Research on replacing seamless titanium tubes with welded titanium tubes

Titanium has the characteristics of low density, high specific strength, corrosion resistance, high elasticity, low damping, non-magnetism and good high and low temperature performance. It is known as “space metal” and “ocean metal” and is widely used in aerospace, seawater desalination, and ships. , chemical industry, automobile, electric power and many other fields of national economy and national defense construction. Due to the low yield of seamless pipes, long production cycle and high cost, while the production process of pure titanium welded pipes is short, low production cost and high production efficiency, vigorously developing thin-walled welded pipes has become a trend in today’s pipe production. In Japan, Europe and the United States, titanium and titanium alloy thin-walled seamless pipes are gradually being replaced by titanium welded pipes. They are used in condensers and condensers in Binhai power stations and nuclear power plants, which not only improves the heat exchange efficiency, but also improves the efficiency of the condenser. And the service life of the condenser unit, the economic benefits are significant.
When welding titanium pipes, you must ensure:
(1) The metal in the welding area is not contaminated by active gases N, 0, H and harmful impurity elements C, Fe, Mn, etc. above 250°C.
(2) Coarse grain structure cannot be formed.
(3) Large welding residual stress and residual deformation cannot be generated. Therefore, the welding process must follow the predetermined construction sequence, strictly follow the process quality management standards, and implement quality control throughout the entire process. The factors of human, machine, material and method are all under good control, so as to ensure the welding quality of titanium pipes within a reasonable construction period.
The burst pressure of the titanium welded pipe was 61.76MPa. A small crack appeared in the longitudinal weld of the titanium welded pipe, and the water in the pipe burst out from the weld. Seamless titanium pipe: The burst pressure was 68.10MPa. A large crack appeared at the connection between the pressure test plug and the pipe. The water in the pipe burst out from the connection between the pipe and the plug. The seamless pipe itself was not damaged. During the use of titanium tube heat exchangers, condensers, condensers, etc., vibrations induced by the shell-side fluid cause vortices, buffeting, elastic excitation and acoustic resonance of different fluids. These oscillations combine to form severe vibrations. , which causes the pipe to continuously hit the baffle, causing the pipe to be cut and penetrated by the edge of the baffle, and the connection between the pipe and the tube sheet to be stretched to leak. Therefore, fatigue vibration tests were conducted on titanium welded pipes and titanium seamless pipes, and the fatigue times of titanium welded pipes and titanium seamless pipes under the same conditions were counted to determine whether replacing titanium seamless pipes with titanium welded pipes would have an impact on the service life of the equipment.
(1) Expansion properties of welded titanium pipes: Under the same expansion pressure, the pull-off force of welded titanium pipes is 2.5-3.1MPa greater than that of seamless titanium pipes. After expansion, the welded titanium pipes are combined with the expansion tooling. Tighter, the expansion joint has better airtightness.
(2) The welded titanium pipe can withstand the maximum design pressure of 35MPa within the defined range of the pressure vessel and maintain the pressure for 3 minutes without leakage.
(3) Titanium welded pipe and titanium seamless pipe at frequency 30.
The principles and processes for rolling welded titanium pipes are the same as those for seamless titanium pipes. However, the presence of weld beads requires additional factors to be considered in order to determine the best and most stable roll expansion process. Among these factors, the most important are the metallurgical quality and surface condition of the pipe. For example, where it is desired to use pipe in the annealed rather than welded condition, excessively protruding filler beads must be avoided for tight-fit connections in tubesheet designs. It has been demonstrated that air-annealed flared pipes with polished rings on the head The bonding strength is better than that of tubes expanded in the bright annealed or bright stress relief annealed state.
On the other hand, rough tube surface or tube sheet surface can improve mechanical strength, but has poor sealing performance. The surface finish of the titanium tube plate holes manufactured by precision machining or machining and hinged tube plate holes is 0.75-1.5-101, and the finish of the tube head polishing ring also reaches the same level, so that the strength and sealing properties can be optimally matched. .

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Analysis of black stripe defects in TC6 titanium alloy rods

