Study on production technology of industrial pure titanium pipe

With the rapid development of modern science and technology and the needs of national economic construction, the demand of various industrial departments for titanium tubes is increasing, and the specifications and technical requirements of titanium tubes are also higher and higher. Especially with the development of market economy, all walks of life require more titanium tubes of various specifications. Although the uses of titanium tubes are different, the overall requirements for titanium tubes are the same, that is, the processing conditions and end use require that titanium tubes have various quality indexes such as size, accuracy, corresponding performance and surface finish required for this purpose. Whether the quality of titanium pipe can meet the various requirements of various user units for titanium pipe will directly affect the economic interests of users or processing industry and the economic benefits and reputation of the unit.
However, the production practice tells us that although the market prospect of industrial pure titanium pipe is promising, the production of industrial pure titanium pipe is difficult, especially whether the pickling, heat treatment and rolling processes are formulated reasonably will have a direct impact on the quality and production efficiency of titanium pipe. Therefore, how to effectively formulate various production processes is a topic worthy of discussion.

Characteristics and application of industrial pure titanium pipe

Titanium is plastic. The elongation of high purity titanium can reach 50 ~ 60% and the reduction of area can reach 70 ~ 80%.
Although high-purity titanium has low strength, pure industrial titanium contains a small amount of impurities and alloy elements, which can significantly strengthen its mechanical properties and make its strength comparable to high strength. This means that the industrial pure titanium tube can have both high strength and appropriate plasticity as long as it contains a small amount of gap impurities and other metal impurities.
The specific strength (strength to weight ratio) of industrial pure titanium pipe is very high in metal structural materials. Its strength is equivalent to that of steel, but its weight is only 57% of that of steel. In addition, the titanium tube has strong heat resistance and can still maintain good strength and stability in the atmosphere of 500 ℃. Titanium tube also has good low temperature resistance. Even at – 250 ℃ ultra-low temperature, it still has high impact strength, high pressure and vibration resistance.
Another remarkable feature of industrial pure titanium pipe is its strong corrosion resistance, which is due to its great affinity for oxygen, which can form a dense oxide film on its surface, which can protect titanium from medium corrosion. Therefore, titanium has good stability in acidic, alkaline and neutral brine solutions and oxidizing media, and has better corrosion resistance than existing stainless steel and other non-ferrous metals.
Industrial pure titanium pipe has a wide range of applications. At present, industrial pure titanium pipe not only has very important applications in aerospace and aviation industry, but also has been widely used in many industrial departments such as chemical industry, petroleum, light industry, power generation and so on.
Because pure industrial titanium pipe has many excellent characteristics such as light weight, high strength, strong heat resistance and corrosion resistance, it is known as “the metal of the future” and is a new structural material with great development prospects.

Discussion on production process of industrial pure titanium pipe

Product outline

• Blank specification: φ 76mm × 7mm
• Finished product specification: φ 45mm × 1.25mm (6m fixed length)
• Dimensional deviation: △ D ± 0.3mm, s ± 0.18mm (as required by the agreement)
• Product standard: GB / T 3625-1995

Preparation of process flow

The process flow of producing industrial pure titanium pipe is as follows:

• Surface treatment – blanking – rolling – degreasing – drying – cutting to length – annealing – straightening – Pickling – rolling – degreasing of finished products – drying – annealing – straightening – cutting to finished products – Pickling – Inspection – packaging

(Note: the above processes can be added or reduced according to product specifications and performance requirements.)
For titanium tube billets lubricated by copper plating process before rolling, a copper removal process is added before surface treatment of this process flow.

