Heat treatment of high strength fasteners according to technical requirements have to be quenched and tempered
Heat treatment of high strength fasteners according to technical requirements have to be quenched and tempered. Heat treatment quenching is to improve the overall mechanical properties of fasteners to meet the product requirements of the tensile strength value and yield ratio. Heat treatment processes have a crucial effect on high-strength fasteners, especially their inherent quality, so advanced heat-treatment technology equipment is required to produce high-quality, high-strength fasteners. Due to high strength bolt production, low cost, the thread part is a relatively fine structure, therefore, require heat treatment equipment must have the ability to produce large, high degree of automation, heat treatment of good quality. Since the 1990s, the production line of continuous heat treatment with protective atmosphere has occupied the dominant position. The vibration floor type and net belt furnace are especially suitable for the heat treatment of medium and small size fasteners. Conditioning line in addition to the furnace sealing performance is good, but also has advanced atmosphere, temperature and process parameters of computer control, equipment failure alarm and display capabilities. High-strength fasteners from the material – cleaning – heating – quenching – cleaning – tempering – coloring to the next line, all automatic control operation, effectively ensure the quality of heat treatment. Thread decarburization will lead to the fastener does not meet the mechanical requirements of the first resistance occurs when the trip, the failure of threaded fasteners, shorten the service life. Due to the decarburization of raw materials, if the annealing is improper, it will deepen the decarburization layer of raw materials. Quenched and tempered heat treatment process, usually brought in from the furnace some of the oxidation gas. Bar wire steel rust or cold-drawn wire rod after the residue on the surface, into the furnace heating will be decomposed, the reaction generates some oxidizing gas. For example, the surface of steel wire rust, its composition is iron carbonate and hydroxide, will decompose after heating into CO2 and H2O, thus aggravating the decarburization. The research shows that the carbon decarburization degree of carbon steel is more serious than carbon steel, while the fastest decarburization temperature is between 700-800 degrees Celsius. Due to the attachment of the steel wire surface under certain conditions decomposition of CO2 and H2O synthesis speed very quickly, if the continuous network with furnace gas control properly, it will cause the screw decarburization tolerance. High-Strength Fasteners When cold heading is used, the raw material and the annealed decarburized layer not only remain but are squeezed to the top of the threads and the required hardness is not obtained for the surface of the fastener to be quenched, which Mechanical properties (especially strength and abrasion resistance) are reduced. In addition, the steel surface decarburization, the surface and the internal organization of different with different expansion coefficient, quenching may have surface cracks. To this end, quenching and heating to protect the top of the thread is not decarbonization, but also on the decarbonization of raw materials fasteners moderate carbon coating, the advantages of the protective atmosphere in the belt furnace transferred to and coated with carbon original parts Carbon content is basically the same, so decarburization fasteners slowly restore the original carbon content, carbon potential set at 0.42% -0.48% is appropriate, the same carbon coating temperature and quenching heating, can not be carried out at high temperatures , So as to avoid coarse grains, affecting the mechanical properties. Fasteners in the quenching and tempering process may occur quality problems are: lack of quenching hardness; quenching hardness uneven; quench deformation over tolerance; quenching and cracking. Such problems often appear on the scene with the raw materials,quenching and quenching and cooling related to the correct development of heat treatment process, regulate the production operation, often can avoid such quality accidents.