Fri, 03 Aug 2018 02:00:47 +0000
High strength fasteners heat treatment process
With the development of automobile products to energy saving, environmental protection, light weight and miniaturization, high strength fasteners used in automobiles are more and more valued. Automobile fasteners are the main parts and functions of automobile, accounting for about 50% of the total number of automobile parts. The key points in the production of high strength fasteners for automobiles are materials and heat treatment. No matter there is any problem, the performance of high strength fasteners will fail. The development and research of high strength automobile fasteners in China is still relatively backward.
For high strength fasteners of automobiles, the tempered soxhlet and tempered totensite tissues with good comprehensive mechanical properties should be obtained, provided that martensite structures should be obtained from the core during quenching, which is closely related to the hardenability of steel. Both carbon steel and alloy steel have similar mechanical properties such as strength, plasticity and toughness when the fasteners are completely quenched and tempered to obtain the same hardness. However, if it is not completely quenched, even if the hardness after tempering is the same as that after quenching, its yield strength, elongation after forging, shrinkage of section and impact toughness are all lower, and the degree of reduction increases with the reduction of quenching penetration.
The quenching heating temperature 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. According to different quenching media, different quenching heating temperature is adopted. For hardened fasteners at the core, the maximum tensile stress due to the combination of tissue stress and thermal stress will be near the surface of the part, leading to the cracking. The size of the quenching danger 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. Proper measures must be taken to prevent the quenching of bolts in dangerous dimensions.
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 effect of metallographic structure on hydrogen embrittlement sensitivity of steel is obvious. The sensitivity of tempered martensite, upper bainite, lower bainite, soxhlet, pearlite and austenite to hydrogen embrittlement decreased successively. The heat treatment of automobile fasteners is carried out at high temperature. In order to reduce the generation of surface oxidation during heat treatment, protective atmosphere is often added in the furnace. If hydrogen compounds are present in the protective atmosphere, it is possible to absorb hydrogen during heat treatment, increasing the risk of hydrogen embrittlement of fasteners. To prevent hydrogen embrittlement, high strength fasteners of 1000 ~ 1300MPa must be hydrogen-driven after electroplating.
Under certain conditions, the quality of raw materials will affect the choice of technical parameters for fasteners production, and directly affect the comprehensive performance and safety of high-strength fasteners for automobiles. In the process of automobile assembly, the development and application of automobile high-strength fasteners also have challenges from raw material suppliers and processing and manufacturing.