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New Development of Heat Treatment Technology for Automobile Fasteners
New Development of Heat Treatment Technology for Automobile Fasteners

New Development of Heat Treatment Technology for Automobile Fasteners

The impact of heat treatment process on improving the fatigue strength of bolts

All along, automotive fasteners have been dominated by a wide range of varieties, various types and specifications of the basic features, and its selection and use involves structural analysis, connection design, failure and fatigue analysis, corrosion requirements and assembly methods, as well as related product quality control and testing, etc. These factors largely determine the final quality and reliability of automotive products.

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 there are almost no signs of bolt fatigue damage, so major accidents can easily occur when fatigue damage is generated. 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.

Automobile Fasteners

The emergence of material fatigue cracking

Fatigue cracks are the first place to start called fatigue source, fatigue source is sensitive to the bolt microstructure organization, fatigue cracks can sprout at a very small scale, generally within 3-5 grain size, the bolt surface quality is the main source of fatigue, most of the fatigue begins at the bolt surface or subsurface. A large number of dislocations and some alloying elements or impurities inside the bolt material crystal, grain boundary strength differences, all these factors may lead to fatigue crack sprouting. Studies have shown that fatigue crack-prone locations are: grain boundaries, surface inclusions or second-phase particles, and voids, all of which are related to the complex and variable microstructure of the material. If the microstructure can be improved after heat treatment, then the fatigue strength of the bolt material can be improved to some extent.

The effect of decarburization on fatigue strength

The decarburization of bolt surface will reduce the surface hardness and wear resistance of the quenched bolt and significantly reduce the fatigue strength of the bolt. GB/T3098.1 standard has a decarburization test for bolt performance and specifies the maximum decarburization layer depth. In the analysis of 35CrMo wheel bolt fracture failure cause, found in the thread and rod intersection because of the presence of decarburization layer. fe3C can react with O2, H2O, H2 at high temperature resulting in the reduction of Fe3C inside the bolt material, thus increasing the ferrite phase of the bolt material, reducing the strength of the bolt material and easily triggering micro-cracking. In the heat treatment process to control the heating temperature, while the controlled atmosphere must be used to protect the heating can be a good solution to this problem.

The effect of heat treatment on fatigue strength

The stress concentration on the surface of the bolt will reduce its surface strength, and when subjected to alternating dynamic loads, the process of micro-deformation and recovery occurs continuously at the notched stress concentration site, and the stress is much greater than the site without stress concentration, which can easily lead to fatigue cracking.

Fasteners through heat treatment and tempering to improve the microstructure, and has excellent overall mechanical properties, can improve the fatigue strength of the bolt material, reasonable control of grain size to ensure low-temperature impact work, but also to obtain a high impact toughness. Reasonable heat treatment to refine the grain, shorten the grain boundary distance can stop the fatigue crack, in the material internal if there is a certain amount of grain whiskers or second particles, these joined phases can be to a certain extent to prevent the slip of the resident slip zone, thus preventing the sprouting and expansion of microcracking.

