Heat treatment of high strength bolt production process


High toughness fasteners need quenching and tempering heat treatment according to technical standards. Heat treatment Heat treatment is to improve the overall physical properties of fasteners to achieve the required strength and yield ratio of the product. The heat treatment process of bolt manufacturers has a very important impact on high-toughness fasteners, especially its internal quality. Therefore, in order to manufacture high-quality high-toughness fasteners, advanced heat treatment technology and equipment are required. Because high-strength bolts have a large output, high quality and low price, and the thread part is relatively small and relatively fine in structure, it is required that the heat treatment equipment must have the capacity of large production capacity, high automation level, and high heat treatment cost performance. In the 1990s of the 20th century, the rotary heat treatment production line with maintenance atmosphere has been dominant. The bottom vibration type and mesh belt furnace are especially suitable for heat treatment of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the heat treatment line also has a professional atmosphere, computer control of temperature and technical parameters, and alarm and display functions for equipment failures. High-toughness fasteners from feeding-cleaning-heating-quenching-cleaning-quenching-coloring to withdrawal, all automatic control systems operate, which reasonably guarantees the quality of heat treatment. The decarburization of the external thread will cause the fastener to break out first when the resistance specified by the physical properties is not achieved, making the external thread fastener invalid and reducing the service life. Due to the decarburization of raw materials, if the quenching is unreasonable, the decarburization layer of raw materials will be aggravated. In the heat treatment process, some oxidation gas is usually carried in from the furnace. The rust of the round rod stainless steel wire or the residue on the surface of the round stainless steel wire after cold drawing will also dissolve after heating and heating, reflecting the conversion into some oxidizing gas. For example, the surface rust of stainless steel wire, its active ingredients are iron carbonate and nickel hydroxide, which will be converted into CO2 and H2O after heating up, which intensifies decarburization. Studies have shown that the degree of decarburization of medium-carbon carbon steel is more serious than carbon steel, and the rapid decarburization temperature is between 700-800 °C. Because the appendages on the surface of the stainless steel wire dissolve quickly to form CO2 and H2O under certain circumstances, if the furnace gas of the rotary mesh belt furnace is not properly controlled, it will also lead to deviations in screw decarburization. When high-toughness fasteners are formed by cold extrusion, the decarburized layer of raw materials and quenching not only still exists, but also is squeezed to the top of the external thread, and the specified strength cannot be obtained for the surface of the fastener that must be quenched , its physical properties (especially hardness and wear resistance) are reduced. In addition, the surface of the stainless steel wire is decarburized, and the surface and internal structure are different and have different thermal expansion coefficients, which may cause surface cracks during quenching. Therefore, to protect the top of the external thread from decarburization during quenching and heating, it is also necessary to properly carbon-coat the fasteners whose materials have been decarburized, and to adjust the advantages of the protective atmosphere in the mesh belt furnace to the advantage of shifting carbon. The initial carbon content of the parts is basically the same, so that the decarburized fasteners can gradually recover to the original carbon content. The carbon potential should be set at 0.42%-0.48%. Carry out at high temperature to avoid crystal growth and affect physical properties. The quality problems that may occur during the heat treatment and quenching process of fasteners mainly include: insufficient quenched strength; uneven quenched strength; deviation of quenching deformation; quenching cracking. These problems that arise on the spot are often related to raw materials, quenching heating and quenching refrigeration. Properly formulating the heat treatment process and standard production operation process can often prevent such quality incidents.

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