Dehydrogenation method for titanium alloy bar

Abstract

The invention discloses a dehydrogenation method for a titanium alloy bar, which is suitable for the titanium alloy bar with the cross section size of 100mm or more, and particularly utilizes the diffusion principle that the titanium alloy can be reversely dehydrogenated to creatively solve the problem that the core part of the large-specification and thick-cross-section titanium alloy bar is difficult to dehydrogenate. On the premise of pretreatment and placement of scrap-shaped and filiform titanium alloy materials in the same furnace, a reverse pressure hydrogen diffusion principle of the titanium alloy materials under different temperatures and different vacuum conditions is utilized; according to the method, the hydrogen element can be smoothly diffused and removed from the surface to the core part of the bar material through the setting of technological parameters of staged step-by-step rate-reducing heating-T2 short-time heat preservation-further rate-reducing heating to T3-T3 low-vacuum long-time heat preservation and argon filling cooling, and finally the hydrogen removal treatment of the titanium alloy bar material is realized; and the excellent structure property quality of the bar can be ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of titanium alloy processing, and particularly relates to a titanium alloy bar dehydrogenation method. BACKGROUND

[0002] Titanium and titanium alloy have the characteristics of small density, high specific strength, good corrosion resistance, no magnetism, etc., and can be used as aircraft, rocket, missile and space structure materials, and can also be applied to various industries such as ships, chemical industry, metallurgy and medical health, and has broad application prospects. Titanium and titanium alloy materials need to be heated for multiple times in the process of hot working such as forging, rolling and ring rolling. High-activity titanium interacts with almost all substances at high temperature, not only reacts with H2O, CO2 and other substances in the heating atmosphere, but also reacts with the heating furnace wall and the parts in contact. When the H content in the heating furnace is relatively high, it is extremely likely to cause hydrogen to penetrate into the titanium and titanium alloy products. The introduction of trace hydrogen into titanium and titanium alloy can cause hydrogen embrittlement to occur, which causes the metal material to break down under the action of less than the yield stress. When the hydrogen content of titanium and titanium alloy products exceeds the standard, effective means for dehydrogenation cannot be taken, and the products can only be remelted and modified or scrapped. The only way to dehydrogenate is vacuum annealing treatment, but there is less research on the parameters of vacuum annealing in the industry. Vacuum annealing of various products is more used for adjusting the product organization and performance (the performance of products with and without vacuum annealing is different), and dehydrogenation is mainly used for powder materials and parts with a thin cross-sectional thickness. When titanium alloy is subjected to vacuum annealing treatment, due to the high affinity of hydrogen and titanium, it is difficult for the internal hydrogen of titanium and titanium alloy with a cross-sectional thickness of 100 mm and above to escape from the titanium alloy material, and it is difficult to achieve good dehydrogenation effect. Vacuum dehydrogenation annealing is generally only suitable for titanium and titanium alloy workpieces (mainly parts) with a cross-sectional thickness of less than 50 mm. Therefore, for titanium alloy bars with a minimum cross-sectional size of 100 mm and above, the existing vacuum annealing method cannot solve the problem of dehydrogenation of thick cross-section titanium alloy. SUMMARY

[0003] In view of the deficiencies of the prior art, the purpose of the present application is to provide a titanium alloy bar dehydrogenation method, which is suitable for titanium alloy bars with a cross-sectional size of 100 mm and above, and can reduce the internal hydrogen content of titanium alloy bars without affecting the quality of the bars.

