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Heat treatment of bolts

Update:24 Jul 2017
Summary:

   High strength fasteners should be quenched and tempe […]

   High strength fasteners should be quenched and tempered according to technical requirements. Heat treatment conditioning is to improve the comprehensive mechanical properties of fasteners to meet the product requirements of tensile strength value and yield ratio;  . The heat treatment process has a crucial influence on the internal quality of high strength fasteners, especially for its high quality fasteners. Advanced heat treatment equipment must be equipped to produce high quality fasteners. Because of the high strength bolt production volume, low price, and the more subtle threaded part relatively sophisticated structure, therefore, requirements of heat treatment equipment must have large production capacity, high degree of automation, good ability of heat treatment quality.

Since 1990s, the continuous heat treatment production line with protective atmosphere has taken the lead, and the shock bottom type and mesh belt furnace is especially suitable for the 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 atmosphere, temperature and process parameters, computer control, equipment malfunction alarm and display function. High strength fasteners can be automatically controlled by feeding, cleaning, heating, quenching, cleaning, tempering, coloring to the assembly line, thus effectively ensuring the quality of heat treatment.

The decarburization of the thread will cause the fastener to trip when the resistance of the mechanical property is not met, which will lead to the failure of the threaded fastener and shorten the service life. Because of the decarburization of raw materials, if the annealing is not good, the raw material decarburization layer will be deepened. During quenching and tempering, some oxidation gases are usually removed from the furnace. The rust of bar wire or the residue on the surface of wire rod after cold drawing will also decompose after heating into the furnace and react to produce some oxidizing gas. For example, the surface rust of steel wire, which is composed of iron carbonate and hydroxide, is decomposed into CO2 and H2O after heating, thus increasing decarburization. The research shows that the degree of decarburization of medium carbon alloy steel carbon steel is serious, and the fastest decarbonization temperature between 700-800 degrees celsius. Due to the adhesion of the steel wire surface under certain conditions, decomposition of synthetic CO2 and H2O speed is very fast, if the continuous mesh belt furnace gas control improperly, it will lead to screw decarburization. High strength bolts when adopting cold forming, decarburized layer materials and annealing not only still exist, and are being squeezed to the top of the thread, fastener surface to be quenched, not required hardness, its mechanical properties (especially the strength and wear resistance decreased). In addition, the surface decarburization of steel wire has different surface and internal structure, and has different expansion coefficient, and it may cause surface crack when quenching. Therefore, to protect the top thread not decarburization in quenching, the moderate carbon coated fastener also for raw materials has been decarburization, took advantage of the protective atmosphere furnace and the carbon coating to parts of the original carbon content is basically the same, the fastener has decarburization slowly return to the carbon content of the original the carbon potential setting should be 0.42% - 0.48%, carbon coating temperature and quenching the same, cannot be carried out at high temperature, so as not to affect the mechanical properties of coarse grains. The quality problems that may occur in the process of quenching and tempering are mainly as follows: lack of quenched hardness, uneven quenching, excessive quenching deformation, and quenching cracking. Such problems appear on the scene, often related to raw materials, quenching, heating and quenching cooling, the correct formulation of heat treatment process, standardize the operation process, often can avoid such quality accidents.http://www.lwhardware.com/