The quenching medium and processing medium for heat treatment

Automotive high-strength fasteners have a series of technical characteristics: high-precision grade; service conditions are harsh, it will be with the host year-round cold and extreme temperature differences, high temperature, low temperature erosion; static load, dynamic load, overload, heavy load and environmental media corrosion, in addition to the role of axial preload tensile load, but also in the work of additional tensile alternating load, transverse shear alternating load or thus The role of the composite bending load, sometimes also by the impact load; additional transverse alternating load will cause the bolt to loosen, axial alternating load will cause the fatigue fracture of the bolt, axial tensile load will cause the delayed fracture of the bolt, as well as high temperature conditions caused by the creep of the bolt.
A large number of failed bolts show that the service is fracture along the transition between the bolt head and the rod part; pulled off along the intersection of the bolt rod thread and the rod part; and also slipped buckle along the threaded part. Metallographic analysis: the bolt surface and the heart have more insoluble ferrite, in quenching austenitization is not sufficient, the matrix strength is not enough and stress concentration is one of the important reasons for failure. For this reason, it is very important to ensure that the bolt section quenching and tissue uniformity.
The function of the quenching oil is to take away the heat of the red hot metal bolts quickly, so that it drops to the martensite transformation temperature to obtain a high hardness of the martensite organization and hardening layer depth, while also taking into account the reduction of bolt deformation and prevent cracking. Therefore, the basic characteristics of quenching oil is the “cooling characteristics”, which is characterized by high temperature stage of the cooling rate is faster, while the low temperature stage of the cooling rate is slower, this characteristic is very suitable for alloy structural steel ≥ 10.9 grade high strength bolts quenching requirements.
Fast quenching oil in use due to thermal decomposition, oxidation and polymerization reactions, resulting in changes in cooling characteristics, oil containing trace amounts of water will seriously affect the cooling performance of the oil, resulting in quenched fasteners brightness decline, uneven hardness, soft spots and even cracking tendencies. Studies have shown that the deformation problems caused by oil quenching are partly caused by water in the oil. In addition, water in the oil also accelerates the emulsification of oil deterioration and promote the failure of additives in the oil. When the water content in the oil ≥ 0.1%, the oil is heated may cause the water gathered in the bottom of the tank to reach the boiling point, the volume of the sudden expansion, easy to cause the oil to overflow quenching oil tank and cause a fire.
For continuous mesh belt furnace using fast quenching oil, according to the quenching characteristics data accumulated in the 3-month interval test, it is possible to establish the stability of the oil and quenching characteristics of the map, to determine the appropriate life of quenching oil, predict the problems associated with changes in quenching oil performance, thereby reducing the loss of rework or scrap caused by changes in quenching oil performance, making it a routine control method of production. The quenching depth directly affects the quality of the bolt after heat treatment, when the material is poorly hardened, the cooling medium is slow and the bolt size is large, it is not possible to quench the bolt heart all quenched into martensite tissue during quenching, reducing the strength level of the heart region, especially the yield strength. This is obviously very unfavorable for bolts subjected to uniformly distributed tensile stresses along the entire cross-section. The lack of hardenability reduces the strength, and metallographic examination reveals the presence of pre-eutectoidal ferrite and reticulated ferrite tissue in the core, indicating that the bolt hardenability needs to be enhanced. It is known that two ways to increase the hardenability, increase the quenching temperature; increase the quenching capacity of the quenching medium, both can effectively increase the quenching depth of the bolt.
Houghto-Quench K2000 is a fast quenching oil developed by Houghto on the basis of Houghto-Quench G, a medium cooling quenching oil, which further increases its hardening capacity and is especially suitable for quenching and cooling of fasteners to obtain satisfactory quenching depth.
Fast quenching oil vapor film phase is short, that is, the high temperature phase of the oil cooled quickly, this feature is conducive to 10B33, 45 steel ≤ M20 bolts, M42 nuts to obtain a deeper hardened layer, while for SWRCH35K, 10B28 steel is thinned to ≤ M12 bolts, M30 nuts to ensure that the heart hardness and surface hardness has a small difference. Analysis from the cooling speed distribution, in addition to the high temperature stage requires cooling fast, the oil of the low temperature cooling rate of high and low to obtain a greater role in the depth of the hardened layer, the higher the low temperature cooling rate, the deeper the hardened layer is often. This is very favorable for high-strength fasteners throughout the cross-section of uniform load, the requirements in the hardened state, before tempering to obtain about 90% of the martensite organization. The indicators assessed are flash point, viscosity, acid value, oxidation resistance, carbon residue, ash, sludge, quenching and cooling rate, quenching brightness and nearly 20 indicators.
For larger size bolts or PAG quenching agent to meet the quenching requirements of most products, PAG quenching agent in the martensite transformation zone in the boiling stage, cooling rate and a large risk, it can be adjusted by the concentration of the key indicators in the cold rate of about 300 ℃, the lower the cold rate at this temperature point, the stronger the ability to prevent quenching and cracking, the more applicable steel grades. The stability of the convection cold rate during use is the most important factor in ensuring the quality of quenching.
In the early failure of the bolt specimen, also visible in the fracture bolt near the fracture of the threads are crack defects, mainly in the bolt rolling process is not appropriate, in the rolling threads of the cold forming process after the convergence of metal plastic rheology and folding; in the bottom of the thread teeth can also be seen in the presence of micro cracks of varying depths, the formation of processing chip tumor stress concentration area. GB/T5770.3-2000 “Fasteners Surface Defects Bolts, Screws and Studs Special Requirements” standard stipulates that folding is allowed for bolts that are subjected to stress with a depth of not more than one-fourth of the threaded tooth height above the mid-diameter; folding and chip tumor at the bottom of the threaded tooth are not allowed defects, and folding is one of the main reasons for bolt fracture. The use of good Fulton extreme pressure lubricant for bolt thread processing can effectively prevent chip tumor and reduce stress concentration, thus helping to improve the fatigue life of the bolt.

The surface protection of automotive fasteners and technology development

Fasteners on the car, especially fastening bolts, pipe clamps, elastic clamps, etc. are in the process of use and its harsh environment, usually more serious corrosion, even due to rust and corrosion lead to disassembly difficulties. Therefore, fasteners must be required to have good anti-corrosion properties. At present, the more common method is to use its surface electroplating zinc, zinc-nickel alloy, phosphating, blackening and Dacromet treatment. Due to the restrictions on the content of hexavalent chromium in the surface coating of automotive fasteners, products that do not meet the standards of the Environmental Protection Directive and contain harmful substances are not allowed to enter the market, which puts forward an unprecedented high standard of environmental protection requirements for the innovative ability of the surface treatment of automotive fasteners.

