Improvement of heat treatment technology for the h

Improvement of heat treatment technology of high-strength fasteners

heat treatment technology has a crucial impact on the internal quality of high-strength fasteners, especially its internal quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are necessary

1 heat treatment status of Dongfeng Automobile high strength fasteners

Dongfeng Automobile high strength fasteners have four performance levels, namely 8.8, 9.8, 10.9 and 12.9. These four grades of high-strength fasteners should be quenched and tempered

heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength value and yield ratio of products. Therefore, the quenching and tempering process has strict requirements for raw materials, furnace temperature control, furnace atmosphere control, quenching medium, heat treatment quality inspection and control

1.1 materials

Dongfeng Automobile high-strength fastener materials mainly include ML35, 35K, ml40cr, ML35CrMo steel, etc., of which 75% ML35CrMo steel is imported. The main problems of materials are box segregation, decarburization, cracks and so on. Before cold forming, the material needs to be spheroidized and annealed in an oil furnace. There is no protective atmosphere in the furnace, and there is decarburization

1.2 heat treatment quenching and tempering furnace

12.9-grade high-strength fasteners are quenched and tempered on the production line of chain casting furnace. The temperature and atmosphere in the furnace are controlled by computer, and Rx gas is used as the protective atmosphere. Heating and anti oxidative decarburization can be effectively controlled, and the quality of fasteners is intact during heating

8.8 are quenched and tempered on the bottom shaking furnace. Affected by the structure of the bottom shaking furnace, the sealing performance of the furnace is poor, the RX atmosphere is not easy to control, the temperature difference of the furnace is large, the parts are heated in the furnace with collision and different length of time, and the quality of the fasteners in the heating process is poor, with decarburization, bruise, uneven heating and other phenomena

1.3 quenching medium

Dongfeng Automobile high-strength fasteners are quenched and tempered. There are two kinds of quenching medium, ML35 is quenched with alkaline water, and medium carbon alloy steel is quenched with ordinary machine oil. Quenching cracking exists when quenching in alkaline water. Quenching in common machine oil has uneven hardness or even unqualified phenomenon, so it has to be repeatedly quenched and tempered in production

1.4 testing

testing of high-strength fasteners of Dongfeng Motor after heat treatment, mainly testing hardness, tensile strength, metallographic structure (qualitative testing), RX gas analysis, 100% magnetic particle fluorescent nondestructive inspection crack of grade 12.9, etc. However, there are no testing methods to measure the cooling performance of quenching medium, quantitative analysis of metallographic structure, internal defects of parts and hardness sorting

2 heat treatment level of foreign high-strength fasteners

2.1 materials

spheroidizing annealing in material restructuring. Kobe Steel Plant in Japan has leading technology. The spheroidized steel provided by Kobe Steel plant does not need to be spheroidized and annealed by the fastener manufacturer, so there is no annealing and decarburization problem. Some manufacturers in Taiwan Province and Russia adopt induction heating for material annealing, and then enter the protective atmosphere furnace for spheroidizing annealing. This process is advanced, and the problem of decarburization can be well solved. Most western countries use the electric heating bell type spheroidizing annealing furnace controlled by protective atmosphere to realize the spheroidizing annealing of materials. The equipment and technology are advanced, the spheroidizing quality is reliable, and there is no decarburization phenomenon

2.2 heat treatment tempering furnace

belt type or cast chain type heat treatment tempering line is widely used abroad. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced functions such as computer control of atmosphere, temperature and process parameters, equipment fault alarm and display. High strength fasteners are automatically controlled from feeding - Cleaning - heating - quenching - Cleaning - tempering - coloring to offline, effectively ensuring the quality of heat treatment

2.3 quenching medium

quenching medium directly affects quenching cooling capacity, quenching cracking and deformation. Water soluble quenching medium (PAG) is widely used for medium carbon steel and rapid quenching oil is widely used for medium carbon alloy steel when quenching and tempering high-strength fasteners abroad