TC6 (Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si) is a martensitic a+/3 two-phase titanium alloy with good comprehensive properties. Its service temperature can reach 450°C and is widely used in important parts of the aviation industry. Structural parts, such as wing blades and aero-engine disks. Since TC6 titanium rod is a two-phase titanium alloy, if the composition of the micro-regions is uneven, it will inevitably cause abnormalities in the macrostructure and microstructure, resulting in a significant difference in hardness between the abnormal area and the normal area, making the overall performance of the material poor. Uneven, fatigue cracks will eventually originate, which will bring great hidden dangers to the safety of parts and reduce the service life of the alloy. In view of the black stripe defect problem found during low-magnification inspection of a certain TC6 titanium alloy bar processing product, in order to accurately determine the type of defect, a metallographic microscope was used to observe the microstructure and determine the abnormal area of ​​the metallographic structure; then scanning Electron microscopy analysis shows that the black stripe area is a chemical composition segregation defect that is rich in molybdenum and poor in aluminum; through microhardness testing, it is determined that the composition segregation in the black stripe area is non-brittle segregation. The test results show that the above method can effectively determine the component segregation and type of TC6 alloy; and it is determined that this type of defect does not affect use and can be delivered after removal. Such defects can be reduced or eliminated by controlling the selection of raw materials for titanium alloy ingots, the mixing and electrode preparation processes, and the voltage and current during the smelting process. The segregation of titanium alloys is classified according to the high and low difference in hardness between the segregation site and the normal area. It can be divided into hard segregation (hardness higher than the normal area, also known as brittle segregation) and soft segregation (hardness lower than the normal area, also known as brittle segregation). Non-brittle segregation) two types. If there is only non-brittle segregation in the product, and all properties meet the requirements of product standards, the product can generally still be delivered for use after removing the segregation; brittle segregation is not allowed to be delivered after removal, and the entire batch should be scrapped. The author discussed the analysis and judgment methods of non-brittle segregation encountered in TC6 alloy rod segregation, aiming to provide reference for product inspection to improve product quality.
1) For the black stripe defects found visually in the TC6 rod, a metallographic microscope was used to observe the microstructure. The defective area was not much different from the normal area, and the type of defect could not be judged; a scanning electron microscope was further used to examine the defective area of ​​the rod. Chemical composition analysis was performed and it was found that the defect area was the segregation of chemical elements rich in giant and poor in aluminum. Finally, combined with the microhardness test, it was determined that the segregation type of the TC6 rod was non-brittle segregation of rich in giant and poor in aluminum. Through microstructure observation, micro-area The method of component analysis and microhardness testing can effectively determine the component segregation and type of TC6 alloy.
2) The segregation in the TC6 alloy rod is non-brittle segregation rich in titanium and poor in aluminum, which does not affect the use and can continue to be delivered after removal; it can be eliminated by controlling the selection of raw materials, mixing and electrode preparation parameters, and the voltage and current during the smelting process. Reduce or eliminate such defects.

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Characteristics of the hot extrusion process of titanium and titanium alloy materials

The thermal conductivity of titanium and titanium alloy billets is low, which will cause a huge temperature difference between the surface layer and the inner layer during hot extrusion. When the temperature of the extrusion barrel is 400 degrees, the temperature difference can reach 200~250 degrees. Under the combined influence of the inhalation strengthening and the large temperature difference in the cross section of the billet, the metal on the surface and center of the billet produce very different strength properties and plastic properties, which will cause very uneven deformation during the extrusion process. A large additional tensile stress is generated in the extruded product, which becomes the source of cracks and cracks on the surface of the extruded product. The hot extrusion process of titanium and titanium alloy products is more complicated than the extrusion process of aluminum alloy, copper alloy, and even steel. This is determined by the special physical and chemical properties of titanium and titanium alloys.

The main factors affecting metal flow during extrusion:
(1) Extrusion method. The metal flows more evenly in reverse extrusion than in forward extrusion, the metal flows more evenly in cold extrusion than in hot extrusion, and the metal flows more evenly in lubricated extrusion than in unlubricated extrusion. The influence of the extrusion method is achieved through changes in friction conditions.
(2) Extrusion temperature. The uneven flow of metal intensifies when the extrusion temperature increases and the deformation resistance of the blank decreases. During the extrusion process, if the heating temperature of the extrusion barrel and the die is too low and the temperature difference between the metal in the outer layer and the center layer is large, the unevenness of the metal flow will increase. The better the thermal conductivity of the metal, the more uniform the temperature distribution will be on the end face of the ingot.
(3) Metal strength. When other conditions are equal, the higher the strength of the metal, the more uniform the metal flow will be.
(4) Die angle. The larger the die angle (that is, the angle between the end face of the die and the central axis), the more uneven the metal fluidity will be. When a porous die is used for extrusion and the die holes are arranged reasonably, the metal flow tends to be uniform.
(5) Degree of deformation. If the degree of deformation is too large or too small, the metal flow will be uneven.
(6) Extrusion speed. As the extrusion speed increases, the unevenness of metal flow intensifies.
Research on the metal flow dynamics of industrial titanium alloys shows that in the temperature zones corresponding to the different phase states of each alloy, the flow behavior of the metal is greatly different. Therefore, one of the main factors affecting the extrusion flow characteristics of titanium and titanium alloys is the billet heating temperature that determines the phase transformation state of the metal.

Compared with temperature extrusion in the a or a+P phase zone, the metal flow is more uniform than temperature extrusion in the p phase zone. It is very difficult to obtain high surface quality of extruded products. Until now, lubricants have been necessary for the extrusion process of titanium alloys. The main reason is that titanium will form a fusible eutectic with iron-based or nickel-based alloy mold materials at temperatures of 980 degrees and 1030 degrees, causing strong wear of the mold. When graphite lubricant is used, deep longitudinal scratches can be formed on the surface of the product. This is a consequence of titanium and titanium alloys adhering to the mold. When glass lubricants are used to extrusion profiles, a new type of defect is caused called “pockmarks”, i.e. cracks in the surface layer of the product. Research shows that the appearance of “pockmarks” is due to the low thermal conductivity of titanium and titanium alloys, which causes the surface layer of the billet to cool violently and the plasticity to drop dramatically.

