An automotive fastener is a key component used to assemble the various components of a vehicle. As such, it must have the strength and rigidity to support the required function. It must also be lubricated to allow for low insertion forces to be achieved. During insertion, the fastener needs to be flexible enough to deform without breaking. Most importantly, it must be easily molded to fill all the fine parts of the fastener.
From the perspective of the development of the entire manufacturing industry, fasteners are known as the “rice of industry” and the “screws” of the national economy. Fasteners in the entire product structure, usually only a small part, but as an organic part of the product, but plays an important role: the product structure of two or more workpiece fastening connected into a whole. Common fasteners are bolts, screws, studs, nuts and their combinations such as washers, gear rings, pins, rivets, connection vice, etc., widely used in automotive, engineering, construction and other fields.
Overview of automotive fasteners
Because the automotive fastener products involve a wide range of technologies, exceptionally complex, and a variety of technologies intertwined and affect each other, so it is necessary to sort out and accurately identify through comprehensive research and in-depth analysis, in order to determine the specific key technology areas, clear subdivision of the development goals and priorities of technology, and then form a good situation to focus on the advantageous resources to accelerate the core technology.
According to ISO898-1, GB/T3098.1 standard service conditions of high-strength bolts, the mechanical properties generally have the following requirements.
- ① higher tensile strength and high flexural strength ratio.
- ② sufficiently high plasticity, especially when using plastic zone tightening.
- ③ can be repeatedly tightened, that is, able to withstand a large stress amplitude loading enough times, with good low circumferential fatigue performance.
- ④ when subjected to alternating loads, should have good high circumferential fatigue performance.
- ⑤ When subjected to impact loading, it should have high impact toughness.
- ⑥ Good delayed fracture resistance.
- ⑦ Good low temperature resistance.
- ⑧ good creep resistance, stress relaxation resistance.
- ⑨ lower notch sensitivity (bolt system multi-notch parts)
- ⑩ Stable surface friction coefficient to obtain stable assembly preload.
Safety of fasteners for automobiles
Fasteners account for a small proportion of the finished car, but the damage caused to the whole car is great, from the point of view of the car accident, due to damage to fasteners caused by more than half of the accident, and some even endanger the lives of people, and investigate the causes of damage to fasteners, mainly depends on the good or bad materials used for fasteners.
Fasteners in various types of mechanical parts or building structural parts, mainly to play the role of connection, when the external load on the structural parts, fasteners can make the structural parts to maintain the original state of connection and not produce failure. As the structural members are subjected to variable forms of load, such as preload, working load and impact load, the fasteners will fail under the action of these loads, mainly fracture and loosening. The fracture and loosening of any fastener connection point in the structural member will cause the failure of the entire structural connection chain, where the fracture will cause the disconnection of the structural connector. The reasons for fracture of fasteners can be analyzed from the following three aspects, the material used in fasteners is not strong enough to fracture under the action of external load; due to unreasonable heat treatment process of the material used in fasteners, there are surface cracks and other defects; due to the fastener in the external long-term load under the action of fatigue fracture.
Performance and organizational requirements
High-quality fasteners used in the performance of steel needs to meet the following requirements: 1) fasteners used in the plastic deformation of steel materials with high elongation and section shrinkage; 2) fasteners in the cold plastic deformation, steel materials used with a smaller deformation resistance, low hardening rate in the process of deformation, with a smaller yield-to-strong ratio. High-quality fasteners used steel in the organization needs to meet the following requirements: 1) fasteners used steel microstructure is mainly composed of ferrite plus pearlite, of which the proportion of ferrite to more than 60%, the remaining pearlite; 2) fasteners used in steel containing non-metallic inclusions, aluminum oxide and spherical oxide size level should be less than 0.5, the sum of inclusions contained should be less than 3; 3) The surface of the steel used for fasteners does not have surface cracks and other defects, such as concave and convex folding, scars, pockmarks, etc., otherwise the fasteners are prone to the risk of cracking in the cold heading process.
In order to produce high-quality fastener products, in the process of selecting the steel used for fasteners, the chemical composition of the steel used must be strictly controlled to reduce the content of harmful elements S, P, O in steel, reduce the impact of oxides, sulfide inclusions on the production process of fasteners cracking; in the process of smelting and rolling, by controlling the process parameters, such as reducing the amount of pressure, reducing the number of rolling and turning steel, so that The steel used for fasteners in the production process to reduce folding and obtain uniform, fine grain organization. In the process of producing fastener products, advanced production equipment is the premise of ensuring high-quality fasteners, the correct choice of rolls and deformation of uniform hole-shaped system, which helps to improve the qualification rate of the finished product and ensure the surface finish of the product; in the fastener heat treatment process, strict control of the temperature, time and atmosphere in the heating furnace to reduce the surface burn and decarburization of the finished fastener; strengthen the billet in the entire production process The quality inspection of billets throughout the production process is strengthened to avoid the existence of cracks and other defects on the surface of billets.
