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China Heat Treatment and Surface Modification Technology Roadmap (II)


Jun 07,2016

China Heat Treatment and Surface Modification Technology Roadmap (II)

Key components are the core of high-end machinery and equipment

1. key components and their "three major problems"

Key components are to determine the main function of mechanical equipment, reflecting the life and reliability of mechanical equipment, failure breeds catastrophic consequences, the main failure components are fatigue of the main load-bearing components. Key components have three major categories: ① rotating components, such as blades, wheels, shafts; ② transmission components, such as gears, bearings; ③ main bearing components, such as aircraft landing gear, docking bolts, bearing joints, bearing springs, etc.. One of the common characteristics of key components is: dynamic! They are in service under dynamic, subject to dynamic load resulting in their fatigue failure. The second common characteristic is: extreme environment! Because the key components are designed for light weight, small size, long life, and high reliability, the performance of the materials selected are used to their limits. Dynamic service in extreme environments often leads to early fatigue failure of critical components. Therefore, for a long time, China's key components have three major problems such as short life, poor reliability and heavy structure. For example, the main bearing life of military aircraft engines is only about 300h, foreign countries have reached 3000h. military helicopter main gear transmission gear overhaul period (TBO) is only about 800 ~ 1200h; foreign reached 6000h, civilian helicopters reached 8000 ~ 10000h. gearbox is the core component of wind turbine, foreign wind turbine gearbox life of more than 20 years, or can not recover the cost. And maintenance is difficult and costly, and two European machine manufacturers have closed down due to gear failure repair. China-made wind turbine gearboxes only 2 years warranty, and gear weight is 20% to 25% higher than the original machine, but also often out of order. According to statistics, an engine level compressor blade life more than 100h, short more than 20 hours; turbine guide blade bench test only 48h crack, low even 5h crack; turbine blade even rarely test. Key components "three major problems" seriously restrict the development of high-end machinery and equipment and safety in service.

2. The main failure mode of key components is fatigue

Fatigue is a kind of cyclic stress and strain under the repeated action, a member or several cracking or fracture of the failure form. Fatigue is a process of accumulated mechanical damage and delayed failure. Fatigue consists of two processes: crack initiation and crack expansion, which occur below the yield strength of the material and fail suddenly without significant deformation. Therefore, compared with other failures, fatigue is a class of the most dangerous form of failure. Usually, the blade for vibration fatigue failure, high temperature turbine blade for hot and cold fatigue failure, transmission gear for bending fatigue failure, butt bolt for pull - pull fatigue failure, bearing for contact fatigue failure, etc.. According to statistics, fatigue failure of aviation key components accounts for more than 80%. Among the 17 incidents of aviation gears, 13 were fatigue failures, accounting for 76%, both bending fatigue fracture of tooth roots and contact fatigue spalling of tooth surfaces. 14 major accidents of aviation engines occurred from 1960s to 1990s, 13 originated from fatigue failures of key components. 80s, a machine failure led to the grounding of 3,000 aircraft, and the cause was found to be fatigue cracks of the main beam of the wing. In 2003, a study of more than 300 major engine failures found that the vast majority came from fatigue failures of key components. Of particular concern is that in the past, fatigue failure of engine turbine blades occurred after a certain period of time in service, but now it appears in the test phase of the bench. The fatigue failure rate of non-aerospace critical components is 50% to 90%. Turbine high-pressure, medium-pressure section blade withstand high temperature, high pressure superheated steam, disturbed steam flow to produce bending stress and dynamic stress, failure mode for fatigue fracture or stress corrosion-induced fatigue fracture. Turbine blade fatigue fracture accounts for about thermal power plant operating accident 1/3. wind turbine service environment is complex and harsh, both to withstand dynamic load impact and corrosive environment, the buttress bolt fatigue failure up to 85%. China's rapid development of railroad transport, with the increase in train speed and the rapid development of high-speed heavy railroads, high-speed heavy line wheel rail and wheel rolling contact fatigue failure is very serious.

3. Key components "three problems" from stress concentration sensitive

The key components are generally made of high-strength aluminum alloy, titanium alloy, ultra-high-strength steel and high-temperature alloy. With the development of high-end machinery and equipment, the strength of key components with alloys will continue to improve. Due to the high fatigue strength of high-strength alloys, its light weight, small size, long life, high reliability. However, the outstanding weakness of high-strength alloys is that fatigue strength is sensitive to stress concentration. When the carburized surface hardness > 60 HRC, the surface scratch stress concentration even exceeds the tensile strength of the material. As the alloy strength increases, the fatigue strength stress concentration sensitivity will be more dramatic. Due to the fatigue strength and life reduction caused by stress concentration, the design of key components had to reduce the allowable stress and increase the safety factor, which led to high structural weight; to the determined design and selection of materials, because there is no anti-fatigue concept and prevention measures, cutting and machining and inevitably caused damage and roughness on the surface of the key components, thus attaching a stress concentration, fatigue strength again reduced, resulting in the key Short life span of the component; poor reliability of the critical component due to uncertainty of the location and degree of additional stress concentration. In a word, fatigue strength stress concentration sensitivity leads to the "three major problems" of critical components. A large number of fatigue failure analysis results of critical components show that more than 80% of fatigue failures originate from surface damage or cutting tool marks discontinuities because of high stress concentration there. The severity and potential harm of fatigue strength stress concentration sensitivity in critical components is evident from the following examples. As we all know, low alloy ultra-high strength steel 300M is an excellent structural steel, it has: σb = 1960MPa, σ0.2 = 1625MPa, δ5 = 10.4%, ψ = 41.6%, K ⅠC = 80MPa-m, fatigue strength σf = 1035MPa comprehensive performance. Therefore, more than 95% of the landing gear of military and passenger aircraft in the world are made of it. However, when the hardness indentation on the surface of the component causes stress concentration, the fatigue strength is reduced to σNf=240MPa; when tested in 3.5% NaCl aqueous solution, the fatigue strength is reduced to σcf=280MPa; when the component with hardness indentation is tested in 3.5% NaCl aqueous solution, the fatigue strength is reduced to σNcf=100MPa. The fatigue strength was reduced to σNcf=100MPa when the members with hardness indentation were tested in 3.5% NaCl aqueous solution. It is obvious that in the stress concentration and corrosive media service environment, the comprehensive performance of ultra-high strength steel 300M actually becomes completely unusable. It can be seen that, without solving the problem of fatigue strength stress concentration sensitive, key components not only can not receive the expected effect of light weight, small size, long life, high reliability, but potentially disastrous consequences.

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