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Effects of various elements on titanium alloys

Classification of titanium alloys

Titanium alloys are generally divided into α-type, α+β-type, and β-type titanium alloys. According to the relationship between the phase composition of titanium alloys after quenching from the β phase region and the content of β stabilizing elements, titanium alloys are divided into the following six types:
α-type titanium alloys: including industrial pure titanium and alloys containing only α-stabilizing elements;
Near α-type titanium alloy: an alloy with a β-stabilizing element content less than C1;
Martensitic α+β titanium alloy: an alloy with a β-stabilizing element content ranging from C1 to Ck. This type of alloy can be referred to as α+β titanium alloy;
Near metastable beta titanium alloy: alloy with beta stable element content from Ck to C3. This type of alloy can be referred to as near beta titanium alloy;
Metastable β-type titanium alloy: an alloy with a β-stable element content from C3 to Cβ. This type of alloy can be referred to as β-type titanium alloy;
Stable β-type titanium alloy: An alloy with a β-stabilizing element content exceeding Cβ. This type of alloy can be referred to as a full β-type titanium alloy.

Titanium alloy elements

Pure titanium has high plasticity but low strength, which limits its application in industrial production. In order to meet the requirements of high strength and corrosion resistance in actual production, some alloying elements are added to pure titanium to form titanium alloys. According to the effects of alloying elements and impurities on the β-transition temperature of titanium, they can be divided into α-stable elements, β-stable elements and neutral elements.

alpha stable elements

Elements that increase the β-transition temperature, expand the α-phase region, and increase the stability of the α-phase are called α-stabilizing elements [see Figure 4(a)]. Alpha stable elements mainly include: alloy elements aluminum, gallium, germanium, boron and impurity elements oxygen, nitrogen, carbon, etc. Aluminum is the most commonly used α-stable element in industry. Through substitution and solid solution strengthening, adding an appropriate amount of aluminum element can improve room temperature and high temperature strength and thermal strength. Therefore, appropriate amounts of aluminum are almost added to various types of titanium alloys at home and abroad. However, when the aluminum content exceeds 7wt.%, the brittle Ti3Al phase is easily formed, which should be avoided in alloy design. Gallium and germanium elements are rarely used in actual production. Boron element is called the vitamin of metal materials. Adding a small amount of boron to titanium alloys can refine the grains and improve the properties of the alloy. The impurity elements oxygen and nitrogen can greatly increase the strength of titanium, but also seriously reduce the plasticity of the alloy. Therefore, their content must be strictly limited in actual production. Carbon element has little effect on the strength and plasticity of the alloy, and is easier to control during production.

Isomorphous β-stable elements

Elements that have the same lattice structure and similar atomic radius as titanium, lower the β transition temperature, infinite solid solution in the β phase, expand the β phase area, and increase the stability of the β phase are called isomorphous β stable elements [see Figure 4(b)]. It includes elements such as molybdenum, vanadium, niobium, and tantalum. Among them, molybdenum has the most obvious strengthening effect, which can improve room temperature and high temperature strength, increase hardenability, and improve the thermal stability of alloys containing chromium and iron. Molybdenum and vanadium are the most widely used. The strengthening effect of niobium is weak, but it is often added to titanium alloys, especially to Ti-Al intermetallic compounds to improve plasticity and toughness. Tantalum has the weakest strengthening effect and is dense, so it is only added in small amounts to improve oxidation resistance and corrosion resistance.

Eutectoid β-stable elements

Elements that lower the β-transition temperature, expand the β-phase area, and also cause eutectoid transformation are called eutectoid-type β-stable elements [see Figure 4(c)]. This type of elements covers a wide range, and the eutectoid reaction rates vary greatly. Among them, elements such as chromium, manganese, iron, etc. react with titanium at low eutectoid temperatures and have extremely slow transformation speeds, making it difficult to transform under general heat treatment conditions, so they are called inactive eutectoid elements; conversely, silicon, copper, hydrogen, Elements such as nickel and silver have extremely fast eutectoid transformation speeds and cannot be inhibited by quenching, so they cannot stabilize the β phase to room temperature. They are called active eutectoid elements. Iron is one of the strongest β-stable elements, but it has poor thermal stability and is prone to segregation during smelting, so it is rarely used. Iron can be added to some low-cost titanium alloys to replace expensive vanadium. Chromium is one of the elements that is widely added. Chromium-added alloys have high strength and good plasticity, and can be strengthened by heat treatment. However, under certain conditions, the plasticity will be reduced due to the precipitation of compounds. Manganese is a widely used element in early alloy design to improve strength and plasticity, but it produces eutectoid decomposition under certain conditions and is unstable. Silicon is one of the important trace elements that improves thermal strength and heat resistance. It is added to most high-temperature titanium alloys, but generally does not exceed 0.5%. As a harmful element, hydrogen must be strictly controlled. The main reason is hydrogen embrittlement caused by the precipitation of hydride. Other elements such as copper, nickel, and silver are rarely used.

Neutral elements

Elements that have little effect on the beta transition temperature are called neutral elements, mainly zirconium, hafnium and tin. Zirconium and hafnium have similar properties to titanium, and their atomic sizes are also very close, and they can be infinitely dissolved in the α phase and β phase. The room temperature strengthening effect of zirconium is weak, but the high temperature strengthening effect is strong, and it is usually used in thermally strong titanium alloys; the room temperature strengthening effect of tin is even weaker, and eutectoid reactions will occur, but it can improve the thermal strength.

