Vacuum heat treatment of titanium alloy
Vacuum oil quenching and vacuum high pressure gas quenching are the main methods of heat treatment. Vacuum oil quenching seriously affects the quality of titanium alloy. The cooling speed of high pressure gas quenching is far from meeting the requirements of solid solution treatment of titanium alloy. Vacuum water quenching is the only key technology to endow solid solution strengthening of titanium alloy. Vertical vacuum water quenching has become a hot spot in the industry due to its fast cooling speed and good solid solution strengthening effect.
According to the research status of vacuum heat treatment of titanium alloy in China, combined with the related development goals in the technical roadmap of heat treatment and surface modification in China, taking the opportunity of a new generation of vertical vacuum water quenching furnace, this paper makes a deep thinking and Discussion on the future development trend of vacuum heat treatment of titanium alloy, and points out that vertical vacuum water quenching is one of the main development directions.
Quenching treatment of titanium alloy
Quenching and aging is the main way of heat treatment and strengthening of titanium alloy. It uses phase transformation to produce strengthening effect, so it is also called strengthening heat treatment. For pure A-type titanium alloy, the heat treatment method is basically ineffective, that is, the heat treatment of titanium alloy is mainly used for α + β type titanium alloy. Generally, water quenching is used as the cooling method, oil quenching and gas quenching are less used, and the quenching process should be rapid to prevent the decomposition of β phase in the transfer process and reduce the aging strengthening effect.
Transformation process of titanium alloy
The basis of heat treatment strengthening is the transformation of titanium alloy during heating and cooling, which is mainly based on the diffusion strengthening obtained by aging decomposition after martensitic transformation. Titanium alloy (take α + β titanium alloy ti-5cr-3al as an example) will undergo the following transformation according to the different cooling rate during self high temperature cooling, as shown in Figure 1.
Fig.1 Time temperature phase diagram of ti-5cr-3al
It can be seen from Figure 1 that when the alloy is rapidly cooled (water quenched) from solution temperature, martensitic transformation occurs, in which metastable β (β ‘) is formed, and hexagonal α’ phase (HCP, martensite) is obtained at room temperature; when the cooling rate is slow (oil quenched), part of β phase is transformed into ω phase; when the cooling rate is lower (pressurized gas quenched), β phase is transformed into α ‘and ω phase; when the cooling rate is lower, β phase is transformed into α’ and ω phase In the range of 520 ℃ ~ 720 ℃, when the cooling rate is very slow, the eutectoid decomposition of β → α + TiCr2 (compound) occurs, that is, the α phase nucleates and grows at the original β phase boundary.
To sum up, in order to achieve solution strengthening of titanium alloy materials, quenching is the premise, and the cooling rate of quenching process plays a decisive role. In order to obtain martensitic transformation, cooling rate should be increased, which can only be achieved by vertical vacuum water quenching.
Heat treatment and strengthening characteristics of titanium alloy
Titanium alloy will undergo phase transformation during heating and cooling. For different alloy systems, different microstructure can be obtained by controlling their respective phase transformation process. Through the cooling test of titanium alloy in different media (as shown in Figure 2), it can be found that the characteristics of heat treatment and strengthening of titanium alloy are as follows:
- (1) Heat treatment strengthening is mainly used for α + β titanium alloy and near β titanium alloy, and other titanium alloys are basically invalid;
- (2) Different from iron and steel, the grain can not be refined by repeated heat treatment;
- (3) The ω phase makes the alloy brittle, and the formation of ω phase should be avoided in the quenching process;
- (4) Martensitic transformation can not strengthen titanium alloy, but only through aging decomposition of stable phase (including martensitic phase) formed during quenching, namely dispersion strengthening;
- (5) After heat treatment and quenching, the hardenability of α + β titanium alloy is not high and the quenching thermal stress is large, which is easy to cause deformation of long rod-shaped parts, so it can only be loaded vertically and enter into the quenching medium longitudinally;
- (6) The α P phase in the fast cooling structure is smaller than that in the slow cooling structure.
