Mechanism and prevention of black spot defect on cast iron cylinder liner of diesel engine

Through comprehensive analysis of the test results, it can be seen that:

(1) The slight difference of graphite morphology in some places will not seriously affect the performance of cylinder liner. In essence, the casting black spot defect is the uneven section structure of the cylinder liner. The coarse and fine inclusions and irregular distribution of the fracture grains can be seen in the macro. In the micro, the overall structure uniformity of the cylinder liner is poor. The graphite morphology of individual parts is different from that of the normal parts, and the hardness of each part is uneven. However, from the graphite morphology inspection, the graphite morphology at the black spot is actually better, and the proportion of type a graphite is higher. According to the national standard requirements of vanadium titanium cylinder liner, the graphite forms a, B, C and D are qualified.

(2) In order to solve the problem of black spots, we must start from many aspects to reduce the influence on the microstructure segregation in the solidification process of cylinder liner as much as possible. From the factors affecting the structural uniformity and hardness difference of cylinder liner, the above test factors, including inoculant, pouring temperature, pouring speed, metal mold temperature and cooling process, will essentially affect the structural segregation of cylinder liner.

(3) Air cooling and forced cooling can effectively control casting black spot defects, but both belong to a means different from conventional production process. From the actual production, the above measures are not desirable. During air cooling, the cooling solidification cycle of molten iron is long and the production rhythm is slow, which seriously reduces the production efficiency; When forced water cooling, the cooling time is short and the production rhythm is faster, but there is a great risk of deterioration of graphite morphology of cylinder liner, extremely low die temperature, serious white mouth and so on. Therefore, appropriate process measures should be taken in combination with the above theoretical analysis to reduce organizational segregation by appropriate means while taking into account production efficiency.

(4) The abnormal vibration of centrifuge spindle is the main cause of casting black spot defect. In the production process, sample and check the cylinder liner section produced by each centrifuge one by one, and the products with casting black spot defects can be found in time. Timely repair the centrifuge to eliminate the abnormal vibration of its main shaft, and the casting black spot defect will be avoided or even eliminated.

Through the test results and comprehensive analysis, the following control measures for casting black spot defects can be put forward:

(1) In production, strictly implement the process code and standardized operation procedures to ensure that the pouring temperature reaches more than 1320 ℃ and reduce the problem of insufficient melting of inoculant due to low temperature.

(2) In the smelting process, after the addition of various alloy elements, sufficient refining time in the furnace must be ensured to ensure the full diffusion of alloy elements in molten iron.

(3) During the first article inspection of each shift, check the section of the cylinder liner produced by each centrifuge. If any abnormality is found in the section, stop the machine for maintenance and eliminate the abnormal vibration of the centrifuge before continuing production.

(4) With reference to the classification standard of casting cloud spot defects in China’s shipbuilding industry standard CB / T 3903-1999 metallographic inspection of centrifugal casting cylinder liner of medium and high power diesel engines, and in combination with product structure and application, the evaluation standard of casting black spot defects is formulated to reduce unnecessary scrapping and loss. After the above control measures were implemented, good results were achieved, the casting black spot defects decreased significantly, and the casting scrap rate of cylinder liner returned to normal.

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