Effect of impurity elements on titanium and titanium alloys

The impurity content in titanium has a great influence on the mechanical properties of titanium. An increase in the impurity content can increase its strength and reduce its plasticity. Oxygen, carbon, and nitrogen are impurities that often exist in titanium. They can increase the strength of titanium and reduce its plasticity. Among them, nitrogen has the greatest impact and carbon has the smallest impact.
The influence of hydrogen on the mechanical properties of titanium is mainly reflected in hydrogen embrittlement. In titanium, when the hydrogen content reaches a certain value, the sensitivity of titanium to notches will be greatly increased, thereby drastically reducing the impact toughness and other properties of notched samples. It is generally believed that the mass fraction of hydrogen in titanium should be lower than 0.007%~0.008%, and not allowed to be higher than 0.0125%~0.015%, because above this content, hydrides will precipitate on the structure and obvious hydrogen embrittlement will occur. .
In addition to oxygen, carbon, and nitrogen, the elements that have a greater impact on improving the strength of titanium are boron, beryllium, and aluminum. Other elements have a less strong effect on the strength of titanium, in order: chromium, cobalt, niobium, manganese, iron, vanadium and tin.

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Analysis of Cracking Causes of TA18 Titanium Alloy Tube in Cold Rolling

TA18 (Ti-3Al-2.5V) titanium alloy is a low aluminum equivalent near α-type α+β-type titanium alloy evolved from TC4 (Ti-6Al-4V) titanium alloy. It not only has good room temperature and high temperature mechanical properties and corrosion resistance, but also has excellent cold and hot processing plasticity, formability, welding performance and corrosion resistance. TA18 titanium alloy pipes are widely used in aircraft engine hydraulic and fuel pipes Road systems, bicycle tripods and handles, golf clubs, fishing rods, casings for oil drilling and heat exchanger tubes, etc.
A certain batch of TA18 titanium alloy pipes had a large number of cracks in the process of cold rolling from Φ70mm×8mm to Φ55mm×6mm. Macroscopically observed that the cracks were distributed along the longitudinal direction of the pipe body. There were no obvious scratches on the surface of the pipe body. The cracks penetrated The pipe wall was damaged and penetrating cracks were formed, resulting in the scrapping of 60% of the batch of pipes. In order to find out the reason for the cold-rolled cracking of the TA18 alloy pipe, samples were taken from typical cracked parts and normal parts, and the chemical composition, microstructure, fracture morphology and microhardness of the structure were analyzed, and the reason for the cracking was analyzed and studied.
Three samples were taken from the cracked part of the fracture and the normal part, and the chemical composition was determined by ICP full-spectrum direct-reading spectrometer and TC-600 oxygen and nitrogen analyzer. The longitudinal and transverse metallographic samples were taken from the cracked part and the normal part respectively. The etching agent (volume ratio) was: hydrofluoric acid: nitric acid: water = 1:4:45, and the microstructure was observed with a Leica MM-6 optical microscope. Samples were taken at the cracked site, and the surface morphology of the cracked section was observed with a JEOL JSM-5610LV scanning electron microscope. Three metallographic samples were taken from the cracked part and the normal part respectively, and the hardness at 5 uniform points was tested on the metallographic sample by HMV-2T Shimadzu microhardness tester, and the test condition was 9.8N/30s.
  In the composition analysis of the cold-rolled cracked parts and normal parts of TA18 titanium alloy pipes, it was found that the content of Fe element exceeded the standard and the content of O element was close to the upper limit of the standard. The microstructure examination found that the normal part was equiaxed, the β phase was diffusely distributed in the α phase in the transverse structure of the cracked part, and the grains were found to be coarse in the longitudinal structure, which tended to transform into Widmanstatten structure. Microscopically, the cracked fracture surface is intergranular brittle fracture. The hardness test results show that the average Vickers hardness of the cracked part is 15% higher than that of the normal part.
The test results show that iron nails are added to the TA18 titanium alloy during the smelting process, which cannot be evenly distributed during the mixing process, resulting in uneven Fe content in the electrode, which eventually leads to local segregation of Fe in the smelted ingot. Due to Fe segregation, the microhardness value in this area will be about 15% higher than that of the matrix, forming a hardened block, which is the main reason for the cracking of the subsequent cold-rolled pipe.
In order to verify the accuracy of the analysis problem, the iron nails were changed to TiFe and VAlFe in the TA18 alloy ingredients, and no cracking was found in the subsequent rolling process, indicating that the problem analysis and improvement measures are effective.

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Various ways of titanium forging

