With the continuous development of marine low-speed diesel engine in the technical direction of high intensity, ultra-long stroke, ultra-high burst pressure and effective pressure, high output power and so on, the performance requirements of cylinder liner materials are also increasing. The traditional gray cast iron cylinder liner has low tensile strength and thermal fatigue resistance due to the notch effect of flake graphite tip, which can not meet the performance requirements of new diesel engine. Compared with gray cast iron, vermicular cast iron has higher strength, excellent heat resistance, thermal conductivity, fatigue resistance and dimensional stability. It can meet the requirements of cylinder liner for material mechanical properties, thermal conductivity and friction and wear resistance, so as to deal with the working conditions of high temperature and high pressure and reciprocating motion of friction pair. For vermicular graphite cast iron cylinder liner, only by obtaining high vermicular rate can its performance advantage be brought into play. However, in the production process of vermicular cast iron, the residual range of vermicular elements (re, Mg) in molten iron is narrow, and its fluctuation has a significant impact on the solidification process of vermicular cast iron, which increases the difficulty of obtaining high vermicular rate.
(1) The peristaltic treatment makes the eutectic point of molten iron move to the right and the primary crystallization temperature increase. 0.019% ~ 0.023% RE residue and 0.006% ~ 0.011% Mg residue make the primary crystallization temperature of molten iron solidify from 1152 7 ℃ rises to 1 163.2 ~ 1 166.6 ℃.
(2) There is a good corresponding relationship between eutectic re glow temperature and graphite creep rate. With the increase of eutectic re glow temperature, graphite creep rate increases. When the re glow temperature is lower than 5 ℃ and the creep rate is less than 50%; When the re glow temperature is higher than 5 ℃, the creep rate is greater than 50%.
(3) Properly increasing the ratio of re to Mg in Vermicular elements is conducive to obtain high vermicular rate. After the RE content increases, high-efficiency inoculant should be selected to inhibit the white tendency of vermicular molten iron and prevent free cementite in the matrix structure.
The cooling curve of liquid iron solidification process is recorded by thermal analysis technology. Based on the characteristic value of the cooling curve, the solidification process and graphite morphology of vermicular graphite cast iron can be effectively studied, which provides an experimental basis for Vermicular and inoculation treatment of liquid iron. Therefore, different vermiculation and inoculation treatments were carried out on the molten iron. The effects of the content of vermicular elements and the type of inoculant on the characteristic values of the cooling curve and the graphite morphology were studied through the solidification cooling curve and microstructure analysis, and the relationship between the graphite morphology and the characteristic values of the cooling curve was established.