Investigation shows that most of the landing gear fractures are caused by surface stress corrosion or fatigue crack propagation.
The atoms in steel are arranged according to certain rules, called lattice, but there are also some disorderly atoms in the lattice that will stand in the wrong line, causing dislocation.
In the process of smelting and solidification of iron and steel, the hydrogen atoms remaining in the steel will actively seek for dislocations due to their lightest mass and smallest volume, and continuously concentrate near the defects in the metal. At room temperature, the atomic hydrogen will combine at the defects to form hydrogen molecules, resulting in huge internal stress, which exceeds the strength limit of steel, causing invisible cracks in the metal, namely "hydrogen embrittlement".
(The material observed under the microscope is cracked due to hydrogen embrittlement)
The higher the strength grade of steel, the more sensitive it is to hydrogen embrittlement; The steel with low carbon content and less impurity elements such as sulfur and phosphorus has low hydrogen embrittlement sensitivity. The steel for aircraft landing gear belongs to low alloy ultra-high strength steel, which is extremely sensitive to hydrogen embrittlement. In order to prevent the occurrence of hydrogen embrittlement, realize the long life, high reliability and structural weight reduction of landing gear, it is necessary to reduce the content of impurity elements and improve the purity of steel, which is also the basis for improving other properties of landing gear steel.
However, the essential disadvantage of the traditional steel production process is instability. Unstable raw materials, unstable process, unstable process temperature, unstable process rhythm, and unstable quality all make it difficult to control the cleanliness.
300M steel adopts vacuum heat treatment technology to avoid hydrogen permeation and improve surface quality. The harsh production conditions and process quality requirements are not only reflected in the entire production process to meet the quality control requirements of international civil aviation products, but also to ensure that the operation steps of each production process are traceable and reproducible.
In 2009, Chinese iron and steel enterprises began to research and develop 300M ultra-high strength steel. Instead of using traditional converter production for 300M steel, they adopted vacuum induction+vacuum consumable remelting process (VIM+VAR). Through vacuum melting, the composition control is more accurate, the impurity content is lower, the inclusion is less, and the ingot structure is good to improve the processing and forming performance.
In addition, in order to ensure the cleanliness of molten steel, all raw materials for steelmaking should be stored in special material racks to prevent pollution of other raw materials.
In addition, the R&D unit, in strict accordance with the quality management requirements of the civil aviation material system, specially prepared the process control documents and successively adjusted and modified them for 13 times, improved the equipment accuracy, established a manufacturing system from the perspective of human, machine, material and law, ensured the stability of the product manufacturing process, and truly formed a full process manufacturing technology with independent intellectual property rights.
After dozens of batches of industrial production and trial production, the development of 300M steel in China has successively broken through the ultra-pure smelting of 40t electric furnace, large electrode ingot casting φ 810mm ingot melting in vacuum consumable furnace φ The 400 mm bar forging process, bar annealing and testing have many technological difficulties. In 2014, it obtained the production license of the most stringent class B 300M steel. In 2016, it successfully passed the supplier certification, and has the qualification of a qualified supplier. It is officially supplied for C919 landing gear manufacturing.
C919 requires two 300M ultra-high strength steel forgings with a diameter of about 400 mm and a length of nearly 3 meters to manufacture a landing gear, which are forged into outer cylinder and piston respectively.
