacuum tempering can soften the steel. Tempering makes the alloy dispersion strengthened;
(2) Steel has only one martensitic strengthening mechanism, while (α+β) titanium alloys with the same composition have two strengthening mechanisms, that is, the β-stable elements contained in the β-phase of high-temperature vacuum quenching are less than the critical concentration, and the quenching transformation is martensitic. During aging, martensite decomposes into a dispersed phase to strengthen the alloy, and the low-temperature vacuum quenching β phase contains stable elements greater than the critical concentration, then vacuum quenching obtains a supercooled β phase, and during aging, the β phase decomposes into a dispersed phase to strengthen the alloy;
(3) The solution treatment and aging process of titanium alloy are basically similar to that of aluminum alloy.
*The principle of intensified vacuum heat treatment is to obtain metastable β, a’, a” phases by rapid cooling, and then decompose them into dispersed a and β phases during the artificial aging process.
*The strengthening effect of the alloy depends on the type, quantity, composition and dispersion of the α and β phase particles formed after aging. The above is determined by the alloy composition and vacuum heat treatment process.
Defects and prevention of titanium alloy vacuum heat treatment:
Typical defects caused by vacuum heat treatment of titanium alloys include the following:
(1) produce coarse grains
(2) The mechanical properties and performance of the alloy deviate from the established technical conditions
(3) Strong gas saturation
(4) Alloying elements evaporate from the surface or along the interface
(5) Distortion of parts or semi-finished products
(6) Prevention and treatment of crack defects due to thermal stress and corresponding stress: the first 5 types are prevented by rationally formulating and strictly controlling the vacuum heat treatment system; the prevention and control methods of the sixth type of defects are: (1) reduce the vacuum quenching temperature; (2) Increase the aging temperature; (3) adopt austempering; (4) introduce pre-high temperature aging before low temperature aging.
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