What is the card on our neck? "Can stainless steel not rust? ”
E-BON E-BON 2018-07-10 11:05:10
Core technology to be overcome
"Can stainless steel not rust?" This is a bit of black humor, almost let the Chinese Aerospace Science and Technology Group Sixth Engine engine expert, Chen Jianhua, deputy chief designer of the Long March 5 carrier rocket, fall into a heart attack.
In the development of China's 120-ton liquid-oxygen kerosene supplemental combustion engine YF-100, Chen Jianhua noticed that several high-strength stainless steels are prone to rust. Since 2011, he has communicated with his old friend, Su Jie, deputy director of the Special Steel Research Institute of the Iron and Steel Research Institute, and the pressure on both sides is very high.
Today, the Long March 6, No. 7 and No. 5 rockets have made their first flight. Chen Jianhua still has not got a completely convincing answer.
Strength and rust resistance are contradictory
Steel for rocket engines needs to have a variety of characteristics, of which high strength is an important indicator that must be met. According to the reporter, the austenitic stainless steel used in our rocket engines in the past has a yield strength of about 300 MPa, and a new generation of launch vehicles. The material used is 2 to 4 times stronger.
However, the strength and rust resistance of stainless steel is a contradiction between fish and bear's paw.
“It is necessary to clarify a concept that stainless steel will rust,” Su Jie said. “In simple terms, the corrosion resistance of steel depends mainly on the content of the alloy element 'chrome', but if the element is added too high, the strength will be on Do not go. Therefore, high-strength materials, anti-rust ability will certainly be poor."
Li Jingyuan, a professor at the School of Materials Science and Engineering at the University of Science and Technology Beijing, added that the common method for improving the strength of stainless steel is precipitation strengthening. In the heat treatment process, some particles are precipitated inside the steel material, and are dispersed in the matrix to cause hardening. "However, there are defects in the position where the particles are precipitated, the alloy elements in the defects are reduced, the structure is different, and the uniformity of the metal material is destroyed, which is prone to corrosion, which is commonly referred to as 'rusting'," she said.
Chen Jianhua said that the rocket engine material is not a big problem if it is only a little rust, but if it is severely rusted, it may have a big impact.
Assuming that the valve spool is rusted, the valve will encounter resistance during the opening and closing process, resulting in a problem with the reaction speed. If the rust is bad, a leak may occur. If the pipe is rusty, the rust block may cause blockage as the fuel flows.
The world's problem: foreign engine materials are also rusty
Chen Jianhua has a deep understanding of the distress caused by the rust problem. "In order to prevent rust, we stipulate that the engine can't see water for more than a few hours. It is inevitable that the engine is wet. For example, do a hydraulic test. Every time you finish, you can quickly pull the engine back into the factory and put it into the oven to dry." He said "The Long March 5 rocket can only be launched in Wenchang, Hainan. How long will it rust when exposed to moisture? I only care about this."
He has a foreign engine material that has been in the warehouse for many years.
Su Jietan Chen, China has a gap with developed countries in the research and development of new materials, and is currently in the imitation stage. At present, most of China's aerospace materials are used in foreign countries in the 1960s and 1970s. However, today's process technology is more advanced, and the same materials produced are better.
Li Jingyuan is quite worried about this: if we encounter high-end products, foreign materials and processes are not disclosed, we will inevitably be "snapped to the neck."
At the same time, she believes that China has first-class equipment, but the level of management is different from that of foreign countries. For example, developed countries will strictly control the impurity content in the production process. If the purity is not up to standard, they will be returned to the furnace, but domestic manufacturers often lack such a strict attitude.
However, Su Jie said that Chen Jianhua’s foreign material was later taken to Hainan, and compared with domestic materials, the result was both rusty. "It is a worldwide problem to rely solely on the material itself to achieve high strength and rust resistance," said Su Jie.
Fish and bear's paw have to rely on foreign aid
Li Jingyuan believes that to achieve both fish and bear's paw, more scientific composition design can be carried out, such as adding corrosion-resistant elements, strengthening elements, etc., and scientifically matching the amount of each element added.
But Su Jie believes that this is a theoretical direction, correct but not applicable. “For high-strength materials such as aerospace power, we have optimized them as much as possible, taking into account the corrosion resistance.” He said, “There are many materials with better corrosion resistance, but our core is not to ensure that there is no rust. It is a match of performance in terms of strength, toughness, etc."
In fact, the anti-rust of aerospace materials is not impossible, but it needs to rely on "foreign aid." Su Jie said that through the combination of materials and processes, on the one hand, the use of the material's own anti-rust ability, on the one hand using surface coating treatment or injection of dry air, nitrogen and other auxiliary means into the material cavity, has solved the problem of rust.
Chen Jianhua said that the Sixth Institute has also carried out research on test strips for rust problems and has fully tested and verified the product adaptability. Through several evaluations of the engine stored for 5 years, it was found that the working condition was good.
But he still has no complete reassurance. Chen Jianhua said that for the reusable rocket engine that is being developed in China, it will last longer, and the interior should be repeatedly subjected to kerosene immersion, and it can not be dismantled. The test of rust prevention ability will be more severe. Another example is China's new generation of launch vehicles, which are now unmanned. In the future, if they are manned, the safety and reliability requirements will be higher. Does the rust problem have any effect? These issues must be clarified.