In order to ensure the demand for high volume production of fasteners, fastener producers need to ensure high quality and stability of fasteners. Despite the strict control of key production processes by the producer, it is still not guaranteed to produce a completely good product and the product qualification rate cannot be improved. Therefore, final sorting and inspection, such as manual selection, eddy current sorting, optical sensing size and profile sorting, must be performed after production is completed to sort out defective products with obvious defects, such as the presence of surface cracks, surface decarburization and dimensional non-conformity, in order to meet customer requirements. Defect-free fasteners are the key to ensure the smooth operation of automotive assembly lines, the safe operation of vehicles and the good reputation of manufacturers. Therefore, in recent years, foreign fastener enterprises have flooded into the country, while Chinese fastener enterprises have both challenges and opportunities. Domestic fastener enterprises need to build their brands, improve quality, ensure higher parts qualification rate and make continuous efforts toward the goal of zero defects.
The development of automotive fastener process technology
Made in China 2025″ specifies that the development direction of energy-saving and new energy vehicles is low carbonization, informationization and intelligence. As an effective measure to improve the quality of automotive products and enhance the competitiveness of automobiles, automotive high-quality, high-efficiency by reducing the process for automotive fastener enterprises to chase the goal.
Material is the material basis of the composition of the car, the process is the material to parts, parts to the realization of the process of the whole car, therefore, the progress of automotive technology to a large extent depends on the development of materials and process technology.
Low carbonization is an important way to save energy in automobiles, which mainly includes lightweight technology, low friction technology and new energy technology. There are 3 aspects: structural optimization design, application of lightweight materials and lightweight manufacturing process.
Automotive lightweight materials are mainly in 2 categories: one is high-strength materials, mainly refers to high-strength steel, fasteners in terms of materials abroad mostly choose Mn-B series steel, high-strength bolts surface treatment mostly using zinc and aluminum coating, zinc-nickel alloy and other ways to improve corrosion resistance while improving the stability of the friction coefficient of fasteners; the other is lightweight materials, mainly including aluminum alloys, magnesium alloys, titanium alloys, plastics carbon fiber composites and other applications.
In the lightweight manufacturing process, the focus is on the forming and connection process, mastering the key technical parameters such as aluminum alloy forging temperature, riveting and bonding, solving the connection problem between different materials, avoiding stress concentration, and using extracted core rivets for some metal brackets, changing the original welded nuts to pull rivet nuts and the original welded bolts to pull rivet bolts. Due to the high production efficiency and low cost of pull riveting, it is worth to promote vigorously.
The pursuit of high quality, reliable and durable technology, the concept of stable friction coefficient (0.08-0.14) for the prevention of “loose, rusty and leaky” faults of connecting parts, the improvement of fastener quality stability, and the systematic research of anti-loosening technology, the development of new assembly processes on drive axles, engines and complete vehicles, the development of a variety of gas-phase anti-rust materials and supporting antirust packaging.
In terms of capacity building, based on the lightweight laboratory, we will focus on the construction of laboratories in the metal field, composite material field and connection field, among which the metal field will focus on building laboratories for element analysis, metallographic inspection, hardness mechanics, nondestructive testing, material fatigue, metal material process verification, etc., to improve the accuracy of process failure determination and the rate of achieving dimensional professional quality standards, etc., and to realize the interconnection of planning, design and production of automotive The interconnection of planning, design and production of automotive fasteners.
The influence of processing process on improving the fatigue strength of bolts
A comprehensive analysis shows that the key factors affecting bolt performance are the following.
- (1) Material chemical composition, inclusions, microscopic metallographic organization, decarburization and carbon enhancement, phosphorus-rich layer.
- (2) Machining process.
- (3) Guaranteed load.
- (4) Tensile strength, yield strength, toughness.
- (5) Hardness (heart, surface and special requirement position).
- (6) Thread tip shape, size and form tolerance.
- (7) Shape and dimensions of the light bar section.
- (8) The chamfer and inclination of the supporting surface under the head, the inner and outer diameter of the supporting surface and the tolerance of the shape and position.
- (9) Excessive shape dimensions of the head rod, closing thread shape dimensions, screw end shape.
- (10) Head opposite side and opposite angle width, trigger screw height (drive size).
- (11) Head solidity.
- (12) Anti-loosening performance.
- (13) Surface protection against corrosion.
- (14) Friction coefficient.
- (15) Fatigue strength.
- (16) Manufacturing defects; such as cracks, etc.
- (17) Other special performance requirements such as high and low temperature resistance.
