Abstract : Through a variety of comparative process experiments with thousands of crankshafts and a large number of investigations in the same industry in the country, the cause of the crankshaft's third main-neck runout was found, and a solution was proposed. The crankshaft is an important part of the diesel engine, it transmits the force and moment necessary for the movement of the car. As the crankshaft in the assembly process, with the axis of the interaction with each other there is a certain requirement, so the higher requirements of the main shaft runout, so as to avoid crankshaft in the engine running "hold" or "burn tile" and other quality accidents. 1 Causes of high runaway rate after crankshaft nitriding In order to find out the real cause of the high jump rate of the crankshaft nitriding in our factory and solve the problem, we have made several comparative experiments of thousands of crankshafts and a large number of research work in the same industry across the country since the second half of 1997. Through the comprehensive analysis of the results of these process experiments and the survey results, the real cause of the crankshaft runout error in our factory is that there is a large casting internal stress before the crankshaft. 2 Analysis of Main Causes of High Inner Stress before Nitriding of Crankshaft The spindle hoop runout and production process of our factory crankshaft and xichai crankshaft are shown in Table 1 and Table 2. Table 1 Comparison of spindle neck runout 3 Measures Based on the actual and theoretical analysis of the process of our plant, we believe that due to the uneven contact between the cooling medium and the workpiece during the normal cooling phase of the crankshaft, the cooling rate of the parts in the cooling process is not even, and the internal stress of the casting is relatively large; Qualitative heating is not uniform, it is difficult to carry out strict temperature control, but also will cause the crankshaft qualitative out of control, can not completely eliminate the internal stress of the casting, which is the root cause of the third main spindle of our factory crankshaft jump. Square Frame Bending Machine,Square Frame Pipe Bending Machine,Aluminum Square Frame Bending Machine,Square Frame Roll Bending Machine JINAN SUCCESS CNC MACHINE , https://www.roll-bending.com
Keywords: crankshaft third main journal runout; casting internal stress; normalizing; qualitative
Classification number: TG162.7 Document code: B
Article ID: 1001-2265 (2000) 02-0039-03
The crankshaft of our factory (Yunnan Internal Combustion Engine Plant) has long been subject to high quality (beyond 20% to 40%) high jump ratio (beyond 0.08mm percentage) after nitriding. At the beginning of the period, most people insisted that the main cause of this quality problem was in the nitriding or machining phase of the crankshaft. However, in terms of machining and nitriding, we have improved the processing requirements (the nitriding of the third main journal of the crankshaft before nitriding is controlled within 0.05 mm with domestic equipment, and the foreign equipment is controlled within 0.03 mm), and the nitriding process is strictly controlled. Parameters, strict check of temperature control instruments for nitridation furnaces, and strict disciplinary inspections did not resolve this quality issue.
The internal stress of the ductile iron is greater than the internal stress of the gray iron. Once the crankshaft is nitrided, the nitriding temperature is 560-600°C (this nitriding temperature zone is within the qualitative temperature range of 530-600°C for releasing the internal stress of the crankshaft). It will be released and redistributed, resulting in a deformation of the crankshaft, that is, crankshaft runout after nitriding. The nitriding process is only a manifestation of the internal stress of the casting. Excessive jerk after nitriding is only a manifestation of the redistribution of stress in the casting during the nitriding stage. Therefore, the nitriding process is just a carrier or an external factor of the runout after the nitriding of the crankshaft, and the fundamental cause of the runaway after nitriding of the crankshaft is that there is a large internal stress of the casting before the nitriding of the crankshaft. .