[0004] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: A titanium alloy bar dehydrogenation method is mainly realized by the following steps: Step 1), pretreatment: the surface of the titanium alloy bar is machined to be bright and clean, and the oxide scale and surface oil stains are removed; The above step 1) removes the surface oxide scale and oil stains of the bar to be dehydrogenated, so as to prevent the existence of a dehydrogenation barrier. Step 2, furnace preparation: the rod processed in step 1 is placed in a vacuum annealing furnace, and whether to use a rack is determined according to the length of the rod. After the rod is placed, a single titanium or titanium alloy material with a thickness of not more than 2 mm is placed at the gap of the furnace. The titanium or titanium alloy material includes but is not limited to one or more than two shapes of scrap, strip, and wire in any proportion; the total weight of the titanium or titanium alloy material placed in the furnace is not less than 5 kg, which is used to absorb part of the hydrogen elements escaping from the inside of the rod during the dehydrogenation process; The above step 2) places scrap, wire, and titanium alloy materials in the furnace during furnace preparation. The scrap, wire, and strip materials have large specific surface area and small thickness size, which makes them more easily absorb hydrogen than the rod with small specific surface area. This feature prevents hydrogen escaping from the rod during the unstable vacuum stage from re-entering the rod; Step 3, pre-vacuum preparation: close the vacuum annealing furnace door and vacuumize to ≤1.0×10 -2 Pa, then start vacuum heating; Step 4, heating at a decreasing rate in stages with holding: First stage, two-step heating at a decreasing rate: first, heat from room temperature to T1 temperature of 250-300℃ at a heating rate of S1=1.5-3.5℃ / min; then heat from T1 temperature to T2 temperature of 460-520℃ at a heating rate of S2=0.5-1.5℃ / min; maintain the vacuum degree ≤3.1×10 -2 Pa, then immediately start the second stage holding; Second stage, short holding: hold at T2 temperature of 460-520℃, holding time is rod diameter D (mm) * holding coefficient K1, K1=0.2-0.4 min / mm, at this temperature part of the gas in the furnace is discharged to ensure the vacuum degree ≤1.5×10 -2 Pa, then start the third stage heating; Third stage, low vacuum heating at a decreasing rate: slowly heat from T2 temperature of 460-520℃ to T3 temperature of 700-800℃ at a heating rate of S3=0.15-0.4℃ / min, maintain the vacuum degree ≤1.0×10 -2 Pa during the heating process, then immediately start the fourth stage holding; Fourth stage, low vacuum holding for a long time: hold at T3 temperature of 700-820℃, holding time is rod diameter D (mm) * holding coefficient K2, K2=4-10 min / mm, the vacuum degree during the fourth stage holding process needs to be controlled to ≤1.0×10 -2 Pa; stop heating after the completion of the fourth stage; As the diffusion of hydrogen element in titanium alloy material is accelerated with the increase of temperature, the diffusion of hydrogen element is accelerated with the increase of temperature and the extension of heating time during the heating process, which easily leads to vacuum instability (increase of vacuum degree). The step 4) in the present application controls the dehydrogenation rhythm by adopting staged step-by-step slow heating and heat preservation: the first stage realizes fast adjustment of the atmosphere and vacuum degree in the furnace by two-step "fast heating-slow rate heating", which makes good preparation for the subsequent dehydrogenation; the second stage is suitable for heat preservation at the intermediate process T2, which is beneficial to the discharge of gas impurities generated by the decomposition of oil stains on the surface of the rod and adjustment of the vacuum degree to a lower level, preventing the increase of vacuum degree and the decrease of reverse pressure difference, which leads to the decrease of hydrogen element escape rate and dehydrogenation difficulty. The step 2) of the third stage can effectively prevent the hydrogen element from entering the rod again during the vacuum instability. The third stage realizes the purpose of fast dehydrogenation of hydrogen element under the pressure difference by slow heating at a slower rate than the first stage and long time heat preservation at T3 temperature. The third stage further increases the temperature, and the activity of hydrogen increases with the increase of temperature. The hydrogen element is beneficial to escape from the surface of the rod under the further low vacuum, but the thermal conductivity coefficient of the titanium alloy material itself is low. Therefore, the further reduction of the heating rate when the temperature approaches the target dehydrogenation temperature T3 is beneficial to the reduction of the temperature difference and hydrogen element difference between the edge and the center of the large-size rod, preventing the problem that the edge rapidly dehydrogenates and the center cannot dehydrogenate under the condition of fast heating under low vacuum. At the same time, the T3 temperature is far away from the solid solution temperature of the titanium alloy, which does not have a great influence on the organization and performance of the material. The fourth stage maintains low vacuum at T3 temperature and carries out ultra-long time heat preservation. At this time, the low vacuum in the furnace and the temperature of the large-size rod edge and center tend to be consistent through the two-step slow rate heating-T2 short time heat preservation-further slow rate heating to T3 in the previous stages. The T3 temperature does not significantly change the material organization. The long time low vacuum heat preservation promotes the hydrogen element in the center or inside of the large-size rod to gradually diffuse and transfer to the surface and slowly escape under the reverse pressure difference of low vacuum, finally achieving the purpose of dehydrogenation. The hydrogen elements in the edge and the center can be smoothly dehydrogenated and restored to normal, avoiding the situation that the surface dehydrogenates and the center cannot dehydrogenate. Step 5), argon filling cooling: after the completion of the fourth stage, argon is filled into the furnace for forced cooling. The argon flow needs to keep the pressure in the vacuum annealing furnace at 80-100 KPa. The pressure in the furnace during the whole cooling period should not be higher than the atmospheric pressure. The total cooling time is not less than 10 hours. The furnace temperature is lower than 60℃, then the furnace door is opened for air cooling or the furnace is cooled to room temperature. After cooling to room temperature, the furnace is discharged, which completes the dehydrogenation treatment. The step 5) is filled with argon and forcedly cooled for a long time, on one hand, the argon is used to replace the atmosphere in the furnace to prevent the hydrogen from entering the bar again, on the other hand, the surface oxidation of the bar during the cooling process can be prevented, and the bar is cooled to below 60 DEG C to ensure that the surface of the bar is still shiny after the dehydrogenation, and the surface machining treatment for removing the oxide skin is not needed.