Forging is a forming processing method that applies external force to titanium metal blanks (excluding plates) to cause plastic deformation, change size, shape and improve performance, and is used to manufacture mechanical parts, workpieces, tools or blanks. In addition, according to the way the slider moves, there are vertical and horizontal movements of the slider (for forging of slender parts, lubrication and cooling, and forging of parts for high-speed production), and the compensation device can increase the movement in other directions. The above methods are different, and the required forging force, process, material utilization, output, dimensional tolerance and lubrication and cooling methods are different. These factors are also factors that affect the level of automation.
According to the way the billet moves, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting. Because there is no flash in closed die forging and closed upsetting, the utilization rate of materials is high. It is possible to complete the finishing of complex forgings with one process or several processes. Since there is no flash, the force-bearing area of the forging is reduced, and the required load is also reduced. However, it should be noted that the billet cannot be completely restricted. For this reason, the volume of the billet should be strictly controlled, the relative position of the forging die should be controlled and the forging should be measured, and efforts should be made to reduce the wear of the forging die.
According to the movement mode of the forging die, forging can be divided into pendulum forging, pendulum swivel forging, roll forging, cross wedge rolling, ring rolling and cross rolling. Rotary forging, rotary forging and ring rolling can also be processed by precision forging. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the pre-process processing of slender materials. Rotary forging, like free forging, is also partially formed, and its advantage is that compared with the size of the forging, it can be formed even when the forging force is small. In this kind of forging method including free forging, the material expands from the vicinity of the die surface to the free surface during processing, so it is difficult to ensure the accuracy. The forging force is used to obtain products with complex shapes and high precision, such as forgings such as steam turbine blades with many varieties and large sizes.
In order to obtain high precision, attention should be paid to prevent overload at the bottom dead center, control speed and mold position. Because these will have an impact on forging tolerances, shape accuracy and forging die life. In addition, in order to maintain the accuracy, attention should also be paid to adjusting the clearance of the slider guide rail, ensuring the rigidity, adjusting the bottom dead center and using the auxiliary transmission device and other measures.
Titanium forging materials are mainly pure titanium and titanium alloys with various components. The original state of the material includes bar stock, ingot, metal powder and liquid metal. The ratio of the cross-sectional area of the metal before deformation to the cross-sectional area after deformation is called the forging ratio. Correct selection of forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount and deformation speed have a great relationship with improving product quality and reducing costs. Generally, round or square bars are used as blanks for small and medium-sized forgings. The grain structure and mechanical properties of the bar are uniform and good, the shape and size are accurate, the surface quality is good, and it is convenient to organize mass production. As long as the heating temperature and deformation conditions are controlled reasonably, forgings with excellent performance can be forged without large forging deformation.

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Causes of Stripe Crack Defects on the Surface of TA17 Alloy Cold-rolled Sheet

TA17 alloy is a ternary titanium alloy with a nominal composition of Ti-4Al-2V, which contains a small amount of α-stable element Al to improve the thermal strength of the alloy, and a small amount of β-stable element V can refine grains and improve Due to the effect of process plasticity, its cold working performance is better than that of TC4 alloy. TA17 alloy has medium strength, excellent welding performance and water corrosion resistance, high static and cyclic strength, heat treatment and subsequent sandblasting have no effect on the strength, plasticity and low cycle fatigue performance of the alloy. TA17 alloy was developed by Prometheus Central Institute of Structural Materials Science of the Soviet Union. Its international designation is ЛТ-3B alloy. It is mainly used in shipbuilding, atomic energy, chemical industry, aviation and other fields, especially as a ship shell material. Its output was once It accounts for 1/3 to 1/2 of the finished products of titanium alloy in the Soviet Union. The main product forms of this alloy in my country are plates, forgings, bars and wires.
Compared with traditional common metal materials, titanium alloys are more difficult to process, so various defects will occur in the production process. Even so, there are few research reports on surface crack defects of TA17 alloy cold-rolled sheets. Researchers analyzed the strip crack defects generated during the cold rolling process of TA17 alloy sheets, and found out the causes of their formation, so as to provide technical support for the production of TA17 alloy sheets with qualified surface quality.
The basic process of TA17 alloy cold-rolled plate production is: alloy smelting→slab forging→hot rolling→cold rolling. Sampling was carried out on the cold-rolled sheet with surface strip cracks for chemical composition analysis, and the surface and cross-section samples of cold-rolled sheet with strip cracks were sampled for morphology and micro-area composition analysis. Stereoscopic observation was carried out on a DM6000 stereoscope, and a JSM-7001 scanning electron microscope and an attached INCA-7557 energy spectrometer were used to analyze the crack surface, cross-sectional morphology and micro-area composition.
  According to the analysis, it is found that the strip cracks contain a large amount of oxygen elements. It can be seen that the appearance of strip cracks on the surface of the cold-rolled sheet has a great relationship with the oxidation of the sheet. The TA17 alloy sheet will not be oxidized during cold rolling, and the sheet oxidation can only occur during the annealing process. Therefore, it is inferred that these oxidation defects were not completely removed during the alkali pickling process after annealing and remained on the plate surface, resulting in strip cracks along the rolling direction in the subsequent further cold rolling process.
First of all, during the annealing process before cold rolling, the surface of the plate is oxidized, and where there are quality defects on the surface of the plate, such as grain boundary outcropping, inclusions and other defects provide channels for oxidation, and the oxidation of these parts is deeper. In the subsequent surface cleaning process such as alkali pickling, the parts with surface defects are deeply oxidized and the alkali pickling time is insufficient, so the oxide layer cannot be completely removed and remains on the board surface. These remaining oxidation defects are elongated along the rolling direction and broken into a series of small holes in the subsequent cold rolling process because of their poor plasticity and cannot cooperate with the deformation of the matrix. The small holes in the steel are further elongated and enlarged, separated from each other, and the brittle oxides partly fall off, finally forming linear jagged strip crack defects. The exposed part of the plate falls off due to a certain amount of solid solution of oxygen, which makes the plasticity worse than that of the matrix. During the rolling process, it cannot produce consistent plastic deformation with the matrix and a large number of microcracks appear, because the oxygen content of this part of the oxygen-enriched layer is low. At the same time, the part with high oxygen content has fallen off, so that the oxygen in the remaining part cannot be detected by energy spectrum, so no obvious oxygen element is found in the energy spectrum analysis part.
The above analysis shows that in order to eliminate the strip cracks of TA17 slab, it is first necessary to ensure that the alloy has no metallurgical inclusion defects, and the vacuum consumable arc melting has limited effect on the removal and purification of the alloy, so the quality of the raw materials must be controlled, that is, to ensure that the titanium sponge And the quality of V-Al alloy. The second is to ensure sufficient alkali pickling time after annealing to remove surface oxidation defects. Third, quality defects such as surface holes caused by metallurgical defects should be cleaned up before cold rolling.