Fig.2 Metallographic picture of titanium alloy solution treatment (a) water cooling; (b) oil cooling; (c) air cooling
Research status of vacuum heat treatment technology of titanium alloy
Vacuum heat treatment technology is to select suitable cooling medium according to the requirements of structure and properties and the phase transformation law of component materials, so that the materials and components heated by vacuum can be cooled to the furnace temperature according to the required cooling speed in the vacuum environment. Common cooling media for vacuum heat treatment include vacuum quenching oil, non oxygen gas and water. Different cooling media are selected with different cooling rates, as shown in Figure 3.
Fig.3 Cooling mode of titanium alloy solution treatment
Vacuum oil quenching cooling technology
The cooling medium used in vacuum oil quenching is vacuum quenching oil. This technology can replace salt bath and atmosphere protection heat treatment. While obtaining ideal core structure and mechanical properties, it can ensure the smooth surface, especially suitable for quenching of medium and high alloy steel.
At present, vacuum oil quenching technology is widely used in heat treatment industry in China. Through long-term improvement of equipment, process and quality of quenching oil, vacuum oil quenching technology is relatively mature. However, due to its slow cooling rate (lower than water quenching), it is easy to form brittle ω phase if it is not properly controlled in the heat treatment process of titanium alloy; on the other hand, after the titanium alloy workpiece is heated into oil at high temperature, it will form carburization at the moment of contact reaction between the workpiece surface and oil vapor, which is not conducive to the improvement of the performance of titanium alloy, so it is not suitable for the vacuum heat treatment of titanium alloy.
Vacuum air quenching cooling technology
The non oxygen type gas used in vacuum gas quenching cooling is used as the cooling medium. Its characteristic is that in a certain range, with the increase of gas pressure in the furnace, the cooling speed can be increased accordingly. The cooling gases commonly used in vacuum gas quenching are: N2, H2, he and ar. their thermal conductivity is quite different. The cooling capacity of H2 is the strongest, followed by he, N2 and ar.
The cooling rate of 40 bar H 2 ultra-high pressure gas quenching can be close to that of water quenching, but the use of H 2 is more dangerous. At the same time, the titanium alloy is easy to absorb h at high temperature, resulting in hydrogen embrittlement, so it can not be used as a cooling medium for vacuum heat treatment of titanium alloy. The cooling rate of 20 bar he ultra-high pressure gas quenching is between oil and water, which can replace vacuum oil quenching. However, the price of he and AR is too high, and it can not be used as a cooling medium after quenching It is necessary to consider recovery, and the cost of heat treatment is too high to be the main cooling medium for vacuum heat treatment of titanium alloy; N2 is cheap and safe, but its cooling capacity is poor, and it reacts strongly with Ti at high temperature, reducing the properties of titanium alloy materials. The technology is not suitable for solution strengthening of titanium alloy.
Vacuum water quenching cooling technology
Vacuum water quenching technology uses water as quenching medium, which is especially suitable for solution treatment of titanium alloy workpiece. After the workpiece is heated in vacuum without oxidation, it is transferred to the quenching water tank to obtain the required solid solution structure.
At present, the developed countries are studying the new technology and equipment of vacuum water quenching of titanium alloy, trying to reduce the production cost of vacuum heat treatment of titanium alloy, improve the performance and improve the product quality.
Status quo and development trend of titanium alloy vacuum heat treatment equipment
Limitations of horizontal vacuum water quenching furnace
At present, the traditional horizontal vacuum water quenching furnace is mainly used for titanium alloy vacuum water quenching. The traditional horizontal vacuum water quenching furnace evolved from the horizontal vacuum oil quenching furnace. The difference is that the quenching medium changed from oil to water, which brings many disadvantages:
- 1. Workpiece transfer time is long (more than 10s, as shown in Figure 4);
- 2. Pollution heating room;
- 3. Emulsification of vacuum pump oil pollutes the environment;
- 4. It needs argon gas to wash the furnace, so the operation cost is high.