[0005] Further, the steps 1) to 5) are repeated for 2 times or more.

[0006] Further, whether the rack is used in the step 1) is determined according to whether the length of the bar is greater than 1.5 m, when the length is greater than 1.5 m, the bar needs to be placed on the rack to prevent the bending deformation during the annealing process, and the rack needs to be free of the oxide skin.

[0007] Compared with the prior art, the present application has at least the following beneficial effects: 1. The titanium alloy bar treated by the dehydrogenation method can effectively realize the dehydrogenation of the large-size titanium alloy bar with a diameter of more than 100 mm, restore the hydrogen content to less than or equal to 0.015%, and has the same structure and performance as before the dehydrogenation treatment, and the surface of the bar does not need to be machined after the dehydrogenation, which can greatly reduce the scrap and modification cost of the bar (the large-size bar with excessive hydrogen content needs to be scrapped or needs to be remelted to prepare a cast ingot and then forged for multiple times to modify the qualified bar, and the production cost is particularly high); 2. The dehydrogenation method for the titanium alloy bar uses the reverse dehydrogenation diffusion principle of the titanium alloy, creatively solves the problem of the dehydrogenation of the large-size and thick-section titanium alloy bar, and under the premise of pretreatment and placing the scrap and filamentous titanium alloy materials in the same furnace, uses the reverse pressure hydrogen element diffusion principle of the titanium alloy material under different temperatures and different vacuum conditions, and innovatively sets the process parameters of the two-step heating rate reduction-T2 short-time holding-further heating rate reduction to T3-T3 low-vacuum long-time holding, so that the hydrogen element can be diffused and removed from the surface to the center of the bar, and finally realizes the dehydrogenation treatment of the titanium alloy bar and guarantees the excellent structure, performance and quality of the bar; 3. The vacuum dehydrogenation method can guarantee that the structure, performance, flaw detection and surface quality of the titanium alloy bar meet the final requirements of the product, can save the scrap caused by the excessive hydrogen content, and does not need to be remelted to prepare a cast ingot and forged to modify the bar, which greatly reduces the scrap and modification cost of the product while guaranteeing the quality, and has good economy. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 It is a drawing of the surface quality of the bar before the dehydrogenation in the first embodiment of the present application. Figure 2 Figure is the surface quality diagram of the rod prepared after dehydrogenation in the embodiment one of the present application; Figure 3 Figure is the diagram of the titanium or titanium alloy material for hydrogen absorption placed in the same furnace for preparation of furnace in the embodiment one of the present application; Figure 4 Figure is the high magnification structure diagram of the rod before dehydrogenation in the embodiment one of the present application; Figure 5 Figure is the high magnification structure diagram of the rod prepared after dehydrogenation in the embodiment one of the present application. DETAILED DESCRIPTION

