Bolts and screws are the most basic mechanical parts, which are widely used in various industries such as automobiles, industrial machinery and construction. Unlike welding and riveting, the biggest feature of bolts is that they can be easily removed even after tightening, and can be reused by tightening again.
1 Manufacturing process Bolt is generally made of coiled wire. After secondary processing such as pickling, lubrication, annealing and wire drawing, cold forging is used to form the bolt head and thread, heat treatment and surface treatment. Due to different product types, some need to be heat-treated before forming the screw part, and some need not be heat-treated or surface-treated. The secondary processing is generally handled by professional processing personnel, but sometimes it is processed in the bolt factory.
1.1 The forming of the screw head generally refers to the process of cold forging and forming the head of the bolt using a forging machine such as a jacking machine or a jacking machine. The so-called cold forging is to process at normal temperature, which is the opposite of the warm processing and hot processing after the billet is heated. The process of forging is to cut the coiled steel to an appropriate length and use several forging die devices to forge the material. Forging includes four processes: upsetting, deep drawing, reverse extrusion, and trimming. Upsetting is the process of squeezing the material from one end to expand it to be larger than the original diameter; deep drawing is the opposite, that is, squeezing the material from one end to reduce it to a process smaller than the original diameter; reverse extrusion is From the end face of the material, squeeze in the forging die smaller than the diameter of the material, so that the material is extruded while being perforated; trimming is the process of removing excess thickness with a forging die. Depending on the shape of the processed product, these processing methods can be used separately or in combination for forming. The more complex the shape of the product, the more processing steps need to be added, and the forming process is slowly carried out, but ordinary bolts can be processed and formed in only 2-5 processing steps.
1.2 The forming process of the threaded part In the screw forming process of the bolt, a roll forming machine is used to cold forge the thread, and the forming method is the same as the forming method of the bolt head. The processing of the thread shape is to hold the blank between two forging dies of 1 group, while turning one of the two forging dies, while rotating the blank, it becomes a thread shape by plastic processing. The forging die used for thread forming is called “roll forming die”. There are three types of roll forming machines: flat forging die roll forming machine, round forging die roll forming machine and planetary roll forming machine. The flat forging die roll forming machine is equipped with two ordinary rolling forming forging dies, one is fixed, and the other is moved back and forth to roll the billet. The circular forging die roll forming machine rotates the two cylindrical rolling forming forging dies installed in parallel in the same direction, and roll forming the billet sandwiched between the two forging dies. The processing method of the planetary roll forming machine is to sandwich the blank between the cylindrical forging die and the sector forging die, and roll forming by rotating the cylindrical forging die.
1.3 Heat-treated cold-forged press-formed bolts are generally made of materials with hardness suitable for plastic processing. Materials with a high carbon content or materials with added alloy elements are hard to process because of their own materials, so some need to be softened by annealing. Most materials cannot meet the required strength under cold forging. Heat treatment is a treatment performed after cold forging. It can make bolts have the required strength and mechanical properties. It is the most important process in the bolt manufacturing process. Bolts can be divided into various strength grades according to the location and use. In order to make the formed bolts have the required mechanical properties, heat treatment is required. 1.4 Surface treatment Bolts used in automobile engines often adhere to lubricating oil, so even if they are not surface-treated, they will not rust. However, such bolts are only a small part, and most bolts are used in corrosive environments, so if they are not surface-treated, they will soon rust. If the rusty bolt is placed here, it will not be able to open after corrosion. In severe cases, the bolt will break and cause a big accident. Therefore, bolts used in corrosive environments need to be surface-treated such as electroplating. The surface treatment of bolts can be roughly divided into plating and coating. The most widely used method is electroplating, which has the advantages of low cost and good corrosion resistance. When the corrosion resistance requirement is higher than that of electroplating, alloy plating such as galvanized iron and galvanized nickel can be used, or a zinc-aluminum composite coating can be applied.
2 The development trend of automotive bolts In recent years, automobile manufacturers have requested to reduce the cost of parts, light weight and high strength. At the same time, in order to cope with environmental problems, it is necessary to reduce CO2 emissions, and to reduce fuel consumption, the body weight must be reduced as much as possible. In the production cost of bolts, the billet cost accounts for the main part, so the most effective way is to reduce the cost of the material itself. In order to reduce the supply cost of materials, Japanese automobile manufacturers are studying the use of cheap materials overseas. In order to meet the requirements of users, bolt manufacturers have also conducted various studies to make bolts lighter. As a measure to reduce the weight of bolts, some use light metals such as aluminum or titanium, and some reduce the bolt size. Reducing the size of the bolt can reduce the weight, but if the size of the bolt of the same strength level is reduced, the tightening force of the bolt will decrease due to the reduction of the bolt cross-sectional area. Therefore, in order to ensure the same degree of firmness and reduce the bolt size, the strength of the bolt must be increased. In the current JIS and other public standards and auto manufacturers’ own standards, only bolts with strength levels below 12.9 are specified. When the strength grade of common quenched and tempered bolts exceeds 12.9, the delayed fracture characteristics will suddenly deteriorate. In places where high-strength grade bolts of 10.9 or 12.9 have been used, if you want to reduce the size to achieve weight reduction, you must use bolts with a strength grade exceeding 12.9 and use some method to improve the delayed fracture characteristics. In order to achieve high strength of quenched and tempered bolts, some alloy elements are added to improve the delayed fracture characteristics. Although the addition of alloying elements can somewhat improve the delayed fracture characteristics, but due to the high price of alloying elements, the increase in cost is inevitable. Another measure is to use non-tempered bolts. The non-quenched and tempered bolts do not need to be tempered (heat treated) after forming, mainly through the work hardening of the material to ensure strength. The metal structure of non-quenched and tempered bolts is completely different from that of quenched and tempered bolts. It has strong resistance to delayed fracture. Non-quenched and tempered bolts use a material with a higher carbon content than quenched and tempered bolts. Through controlled cooling and heat treatment at the blank stage, high cross-sectional shrinkage can be obtained. After drawing and forming to work hardening, the bolts are further formed Work hardening ensures the strength of the bolt. Although ordinary bolts are not tempered after forming, in order to eliminate the internal strain of the bolts by cold forging, it is necessary to carry out bluing treatment. The disadvantage is that the hardness of the material is higher than that of the quenched and tempered type, which is very difficult to form. At present, the strength of non-quenched and tempered bolts for engines is 1600MPa, and the strength of non-quenched and tempered bolts for car bodies has reached 1400MPa. It can be expected that the demand for such high-strength bolts will expand further in the future.
Production Of Titanium Bolts And Titanium Screws