Water-based zinc and aluminum coating Jumite

Based on more than 30 years of experience in surface rust prevention technology of Dacromet, and after years of research and development, EnOufor Group has developed a new surface treatment technology completely free of chromium — GEOMET.
The rust prevention mechanism, the skin structure of GEOMET treatment is the same as that of Dacromet treatment, in which the metal pieces are overlapped in layers, forming a skin film that is combined with a silicon adhesive, thus covering the base material.
Advantages of Jumite: Electrical conductivity, the high strength metal sheet makes Jumite bolts electrical conductivity. Paint adaptability, Kumite can be used as a primer for most paints, including electroplating. Environmentally friendly, water-based solution, does not contain chromium and no waste water is produced, no harmful substances are emitted into the air. Excellent corrosion resistance, only 6-8μm film thickness is required to achieve salt spray test over 1000h. Heat resistance, inorganic skin film, and the skin film does not contain moisture. No hydrogen embrittlement process, the coating process without acid and electrolysis, avoiding hydrogen embrittlement as with ordinary plating process.
The stability of the coefficient of friction is essential for the assembly of automotive fasteners. The solution of water-based zinc-aluminum flake coating for the coefficient of friction is to apply a water-based inorganic surface coating with lubricating function – PLUS on top of zinc-aluminum coating, the types of PLUS and the range of coefficient of friction are shown in Table 1.
Table.1 PLUS type and range of friction coefficient

PLUS types Friction coefficient range (ISO16047)
PLUS XL 0.06-0.09
PLUS L 0.08-0.14
PLUS VL 0.09-0.14
PLUS ML 0.10-0.16
PLUS M 0.12-0.18

Electrodeposition coating technology

In recent years, some fasteners of some automobile companies use electrophoretic coating instead of passivation treatment after electroplating. The principle of electrophoretic coating is simply “opposites attract”, just like a magnet. Anodic electrophoresis is applied to the bolt at the anode, and the paint is negatively charged; while cathodic electrophoresis is applied to the bolt at the cathode, and the paint is positively charged. It is well known that electrophoretic coating is highly mechanized, environmentally friendly, and the paint film has excellent corrosion resistance. Water recycling and reuse, reduce emissions; strengthen heavy metal recycling to reduce emissions; reduce VOC (volatile organic compounds) emissions; reduce energy consumption (water, electricity, fuel, etc.), meet the premise of environmental protection requirements, reduce costs and improve quality.
It has been used in auto parts and fasteners for several years, and the electrophoretic coating process is more mature, and it is a product to replace electroplating, PPGElect ropolyseal fastener special electrophoretic coating material, EPll/SST 120-200h anodic electrophoresis, EPlll/SST 200-300h cathodic electrophoresis, EPlV/SST 500-1000h cathodic electrophoresis EP V/SST 1000-1500h cathodic electrophoresis; and ZiNC Rich coating zinc-rich organic coating (conductive).
With the development of technology, besides cathodic electrophoresis coating with excellent corrosion resistance, anodic electrophoresis coating with certain weather resistance and edge corrosion resistance cathodic electrophoresis coating have also been practically applied on the production line. At present, PPG’s electrophoretic coating series has been recognized by several automobile manufacturing companies, and a series of specifications have been adopted as a unified standard with S424 changed to S451, such as Ford’s WSS-M21P41-A2, S451; GM’s GM6047 code G; Chrysler’s PS-7902 Mcthod C.
The benefits of using electrophoretic coating are good for environmental protection. Electrophoretic coating using water-based paint, passivation using trivalent chromium; improve the corrosion resistance of the product, excellent adhesion; no plug holes, no plug screw teeth, uniform film thickness, consistent torque value; traditional plating + passivation process, salt spray test up to about 144h. After using zinc phosphating + zinc-rich primer + cathodic electrophoresis coating process, the salt spray test can reach more than 1000h, and if using electroplating + cathodic electrophoresis coating process, the salt spray test can reach more than 500h (see Table 2)
Table.2 Comparison of salt spray test results of several film layers

Workmanship Zn phosphating+anodic electrophoresis Zn phosphating+cathodic electrophoresis Zn phosphating+zinc rich primer+cathodic electrophoresis Electroplating+cathodic electrophoresis+topcoat
Salt spray test/h 120 200-300 1000-1500 500-1000


The future development of automotive fasteners will be more personalized, heat treatment process to highlight the service characteristics, intelligent, green, lightweight technology will play an important role. The development of technology and equipment is the basis for the development of advanced manufacturing industry, there is still a lot of room for development, to narrow the gap with the advanced foreign level, the task is still very difficult, there is a long way to go.

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