(≤0.08mm
Percentage)
(>0.08mm
Percentage) Overshoot Maximum
(mm) Excessive crankshaft treatment method Our factory crankshaft ≤0.05mm ≤0.08mm 60%～80% 20%～40% 0.85 Thermal school xichai crankshaft ≤0.04mm ≤0.08mm 95% 5% 0.25 Thermal school
Table 2 Comparison of route
(1) Normalized crankshaft cooling unevenness The normalizing cooling method of the crankshaft of our factory is fog cooling. The spray source is single and sprays in a fixed direction. The crankshaft is also fixedly placed on the ground. This kind of spraying method can easily cause the cooling of the crankshaft toward the side of the spray source when cooling, and the cooling of the crankshaft when the side facing away from the spray source is slow. Because the normal cooling of the crankshaft is not uniform, hardness unevenness and pearlite content non-uniformity are generated, resulting in greater internal stress of the casting.
1 In daily crankshaft production, whether or not it is qualitative, its body hardness is in the range of HB170-250, and both are low or the drawing requirements cannot be met (drawing requirements are HB240-320). In the sampling inspection, it has been found many times that the same crankshaft has a phenomenon of serious uneven hardness, and some crankshafts only have a hardness difference of HB60 to 80 on the large end face;
2 The pearlite content was also found to be seriously uneven. For example, in a single fatigue test, it was found that the pearlite content of the same crankshaft differed by as much as 30%.
(2) Non-uniform heating of the crankshaft's qualitative equipment The character of the crankshaft of our factory is that there is no rack code on the bench, and the reverberation furnace is used for direct heating. There is only one temperature control point. The temperature display value cannot represent the actual temperature in the crankshaft stack. Qualitative overburning has also occurred several times in practice. Because the temperature cannot be accurately controlled, the elimination of internal stress in this process is in an uncontrollable state.
The following is a comparative analysis of the normalizing equipment and qualitative equipment of the Xichai crankshaft, and the normalizing equipment and qualitative equipment of the crankshaft of our company.
1 Analysis of the Normalizing Equipment and Qualitative Equipment of the Xichai Crankshaft The xiecha crankshaft has a low jump rate after nitriding, which is closely related to the normalizing equipment and qualitative equipment used. The normalizing equipment it uses is a continuous normal furnace. The furnace is divided into four temperature control zones. The workpiece is heated evenly and the temperature control is strict. During the cooling process after heating, the crankshaft is hung in the furnace and can be sprayed or blown around. The entire cooling process is very uniform. In order to eliminate the internal stress and machining stress of castings, Xichai uses a dedicated continuous resistance furnace in the qualitative processing after rough machining. The furnace is divided into six temperature control zones and the temperature is 600°C. Each of the six qualitative crankshafts is divided into two layers (a few), which are fed into the furnace by a trolley (the production cycle is to run a car every half hour). The characteristics of the furnace are: uniform heating temperature uniformity, strict temperature control, the workpiece is heated evenly, can completely reduce or eliminate the casting internal stress and machining stress, in order to achieve the purpose of qualitative.
2 Analysis of the Normalizing Equipment and Qualitative Equipment of the Crankshaft of Our Factory The normalizing equipment of our crankshaft has been heated by the electric furnace since May of this year, and the heating is relatively uniform. Qualitative equipment is a reverberatory furnace. Its heating method is to burn the bituminous coal to heat it. The degree of uniformity of the heating depends largely on the man-made operation. The furnace of the reverberatory furnace is large, and a furnace can be used to characterize 800 to 900 crankshafts, but its temperature measurement system has only one temperature zone, and a thermocouple is used for temperature measurement. Therefore, such a temperature measurement system is used in such a reverberatory furnace. The measured temperature can only represent the temperature near the thermocouple in the furnace and cannot represent the heating temperature of the workpiece in the entire furnace. Obviously, difficult to control the temperature is an important cause of uneven heating. In addition, it is worth mentioning that the quality of the crankshaft of our factory is usually 800 to 900 pieces, and the placement of the crankshaft in the furnace is stacking, so that the workpiece is more unevenly heated when it is qualitative, and it also causes the crankshaft. In the hot state, mutual extrusion and other undesirable phenomena increase the deformation tendency of the crankshaft.