The fatigue life of automotive high-strength bolts has always been an important issue, and data show that the vast majority of bolt failures are caused by fatigue damage, and fatigue damage when the bolt is almost no signs, so major accidents can easily occur when fatigue damage occurs.
Heat treatment can optimize the fastener material properties, so that its fatigue strength, for the use of high-strength bolts increasingly high requirements, heat treatment to improve the fatigue strength of the bolt material is very important.
With the development of the automotive industry, energy saving, environmental protection, lightweight, miniaturization, thin-walled requirements continue to grow, and fasteners for automobiles are increasingly important. Steel used in the manufacture of automotive fasteners need to have good strength and plasticity, after heat treatment can be obtained after the expected comprehensive performance, and resource-rich price is relatively low, and has a certain degree of renewable use, compared with other materials have a higher cost performance. Heat treatment process is one of the keys to obtain good overall performance of automotive fasteners with steel, unreasonable heat treatment process will directly lead to the failure of the fastener.
Fastener industry heat treatment profile
Fasteners used in steel are subject to different processes of heat treatment to achieve the requirements of the mechanical properties of fasteners. According to the mechanical properties of the steel used in fasteners and the sequence of the heat treatment process, the heat treatment is divided into three steps, the first step for the heat treatment of steel, including softening annealing and spheroidal annealing; the second step for the intermediate heat treatment, including recrystallization annealing and low-temperature annealing; the third step for the heat treatment of finished fasteners, for the tempering treatment.
According to statistics, by the end of 2007, China has a total of more than seven thousand fastener enterprises, with an annual output of more than 5.2 million tons, of which about 2.2 million tons of fasteners above grade 8 are produced, requiring heat treatment of up to 40% of high-strength fasteners. There are more than 2,700 sets of heat treatment equipment in Chinese fastener production enterprises, including more than 1,600 sets of controlled atmosphere continuous heating mesh belt furnace. Tempering treatment of finished fasteners using continuous mesh belt furnace, the average heat treatment price of fasteners is about 725 yuan / t, the annual processing income of fasteners is about 1.55 billion yuan. According to the electricity cost of fastener heat treatment accounted for 70% of the annual processing income, the electricity cost of heat treatment is about 1.085 billion yuan, according to the national average electricity price of 0.7 yuan / kW, the annual electricity consumption of fasteners is about 15.5 × 109kW-h, the average processing energy consumption of fasteners is about 775 kW-h / t, while the average processing energy consumption of fasteners in Europe and the United States is about 425 kW-h / t The average processing energy consumption of fasteners in Europe and the United States is only half of the average processing energy consumption of domestic fasteners. Fastener heat treatment auxiliary materials, such as heat treatment with various types of quenching medium, blackening agent, cleaning agent, the annual consumption of more than 15 million yuan; fastener heat treatment accessories, such as heat-resistant steel spare parts, fixtures, etc., the annual consumption of more than 7 million yuan; the average profit margin of about 12.5%.
Technology of fastener heat treatment
In order to obtain high-quality automotive fasteners, fasteners in the process of tempering heat treatment, advanced heat treatment equipment and advanced production technology must be used to ensure the smooth conduct of heat treatment, so that fasteners to obtain a high tensile strength and yield strength ratio. The tempering heat treatment process mainly includes three steps of heating, quenching and tempering, of which heating and tempering are required before a thorough cleaning of the raw materials, and the entire step can be colored after the completion of the line. For example, the fasteners of Dongfeng Motor, through tempering heat treatment need to achieve four strength levels of fasteners: 8.8, 9.8, 10.9 and 12.9, and in the heat treatment process, the raw material material, heat treatment and tempering furnace temperature, furnace atmosphere, quenching medium, finished products after the quality inspection are the key factors affecting the quality of fasteners.
Domestic fasteners used in the process of steel bias, decarburization, cracking and other problems, which is mainly due to two factors, on the one hand, fasteners used in the raw material of steel before cold forming without spheroid annealing; on the other hand, the annealing process annealing furnace does not use atmosphere protection, so that the surface of the raw steel decarburization. And some foreign enterprises from the material and annealing protection atmosphere from two aspects, a good solution to the problem of decarburization of steel materials. Japanese Kobe enterprises use spheroidized steel as the steel raw material used for fasteners, fastener manufacturers do not need to spheroidize and anneal the steel raw material, so there is no decarburization problem; Russian fastener enterprises induction heating of steel raw materials, and then spheroidized and annealed in a protective atmosphere, the process is a good solution to the problem of decarburization of fastener steel raw materials.
