A feasible method for reducing surface contamination of titanium screws and titanium workpieces

Russia is an international leader in the research and manufacturing technology of building titanium alloy nuclear submarines, and is also the first country to build pressure hulls with titanium alloys. During the peak period, the annual output of titanium alloy plates and pipes for submarines was as high as 10,000 tons, accounting for 1/3 to 1/2 of the annual output of titanium alloy processed materials. Titanium plates for submarine heat exchangers require good thermal conductivity. The shell material is required to have good toughness to resist the shock wave caused by the depth charge explosion. The titanium plate (thick plate) for the submarine shell is produced in St. Petersburg, and the ingot is provided by the Upper Salda Metallurgical Production Joint Company. In terms of the development level of the titanium industry and the scale of use of titanium plates and wires in the shipbuilding industry, Russia is far ahead of all other countries in the world. Because Russia is ahead of competitors such as the United States in the field of ship materials research, its scholars have called on the government to formulate a near-term shipbuilding plan to avoid losing the leading position and development potential it has achieved, and at the same time to ensure that the Russian Navy will continue to lead in the 21st century. In other countries.

Since the 1960s, there have been 4 generations of nuclear submarines developed by Russia. The world’s first K162 all-titanium nuclear submarine was launched in December 1968 and has been in operation for more than 30 years. It has been to various oceans and sea areas and has withstood different loads and loads. Environmental assessment, there has never been any accident. Russia built the first "ALFA" class nuclear submarine in 1970, and continued to build 6 ships from the 1970s to the 1980s. Each of them uses titanium rods, titanium wires, and titanium plates about 3000t. The maximum diving depth is 914m, which is light and fast. .

In the 1980s, six "Typhoon" class nuclear submarines with a capacity of 9000t of titanium rods, titanium wires and titanium plates were manufactured. The first "Typhoon" class nuclear submarine was built in 1980 and commissioned in 1984. Its underwater displacement With a speed of 33800t and an underwater speed of about 27 knots, the "Typhoon" class nuclear submarine is the world's largest submarine. It has a double hull structure. The non-pressure hull is made of high-strength and low-magnetic steel, and the pressure hull is made of titanium alloy. It can carry 20 strategic missiles. The launch of the sixth "Typhoon" submarine was at the end of 1989, and the "Typhoon" series of submarines will be decommissioned at the beginning of the 21st century.

The first submarine in the new series of "The God of the North Wind", which was commissioned in 2003, is the "Yuri Dolgoruky". It is a new fourth-generation submarine, and its combat performance should exceed the existing ones of the same level. All submarines. Its underwater displacement is greater than that of the "Detal" class (11740t), but less than the "Typhoon" class (33800t), and the construction cost requires several trillion rubles.

The "Ohio" missile submarine that the United States entered service in 1980 has an underwater speed of 20n mile/h (knots).
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Chemical properties of titanium materials such as titanium rods and titanium wires

Whether it is the development of the marine economy or the development of the modern navy, it is necessary to develop a series of marine engineering equipment. Practice has shown that advanced offshore equipment, whether it is deep-sea oil and gas exploration equipment, nuclear submarines, deep submersibles and other equipment, are all titanium-related equipment. Lightweight and corrosion-resistant titanium and titanium alloy materials can be used to achieve high-efficiency offshore equipment. , Long life and high reliability make a major contribution.

At present, the time is ripe to accelerate the development of titanium for marine engineering. my country is already a big country in the world's titanium industry. It has a relatively complete titanium R&D-production-application system, large-scale production capacity and many application technology reserves. It is not only necessary but also feasible to accelerate the development of titanium for marine engineering. Compared with commonly used materials such as steel, stainless steel, copper and aluminum, the most prominent features of titanium are low density, high specific strength, and strong corrosion resistance. At the same time, it is also resistant to seawater erosion, non-magnetic, non-cold brittleness, and high sound permeability. The coefficients and other properties are convenient for forming, casting, and welding, making it widely applicable to various marine engineering.