[0009] The present application will be described in detail below with reference to the embodiments, so that the advantages and features of the present application can be more easily understood by those skilled in the art, and the protection scope of the present application can be more clearly defined. Any same or similar schemes without departing from the concept of the present application shall fall within the protection scope of the present application. And the parts not described in detail in the text are carried out in the conventional manner in the art. And in the following text: "Φ" refers to the diameter of the blank with a circular cross section.

[0010] It should be noted that the element content in the embodiments, if not specially limited, is the mass content.

[0011] Embodiment one (Φ300mm TC11 rod) The H content in the TC11 alloy product standard is generally required to be ≤0.012%, and the normal delivery product hydrogen content is generally ≤0.006%. The Φ300mm TC11 titanium alloy rod used in the present embodiment has an overall hydrogen content exceeding the standard due to abnormal heating equipment in the production process. The hydrogen content H measured from the rod after machining and lightening before dehydrogenation is 0.015% and 0.014%. The rod dehydrogenation is carried out according to the method of the present application, and the specific implementation steps are as follows:

[0012] Step 1: clean the surface of the TC11 rod after machining and lightening, remove the surface oxide scale and oil stains, and the surface quality of the pretreated rod is shown in Figure 1 ; Step 2: hoist the rod treated in step 1 into the vacuum annealing furnace. When the length of the rod is greater than or equal to 1.5m, it should be placed on the rack to prevent bending and deformation during annealing. The rack should not contain oxide scale. When the length is less than 1.5m, it is directly placed in the vacuum annealing furnace. After the rod is hoisted, place the titanium or titanium alloy scrap, strip and filamentous material (as shown in Figure 3 ) with a thickness not exceeding 2mm in the same furnace at the gap in the furnace for absorbing part of the hydrogen elements released during dehydrogenation; Step 3: close the door of the vacuum annealing furnace, and vacuumize to ≤1.0×10 -2 Pa, and then start heating; Step 4: Heating and holding in four stages: The first stage of heating starts from room temperature and rises to T1 (280℃±10℃) at a heating rate controlled at S1 = 2.5℃ / min. Then, the heating rate is reduced from T1 (280℃±10℃) to T2 (490℃±10℃) at a heating rate controlled at S2 = 1.0℃ / min. Throughout the entire first stage of heating, a vacuum degree ≤ 3.0 × 10⁻⁶ is maintained. -2 Pa, then immediately begin the second stage of heat preservation; the second stage of heat preservation is to first maintain the temperature at T2 (490℃±10℃), and the heat preservation time is equal to the diameter of the bar (300 mm) * the heat preservation coefficient K1, where K1 = 0.2-0.4 min / mm. The actual heat preservation time is 60 min. At this temperature, some of the gas inside the furnace is discharged, ensuring that the vacuum degree is adjusted to ≤1.5×10. -2 Pa, then the third stage of heating is initiated; the third stage of heating involves slowly increasing the temperature from T2 (490℃±10℃) to T3 (750℃±10℃) at a rate controlled at S3=0.25℃ / min, while maintaining a vacuum degree ≤1.0×10⁻⁶ throughout the heating process. -2 Pa, followed immediately by the fourth stage of heat preservation; the fourth stage of heat preservation is a long-term heat preservation at T3 temperature (750℃±10℃), the heat preservation time is 300 (mm) of the bar diameter * heat preservation coefficient K2, K2=4-10min / mm, the actual heat preservation time is 1600min, and the vacuum degree is controlled ≤1.0×10 during the fourth stage of heat preservation. -2 Pa. Heating should be stopped after the fourth stage is completed; Step 5: Introduce argon into the furnace and start forced cooling. The argon flow rate should be such that the pressure inside the vacuum annealing furnace is maintained at 80-100 kPa. The pressure inside the furnace should not exceed atmospheric pressure at any time. The total cooling time should not be less than 10 hours. After the furnace temperature drops below 60°C, the furnace door can be opened for air cooling or the furnace can be cooled to room temperature. After cooling to room temperature, the furnace can be removed, thus completing the dehydrogenation process.