Preparation of pickling process

In the production process of industrial pure titanium pipe, pickling is a very important process, which can not be ignored. Because in the process of thermal processing or heat treatment, titanium pipe will chemically react with oxygen and other gases in furnace gas without protective atmosphere to form oxide layer. The oxide layer formed on the surface of titanium pipe is very harmful to all processes after pickling and the quality of final products If titanium pipe is rolled with oxide layer, the pipe will have defects. At the same time, due to the existence of oxide layer, the oxide layer will be pressed into the pipe surface, resulting in unsatisfactory surface, which will directly affect the product quality, material consumption, productivity and economic benefits of titanium pipe.
As we all know, pickling refers to the process of removing the surface oxide layer by treating metals in acid, alkali, salt and their solutions. There are many methods to remove the oxide layer. The traditional method is to use inorganic strong acids, such as sulfuric acid H2SO4, hydrochloric acid (HCI), nitric acid (HNO3) and hydrofluoric acid (HF) Etc. this kind of method uses the chemical reaction between the oxide layer and acid to remove the oxide layer. For different materials, one of them can be adopted according to their performance characteristics.
The oxide layer of industrial pure titanium tube is mainly titanium dioxide (TiO2) Secondly, there are many low-priced titanium oxides, such as TiO, Ti2O3 and tio5. In addition, there are high-priced titanium oxides, such as TiO3. The research shows that at room temperature, when the titanium pipe is pickled in < 5% dilute sulfuric acid pickling solution, a protective oxide layer is formed on the surface of the titanium pipe to protect the titanium from further erosion by dilute sulfuric acid, but pickling in > 5% sulfuric acid has an obvious reaction with titanium. At room temperature, about 40% sulfuric acid reacts with titanium The corrosion rate is the fastest. Hydrochloric acid with concentration < 5% does not react with titanium at room temperature. When the temperature increases, even dilute hydrochloric acid will corrode titanium, but more than 10% hydrochloric acid will significantly corrode titanium at 70 ℃ and 1% hydrochloric acid at 100 ℃. However, when there is an oxidation layer or metal ions (such as copper, etc.) in hydrochloric acid solution, the corrosion effect of hydrochloric acid on titanium can be reduced. Hydrofluoric acid (HF) It is the strongest solvent of titanium. Even hydrofluoric acid with a concentration of 1% can react violently with titanium:

• 2Ti+6HF=2TiF3+3H2

When there are metal ions such as iron and copper in hydrofluoric acid solution, the dissolution of titanium can be accelerated.
Titanium tubes with rough surface are most likely to react with cold and hot dilute nitric acid (HNO3):

• 3Ti+4HNO3+4H2O=3H4TiO4+4NO
• 3Ti+4HNO3+H2O=3H2TiO3+4NO

In view of the corrosion characteristics of various acids on titanium pipe, in order to effectively remove the oxide layer on the surface of titanium pipe, relevant data introduce that nitric acid and hydrofluoric acid are used for pickling of titanium pipe, and its formula and process are as follows:
The solution concentration ratio is:

• (35% ~ 40%) HNO3 + (5% ~ 7%) HF + H2O (remaining margin)

Solution temperature: 30 ℃ ~ 50 ℃
The test shows that the above formula and process have some shortcomings: first, the reaction of titanium pipe entering acid liquid is too intense, which makes the acid liquid temperature rise rapidly, and even makes the acid liquid boil. This situation is very easy to cause oxidation and combustion of titanium pipe matrix, resulting in increased loss of titanium pipe matrix; second, the acid liquid splashed during such violent reaction is quite harmful to operators; third The reason is that the operating environment of the staff is poor and the pollution is too great. In view of the above reasons, the acid washing formula and process of titanium pipe are adjusted as follows:

• Main components of solution: nitric acid (HNO3) + hydrofluoric acid (HF)

The solution concentration ratio is: (25% ~ 28%) HNO3 + (3% ~ 5%) HF + H2O (remaining balance)
Solution temperature: ≤ 30 ℃
Pickling time: 10 ~ 20 minutes
The practice shows that the adjusted pickling condition is good, the reaction of titanium pipe entering acid solution is stable, and the pickling loss rate of titanium pipe matrix can be reduced by 30% ~ 50%.
It should be noted that the pickled titanium pipe must be washed and dried. Although the purpose of removing the oxide layer is achieved after the titanium pipe is pickled in the acid solution, there is more or less a part of residual acid on the surface of the titanium pipe eroded by acid. If this part of residual acid is not removed, a part of hydrogen will diffuse into the titanium pipe matrix due to the chemical action of residual acid, which will reduce the plasticity of the titanium pipe. At the same time, the plasticity of the titanium pipe will be reduced The presence of residual acid on the surface will cause premature wear of rolls during rolling and waste products of titanium tubes. Therefore, residual acid is not allowed on the surface of titanium tubes after pickling.
The purpose of drying is to drive out the hydrogen atoms in the metal lattice in time, so as to avoid hydrogen embrittlement during rolling and improve the lubrication performance.

Preparation of rolling process

Production practice has proved that different rolling processes for the same kind of steel pipe will get different results. Therefore, reasonable formulation of rolling process is the main condition to ensure the high yield and high quality of industrial pure titanium pipe, and it is a basic comprehensive technical subject in the production of industrial pure titanium pipe.
In the past, the process formulation of products that have not been produced mainly depends on the experience of brother units and consulting data. The experience obtained from the success or failure of production practice is undoubtedly very valuable and reliable. However, its scope of application has locality, such as rolling mill equipment, another part strength, elastic deformation, wear, plastic deformation, estimation of mechanical parameters, etc. Therefore, for titanium The formulation of tube rolling process is determined by integrating the performance, characteristics, tube blank status, finished product delivery requirements and standards, rolling mill equipment capacity and production conditions of industrial pure titanium tube.
about φ 76mm × 7mm industrial pure titanium pipe rolled φ 45mm × For 1.2mm finished products, the rolling process is determined as follows:

• φ 76mm × 7mm→ φ 51mm × 3mm→ φ 48mm × 2mm→ φ 45mm × 1.2mm

It should be noted that during rolling φ 51mm × In case of 3mm industrial pure titanium pipe, the inner and outer walls of the raw pipe shall be lubricated with MoS2 lubricant, and chlorinated paraffin shall not be used as lubricant, so as to prevent the pipe from reacting with chloride ion at high temperature during rolling and causing corrosion of pipe matrix.

