The manufacturing process of high-end bolts in China from the perspective of C919

On May 5, 2017, the domestic large passenger aircraft C919 took off at Shanghai Pudong airport. The aircraft is high-tech in terms of aerodynamic shape and fuselage material, so it has also won many orders from airlines in the past two years.

However, the pedigree of C919 has always been a controversial topic. Some people think that C919 is a complete assembly platform, and some people think that it is made in China by itself. For this, the small editor specially checked the distribution table of suppliers. The materials such as the nose, fuselage, tail and wing of the aircraft are all from China Chengfei Shenfei and Xifei. Other key parts are from foreign enterprises, such as engine, avionics, flight control, etc , landing gear, etc., but the C919 is still a domestic aircraft.

Because its overall design is completed by Chinese engineers, from the beginning of drawing design, aerodynamic shape, calculation experiment to the final manufacturing and system integration.

Why do you say that China has independent property rights?

Because there is no company in the world that can produce all parts of the aircraft independently, including famous Boeing and Airbus, the standard of aviation industry is: as long as any country can design as a whole, the aircraft manufactured will have its own localization, but C919 is not without defects, because it does use more imported parts and materials than its peers, and even bolts are not made in China.

Why can’t China make high-end bolts?

Bolts are the most widely used parts in mechanical equipment at present. There are 10 kinds of bolt performance grades in common use, among which high-end bolts are also called high-strength bolts in 4 kinds, which are generally used in military, aviation, navigation and other fields.

As a big manufacturing country, China produces and exports a large number of middle and low-end bolts every year, but the high-end bolts are basically imported, and the price of imported bolts is 4 to 10 times higher than that of export. Take the United Kingdom as an example, the unit price of bolts imported from the United Kingdom is 20718 US dollars per ton, but the price of middle and low-end bolts exported to the United Kingdom is only 1840 US dollars per ton, with a difference of 10 times.

This is due to the fact that China has not fully mastered the technology of high-end bolts so that the import price is very high. At present, the technical requirements of high-end bolts have four characteristics: corrosion resistance, high strength, small tolerance and consistent tightening torque. At present, there are still many problems with domestic bolts.

• First of all, the processing technology is not good, and the tolerance range is too large. The tolerance of high-end bolt is required to be controlled at 0.01mm level; secondly, the surface treatment is not in place, and fasteners of the same model and specification may even fail to be replaced generally;
• Take the aviation industry as an example, when the aircraft takes off and lands, it will produce violent vibration. In addition, the temperature difference in the high altitude reaches nearly 100 ℃, accompanied by wind, frost, snow and air flow. If the bolts are not qualified, it may cause major safety hazards.

What are the problems in the manufacturing of high-pressure bolts?

We should start from his manufacturing process:

For bolt manufacturing, steel with standard diameter shall be purchased from steel works first, and then drawn to different diameters according to different models and sizes of bolts. There is no problem in China at present. Domestic Baosteel and TISCO group can provide high-quality raw materials. In addition, drawing dies and processes are at the leading level in the world.

The next step is the bolt cold heading forming process, which requires the use of a cold heading machine. The cold heading machine breaks the bolt according to the length of the bolt just now, and then creates a bolt without thread through several processes. These processes need to control key dimensions such as length and diameter, and the error needs to be controlled within 0.01 mm.

This puts forward the super-high requirements for the cold pier machine, which is not only high precision, but also stable. It is equivalent to the bolt mold. However, the current situation in China is that it can basically be made in China, but the performance requirements are not up to the current high-end cold heading machine, which is basically monopolized by the Netherlands, the United States, Japan and Germany, while the domestic cold heading machine basically monopolizes the middle and low-end market.

The third process is thread rolling, which will use a thread rolling machine, which is also a key process, with high precision requirements. However, the high-end thread rolling machine has been basically monopolized by Europe, the United States and Japan. There is also a heat treatment process, which is related to the mechanical properties of bolts, so the temperature control requirements are very high, and also monopolized by foreign countries.
Finally, the bolt surface treatment is generally electroplated. At present, some professional electroplating manufacturers in China can meet the specification requirements.

Among the above four processes, China has only mastered two. The other two processes are due to the lack of high-precision equipment. In addition, China has the ability to make high-precision bolts, but it is unable to carry out mass production. The aircraft carrier fighters in the military field in China are all made in China. This is because the demand in the military field is small, while the output in the traditional machinery industry is high, the demand for bolts is high, and the mass production progress Unable to determine.

C919 can also use domestic bolts, but China’s aviation industry is in its infancy, not afraid of 10000 yuan, just in case, so simply import, China’s industry started late, there will be a certain gap with the West. In addition, the import of high-end bolts does not reflect the problem that Chinese standard parts cannot be manufactured,
However, many high-end industry precision equipment is still in the hands of the west, which will take a long time to make up. Of course, China’s high-end bolt manufacturing still needs to be paid attention to. It can’t stay in the low-end industry all the time, and it needs to move towards the high-end field.

Source: Best China Bolts Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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