[0013] Depend on Figure 2 It can be seen that there is no obvious oxidation on the surface of the dehydrogenated rod. Thirty samples were cut from each end of a Φ300mm TC11 rod after dehydrogenation, and samples were taken from these samples for low-magnification, high-magnification, hydrogen content, and mechanical property testing. The rods were also subjected to ultrasonic flaw detection according to standards. High-magnification microstructure images of the rod before and after dehydrogenation (e.g., ...) are shown in the image. Figure 4 , Figure 5Compared with the contrast between before and after the dehydrogenation, there is no obvious change, which shows that the microstructure and properties of the rod after the dehydrogenation treatment will not be obviously changed, and the beneficial effects described in the application can be achieved. Table 1 is the comparison of the hydrogen content detection results of the rod prepared before and after the dehydrogenation in Example 1. After the dehydrogenation treatment, the hydrogen content of the rod is restored to normal, the hydrogen content from the edge to the center of the rod tends to be consistent and can meet the standard requirements; Table 2 is the mechanical property detection results of the rod prepared after the dehydrogenation in Example 1, which meets the standard requirements. In summary, after the dehydrogenation treatment by the method of the application, the surface of the rod does not occur obvious oxidation and does not need to be machined again, the microstructure and properties of the rod are equivalent to those before the dehydrogenation and do not change obviously, the hydrogen content of the rod can be obviously reduced, and the dehydrogenation effect is good.

[0014] Table 1 Comparison of hydrogen content detection results of the rod prepared before and after the dehydrogenation .

[0015] Table 2 Mechanical property detection results of the rod prepared after the dehydrogenation .

[0016] Comparative Example 1 is a TC11 alloy Φ200 mm rod which is not treated by the technical method of the application. The length of the rod is about 1 m. The TC11 titanium alloy Φ200 mm rod used in the comparative example is out of standard due to the abnormal heating equipment in the production process. The hydrogen content measured from the rod after machining and lightening before the dehydrogenation is H=0.015%, 0.014%, which is consistent with the level before the dehydrogenation in Example 1. The vacuum dehydrogenation annealing process parameters of the comparative example are set according to GB / T 3763-2004, and the specific implementation steps are as follows: Comparative Example 1 Step S1: clean the surface of the TC11 rod which has been machined and lightened, remove the surface oxide skin and oil stains; Step S2: hoist the rod treated in step 1 into the vacuum annealing furnace; Step S3: close the door of the vacuum annealing furnace, and vacuumize to ≤6.7×10 -2 Pa, and then start heating; Step S4: use the heating rate S=2.5 ℃ / min to start heating from room temperature to T=750 ℃, and then keep 750 ℃ for 12 h (equivalent to 720 min), control the vacuum degree to be ≤6.7×10 -2 Pa, stop heating after the holding time is completed; Step S5: after the furnace is cooled to below 200 ℃, the rod is taken out and air-cooled, that is, the dehydrogenation process is completed.