Figure.4 Schematic diagram of workpiece transfer process of horizontal vacuum water quenching furnace
In view of this, Dingli technology has developed a new generation of vvwq series vertical vacuum water quenching furnace equipment, which is specially used for vacuum heat treatment of titanium alloy materials. In addition to high vacuum heating and solid solution quenching in water medium, the equipment can also accurately control the transfer time of the treated titanium alloy parts from the heating chamber to the quenching chamber, and minimize the heat treatment distortion of the parts. After the vacuum heat treatment, the surface of titanium alloy workpiece is bright, which provides equipment guarantee for the heat treatment and strengthening technology of titanium alloy materials.
Structure and characteristics of vertical vacuum water quenching furnace
The structure of vertical vacuum water quenching furnace is shown in Figure 5:
Fig.5 Structural diagram of vertical vacuum water quenching furnace
Structure and characteristics of heating chamber
The vertical vacuum water quenching furnace adopts a metal shield heat field without carbon element and a high vacuum heating environment. The furnace liner is an integral structure design, and the heating element is made of molybdenum lanthanum alloy belt. The heat shield is a composite heat shield composed of molybdenum shield and stainless steel shield. The maximum heating temperature can reach 1350 ℃, and the average temperature is less than ± 5 ℃, which is a stable and clean heating field for titanium alloy. The vacuum obtaining system of the heating chamber is equipped with three-stage pump set, i.e. mechanical pump + roots pump + diffusion pump, which can achieve considerable results When the impurity content is less than or equal to 0.01 ppm, the vacuum degree is less than or equal to 5 × 10-3pa, which can guarantee the bright and non oxidation heat treatment of titanium alloy.
Structure and characteristics of quenching chamber
The quenching chamber adopts a special cylindrical structure and is equipped with a set of mechanical transmission system for lateral transfer, which can realize the three station operation of the quenching chamber. The most important operation station is the docking station with the heating chamber. Quenching in water medium can be completed only after the docking of two chambers is completed; a set of material elevator transportation system is installed inside the quenching chamber for the transportation of titanium alloy workpieces (from charging chamber to charging chamber Hot chamber) and transfer (from heating chamber to quenching chamber), transfer time < 6S, and no water vapor pollutes the vacuum pump set; quenching chamber is equipped with an independent vacuum system, which can provide rough vacuum state (vacuum degree ≤ 10Pa), to ensure that the workpiece is not oxidized in the process of transferring from heating chamber to quenching chamber; quenching chamber is equipped with water temperature sensor, heater, refrigeration system, water level sensor And a set of water agitation system can realize the precise control of the quenching medium temperature (the water temperature is maintained at about 10-15 ℃), the water agitation system can strongly stir the quenching medium to improve the cooling effect, and give the titanium alloy workpiece more even cooling.
Advantages of vertical vacuum water quenching furnace
After the completion of heating and heat preservation of titanium alloy workpiece, the heating chamber enters the water medium until the workpiece is completely immersed. The whole transfer process of solid solution treatment takes the time of quenching delay. Theoretically, the smaller the quenching delay time is, the better. Otherwise, with the increase of the delay time, the temperature on the surface of the titanium alloy workpiece will decrease, which will be very detrimental to the martensitic transformation and microstructure of the titanium alloy material, and further affect the properties. Especially for the α + β titanium alloy, the practical experience shows that the quenching delay time should be controlled below 8s, otherwise the α phase on the structure will nucleate and grow at the original β phase boundary first, which seriously affects the mechanical properties of the titanium alloy in the quenching state.
Fig. 6 is the trace diagram of the workpiece transferred from the heating chamber to the quenching chamber in the vertical vacuum water quenching furnace. The diagram shows that the workpieces only need to fall vertically to complete the solution treatment from the vertical vacuum water quenching furnace, without π type running track, the workpieces’ running time is relatively shorter, which can reach < 6S, and the vacuum pump group is polluted by anhydrous steam, which shows great advantages compared with the horizontal vacuum water quenching furnace. In addition, the vertical structure is very suitable for solution heat treatment of α + β titanium alloy parts with low hardenability and long rod structure, and the distortion is very small.
