Usually, the bolt head is formed by cold heading plastic processing, compared with cutting processing, the metal fiber (metal wire) along the shape of the product is continuous, without cutting in the middle, which improves the strength of the product, especially the excellent mechanical properties.Cold heading forming process includes cutting and forming, single – click, double – click cold heading and multi – position automatic cold heading.An automatic cold heading machine is used for stamping, upsetting, extruding and reducing diameter in several forming dies.Simplex bit or multi-station automatic cold heading machine using the processing characteristics of the original blank is made up of material size 5 to 6 meters long bar or weight is 1900-2000 kg of the size of the wire rod steel wire, the processing technology is the characteristics of cold heading forming is not the cut sheet blank in advance, but USES the automatic cold heading machine itself by bar and wire rod steel wire cut and upsetting the blank (if necessary).Before the extrusion cavity, the blank must be reshaped.The blank can be obtained by shaping.The blank does not need shaping before upsetting, reducing diameter and pressing.After the blank is cut, it is sent to upsetting work station.This station can improve the quality of the blank, reduce the forming force of the next station by 15-17%, and extend the life of the mould.The precision achieved by cold heading forming is also related to the selection of forming method and the process used.In addition, it also depends on the structural characteristics of the equipment used, process characteristics and their state, tool precision, life and wear degree.For high alloy steel used in cold heading and extrusion, the working surface roughness of hard alloy die should not be Ra=0.2um, when the working surface roughness of such die reaches Ra=0.025-0.050um, it has the maximum life.
The bolt thread is usually processed by cold process, so that the screw blank within a certain diameter is rolled through the thread plate (die), and the thread is formed by the pressure of the thread plate (die).It is widely used because the plastic streamline of the screw thread is not cut off, the strength is increased, the precision is high and the quality is uniform.In order to produce the thread outside diameter of the final product, the required diameter of the thread blank is different, because it is limited by the thread precision, whether the material coating and other factors.Rolling (rolling) pressing thread is a method of forming thread teeth by plastic deformation.It is with the thread with the same pitch and conical shape of the rolling (rolling wire plate) die, one side to extrude cylindrical shell, the other side to make the shell rotation, the final rolling die on the conical shape transferred to the shell, so that the thread forming.Rolling (rub) pressure thread processing common point is that the number of rolling revolution is not too much, if too much, the efficiency is low, the surface of the thread teeth easy to produce separation or disorderly buckle phenomenon.On the contrary, if the number of revolutions is too small, thread diameter is easy to lose the circle, rolling pressure abnormal increase in the early stage, resulting in shortened die life.Common defects of rolling thread: some surface cracks or scratches on the thread;Disorderly buckle;The thread is out of roundness.If these defects occur in large Numbers, they will be found in the processing stage.If a small number of these defects occur, the production process will not notice these defects will flow to the user, causing trouble.Therefore, the key issues of processing conditions should be summarized to control these key factors in the production process.
High strength fasteners shall be tempered and tempered according to technical requirements.The purpose of heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength value and bending strength ratio.Heat treatment technology has a crucial impact on the internal quality of high-strength fasteners, especially its internal quality. Therefore, in order to produce high-quality high-strength fasteners, it is necessary to have advanced heat treatment technology equipment.Due to the large production capacity and low price of high-strength bolts, as well as the relatively fine and precise structure of the screw thread, the heat treatment equipment is required to have large production capacity, high degree of automation and good quality of heat treatment.Since the 1990s, the continuous heat treatment production line with protective atmosphere has been in a dominant position. The shock-bottom type and net-belt furnace are especially suitable for the heat treatment and tempering of small and medium-sized fasteners.Tempering line besides the furnace sealed performance is good, but also has advanced atmosphere, temperature, and the process parameters of the computer control, equipment failure alarm and display functions.High-strength fasteners are operated automatically from feeding – cleaning – heating – quenching – cleaning – tempering – coloring to the offline line, effectively ensuring the quality of heat treatment.The decarburization of screw thread will cause the fastener to trip first when it fails to meet the resistance of mechanical performance requirements, which will make the screw fastener lose efficacy and shorten the service life.Because of the raw material decarbonization, if the annealing is not appropriate, will make the raw material decarbonization layer deepened.During the quenching and tempering heat treatment, some oxidizing gases are usually brought in from outside the furnace.The rust of the bar steel wire or the residue on the wire wire after cold drawing will decompose after heating in the furnace, generating some oxidizing gas.Steel wire surface rust, for example, is it made of iron carbonate and hydroxide, after the heat will be broken down into CO ₂ and H ₂ O, thus aggravating the decarburization.The results show that the decarburization degree of medium carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700 and 800 degrees Celsius.Because the attachment on the surface of steel wire decomposes and combines into carbon dioxide and water at a fast speed under certain conditions, if the continuous mesh belt furnace gas control is not appropriate, will also cause the screw decarbonization error.When a high-strength bolt is cold headed, the raw material and the annealed decarburizing layer not only still exists, but is extruded to the top of the thread, resulting in reduced mechanical properties (especially strength and abrasion resistance) for the surface of fasteners that need to be hardened.In addition, the surface decarburization of steel wire, surface and internal organization are different and have different expansion coefficient, quenching may produce surface cracks.Therefore, to protect the thread at the top of the decarburization in heat quenching, but also for raw materials has been moderately coated carbon decarburization of fasteners, turn the advantage of the mesh belt furnace protective atmosphere in the basic equal to the original carbon content and carbon coating parts, already decarburization fasteners slowly back to the original carbon content, carbon potential is set in 0.42% 0.48% advisable, nanotubes and quenching heating temperature, the same cannot under high temperature, in order to avoid coarse grains, affect the mechanical properties.The main quality problems of fasteners in quenching and quenching process are: quenching hardness is insufficient;Uneven hardening hardness;Quenching deformation overshoot;Quenching cracking.Such problems in the field are often related to raw materials, quenching heating and quenching cooling. The correct formulation of heat treatment process and the standardization of production operation process can often avoid such quality accidents.
Post time: May-31-2019