Titanium can be used in ocean engineering, but there are also some shortcomings and problems worth noting. Titanium alloy materials for marine engineering face many challenges, mainly in five aspects:

1. Titanium production

Due to the high melting point of titanium (1668°C), high resistance to high temperature deformation, and narrow thermal processing temperature zone, it is difficult to produce titanium materials, especially large-size, high-performance titanium materials. Not only a large vacuum melting furnace (vacuum electric arc furnace, electron beam cooling bed furnace, etc.) is required, but also heavy pressure processing equipment (forging press, rolling mill, extruder, etc.) is required. Titanium production unit product investment is huge, up to 300,000 ~ 400,000/ton. The equipment capacity of titanium production is equal to or greater than that of steel of the same specification, and its output and equipment utilization are only a few tenths of it, resulting in high production costs.

2. Product design

Titanium has a high flex-strength ratio (above 0.9), a high welding strength coefficient (above 0.9), and a low modulus of elasticity, thermal conductivity and damping coefficient. Under certain conditions, titanium will have crevice corrosion, contact galvanic corrosion and hydrogen embrittlement. Due to these physical, chemical and mechanical characteristics of titanium, the design of titanium equipment is required to adopt new design specifications and technical specifications (such as safety factor, corrosion margin, fire and explosion prevention measures, structural form, weld form, etc.). Titanium materials have a small damping coefficient (except for TiNi shape memory alloys), and they will vibrate during use. Therefore, anti-vibration and vibration reduction measures must be taken. Titanium has only a few decades of history in the industrial field, there is little design experience, and many issues remain to be explored.

3. Product manufacturing　　

Due to the low modulus of elasticity of titanium, large springback during cold working, low thermal conductivity, easy abrasion and scratches on the surface, etc., it brings certain difficulties to the molding, heat treatment, and machining of titanium parts. Mature technology It takes a long time to explore.

4. Titanium material application

Due to the strong corrosion resistance of titanium, many titanium equipment are "permanent" or semi-permanent equipment, and the equipment assessment cycle is very long, so that the first, second, and third generation engineers and technicians of the equipment user can not fully grasp the application history data of a certain equipment , It is impossible to make a comprehensive and objective evaluation of the use effect of titanium equipment.

5. Material selection concept

It is still difficult for most people to break through the inertial thinking that "titanium is too expensive". In the case of limited investment capacity, it is difficult to use titanium, which is 5-10 times more expensive than hull steel, to replace steel or copper to manufacture offshore equipment. Relatively speaking, the one-time investment of titanium equipment is indeed very large, and the technical and economic advantages of titanium equipment are mainly reflected by the "full life cost", that is, mainly by long-term benefits. Titanium should be expanded in marine engineering, and a lot of work must be done in benefit evaluation and publicity.
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Surface treatment technology for titanium and titanium alloy materials is widely used in energy conservation and environmental protection

There are generally three process methods for the manufacture of titanium equipment such as titanium coils, titanium heat exchangers, and titanium reboilers: forming method, casting method, and powder metallurgy method.

The molding method can prepare large-scale titanium equipment of any degree of complexity and is the most important process method. The casting method and powder metallurgy method are suitable for equipment and parts with small size and not very complicated shape, but they have the characteristics of short process flow and low cost in mass production. Large-scale titanium equipment generally needs to undergo pressure forming, machining, welding, surface treatment, and other processes. Small single equipment such as titanium pumps, titanium valves, and titanium impellers are often produced by casting. For products with special functions such as titanium filters, powder metallurgy is considered.

The first process of material preparation in the manufacture of titanium equipment is scribing. The purpose of scribing is to draw the boundary line of sheet metal cutting and processing, which will have a great impact on the subsequent assembly and welding procedures. Scribing includes marking the material line, processing line, and position line and inspection line, etc., and marked with the necessary signs. The titanium material can be cut by cutting, punching, cutting (gas cutting, water cutting or plasma cutting, etc.), or cutting. For pipes and bars, mechanical methods such as sewing machines, pipe cutting machines, and grinding wheel cutting machines can be used to cut. Iron ion contamination must be prevented in each process, and mechanical cutting must be used for edge processing of titanium materials.
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Titanium wire classification standard, application, surface performance treatment

