Automotive fastener heat treatment processing technology
Automotive high-strength fastener heat treatment processing technology
With the development of energy-saving, environmental protection, lightweight and miniaturization of automobile products, more and more attention has been paid to high-strength fasteners for automobile. Automobile fasteners are the main connecting parts and functional parts of automobile, accounting for about 50% of the total number of automobile parts. The key point of producing automobile high strength fasteners is material and heat treatment processing. Any problem will lead to the failure of high strength fasteners. The research and development of high strength automotive fasteners in China is still in a relatively backward state.
In order to obtain the tempering sorbite and tempering tortonite structure with good comprehensive mechanical properties for automobile high strength fasteners, the premise is to ensure the martensite structure in the core during quenching, which is closely related to the hardenability of steel. Whether it is carbon steel or alloy steel, in the case of full quenching, fasteners after high temperature tempering to get the same hardness of the finished product, their mechanical properties such as strength, plasticity and toughness are almost the same. However, if it is not completely hardened, even if the hardness after tempering is the same as that after quenching, its yield strength, elongation after forging, section shrinkage and impact toughness are lower, and the degree of reduction increases with the degree of quenching.
The heating temperature of quenching is mainly determined according to the chemical composition of steel and the specific process. The chemical composition of steel is the main factor to determine the quenching temperature. For the core hardened fasteners, due to the combined effect of the microstructure stress and thermal stress, the maximum tensile stress will be generated near the surface of the parts, resulting in the quenching, the size of the risk of quenching is related to the quenching medium used. For bolts, the diameter of water quenching is 8 ~ 10mm, and that of oil quenching is 20 ~ 39mm. When bolts of dangerous size are quenched, proper measures must be taken to prevent cracking.
Automobile lightweight development, energy conservation, the engine and the design of the power system puts forward higher requirements, with the improvement of strength, caused by hydrogen embrittlement fracture, is mainly by the quenching tempering martensite steel, occurs in the yield strength 620 mpa > > 31 HRC hardness value, high strength material, the higher the tensile strength, is sensitive to the more hydrogen embrittlement, materials easier to absorb hydrogen, hydrogen and flooding is not easy.
The influence of microstructure on hydrogen embrittlement sensitivity of steel is obvious. The sensitivity of tempered martensite, upper bainite, lower bainite, sorbite, pearlite and austenite to hydrogen embrittlement decreased successively. In order to reduce the formation of surface oxidation in the heat treatment process, protective atmosphere is often added into the heating furnace. If the protective atmosphere contains hydrogen compounds, it is possible to absorb hydrogen during heat treatment, increasing the risk of hydrogen embrittlement in fasteners. To prevent hydrogen embrittlement, 1000 ~ 1300MPa high strength fasteners must be hydrogenated after electroplating.
Under certain conditions, the quality of raw materials will affect the choice of production process parameters of fasteners and directly affect the comprehensive performance and safety of high strength fasteners used in automobiles. In the process of automobile assembly, the development and application of automotive high-strength fasteners still face challenges from raw material suppliers and processing and manufacturing.
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