Figure.6 Schematic diagram of workpiece transfer process of vertical vacuum water quenching furnace
A company in China uses vvwq3030 vertical vacuum water quenching furnace to treat TC4 titanium alloy. The size and shape of workpiece are Φ 20 × 200mm, and the process content is solution + aging. Good results have been achieved.
The results show that:
Appearance: after the workpiece is processed, the surface is bright without oxidation color; hardenability: all hardenings in Φ 20mm section; distortion: small, the total run out increases by 0.02mm.
Working pressure of workpiece in vacuum heat treatment furnace
One of the main parameters of vacuum heat treatment is the working pressure of vacuum. We have carried out experiments of different working pressure of vacuum. The test results show that no oxidation is found in the vacuum heat treatment of titanium alloy at 850 ℃ ~ 900 ℃ and the vacuum working pressure is 10 × 10-2 PA, so the vacuum working pressure of titanium alloy is controlled at no more than 6.7 × 10-2 PA. In addition, when the titanium alloy is heat-treated at 950 ℃ and the vacuum working pressure is 2 × 10 – 3 Pa, the alloy elements will not be depleted. Therefore, in the process of vacuum annealing, in order to avoid the corrosion of vacuum surface due to the low vacuum pressure, the working pressure of vacuum should be controlled at not less than 2 × 10-3 PA generally, and high-purity argon is also used for partial pressure control.
Inert gas in vacuum heat treatment furnace
The inert gas can be used as the heating medium of the protective atmosphere or the quenching medium of the vacuum solid solution treatment. Due to the high chemical activity of titanium, there is a higher requirement for the purity of argon. Argon with purity less than 99.99% in vacuum heat treatment furnace is generally not suitable for heat treatment of titanium alloy. During the production of TC16 self-locking nuts, the machined TC16 self-locking nuts were treated with 800 ℃× 2H solid solution in pure argon (99.99%) atmosphere. During the closing deformation of the upper part of the nut, the inner surface of the elliptical stub shaft at the top of the nut was deformed due to the surface pollution The self-locking nuts of the assembled parts have to be disassembled because of the cracks on the surface, which causes great losses. Foreign standards stipulate that the dew point of inert gas (helium and argon) shall not be higher than – 54 ℃. According to the navigation standard, high purity argon conforming to GB / t10624-1995 “high purity argon” shall be used, and the purity shall not be less than 99.999%.
Temperature of workpiece when it is discharged from vacuum heat treatment furnace
In order to determine the process parameters, four titanium alloys, TA2, Tc1, TC4 and TB5, were treated with air at 200, 250 and 320 ℃ for one hour. The results show that the surface of the four alloys is still bright after treatment at 200 ℃, the four titanium alloys are slightly oxidized in varying degrees after treatment at 250 ℃, and the workpiece is obviously oxidized after treatment at 320 ℃. According to this result, the navigation standard stipulates that the workpiece after vacuum heat treatment should be air cooled from the vacuum furnace at a temperature below 200 ℃.
It is not necessary to remove the light yellow oxide film on the surface of titanium alloy heated under inert gas protection or vacuum; however, if the light blue, blue or gray oxide film appears, it must be removed according to the regulations of air furnace.
Allowable delay time of solution treatment and quenching
The cooling of titanium alloy after solution treatment and heating should be carried out in strict accordance with the quenching requirements. If the cooling rate is too low, the phase composition of the alloy will obviously diffuse, which will seriously affect the aging strengthening effect.
In the process of vacuum heat treatment of titanium alloy, the main factors affecting the performance of workpieces are these four items. As long as we do according to the requirements of process parameters in the actual production process, we can ensure that the performance of workpieces from vacuum heat treatment furnace can meet the needs of practical application.
Source: China Titanium Pipe Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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