In the chemical industry and other application fields, high requirements are placed on semi-finished products and processed parts of titanium or titanium alloys. Therefore, in the fields of aviation and aerospace, the cost of developing inspection instruments and monitoring devices is particularly high. The price of the parts has a big impact. Titanium alloy has the highest tensile plasticity and can be welded in various ways. It can be used for a long time at a temperature of up to 250 degrees Celsius. It is mainly used to manufacture various structural parts of aircraft and engines that are not stressed. Industrial pure titanium has good plasticity, can form various sheet metal stamping parts in cold state, and has relatively high corrosion resistance. Ti5Al2.5Sn titanium alloy has a moderate room temperature tensile strength (800 degrees Celsius 1000MPa and good welding performance. Compared with industrial pure titanium, the new titanium alloy mainly includes various grades of industrial pure titanium and widely used Ti5Al2.5Sn For titanium alloys, the room temperature tensile strength of industrial pure titanium fluctuates in the range of 350 degrees Celsius and 700 MPa. Ti5Al2.5Sn alloy has a slightly lower plasticity and higher thermal strength, and the long-term working temperature can be as high as 450 degrees Celsius.

With the rapid development of cutting-edge science and technology such as aviation, aerospace, nuclear energy, etc., the requirements for materials are becoming more and more stringent. Not only are the materials used for manufacturing these equipment parts to be corrosion-resistant, wear-resistant, and anti-fretting, but also require high-end resistance. temperature. It is necessary to pay attention to the long-term test, in many places, before the large-scale application of titanium to the chemical industry. Under the test conditions, cooperate to test its service life and material structure. If the lack of safety (immaturity) due to the use of conventional structural data is mostly indicated and the economic benefits are not great, then the first step is to gradually develop titanium and titanium alloys, as well as the development of high-level technology in the field of structural data in recent decades. Various other mature new materials. Therefore, the military sector has developed faster in the application field of titanium and its alloys than in the civilian field.

In many industrial media, rare earth metals and precious metals are often mainly used for stability, or materials such as stainless steel can only reach a certain limit in corrosion resistance. Most application fields use titanium to obtain benefits due to its low density, corrosion resistance and high strength. So far. Moreover, the consumption cost is relatively high, so the application of titanium or titanium alloy can obtain relatively high corrosion resistance. The creep characteristics of hard titanium at temperatures exceeding 150T surpass that of aluminum and its alloys. Considering that compared with other materials, titanium alloys have the advantages of unique creep characteristics under low density conditions. It is found that hard titanium is used in aircraft manufacturing and missile manufacturing. The importance of application. The earliest application of titanium and titanium alloys is the aviation industry. Recently, the aviation industry has become increasingly urgent for high-strength and low-density materials, which greatly promotes the development of titanium manufacturing. In the early 1950s, the United States successfully used titanium in aircraft. At that time, although an E aircraft only used 1% of the structural weight of titanium, it opened up a pioneering approach to the use of titanium in the aviation industry. At present, titanium alloys are widely used as structural materials in many high-speed aircrafts in the world.
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Main applications of titanium alloy materials such as TC4 titanium rod and TA1 titanium rod in aviation industry

When welding titanium and titanium alloys, the possibility of hot cracks in the welded joint is very small. This is because the content of impurities such as S, P, and C in titanium and titanium alloys is small, and the low melting point eutectic formed by S and P is not easy to appear in On the grain boundary, in addition to the narrow effective crystallization temperature range, the shrinkage of titanium and titanium alloys is small during solidification, and the weld metal will not produce thermal cracks. However, when welding titanium and titanium alloys, cold cracks may appear in the heat-affected zone, which is characterized by cracks that occur several hours or even longer after welding and are called delayed cracks. During the welding process, hydrogen diffuses from the high-temperature deep pool to the lower-temperature heat-affected zone. The increase in hydrogen content increases the amount of TiH2 precipitated in this zone, which increases the brittleness of the heat-affected zone. In addition, the volume expansion during the precipitation of hydrides causes larger structural stress. In addition, hydrogen atoms diffuse and accumulate to the high-stress parts of the region, resulting in the formation of cracks.

