The correct heat treatment process must have the corresponding heat treatment equipment to ensure that the Shanghai brand car, Shanghai jeep and truck rear axleproduced by Shanghai automobile gear factory are often found to have rough tooth surface, early wear and short service life, which is far from the quality of the hyperbolic gear of the rear car of Changchun Automobile Factory Red Flag car. The reason is that kerosene or triethanolamine is dripped into the gears produced by Shanghai automobile gear factory, and gas carburizing or cyanate liquid carburizing is carried out in the well type gas carburizing furnace imitating the Soviet Union’s 1940s. Due to the aging of the equipment, the process can not be guaranteed, resulting in decarburization layer on the surface of the gears after carburizing and quenching, as well as internal oxidation black structure (i.e. non martensitic structure), Generally, after carburizing, the decarburization layer exceeds the allowable range of 0.01mm, but reaches 0.03mm, and the decarburization layer is uneven. These decarburized layers and non martensitic structures are the important reasons for the early surface roughening, peeling and abrasion of spiral bevel gears, and accelerating the early damage of gears. Changchun No.1 Automobile factory adopts the box furnace with endothermic controllable atmosphere seal to carburize and quench the gears of Hongqi car. Due to the good sealing performance of the equipment and the protection of controllable atmosphere, the internal oxidation structure and decarburization layer of carburized gears are prevented to some extent, and the service life of spiral bevel gears is improved.
Although our domestic Changchun automobile factory and Luoyang Tractor Factory adopt endothermic controlled atmosphere carburizing, the service life of automobile tractor gear is still not up to the advanced level of foreign countries. Of course, there are many reasons, but from the point of view of heat treatment factors, we don’t have a good command of the endothermic controlled atmosphere carburizing furnace. In addition to the carbon potential control of the endothermic gas generator, strict carbon potential control is also carried out for each section of the carburizing furnace in foreign countries. Because our continuous gas carburizing furnace is through type, the carbon potential control can’t be carried out because the atmosphere in each section of the furnace interferes with each other, The uncontrollable atmosphere has been formed, which has affected the improvement of gear life of automobile and tractor. For carburized gears, the main reason for tooth surface wear is that the wear resistance of tooth surface is not enough. The reduction of carbon concentration on the surface obviously reduces the wear resistance of the gear surface, and the surface hardness also decreases, which makes the tooth surface prone to plastic deformation, causes the gear to twist and produce incorrect engagement, thus increasing the wear degree. The first automobile factory in Changchun once stipulated that the carbon content of the gear surface was 0.8 ~ 1.05%, but there was a certain deviation in the actual carbon content of the gear surface due to the lack of precise control of the carbon potential in the furnace. The carbon content on the surface of gears is 0.8-0.9% according to the regulations of the United States and West Germany. Due to the precise control of carbon potential in the furnace, it is reported that the carbon concentration on the gear surface can be precisely controlled within ± 0.05% C, thus ensuring that the carbon concentration on the gear surface is in a very precise range.
It can also be found from the metallographic structure of gears in West Germany, America and Japan that the carbon concentration on the tooth surface is about 0.9% C, with little fluctuation, and the concentration gradient of carburized layer is very gentle, without a sharp drop transition zone like that of domestic gears. It is not difficult to see that in order to improve the service life of carburized gears for automobiles and tractors, it is an important step to continuously reform the old heat treatment furnace in the heat treatment workshop of domestic gear plants and adopt the advanced carburizing furnace with controllable atmosphere, so as to truly control the carbon content on the surface of gears accurately. Recently, Changchun No.1 Automobile factory changed the carburizing process ofgear into carbonitriding process. The service temperature decreased from 940 ℃ to 870 ℃, which not only shortened the production cycle, increased the labor productivity, reduced the production cost, but also reduced the gear deformation. Due to the formation of nitride on the gear surface, the contact fatigue life of the gear surface increased by 48%, and the bending strength increased by 34%, The impact toughness is increased by 30%, and the gear can still be used normally after 200000 km of driving test results. This is three times longer than the service life of the gas carburizing gears of 70000-80000km in the past. That is to say, at present, the use of carbonitriding technology has made three gears on the top of one gear be used. If the service life of automobile and tractor gears in China can be tripled, we can save 100000 tons of alloy steel and produce 160000 fewer gears, so this is a very meaningful work, Enough attention should be paid to it.
Based on the investigation of heat treatment process and equipment of gear plants in some areas of China, it is found that the production of heat treatment of gears in China can not keep up with the development of automobile and tractor industries. Many agricultural machinery factories and auto parts factories still adopt solid carburizing process without metallographic examination and poor production management, which affects the improvement of product quality.
In the light of small and medium-sized gear plants, well gas carburizing furnace and cyanate liquid carburizing process are still used for gear heat treatment. Due to the old equipment and backward technology, the metallography and deformation of the gear after heat treatment can not be guaranteed, which shows in the short service life. Therefore, measures must be taken to eliminate the backward solid carburizing process and the toxic liquid carburizing process with three wastes pollution. The well type gas rest carburizing furnace used in most factories at present should be reformed appropriately, the sealing of furnace cover and fan shaft should be strengthened, the temperature in the upper and lower areas of the furnace should be uniform, and the circulation mixing of the circulation fan should be strengthened. At the same time, the drop controlled atmosphere is used instead of kerosene drop carburizing, so that the carbon potential of the atmosphere in the furnace can be controlled to ensure the product quality.
Factories with conditions shall strive to develop controllable atmosphere equipment. For factories that have used controllable atmosphere at present, it is necessary to strengthen experimental research on the atmosphere in the control furnace, so as to make controllable atmosphere truly controllable, and formulate new metallographic technical standards for gear heat treatment. The carburizing conditions and carbon concentration gradient standards should be clearly defined to give full play to the role of controllable atmosphere. Some attention should be paid to the carbonitriding process and the vacuum carburizing process.
Chairman Mao taught us that “all products should not only be large in quantity, but also be of good quality, durable and durable”. The quality issue is a political issue and a line issue. Therefore, we must fully understand the importance of heat treatment, grasp the heat treatment work, reform the backward equipment, develop new equipment, adopt new heat treatment process, grasp the quality of gears as soon as possible, and make contributions to the development of China’s automobile and tractor industries.