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New Development Of Heat Treatment Technology For Automotive Fasteners

May 21, 2018

First, the effect of heat treatment process on improving the fatigue strength of bolts

For a long time, automotive fasteners have been dominated by a wide variety of different types and basic specifications. Their selection and use involved structural analysis, connection design, failure and fatigue analysis, corrosion requirements and assembly methods, and related The quality control and testing of the products, to a large extent, determine the final quality and reliability of automotive products. The

The fatigue life of automotive high-strength bolts has always been an issue of concern. The data shows that the majority of bolt failures are due to fatigue failure, and that bolts have almost no signs of fatigue failure, so major accidents can easily occur during fatigue failure. Heat treatment can optimize the performance of fastener materials and increase their fatigue strength. It is more important to increase the fatigue strength of bolt materials through heat treatment for higher and higher requirements of high-strength bolts. The

1. The initiation of material fatigue cracks

The place where the fatigue crack starts first is called the fatigue source. The fatigue source is very sensitive to the microstructure of the bolt and can initiate fatigue cracks on a very small scale. Generally within 3 to 5 grain sizes, the surface quality of bolts is the main problem. The source of fatigue, most of the fatigue begins on the bolt surface or subsurface. There are a large number of dislocations and some alloying elements or impurities in the interior of the bolt material crystal, and the grain boundary strength is different. These factors may cause fatigue crack initiation. Studies have shown that the location of fatigue cracks is: grain boundaries, surface inclusions, or second-phase particles, voids, all of which are related to the complex and varied microstructure of the material. If the microstructure can be improved after heat treatment, the fatigue strength of the bolt material can be improved to some extent. The

2. Effect of decarburization on fatigue strength

Decarburization on the surface of the bolt will reduce the surface hardness and wear resistance of the bolt after quenching, and significantly reduce the fatigue strength of the bolt. The GB/T 3098.1 standard has decarburization tests for the performance of bolts and specifies the maximum decarburization depth. When analyzing the cause of 35CrMo hub bolt failure, it was found that there was a decarburized layer at the intersection of the thread and the shank. Fe3C reacts with O2, H2O and H2 at high temperature, which leads to the reduction of Fe3C inside the bolt material, which increases the ferrite phase of the bolt material, reduces the bolt material strength, and easily induces microcracks. The control of the heating temperature during the heat treatment, and the need to use a controlled atmosphere to protect the heating can well solve this problem. The

3. Effect of heat treatment on fatigue strength

The stress concentration on the surface of the bolt will reduce the surface strength. When subjected to alternating dynamic loads, the process of micro-deformation and recovery will continue to occur in the notched stress concentration area, and the stress applied to the surface of the bolt will be far greater than that of the part without stress concentration. Causes fatigue cracks. The

The fasteners improve the microstructure through heat treatment and tempering, and have excellent comprehensive mechanical properties, can improve the fatigue strength of bolt materials, reasonably control the grain size to ensure low-temperature impact work, but also can obtain higher impact toughness. Reasonable heat treatment refines the grain and shortens the distance between the grain boundaries to prevent the formation of fatigue cracks. If there is a certain amount of whiskers or particles in the interior of the material, these added phases can prevent resident slippage to some extent. The slippage of the belt prevents the initiation and expansion of microcracks. The

Second, heat treatment with quenching media and processing media

Automotive high-strength fasteners have a series of technical characteristics: high-precision level; harsh service conditions, it will be with the host year-round exposure to cold and extreme temperature differences, withstand high temperature, low temperature erosion; static load, dynamic load, overload, Corrosion of heavy loads and environmental media, in addition to being subjected to axial pretensioned tensile loads, can also be subjected to additional tensile, alternating shear, or combined bending loads during operation. The effect of the load is sometimes subjected to impact loading; the additional transverse alternating load will cause the loosening of the bolt, the axial alternating load will cause the fatigue fracture of the bolt, the axial tensile load will cause the delayed fracture of the bolt, and the high temperature conditions Creep of bolts, etc. The

A large number of failure bolts indicate that the service is fractured along the transition between the head of the bolt and the shank; the point where the thread of the bolt shank meets the shank is broken; and there is a slip along the threaded portion. Metallographic analysis: There are more undissolved ferrites on the surface and the center of the bolt, and insufficient austenitizing during quenching. Insufficient matrix strength and stress concentration are one of the important causes of failure. For this reason, it is very important to ensure the hardenability of the bolt section and the homogeneity of the structure. The

The function of the quenching oil is to rapidly take away the heat of the red hot metal bolts and reduce it to the martensitic transformation temperature to obtain a high hardness of the martensitic structure and the depth of the hardened layer. At the same time, it must also take into account the reduction of the deformation and prevention of the bolts. Cracking. Therefore, the basic characteristics of quenched oil is "cooling characteristics", which is characterized by faster cooling speed in the high temperature phase, and slower cooling speed in the low temperature phase, this feature is very suitable for quenching requirements for high strength bolts of alloy structural steel ≥10.9 or above. The

Rapid 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 decreased brightness after quenching fasteners, uneven hardness, Produces soft spots or even cracking tendencies. Studies have shown that the deformation caused by oil quenching is partly caused by the presence of water in the oil. In addition, the water in the oil also accelerates the emulsification of the oil and promotes the failure of additives in the oil. When the water content in the oil is ≥0.1%, when the oil is heated, the water accumulated at the bottom of the oil tank may reach a boiling point, and the volume suddenly expands, which easily causes the oil to overflow the quenching oil tank and cause a fire. The

For fast quenching oils used in continuous mesh belt furnaces, according to the data of the quenching characteristics accumulated in the 3-month interval test, it is possible to establish the oil stability and quenching characteristics map, determine the appropriate service life of the quench oil, and predict and quench oil performance. Relevant problems are changed to reduce the rework or waste loss caused by changes in performance of the quenched oil, making it a routine control method for production. The depth of hardenability directly affects the quality of the bolt after heat treatment. When the hardenability of the material is poor, the cooling rate of the cooling medium is slow, and the size of the bolt is large, the hardened part of the bolt cannot be completely quenched into martensite. The organization reduces the intensity of the heart area, especially the yield strength. This is obviously very disadvantageous for bolts that are subjected to a uniform distribution of tensile stress along the entire section. The lack of hardenability reduces the strength. Metallographic examination found that the presence of pre-eutectoid ferrite and reticulate ferrite in the heart shows that the hardenability of the bolt needs to be strengthened. As we all know, two ways to increase the hardenability, increase the quenching temperature; increase the quenching capacity of the quenching medium, can effectively increase the depth of the hardened bolt.

Manager: Allen Liang

Tel: +86-573-80705030


E-mail: allen@dyfastener.net

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