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High-strength bolt processing technology

Update:31-12-2020
Summary:

High-strength fasteners must be quenched and tempered a […]

High-strength fasteners must be quenched and tempered according to technical requirements. Heat treatment quenching and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength value and yield ratio of the product. The heat treatment process has a vital influence on high-strength fasteners, especially its internal quality. Therefore, to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are necessary. As high-strength bolts are produced in large quantities, low in price, and the threaded part has a relatively fine and relatively precise structure, the heat treatment equipment must have a large production capacity, a high degree of automation and good heat treatment quality.

Since the 1990s of the 20th century, continuous heat treatment production lines with protective atmosphere have been dominant. Shaking bottom type and mesh belt furnaces are especially suitable for heat treatment and tempering of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced computer control of atmosphere, temperature and process parameters, equipment failure alarm and display functions. High-strength fasteners are automatically controlled and run from loading-cleaning-heating-quenching-cleaning-tempering-coloring to off-line, effectively ensuring the quality of heat treatment. The decarburization of the thread will cause the fastener to trip first when the resistance required by the mechanical performance is not reached, which will cause the threaded fastener to fail and shorten the service life. Due to the decarburization of the raw materials, if the annealing is not proper, the decarburization layer of the raw materials will deepen. During the quenching and tempering heat treatment process, some oxidizing gas is usually brought in from outside the furnace.

The rust of the bar steel wire or the residue on the surface of the wire rod steel wire after cold drawing will also decompose after being heated in the furnace and react to generate some oxidizing gas. For example, the surface rust of the steel wire, its composition is iron carbonate and hydroxide, will decompose into CO after heating? And H? O, thereby aggravating decarburization. Studies have shown that the degree of decarburization of medium-carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700-800 degrees Celsius. Because the attachments on the surface of the steel wire decompose quickly into carbon dioxide and water under certain conditions, if the continuous mesh belt furnace gas is not properly controlled, it will also cause the screw to decarbonize excessively. When high-strength bolts are formed by cold heading, the raw material and the annealed decarburized layer not only still exist, but are also extruded to the top of the thread. For the surface of the fastener that needs to be quenched, the required hardness cannot be obtained, and its mechanical properties (Especially strength and wear resistance) decrease. In addition, the surface of the steel wire is decarburized, and the surface layer and the internal structure have different expansion coefficients, and surface cracks may occur during quenching.

For this reason, it is necessary to protect the top of the thread from decarburization during quenching and heating, and to appropriately carbonize the fasteners whose raw materials have been decarburized, and adjust the advantage of the protective atmosphere in the mesh belt furnace to the original carbon-coated parts The carbon content is basically the same, so that the decarburized fasteners slowly return to the original carbon content. The carbon potential is set at 0.42%-0.48%. The carbon coating temperature is the same as the quenching heating and cannot be carried out at high temperatures. , So as to avoid coarse grains and affect mechanical properties. The quality problems that may occur in the quenching and quenching process of fasteners mainly include: insufficient hardness in the quenched state; uneven hardness in the quenched state; excessive quenching deformation; quenching cracking. Such problems on site are often related to raw materials, quenching heating and quenching cooling. Correctly formulating heat treatment processes and standardizing production operations can often avoid such quality accidents.

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