Preparation of heat treatment process

Heat treatment is a processing method different from rolling. It does not change the size and shape of industrial pure titanium pipe, nor damage the titanium pipe, but essentially changes or improves the performance of titanium pipe, that is, it uses industrial pure titanium pipe to heat, keep warm and cool in the solid range to change its structure, so as to obtain the required performance.
In the rolling process of industrial pure titanium pipe, like other steel pipes, there is a basic characteristic, that is, the rolled pipe will produce a change in shape and size that cannot be recovered by itself – plastic deformation.
In the process of plastic deformation, the microstructure and properties of industrial pure titanium pipe will also change. The most important is work hardening, that is, with the increase of deformation degree, the deformation resistance increases, the strength and hardness increase, and the plasticity and toughness decrease, so that the dynamic consumption of titanium pipe during cold rolling increases and the rolling amount is limited.
The purpose of heat treatment is to transform and recrystallize the structure of industrial pure titanium pipe, so as to restore its plasticity before rolling and eliminate the “work hardening” produced in the rolling process Or the residual stress caused by the uneven heat dissipation in the cooling process during hot extrusion, and the existence of this residual stress reduces the plasticity of titanium pipe, or eliminates the microstructure and other defects caused by improper hot extrusion process, so that the rolling can continue. It can be seen that heat treatment is an indispensable and important process in the rolling process.
Industrial pure titanium tubes annealed in unprotected atmosphere will produce oxide layer, which is mainly composed of titanium dioxide (TiO2).
The research shows that the oxidation reaction of titanium in air is very slow when it is less than 100 ℃, and only the surface is oxidized at 500 ℃. With the increase of temperature, the surface oxide layer begins to dissolve in titanium and oxygen begins to diffuse to the internal lattice of metal, but at 700 ℃, a large amount of oxygen has not entered the internal lattice of metal. When it exceeds 700 ℃, the diffusion of oxygen to the internal of metal accelerates, and titanium begins to interact with oxygen in air at high temperature The thickness of the oxide layer is greatly related to the annealing temperature and time.
During the annealing process of industrial pure titanium pipe, with the increase of temperature, the oxide layer becomes thicker and thicker, and the oxidation degree also increases, which causes some difficulties for reprocessing. Therefore, the formulation of heat treatment process should avoid high temperature.
The heat treatment of titanium tube usually adopts the chamber furnace jacket to pass argon, but the furnace temperature of the chamber annealing furnace is extremely uneven, coupled with the poor sealing performance of the jacket, although argon protection is adopted, the surface oxidation of titanium tube occurs from time to time. In serious cases, the color of titanium tube surface is red gray or gray. This brings some difficulties to pickling, and under pickling will occur if it is not operated properly In addition, the consumption of fuel and argon is also large, resulting in extremely high production cost. Therefore, in order to improve the heat treatment effect of titanium pipe and reduce production cost, continuous roller hearth furnace is used instead of chamber furnace for heat treatment. The process is as follows:
Furnace inlet temperature 720 ℃; speed: voltage: 4.5V, time: 40 minutes; furnace outlet temperature 250 ℃; speed: slow, air cooling.
The test shows that the surface color of titanium pipe treated by this furnace type is light yellow after coming out of the furnace, gradually turns golden yellow, and finally turns light blue. It is very easy to clean after pickling.

Conclusion

Although there are some difficulties in the production of industrial pure titanium pipe, as long as we understand the characteristics of industrial pure titanium pipe and correctly formulate relevant processes in the production process, we can produce high-quality industrial pure titanium pipe. This paper makes a beneficial exploration on the traditional production method and production process of titanium pipe.
The revision of pickling process and the use of continuous roller hearth furnace instead of chamber furnace for heat treatment of industrial pure titanium pipe have certain reference for how to change the working environment of industrial pure titanium pipe, reduce the corrosion of titanium pipe matrix, make full use of the capacity of heat treatment equipment, reduce production processes, reduce production costs, improve productivity and improve product quality It is a subject worthy of further discussion.
Source: China Titanium Pipe Manufacturer: www.titaniuminfogroup.com