2.4 testing

foreign countries use direct reading spectrometer to quickly detect the chemical composition of raw materials, and carry out non-destructive detection of cracks on reformed materials. Ensure that the materials are in good condition before heat treatment

when parts are quenched and tempered, carbon potential control, temperature control and process parameter control are adopted. The quenching medium shall be subject to temperature control and cooling rate monitoring, hardness measurement of quenching and tempering, metallographic measurement of quenching and tempering, quantitative measurement if necessary, 100% fluorescent magnetic particle nondestructive testing of parts after quenching and tempering, 100% hardness of parts, internal defect sorting and mechanical property measurement, etc

3 technology improvement

to sum up, there is a certain gap between the heat treatment technology and equipment of Dongfeng Automobile high-strength fasteners and foreign advanced heat treatment technology and equipment. In order to further develop domestic and foreign markets, Dongfeng Automotive Fasteners Company has made great efforts in improving heat treatment technology

3.1 the realized technology improvement project

3.1.1 update the metallographic microscope, import 2 metallographic microscopes, and purchase the metallographic map analysis software system. Thus, the metallographic analysis and failure analysis of high-strength fasteners have reached the quantitative level, and the grade standard of ferrite content in the quenched and tempered structure of high-strength fasteners has been formulated. In the future, more research should be done to ensure that the deep-seated internal quality is under control

3.1.2 replace quenching oil and purchase rapid quenching oil to realize all rapid quenching oil of medium carbon alloy steel high-strength fasteners. The quenching quality has been significantly improved, which is reflected in the high and uniform quenching hardness, the deepening of the quenching layer, the obvious reduction of ferrite in the heart, the stable mechanical properties, and the absence of parts repair and scrap

3.1.3 use water-soluble quenching medium to purchase water-soluble quenching medium instead of alkaline water for quenching, tempering and quenching of some medium carbon steel high-strength fasteners. The quenching effect is very good, no quenching cracking is found, the quenching hardness is uniform, and the quenching quality is good. Instead of ordinary quenching oil, it can solve the problem that quenching oil is easy to produce greasy dirt and affect the quality of subsequent electroplating. (using Jinyu quenchant)

3.2 the technology improvement project

3.2.1 is purchasing the quenching and tempering line. The focus of Dongfeng Automobile high-strength fastener heat treatment technology improvement is the heat treatment and tempering line. Through the bidding, a powerful and high-level enterprise in Taiwan Province has set up an oil drain behind the oil tank to raise the control cabinet to two heat treatment and tempering lines manufactured by the meter. The line has the functions of computer control of atmosphere, temperature and process parameters, equipment fault alarm and display. The equipment has good sealing performance, high degree of automation, and the technical level has reached the foreign advanced level

3.2.2 completely replace the quenching medium and add two new quenching and tempering lines, one is oil furnace, the medium is rapid quenching oil, and the other is water furnace, the medium is water-soluble quenching medium, which creates good conditions for the safe use of materials. In order to ensure stable quenching and cooling performance, only one imported advanced portable quenching medium is tested, so that the quenching medium can be monitored regularly to ensure quenching quality

3.2.3 the quality of Rx gas is better than that of the protective gas produced by methanol cracking. RX gas is used as the protective gas for grade 12.9 Dongfeng Automobile high-strength fasteners to improve and ensure the quality of grade 12.9 high-strength fasteners. The water boiler uses methanol cracking gas as the protective gas. When a protective gas has gas supply problems, the two quenching and tempering lines can also share a protective gas, which is very convenient to use

3 so as to better use materials 2.4 the imported eddy current sorter uses the advanced eddy current sorter to detect 100% of the high-strength fasteners of Dongfeng Automobile, so as to ensure the quality of the products

3.2.5 Dongfeng Automobile high-strength fastener materials have been tested on a new type of spheroidized steel for cold forming, which has good performance in the process of cold forming and heat treatment. It is also good to try it on Cummins engine. Using spheroidized steel can eliminate the annealing process, avoid environmental pollution and decarburization caused by oil furnace annealing, and reduce mold consumption and quenching cracking. (end)