Heat treatment tempering furnace
Foreign heat treatment tempering lines are widely used cast chain and mesh belt type, which have obvious advantages in terms of furnace sealing, computer control and fault display. High-strength fasteners from heating, quenching and tempering, which includes the heating and tempering before the cleaning process, the whole step after the completion of the coloring and down the line, are automatically controlled by the computer throughout the operation, which can effectively ensure the stability of the quality of material heat treatment.
Most of the domestic enterprises are not highly automated, taking Dongfeng Motor’s high-strength fasteners of grade 8.8 as an example, its tempering treatment is carried out on the shock bottom furnace, which is limited by the conditions of the heat treatment furnace equipment, the furnace is poorly sealed, the purity of the protective atmosphere is not easy to control, the temperature difference of the furnace temperature is large, and the actual heating time of the parts is uncertain, and there is the phenomenon of collision and friction during the heating of the parts in the furnace, so there will be material surface decarburization, uneven local heating, parts bruising each other and other phenomena.
Quenching medium used after the tempering heat treatment plays a very important role for heat treatment, which directly affects the cooling capacity of the steel raw materials used in fasteners after heat treatment, will directly lead to quenching cracking and deformation and other phenomena. Foreign high-strength fasteners in the quenching heat treatment, quenching of medium carbon steel using water-soluble quenching medium (PVG) or fast quenching oil for quenching. Most domestic fastener companies use two quenching medium: alkaline water and ordinary mechanical oil. Quenching in alkaline water, the fastener surface is prone to quenching and cracking phenomenon, and quenching in ordinary mechanical oil, the fastener surface hardness uneven phenomenon, so in actual production had to be repeatedly tempered heat treatment of fastener steel raw materials.
In order to ensure the absolute reliability of the quality of fasteners, perfect testing equipment is a quality assurance prerequisite, foreign countries use direct-reading spectrometer for fastener steel raw material chemical composition for rapid detection; and whether the raw materials exist cracks and other defects in online nondestructive testing, to ensure that the raw materials intact before entering the heat treatment; fasteners in the tempering heat treatment, the carbon potential control, temperature control and process parameters control; quenching medium to implement the temperature and cooling rate monitoring, quenching, tempering hardness measurement and metallographic determination. If necessary, quantitative measurement, fluorescence magnetic particle non-destructive testing of the tempered parts, the parts of the intrinsic defects sorting and strength, hardness and other mechanical properties of the determination.
And domestic enterprises after heat treatment of fasteners mainly for hardness, tensile strength, metallographic organization of the test and protective atmosphere analysis, only part of the high-level fasteners for magnetic particle fluorescence nondestructive examination of internal cracks and other defects in the parts. The quenching medium cooling temperature and cooling rate determination, the quantitative analysis of metallographic organization, but not up to the detection requirements.
Fastener heat treatment equipment updates
The Chinese government vigorously advocates the construction of a resource-saving society, put forward a hard target of saving 20% of energy consumption within three years, encouraging enterprises must carry out technological transformation and equipment renewal. And with the increasingly fierce competition in the fastener industry, customers on fastener product quality, such as lightweight, environmental protection, long life and other aspects of the increasing requirements, the enterprise’s equipment will continue to transform and update, the investment in advanced heat treatment equipment in a considerable period of time will still have a large space for development to adapt to the development of foreign and domestic markets and needs.
Fastener enterprise heat treatment equipment transformation and update mainly tends to use controlled atmosphere mesh belt furnace and multi-purpose furnace, to replace the traditional well furnace and mesh belt furnace. Compared with electric heating furnace, gas heating furnace has higher heating efficiency. With electric heating, the utilization rate of heat energy is only about 30%; while with gas heating, the utilization rate of heat energy can reach about 60%. The heating zone of continuous mesh belt furnace production line adopts gas heating and the heat preservation zone adopts electric heating, which can give full play to the advantages of gas heating and electric heating, and this technology will be widely promoted and applied in the areas where natural gas resources are abundant in China.
Sealed box-type multi-purpose furnace process is flexible, can be quenched on long rod fasteners, the working temperature can be increased to 1050 ~ 1100 ℃, with high efficiency, energy saving, low cost and clean production characteristics, is a small and medium-sized fastener enterprises technical transformation and equipment renewal of the applicable furnace type.
Although domestic automotive fastener enterprises and foreign advanced level compared to the obvious gap, but China’s fastener enterprises after a long period of development, with a certain industrial base and advantages. With the increasingly fierce market competition in the automotive industry, the requirements for lightweight, environmental protection and long life of automotive fasteners have been increasing. China as the world’s largest automotive production and sales country, automotive fastener enterprises continue to develop and grow rapidly. From the perspective of the world pattern, China’s fastener industry has a broad space for development, both in terms of horizontal scale development and vertical technology upgrading, and the road of development and revitalization of automotive fasteners is a long way to go.
Source: China Automotive Fasteners Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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