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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Titanium rod rolling process analysis and rolling mill pass design

The titanium alloy rod continuous rolling production unit is an 8-stand three-roller Y-type rolling machine. The titanium alloy grade is TC4. The blank is a titanium rod with a diameter of 25mm. It is rolled through 8 passes to make a finished product with a diameter of 15mm. The original pass shape is flat. Triangle-circle-flat triangle-circle-flat triangle-circle-circle-round hole type. However, in actual production, the finished titanium alloy bars often have many problems such as ears that run through the entire length of the product, substantial torsion of the rolled piece, and poor roundness of the finished bar, which ultimately leads to substandard dimensional accuracy of the product. In view of the above problems, an idea is put forward: whether the flat triangular hole shape can be improved so that the improved flat triangular hole shape can reduce the torsion of the breast piece caused by the uneven circumferential flow of the breast piece metal, and at the same time reduce the distortion of the breast piece metal. The flow amplitude towards the roll gap in the round hole type.

  1. Based on the improved “flat triangle-circle” pass system and combined with ANSYS/LS-DYNA nonlinear finite element software, the deformation process of titanium alloy bars in the continuous rolling mill is vividly simulated.
  2. At the same time, the existence of the trapezoidal block also restricts the circumferential flow of the metal in the rolled piece, forcing the metal to extend axially, thereby increasing the pass elongation coefficient and increasing the pass reduction, which to a certain extent solves the problem of reduction due to the reduction. There are many problems caused by oversize. Existing equipment can also be fully utilized to increase the size of the blank and thereby enhance the processing capacity of the blank.
  3. From the final simulation results, it can be intuitively seen that the improved “flat triangle-circle” pass system can well improve the roundness of titanium alloy rods, thereby improving the dimensional accuracy of the product.
  4. The improved flat triangular pass type is equivalent to installing a circle of trapezoidal blocks at the bottom of the traditional flat roller. In this way, during rolling, the trapezoidal blocks are pressed into the rolled piece and the part where the rolled piece is pressed appears in the next pass. The sub-circular pass roll gap reduces the flow amplitude of the rolled metal to the roll gap, thereby reducing the occurrence of ears.
  5. The trapezoidal block pressed into the rolled piece also plays a role in positioning the rolled piece, which greatly reduces the problems caused by the uneven metal density of the rolled piece, the elastic recovery of the rolled piece, and the vibration of the rolling mill in the traditional flat triangular pass. The non-uniformity of the circumferential flow of metal in the rolled piece greatly reduces the probability of torsion of the rolled piece that is common in the traditional “flat triangle-circle” pass system.
    For titanium alloy rods, the traditional method is to use a horizontal rolling machine to reciprocate. In order to improve the mechanical properties and processing technology of titanium rods, it is necessary to use hot emulsion technology to produce small-sized titanium alloy rods. With the promotion of application scope and increase in demand, the quality requirements for titanium alloy products have further increased, which requires further optimization of the production and processing technology of titanium alloys.
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Introduction to hydrogen treatment technology of titanium alloy materials

Hydrogen treatment technology of titanium and its alloys is a relatively active research direction in the field of materials science and engineering. At present, hydrogen treatment technology has been applied in research on thermal processing, mechanical processing, powder consolidation, composite material preparation, microstructure refinement, etc. of titanium alloys, and has formed a unique research field. The use of hydrogen treatment technology to improve the superplastic properties of titanium alloys is an important research aspect. So far, many scholars have used the hydrogen treatment effect to improve the superplastic properties of cast titanium, deformed titanium alloys and titanium-aluminum intermetallic compounds.
At present, there are two ways to use hydrogen treatment technology to improve the superplasticity properties of titanium alloys:
(1) Utilize the plasticizing effect of hydrogen, add an appropriate amount of hydrogen before superplastic forming of titanium alloy, increase the proportion of B phase in titanium alloy, reduce the flow stress during superplastic deformation, and achieve the purpose of improving the superplastic properties of titanium alloy.
(2) Hydrogen treatment is used to refine the microstructure of titanium alloys, and combined with plastic deformation technology to prepare ultra-fine-grained titanium alloys, so that titanium alloys have excellent superplastic properties at lower deformation temperatures and higher deformation rates.
Modern superplastic deformation theory believes that grain boundary slip is the main mode of superplastic deformation, and diffusion and dislocation movement within grains and grain boundaries are the main coordination mechanisms of grain boundary slip. In the superplastic forming of titanium alloys, the B phase is dominated by diffusion creep or dislocation creep; the A phase is dominated by grain boundary slip, coordinated through diffusion and dislocation motion; the flow between the A and B phases is dominated by A Completed with B phase boundary migration. Hydrogen mainly plays the following roles in the superplastic forming of titanium alloys:
(1) The addition of hydrogen improves the diffusion ability of alloy elements, leading to the enhancement of diffusion creep of B phase and intergranular slip of A phase.
(2) The diffusion of hydrogen activates the pinned dislocations, promotes the climbing and sliding of dislocations, improves the sliding ability of B grains, and is conducive to the dislocation coordination required for A/A grain boundary sliding.
(3) The weak bond effect caused by hydrogen reduces the diffusion activation energy, enhances the atomic diffusion ability, and improves the superplastic flow ability.
(4) It can be seen from the Ti2H phase diagram that the addition of hydrogen significantly reduces the B\A+B transition temperature and increases the volume fraction of the B phase, which directly leads to the improvement of plasticity and the reduction of flow stress, allowing titanium alloys to Superplastic forming is performed at lower deformation temperatures and higher deformation rates.

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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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