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Oxidation of titanium materials and method for improving oxidation resistance

The rapid oxidation of titanium at high temperature forms titanium-silicon compound and titanium-aluminum compound on the surface of titanium, which can prevent the oxidation of titanium at a temperature above 700 °C. This surface treatment is very effective for high-temperature oxidation of titanium. Perhaps the coating of such compounds on the surface of titanium is beneficial to the bonding of titanium and ceramics, and further research is still needed. From the perspective of oxidation resistance, the service temperature of titanium alloy should not exceed 500°C. Generally, the difference between the shrinkage of area of the sample with oxide film and the sample without oxide film is measured as the index of oxidation degree. Oxidation is the limit of high temperature of titanium materials. One of the main reasons to use it. The process of titanium material and oxidation synthesis oxide. Titanium materials are stable in air at room temperature, but are easily oxidized when heated in air or in an oxidizing atmosphere. The degree of oxidation depends on the characteristics of the titanium material itself and the concentration of oxygen in the environment, heating time and heating temperature, etc. At high temperatures, titanium materials oxidize rapidly, resulting in brittle alloys and deterioration of mechanical properties.
When studying the phase composition of titanium anode films, it was determined that the oxide films are generally Roentgen (X-ray) amorphous films, which when formed have a low potential breakdown level. According to some authors, under certain voltage conditions The anodic film breakdown found during film formation is always accompanied by crystal formation. Titanium on the surface of the anodized film has a high degree of oxidation resistance. Low-molecular-weight titanium oxides are found under titanium dioxide (TiO2), and the thickness increases when the oxide formation potential is raised, and the low-molecular-weight titanium oxides in the film partially decrease. In the potential region of titanium oxide formation, titanium oxides of composition Ti50B to titanium oxide are found on the surface of titanium. This range of titanium oxides is transformed into titanium dioxide Ti02 (octoside stone) as the anode potential increases. Since then, according to the anode As the potential increases, the composition of the oxide film of titanium changes, and the degree of oxidation changes from zero to very high.
Because the spark temperature is sufficient for the polymorphic transformation of fausidendite to rutile, it has not been elucidated why the quasi-stable variant (variant) of the oxide is observed in the coating. Even as a result of plasma spraying alumina, although the temperature of the sprayed oxide is very high, a low temperature modification is still obtained. This is the case where plasma spraying and micro-arc oxidation are used, the same can be found in the process of forming the coating. It seems that one of the main reasons may be that a small part of the oxide layer melts during the micro-arc oxidation, and the melt in this region cools violently when the micro-arc moves. Short-term discharges help to form the amorphous phase in the coating. As a result of the rapid cooling rate in the electrolyte of the anode micro-segments responsible for the breakdown, quenching of the thin film material occurs without reaching thermodynamic equilibrium and without the formation of a fully crystalline phase. The study of Roentgen’s amorphous form of this film found that there is a polycrystalline structure of deformation (octahedral stone) in the amorphous matrix, and the film formed in sulfuric acid or phosphoric acid is composed of crystalline phase TiO2. This phase is a phase that crystallizes under the voltage increasing condition. Using the crystallography method, it is clarified how brookite forms crystalline products under low-density current conditions. It has been determined that the anode thin film TiO2 is in the modified state of anatase (octahite). Under the condition of forming voltage, the inclusion rutile of modified titanium oxide was found by thin film spark voltage asymptotic method. When the film voltage is further increased, it is completely transformed into rutile.
Improving the oxidation resistance of titanium materials can be achieved by coating and developing more oxidation-resistant alloys. The coating can use surface processing technology to coat a protective metal layer (such as aluminum, platinum, gold, etc.) or a metal-oxide mixture layer (such as Al+SiO2) on the surface of the titanium material to improve the oxidation resistance of the titanium material performance. Using platinum ion plating, Ti-6Al-2Sn-4Zr-2Mo does not oxidize for a long time at 590°C. Using tungsten and platinum as the bottom layer of the coating respectively, the anti-oxidation temperature can be increased to 700°C. Adjusting the composition of titanium alloy can also improve the oxidation resistance of titanium materials. The Pilling-Bedworth ratio of selected alloying elements should be greater than 1, and the Gideon free energy is lower than that of titanium, which is in line with Hauffe’s law. Alloying elements that improve oxidation resistance include: niobium, aluminum, molybdenum, tungsten, tin, silicon, etc. Add these alloying elements to obtain titanium alloys with good oxidation resistance, such as Ti-5A1, Ti-5Al-2.5, Ti-4Al-3Mo-1V, Ti-5.8Al-4Sn-3.5Zr-O.5Mo-O.7Nb -O.35Si-0.06C etc. Ti3A1, Ti-Al, Ti-Al-Nb and other intermetallic compounds have higher anti-oxidation ability. The anti-oxidation temperature of Ti3A1 can reach above 750°C, and that of TiAl can reach above 900°C. The anti-oxidation ability of Ti-A1-Nb is higher than that of TiAl is higher.
When the temperature is higher than 800°C, the oxide film will decompose, and oxygen atoms will enter the metal lattice through the oxide film, resulting in embrittlement. In general, the oxidation kinetics of titanium follows a parabolic law at low temperatures and a linear law at high temperatures. The molecular volume of the oxide film formed by titanium is larger than the volume of metal atoms consumed to form the oxide film, so the formed oxide film can cover the entire surface of the metal. At 500°C, the oxide film formed on the surface of the titanium material has a protective effect, can prevent the penetration of oxygen, and prevent the titanium material from continuing to oxidize. As the temperature continues to rise, the oxide film loses its protective effect, and intense oxidation occurs. Oxygen diffuses through the oxide film to the inside of the metal, forming an obvious gas permeation layer.

