Formation mechanism of black spot defect on cast iron cylinder liner of diesel engine

The cast iron cylinder liner of diesel engine is generally produced by metal mold centrifugal casting. In production, it is found that the brightness of individual cylinder liners is uneven in processing; These parts with uneven brightness are randomly distributed on the inner and outer wall surfaces of the cylinder liner, and there are irregular boundaries in the bright and dark parts. This casting defect is called cloud spot defect. Casting cloud spot defect is common in centrifugal casting of cylinder liner, especially in large cylinder liner. In China’s shipbuilding industry standard CB / T 3903-1999 metallographic inspection of centrifugal cast cylinder liner of medium and high power diesel engine, there are special definitions and classification standards for cloud spot segregation and white spot structure.

In order to improve the appearance quality of cylinder liner, increase the feed speed and speed during machining, and the cylinder liner looks brighter as a whole. During the processing, a phenomenon different from the cloud spot structure gradually appeared: some cylinder liners appeared irregular block dark areas after the outer wall processing, resulting in an increase in the scrap rate of about 2 percentage points. Except that the color of the block area is darker than the normal part of the cylinder liner, which is different from the casting cloud spot defect, the other morphological characteristics are consistent with the casting cloud spot defect. In order to distinguish, this casting defect is called black spot. Figure 1 shows the normal surface of the cylinder liner appearance and the defects of cloud spots and cast black spots.

In terms of appearance, cloud spots (white spots) are shown in the irregular white bright areas on the normal gray color of the cylinder liner surface, while black spots are on the contrary, they are shown in the irregular gray dark areas on the normal bright color of the cylinder liner surface. Peer enterprises have carried out relevant research on cylinder liner cloud spot and casting black spot defects. Zhao Minggang pointed out that the uneven structure and composition are the important reasons for the formation of black spots. At the same time, he put forward some measures and suggestions to solve the casting defects of black spots: strengthening the sequential solidification from outside to inside, improving the carbon silicon equivalent and manganese content, controlling raw materials and reducing the influence of heredity. Tao Xiaoming described the macroscopic and microscopic characteristics of cloud spot casting defects of large marine cylinder liner. It is considered that the cloud spot is mainly caused by graphite distortion caused by large end face of large diameter cylinder liner, slow cooling speed and long eutectic residence time; In addition, chemical composition segregation, excessive or uneven distribution of residual inoculant and process conditions are also important factors affecting cloud spots. It is considered that the solidification condition of cylinder liner should be improved and reasonable inoculation treatment process should be adopted to eliminate graphite distortion; Strengthen desulfurization and charge management in smelting process; The mold coating shall be dried, etc. Wang Mingang and Zhao Shixiong [4] analyzed the causes of casting defects of white spots and clouds on the cylinder liner of a large marine diesel engine, and pointed out that the white spots were formed by the precipitation of austenite dendrites from the surrounding molten iron and the segregation of coarse austenite from the supercooling of molten iron when the iron beans gathered at the bottom of the pouring groove fell into the cavity during the pouring process, The cloud spot is formed by the abnormal structure caused by the segregation and uneven distribution of precipitated graphite; The occurrence of white spots can be significantly reduced by reducing Mn, P, s and other elements prone to segregation, increasing infinite solid solution elements such as Cu and Ni, changing bottom leakage centrifugal pouring to side pouring and adding 0.2% 75fesi during pouring; At the same time, the tensile strength of the cylinder liner body is increased by about 20 ~ 30 MPa, and the coarse graphite and coarse phosphorus eutectic in the metallographic structure are significantly reduced. Zhao Yuezhong pointed out that the tensile strength of cylinder liner with cloud spot segregation will be lower than that of normal cylinder liner without casting defects, and the reduction range is directly related to the degree of cloud spot segregation; Compared with CS1 and CS2 segregation areas, the hardness value has little difference from the normal value, but compared with CS3 and CS4 segregation areas, the hardness difference is more than hb40, or even hb100.

Since the hardness value is an index reflecting the wear resistance of the cylinder liner, if the hardness difference of the inner hole of the cylinder liner is large, there will be eccentric wear, abnormal wear or cylinder pulling, so the cloud spot segregation must be controlled. Cloud spot segregation has no obvious difference on its corrosion resistance. Cheng Xianjun] pointed out that for the large marine cylinder liner produced by centrifugal casting, due to its large wall thickness, the cylinder liner surface is easy to produce patterns, and the pattern casting defects are caused by phosphorus eutectic segregation. In addition, Deng Han, Yang Zhigang and Wang Rui also pointed out that the long cooling and solidification time caused by component segregation and thick local structure will promote the occurrence of component segregation in ductile iron. Park et al. Also pointed out in the research on the segregation of Mn in gray cast iron that there is negative segregation of Mn in cast iron along the thickness direction, which has a certain impact on the structural uniformity of castings. Wang Ling, Dong Jianxin and Yang Chunjun studied the formation mechanism of black spot casting defects in large-size castings and pointed out that the black spot is the channel segregation formed by the convection of solute rich liquid in the paste solidification zone during the cooling and solidification of liquid metal. From the above research on the macro and micro morphological characteristics of cloud spots (white spots) and black spots, patterns and gray spots, it can be seen that cloud spots (white spots) and black spots are essentially caused by the non-uniformity and segregation of the microstructure of the cylinder liner. Therefore, in essence, black spot casting defect is essentially a kind of cloud spot casting defect.

However, the above research has not conducted a systematic comparative study on the causes of cloud spot (white spot) and black spot casting defects, which is not enough to guide the actual production. Therefore, taking the black spot of metal mold centrifugal casting vanadium titanium alloy gray cast iron cylinder liner as the research object, through the method of comparative experiment, the possible causes of black spot casting defects of large and medium-sized cylinder liner are systematically studied, the main causes of black spot casting defects are locked, and effective prevention and control measures are put forward, It provides theoretical basis and technical reference for preventing and controlling black spot casting defects of diesel engine cast iron cylinder liner.

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