Heat treatment technology has a crucial impact on high-strength fasteners, especially its intrinsic quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are required.
1 Dongfeng Motor high-strength fastener heat treatment status
Dongfeng Motor's high-strength fasteners have four performance grades, namely 8.8, 9.8, 10.9, and 12.9. These four grades of high-strength fasteners are subject to conditioning.
Heat treatment and quenching and tempering is to improve the comprehensive mechanical properties of the fastener to meet the tensile strength and yield ratio specified by the product. Therefore, the quenching and tempering process has strict requirements on raw materials, furnace temperature control, furnace atmosphere control, quenching medium, heat treatment quality inspection and control.
1.1 Materials
Dongfeng Motor's high-strength fastener materials mainly include ML35, 35K, ML40Cr, ML35CrMo steel, etc., of which 75% ML35CrMo steel is imported. The materials mainly have problems such as box segregation, decarburization and cracking. The material needs to be spheroidized and annealed before cold forming, and the annealing is carried out in an oil furnace. The furnace has no protective atmosphere and decarburization exists.
1.2 heat treatment quenching furnace
12.9 grade high strength fasteners are tempered on the cast furnace furnace production line. The temperature inside the furnace and the atmosphere are controlled by computer. Rx gas is used as the protective atmosphere. Heating and anti-oxidation decarburization can be effectively controlled, and the quality of the fasteners in the heating process is intact.
The 8.8-grade high-strength fasteners are quenched and tempered on the vibration hearth furnace. Due to the influence of the structure of the vibration hearth furnace, the furnace sealing performance is poor, the Rx atmosphere is difficult to control, the furnace temperature difference is large, and the parts are heated in the furnace. The collision and the length of time are not-caused, the quality of the fastener in the heating process is poor, there are decarburization, bumping, uneven heating and so on.
1.3 quenching medium
Dongfeng Motor has high-strength fasteners for quenching and tempering. There are two kinds of quenching media. ML35 is quenched with alkaline water, and medium carbon alloy steel is quenched with ordinary mechanical oil. Quenching in alkaline water has quench cracking. Quenching in ordinary mechanical oil has uneven hardness or even unqualified phenomenon, and it has to be repeatedly tempered in production.
1.4 detection
After the heat treatment of Dongfeng Automobile high-strength fasteners, the main tests are hardness, tensile strength, metallographic structure (qualitative test), Rx gas analysis, and 12.9 grade 100% magnetic powder non-destructive inspection cracks. The quenching medium cooling performance measurement, quantitative analysis of metallographic structure, intrinsic defects of parts and hardness sorting have not been detected.
2 foreign high-strength fastener heat treatment level
2.1 Materials
Spheroidizing annealing in material restructuring, Japan Kobe Steel Plant has leading technology, spheroidized steel provided by Kobe Steel Plant does not require re-spheroidizing annealing in the fastener manufacturer, so there is no annealing decarburization problem. The materials of some manufacturers in Taiwan Province of China and Russia are annealed by induction heating, and then enter the spheroidizing annealing of the furnace in the protective atmosphere. The process is advanced and the decarburization problem can be solved well. Most western countries use electric heating hood spheroidizing annealing furnace controlled by protective atmosphere to achieve spheroidizing annealing of materials, advanced equipment and technology, reliable spheroidizing quality and no decarburization.
2.2 heat treatment quenching furnace
Mesh belt or cast chain heat treatment and quenching and tempering lines are widely used abroad. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced atmosphere, computer control of temperature and process parameters, equipment fault alarm and display. High-strength fasteners from the feeding-cleaning-heating-quenching-cleaning-tempering-coloring to the lower line, all automatic control operation, effectively guarantee the quality of heat treatment.
2.3 quenching medium
The quenching medium directly affects the quenching cooling capacity, quenching cracking and deformation. When quenching and tempering high-strength fasteners abroad, water-soluble quenching medium (PAG) is widely used for medium carbon steel, and fast quenching oil is widely used for medium carbon alloy steel.
