With the development of the automotive industry, the requirements for automotive fasteners have become higher and higher. The main kind of high-intensity automotive fasteners is bolts which all need to be treated with heat treatment to improve the comprehensive mechanical performance of the product and achieve the specified tensile strength. Heat treatment process technology is the key factor in the production of automotive fasteners.

Factors affecting heat treatment process of automotive fasteners

Heat treatment equipment

The continuous mesh radiant-tube furnace is most widely used. It has the advantages of uniform heating, stable quality, good working environment, reducing collision of parts, less gas consumption, and small energy consumption.

According to the requirements of the process, dephosphorization function can be added on the cleaning equipment of production line, to remove the phosphorurative layer of the fastener surface before heating. For large size high-strength bolts, to prevent quenching distortion, the UNICASE series mult-purpose chamber furnace should be adopted to heat treatment process.

The heat treatment process of continuous mesh radiant-tube furnace production line: loading-cleaning-heating-quenching-cleaning-tempering-coloring-rolling out, can realize carbon potential control and automatically working.

Heat treatment raw material

The mostly automotive fastener is made of carbon and alloy steel. In addition to the high -intensity hexagonal bolt, according to the GB/T1231 requirements, the materials with specifications below m30 are specified, the other bolts are not clearly specified in international standards. It is only a guide to indicate the alloy elements to be added.

Control methods of heat treatment process for automotive fasteners

The principle of heat treatment process

For the products with the same materials, when the size of the section is different, the quenching degree is different. Although the same quenching and tempering process is adopted, its mechanical properties are different. The larger the cross -section is, the worse the quenching ability is, and the worse the mechanical properties are. At the same time, the same raw materials allow their chemical composition to fluctuate within a certain range. Especially carbon steels will vary from steel mills as the different smelting technology and standards. The finished products produced by the same process must have performance differences.

The quenching heating temperature is mainly determined according to the chemical composition of the steel which is the main factor and the specific process. According to the different quenching medium, the quenching heating temperature is also different.

Due to the comprehensive effect of the organizational stress and thermal stress, the maximum tensile stress will be located near the surface of the parts, which may cause hardening crack. This size is related to the used quenching medium. For the bolts, the diameter of water quenching is 8-10mm and the diameter of oil quenching is 20-39mm. When the bolts in dangerous diameter are quenched, appropriate measures must be taken to prevent hardening crack.

Hydrogen relief treatment

The energy saving and lightweight development of automotive industry has high requirements for the engine design and dynamical system, which includes weight reduction and highly strengthened of bolts and nuts. But as the intensity increases, the fracture caused by hydrogen embrittlement has become a very prominent problem. During the heat treatment process, hydrogen relief treatment must be performed.

Tempered martensite is most sensitive to hydrogen embrittlement. Therefore, the heat treatment process can be adjusted to reduce the generation of Tempered martensite structure.

To prevent hydrogen embrittlement, the high -strength fastener under 1000-1300MPa must be done hydrogen relief treatment after the electroplating. During hydrogen removal, the fastener is heated to a certain temperature and maintains for a period of time and the hydrogen in the material will gather is to form hydrogen molecules. There are two main reasons for incomplete hydrogen removal. One is that there is no timely hydrogen removal, and the other is that the hydrogen removal time is too short.

Strengthen the measurement of fastener center structure

During the heat treatment process of automotive fasteners, as the effects of factors such as quenching heating temperature, quenching ability of materials, and quenching cooling speed will produce ferritic structure in the center of the fastener. Too much ferritic structure will reduce the hardness and strength of the bolt and affect the applications of fasteners. In actual production, For bolts with a large amount of ferrite in the center, when the hardness of the center structure is lower than the product requirements, the improvement of tensile strength is often adopted to control the quality of the product.

In order to control the heat treatment quality of the automotive bolt, it is necessary to measure the ferrite content in the center part of bolts, checking the hardness of the head or tail after quenching and tempering respectively.

What should be paid attention to for heat treatment process of automotive fasteners

Adopting non annealing cold heading steel, non-quenched and non-tempered steel

In order to ensure the performance of cold forming for the automotive fastener, cold heading steel needs spheroidizing annealing in advance, adjusted through the chemical composition of the steel. During the rolling process, the rolling and cooling are controlled to reduce the deformation resistance in the process of cold forming.

Improving the technical documents of thermal treatment process

The heat treatment process is a special process that can’t ensure the quality of the product through subsequent inspections and testing. Therefore, it must have a reasonable system to achieve its control. First of all, identify the factors that may cause process failure, and do a good job of formulating heat treatment process technology. Secondly, establish a comprehensive heat treatment process management system with the documents which clearly indicates the operation methods, process parameters and inspection methods. At the same time, it should be particularly emphasized in the furnace loading method and the batch of furnace and test the raw materials and process conditions of fasteners before heating. It should adopt statistical technology to analyze, calculate and measure the heat treatment parameters, and look for the influence rules of heat treatment parameters on the product quality. Then adopt professional knowledge to continuously improve the heat treatment process technology.

Developing new heat treatment equipment

New heat treatment equipment should be developed to further improve the furnace temperature and carbon potential control technology, improving the cooling effect and cooling uniformity. It should be paid attention to the technologies of waste heat utilization, energy saving and emission reduction, which will reduce the production cost of heat treatment process.

Summary

Automotive fastener manufacturer should pay attention to the update of heat treatment equipment, selecting appropriate raw materials and continuously improve the heat treatment process system in production, strengthening dehydrogenation treatment, improving the quality of automotive fasteners.