[0017] After dehydrogenation, 50mm was cut off from each end of the Φ200mm TC11 rod, and 30 samples were cut from the rod. The hydrogen content of the samples was detected. Table 3 shows the hydrogen content of the rod before and after dehydrogenation in the comparative example 1. After dehydrogenation, the hydrogen content of the edge of the rod slightly decreased (the hydrogen content of the edge of the rod after dehydrogenation in the comparative example 1 was 0.010%, which was still higher than the hydrogen content of the edge of the rod in the example 1, which was 0.005%), but the hydrogen content of the core of the rod did not change obviously (the hydrogen content of the core of the rod before dehydrogenation was 0.014%, and the hydrogen content of the core of the rod after dehydrogenation was 0.013%). The hydrogen in the core of the rod did not dehydrogenate smoothly, and the dehydrogenation effect of the comparative example 1 was far less than that of the example 1.

[0018] Table 3 shows the hydrogen content of the rod before and after dehydrogenation in the comparative example 1. .

Claims

1. A method of dehydrogenating a titanium alloy bar, characterized by, The method is realized by the following steps: Step 1), pretreatment: the surface of the titanium alloy bar is machined to see light, and cleaned to remove the oxide scale and surface oil stains; Step 2), furnace preparation: the bar treated in step 1) is placed in a vacuum annealing furnace, and after the bar is placed, one or more than one of titanium or titanium alloy materials in the shape of scrap, strip and wire, etc. is placed at the gap of the furnace, the total weight of which is not less than 5 kg, and the thickness of each titanium or titanium alloy material is not more than 2 mm; Step 3), Pre-vacuum preparation: Close the door of the vacuum annealing furnace and evacuate to ≤ 1.0 x 10 -2 Pa, and then open the vacuum heating; Step 4), heating and holding at a gradually reduced rate in stages: The first stage is two-step rate reduction heating: first, from room temperature to T1 temperature of 250-300℃, the heating rate is controlled to S1=1.5-3.5℃ / min; then from T1 temperature to T2 temperature of 460-520℃, the heating rate is controlled to S2=0.5-1.5℃ / min; the vacuum degree of this stage is kept ≤3.1×10 -2 Pa, and then the second stage of holding is started immediately; Second stage short time holding: holding at T2 temperature 460-520℃, holding time is rod diameter D*holding coefficient K1, K1=0.2-0.4 min / mm, ensuring vacuum degree ≤1.5*10 -2 Pa, then open the third stage heating; Third stage: heating from T2 temperature 460-520℃ to T3 temperature 700-800℃ at a heating rate controlled at S3=0.15-0.4℃ / min, the vacuum degree is kept at ≤1.0×10 -2 Pa during the heating process, and then immediately start the fourth stage. Fourth stage low vacuum and long time holding: holding at T3 temperature 700-820℃, holding time is rod diameter D*holding coefficient K2, K2=4-10min / mm, vacuum degree should be controlled ≤1.0*10 -2 Pa during the fourth stage holding process; stop heating after the fourth stage holding process is completed; Step 5), argon filling cooling: after the completion of the fourth stage, argon is filled into the furnace for forced cooling, the argon flow rate needs to keep the pressure in the vacuum annealing furnace at 80-100 KPa, the total cooling time is not less than 10 hours, and after the furnace temperature is lower than 60℃, the furnace door is opened for air cooling or the furnace is cooled to room temperature, and then the furnace is discharged after cooling to room temperature, that is, the dehydrogenation treatment is completed.

2. The method of claim 1, wherein, Whether the rack is used in step 1) above is determined by whether the length of the bar is greater than 1.5 m, when the length is greater than 1.5 m, the bar needs to be placed on the rack to prevent bending and deformation during annealing, and the rack should not contain oxide scale.

3. The method of claim 1 or 2, wherein, The steps 1) to 5) above are repeated 2 times or more.

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