2.4 detection
Foreign countries use a direct reading spectrometer to quickly detect chemical components, and perform online non-destructive testing of cracks on the modified materials. Make sure the material is in good condition before heat treatment.
When the parts are tempered, carbon potential control, temperature control and process parameter control are used. The quenching medium is subjected to temperature control and cold speed monitoring, quenching and tempering hardness measurement, quenching and tempering metallographic determination, and quantitative determination if necessary. After quenching and tempering, 100% of the fluorescent magnetic powder is non-destructively tested, 100% of the hardness of the part and Intrinsic defect sorting and mechanical properties determination.
3 technology upgrades
In summary, there is a certain gap between the heat treatment technology and equipment of Dongfeng Motor's high-strength fasteners compared with foreign advanced heat treatment technology and equipment. In order to further develop domestic and foreign markets, Dongfeng Auto Fasteners Co., Ltd. has made great efforts in improving heat treatment technology.
3.1 Implemented Technology Enhancement Project
3.1.1 Update the metallographic microscope imported from the metallographic microscope, and purchase the metallographic map analysis software system. Therefore, the metallographic analysis and failure analysis of high-strength fasteners have reached a quantitative level, and the ferrite content level standard in the high-strength fastener quenched and tempered structure has been established. Further research is needed to ensure deep in the future. The intrinsic quality of the hierarchy is in a controlled state.
3.1.2 Replace the quenching oil and purchase the fast quenching oil to realize the high-strength fasteners of the medium carbon alloy steel. The quenching quality is obviously improved, which is reflected in the high quenching hardness and uniformity, the hardened layer is deepened, the ferrite in the core is obviously reduced, the mechanical properties are stable, and there is no part repair and waste.
3.1.3 Use water-soluble quenching medium to purchase water-soluble quenching medium instead of alkaline water for quenching and quenching of some medium carbon steel high-strength fasteners. The quenching effect is very good, no quenching and cracking phenomenon is found, the quenching hardness is uniform, and the quenching quality is good. Instead of the ordinary quenching oil, it can solve the problem that the quenching oil is easy to produce oil stain sintering and affect the quality of the subsequent plating. (Use this 8-20 quenching agent)
3.2 Technology improvement project
3.2.1 Purchase of quenching and tempering line Dongfeng Motor's high-strength fastener heat treatment technology is focused on heat treatment and quenching and tempering lines. Two heat treatment and quenching lines designed and manufactured by a Taiwanese company with strong strength and high technical level have been selected through bidding. The line has the functions of computer control of atmosphere, temperature and process parameters, equipment fault alarm and display, equipment sealing performance, high degree of automation, and the technical level has reached the advanced level in foreign countries.
3.2.2 Complete replacement of quenching medium and addition of quenching medium tester Two new quenching and tempering lines are oil furnace, medium is quick quenching oil, and the other is water furnace. The medium is made of water-soluble quenching medium. In order to ensure stable quenching and cooling performance, only one imported advanced quenching medium is measured, so that the quenching medium can be monitored regularly to ensure the quenching quality.
3.2.3 Protective atmosphere selection Because the Rx gas quality is better than the protective gas produced by methanol cracking - some, the 12.9 Dongfeng high-strength fastener uses Rx gas as the shielding gas to improve and ensure the high-strength tightening of 12.9 The quality of the pieces. The water furnace uses methanol cracking gas as the shielding gas. When a protective gas has a gas supply problem, the two quenching and tempering lines can also share a kind of shielding gas, which is very convenient.
3.2.4 Imported vortex sorter Using the advanced vortex sorter, 100% of the factory's Dongfeng auto high-strength fasteners are in-line tested to ensure the quality of the products.
3.2.5 Dongfeng Motor's high-strength fastener materials have been tested on new cold-formed spheroidized steel, which has a good performance in cold forming and heat treatment. It is also very good to try on the Cummins engine. The use of spheroidized steel can eliminate the annealing process, avoid environmental pollution caused by oil furnace annealing, decarburization, and reduce mold consumption and quenching cracking.
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