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Cause Analysis and Control of Cold-rolled Titanium Strip Bell Annealing Oxidation

Titanium metal has the advantages of low specific gravity, high strength, strong corrosion resistance, and good biocompatibility, and is widely used in aerospace, ships, metallurgy, petrochemical, medical and other fields. Cold-rolled titanium strip is made of high-quality sponge titanium, produced and supplied in rolls, and the commonly used thickness specification is 0.5mm~3.0mm. It has the advantages of large output, good surface quality, and uniform structure.
During the processing of cold-rolled titanium strips, in order to eliminate work hardening, improve the plasticity and toughness of titanium strips, and obtain better mechanical properties, it is necessary to anneal titanium strips. Since titanium is a metal with relatively active chemical properties, it is easy to react with O2, N2, H2 and water vapor when heated. In order to ensure the production of high-quality titanium coils, the titanium coils are annealed under the protection of vacuum or inert gas. At present, there are mainly three kinds of annealing equipment at home and abroad: vacuum annealing furnace, argon continuous annealing line, and argon-protected bell annealing furnace. Among them, bell furnace annealing has the advantages of low cost, short cycle, and good uniformity. However, when titanium coils are annealed in bell furnaces, due to various reasons, oxidation after annealing often occurs, and the oxidation of titanium coils seriously affects the surface of titanium strips. The quality and yield will even cause the titanium coil to be scrapped. This paper analyzes the causes and control methods of titanium coil annealing and oxidation.
1 Titanium coil annealing process
The bell annealing furnace is mainly composed of a hearth, a heating mantle, a cooling mantle, an inner mantle, a vacuum system, a heating system, a cooling system, an electrical system, etc. During annealing, one furnace with two rolls is used, and the two rolls are placed vertically.
The operation process of the titanium coil bell annealing furnace is as follows: Titanium coil loading furnace → install the inner cover, heating cover → vacuumize → fill with argon gas → heat → keep warm → remove the heating cover → install the cooling cover → cool → remove the cooling cover, the inner cover → Titanium rolls to cooling table.
The raw material for bell annealing is pure titanium coil (TA1 or TA2), the specification is (0.5~3.0) mm×1250mm×L, and the weight of a single coil is 6t. The titanium coil is hoisted onto the hearth by the material rack, and the inner cover is hoisted in place. A mechanical locking device is used to press the water-cooled rubber ring between the inner cover and the hearth to form a sealed cavity. When the protective atmosphere and working pressure required in the furnace are reached, the heating system and circulation fan start to work to make the protective gas in the furnace circulate strongly, and the temperature is uniform, and all processes such as heating, heat preservation, air cooling, and water cooling are completed under the control of the automatic electrical control system process.
2 Annealing oxidation defects and cause analysis
In order to obtain a bright surface quality, titanium coil annealing is carried out in a vacuum and a protective atmosphere, and titanium coil annealing generally uses argon as a protective gas. However, due to equipment failure, improper operation, surface pollution and other problems, when the titanium coil is annealed, the content of oxidizing media such as oxygen, carbon dioxide and water vapor in the atmosphere is relatively large, and the surface of the titanium coil will be oxidized to form a dense oxide film. . The oxidation reaction equation of titanium is Ti+O=TiO2, and the reaction condition is high temperature heating.
When the heating temperature is low, the oxide film on the surface of titanium is almost transparent, which is difficult for people to detect with the naked eye, but when the temperature rises, the oxide film will gradually thicken and interfere with light. The eyes will also show different colors. Generally, titanium coil annealing oxidation presents a light yellow, tan or blue imprint gradually changing from the edge to the middle, and the contour line is arc-shaped and irregular. Generally, the edge is more serious, as shown in Figure 2. After analysis, the following reasons usually cause the increase of the oxidizing medium in the furnace atmosphere, which leads to oxidation on the surface of the titanium coil after annealing.
2.1 Equipment problems
2.1.1 Equipment leaks
Before annealing, the hood-type annealing furnace needs to be evacuated to a certain degree of vacuum, and the equipment is in a sealed state, but the long-term use and aging of the equipment will cause air leakage in the inner cover, pipeline or seal of the equipment, oxygen will enter the furnace, and the vacuum degree in the furnace will drop. Titanium coils are oxidized during high temperature annealing.
2.1.2 Equipment pollution
When the titanium coil is annealed in the furnace, the material tray, inner cover, bracket and other parts are heated together in the furnace, and the diffusion pump is connected with the atmosphere in the furnace through a pipeline. If the furnace inner cover, material tray and other parts (as shown in Figure 3) There are pollutants such as oil, or there is oil leakage in the diffusion pump. When the temperature rises during annealing, the oil is carbonized to produce oxidizing media such as carbon monoxide, carbon dioxide, and nitrogen oxides. The oxidizing media reacts with the titanium coil at high temperature, causing the titanium coil to oxidize.