Porosity is a common problem encountered in titanium rod processing and welding of titanium and titanium alloys. The root cause of the formation of pores is the result of the influence of hydrogen. The formation of pores in the weld metal mainly affects the fatigue strength of the joint. Hydrogen is the main cause of cold cracks and pores. Because hydrogen is less than 300, the solubility in α phase is very small, and the limit solubility is only 0.002% at room temperature. When the weld or heat-affected zone cools below 300 after welding, supersaturated hydrogen is precipitated in the form of titanium hydride (γ phase). The volume increases and produces intergranular stress, and the development of this stress will cause intergranular microcracks. The intergranular microcracks will expand into cracks under the action of external stress.

When welding titanium alloy, when the temperature is higher than 500~700, it is easy to absorb oxygen, hydrogen and nitrogen in the air, which seriously affects the welding quality. Therefore, when welding titanium alloys, the entire molten pool and the weld area of ​​the high temperature part (above 400~650) must be strictly protected. For this reason, special protective measures must be taken when welding titanium and titanium alloys. Therefore, the method of argon arc welding is used, and a larger welding torch is used to enlarge the area of ​​the gas protection zone. When the nozzle is not enough to protect the weld and the high temperature metal near the seam, it is necessary to supplement the argon protective drag cover.
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Effect of Different Forging Processes on Microstructure and Mechanical Properties of TC4 Titanium Rod and Titanium Alloy Forgings

Titanium alloy material is the heart of aircraft, and aero-engine has always been the focus of research and investment in various countries. Titanium alloys have been widely used in aero engines due to their excellent thermal strength and high specific strength. Over the years, in order to meet the needs of high-performance aero-engines, developed countries in the aviation industry such as Europe, America, and Russia have attached great importance to the research and development of materials, and have successively developed titanium alloys used in 350-600. Titanium alloy is the second most commonly used material for jet engines after nickel-based superalloys, accounting for more than 1/3 of the engine's structural mass. Calculated by volume, titanium alloy is the most used material in engines.
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Application of titanium rod, titanium wire and titanium plate in shipbuilding industry

When titanium rods and titanium alloy rods are heated to β phase, a → β polymorphic transformation occurs. Sometimes this process is also called recrystallization. The transition temperature of a→β of high purity titanium is 875+-5. But until the β phase is completely formed, it is difficult to observe this process with metallographic methods. The reason why phase β appears at low temperature is unclear. But the experiment pointed out that a and β mutually transform, whether it is heating or cooling, a phase and β phase always maintain a certain Bragg orientation relationship. There has been little research on the polymorphic transformation during heating in titanium alloys.

1. The transformation of titanium rods and titanium alloy rods during slow cooling

When titanium rods and titanium alloy rods are slowly cooled from β phase region to a+β phase region, a polymorphic transition of β→a will occur. It has been confirmed in high-purity titanium that the nucleation of phase a is martensitic at this time, and the growth depends on the thermal activation process. During the nucleation, the surface of the sample also has the relief that is usually caused by the martensitic transformation, and it also maintains a strict orientation relationship with the parent phase.

2. Transition in rapid cooling

The transformation of titanium rods and titanium alloy rods during rapid cooling from the β phase region and the transformation products are different with the content of β stabilizing elements. Martensitic transformation: 1.a`Block martensite cannot measure the orientation relationship; acicular martensite a` and β phase maintain the Bragg orientation relationship. The inertia plane is (334) β or (344). ) Β. 2.a" is found in Ti-Mo, Ti-W, Ti-Re, but not in Ti-V system; a" lattice parameter changes with composition; a" means that the plasticity of titanium alloy decreases.

Quenching ω phase formation: ω is a very small particle, which can only be observed by electron microscopy; ω increases the elastic modulus and hardness, and decreases the plasticity. The measurement of recrystallization mainly adopts the method of combining metallographic observation and X-ray diffraction. When recrystallization occurs, fine equiaxed grains appear on the deformed fibrous structure, and at the same time, the diffraction rings on the X-ray back-reflected Laue diagram begin to become unconnected spots. For heat-treatable β alloys, incomplete aging (500/4 to 8 hours, air cooling) can also be used to display the recrystallized structure, and the unrecrystallized grains after incomplete aging will appear dark after corrosion.
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