If our factory crankshaft production is still normalizing + qualitative + nitriding process. Then, to solve the problem of high jump rate after nitriding, and at the same time to meet the production plan of our factory, the key is to solve the following two problems:
(1) Solve the problem of crankshaft normalizing quality. While ensuring that the crankshaft's hardness and pearlite content meet the design requirements of the drawings, we should strive to make the crankshaft have a more uniform normal cooling process, so that the obtained hardness and pearlite content are also relatively Uniform, so that the crankshaft formed in the normalizing phase of the internal stress is smaller and more uniform distribution.
(2) Solve qualitative problem of crankshaft process Qualitative process is a key process to reduce or eliminate internal stress of casting. Due to the reasons on the equipment and the reasons for the placement of the crankshaft in the furnace, the crankshaft of our factory hardly played a "qualitative" role. Therefore, to solve the problem of qualitative quality of the crankshaft of our factory, we must start with the placement of qualitative equipment and workpieces in the furnace. Qualitative equipment must satisfy the uniform heating of the workpiece in the furnace, and the temperature control system can truly reflect the temperature variation of the workpiece in the furnace. With reference to the specific continuous resistance furnace used by Xichai, from the "thought that the temperature must be uniform, the workpiece must be heated uniformly, the placement must be reasonable, while improving the qualitative temperature to make it slightly higher than the nitriding temperature" Solve the selection of qualitative equipment. At this stage, it is necessary to change the way in which a large number of crankshafts are stacked and pressed against each other in the furnace. This can be considered from the tooling. In addition, under the condition that the qualitative equipment and tooling are solved, we believe that the qualitative process of the crankshaft is more appropriate after rough machining.
After identifying the cause of the high jump rate caused by the nitriding of our plant's crankshaft, we have passed the process experiment (4100QB copper-blind crankshaft adopts "air-cooled" normal cooling method and electric furnace tempering process under the condition of existing equipment. Solution) Seek effective solutions. The experimental idea of ​​the process is: first add the appropriate amount of copper to the crankshaft for alloying during casting, so that when forming the as-cast crankshaft, it can form a large amount of pearlite (about 70% to 75%), During the subsequent normalizing phase of the cooling process, relatively uniform cooling with a slower cooling rate and easier control was used. In this way, not only the pearlite content of the crankshaft structure is satisfied, but also the internal stress of the casting is formed during cooling of the normalized crankshaft. The internal stress of these smaller castings is subsequently controlled in the tempering of the electric furnace. Because the temperature is uniform and easy to control (600°C, slightly higher than the nitriding temperature of 590°C), the crankshafts in the tempering are isolated from each other without extrusion, and the internal stress can be fully released. Uniform redistribution, so as to achieve the purpose of eliminating residual internal casting stress. The final result of this process experiment was relatively satisfactory. After the nitridation of 146 experiments, the third main journal runout rate was 5.36%. This value is the most ideal one since the third main journal runout experiment. It solved the problem for me. The long-standing problem of the plant crankshaft has provided a powerful theoretical basis and experimental data.
In addition, the crankshaft is deformed due to the release and redistribution of stresses in the casting during the nitriding phase. After the crankshaft is deformed using a nitriding process (the nitriding process must be placed on the last step of the crankshaft machining process), permanent deformation will result because it cannot be corrected. Therefore, it is suggested that the strengthening treatment of the crankshaft fatigue strength be carried out using the medium frequency quenching + fillet rolling process, so that the deformation caused by the crankshaft in the preceding process can be properly corrected in the final refining stage. This process method has been widely used in some large automobile manufacturers such as FAW, Second Automobile, Shennong Automobile Factory and Tianjin Xiali Automobile Factory.
Also, for crankshafts that are below 0.30mm and that are already out of tolerance, we use thermal corrections before we purchase the qualitative tempering furnaces. The overheated crankshafts of these thermal schools are sampled for metallographic analysis and fatigue tests, all of which can meet the design technical requirements.
(Process experimental results and analysis omitted)