2.2 Argon pollution
Argon is an inert gas that does not react chemically with other substances at room temperature and is insoluble in liquid metals at high temperatures. It is very inactive in nature and can neither burn nor support combustion.
During the annealing process of the titanium coil, the furnace should be filled with argon for protection. If the purity of the argon is not enough, or the argon pressure is not enough during the argon filling operation, the gas in the furnace is mixed with oxygen, and the titanium coil will be oxidized during high temperature annealing.
2.3 Titanium Coil Contamination
Since the bell annealing is vacuum argon-filled annealing, the titanium coil needs to be degreased and cleaned before annealing to remove the surface oil. The degreasing process is to remove the surface oil by spraying the degreasing liquid and brushing the roller brush. After washing with rinsing water and drying with hot air, it meets the requirements of bell annealing. The degreasing of titanium coils is not clean (as shown in Figure 4), which will lead to oxidation defects of titanium coils, mainly including:
(1) The cleaning ability of the degreasing liquid is insufficient, and the oil and degreasing on the surface of the titanium coil are not clean;
(2) The roller brush of the degreasing equipment is aging, and the scrubbing ability is insufficient, so the surface oil cannot be effectively removed;
(3) The rinse water was not replaced in time, and the degreasing solution on the surface of the titanium coil was not rinsed;
(4) The hot air blowing ability is insufficient, and the titanium coil is not completely dried after cleaning, and there will be water vapor on the surface of the titanium coil. When the polluted titanium coil is loaded into the furnace, the oil and water vapor will volatilize in the furnace at high temperature, increasing the oxidizing medium and causing oxidation on the surface of the titanium coil.
2.4 Poor roll shape
A large number of production data show that when the shape of the coil is poor after rolling, the oxidation will increase. After analysis, when the coil has side waves, the gap between the layers of the titanium coil increases after coiling (as shown in Figure 5), and the gas circulation in the furnace is not smooth during annealing, and it is easy to generate eddy currents and other phenomena, thereby creating a negative pressure zone , It is easy to cause the gas outside the furnace to infiltrate into the furnace, and the gas is easy to infiltrate into the interior of the coil along the edge wave area to cause oxidation.
2.5 Operational issues
Improper operation during the bell annealing process will also cause oxidation of titanium coils. Improper operation has the following aspects:
(1) During the process from degreasing and cleaning the titanium coils to loading into the furnace, it is necessary to ensure that there is no pollution. If the operator does not wear clean gloves during the furnace loading process, the tooling sling has oil, which will cause secondary pollution of the titanium coils. oxidation.
(2) During the annealing process, the operator does not operate according to the process requirements, the vacuum degree in the furnace is not up to standard or the argon filling pressure is not enough during annealing, which will cause annealing and oxidation of titanium coils.
(3) The implementation of equipment maintenance and spot inspection is not in place, and the furnace table, inner cover, material tray and other parts are not cleaned cleanly, resulting in annealing and oxidation of titanium coils.

3 Oxidation control of titanium coils
According to the analysis of the above reasons, in order to avoid or reduce the occurrence of titanium coil annealing and oxidation, it can be controlled from the following aspects:
3.1 Equipment Maintenance
Do a good job in regular maintenance and daily spot inspection of equipment, regularly check the working conditions of each component of the equipment, and pass the pressure rise rate test and equipment leak detection to ensure that the equipment does not leak air or oil; Clean important parts such as discs, inner covers, vacuum chambers, etc., and confirm before the titanium coil is loaded into the furnace to avoid oxidation defects caused by oil contamination of the titanium coil.
3.2 Argon gas control
Argon with a purity of 99.99% or above is used as the protective gas, and argon is always kept in the pipeline to avoid air entering the pipeline. The pipeline pressure is guaranteed during the argon filling operation, and the argon pressure in the furnace meets the process requirements.
3.3 Titanium Volume Control
Strengthen the degreasing and drying effect of the degreasing equipment, do a good job in spot inspection and maintenance of the roller brush of the degreasing equipment, avoid the titanium coil degreasing due to the aging of the roller brush, and formulate the degreasing inspection standard of the titanium coil to ensure that the titanium coil is degreased clean and pollution-free , meeting the requirements of bell annealing.
Control the rolling plate shape of titanium coils, and eliminate edge waves by leveling or straightening after rolling to ensure that there are no obvious gaps between layers and no overflow at the ends of titanium coils after degreasing, which is conducive to the circulation of gas in the furnace and the discharge of exhaust gas during annealing.
3.4 Personnel training
Formulate bell-type annealing operation standards, strengthen personnel training, strictly implement operating standards and process document requirements for furnace loading and annealing processes, standardize equipment maintenance, and check and confirm maintenance results. During the whole annealing process of titanium coils, the changes of vacuum degree and temperature in the furnace are constantly observed, and abnormal situations are reported and resolved in time.
Through the above control measures, the titanium coil can appear bright silvery white after annealing, which can greatly improve the surface quality of the titanium coil and increase the product yield.

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