【作者】 李青春；

【导师】 翟启杰； 常国威；

【作者基本信息】 上海大学 ， 钢铁冶金， 2008， 博士

【摘要】 针对水冷金属型离心铸造法生产球墨铸铁管高温退火中存在的高温耗能、易变形等问题,本文将脉冲电流处理技术用来加速球墨铸铁的固态石墨化过程。首先从理论上探讨了脉冲电流作用时球墨铸铁的固态石墨化行为,然后通过实验验证了理论研究结果的正确性,从而系统地阐述了脉冲电流作用下球墨铸铁的固态石墨化机理。理论研究表明,对铸态球墨铸铁进行脉冲电流处理,在渗碳体周围产生的位错塞积使其自由能增加而稳定性降低,渗碳体石墨化的驱动力增加,为渗碳体的快速石墨化提供了有利的热力学条件。本文建立了电流密度与渗碳体分解速率和石墨形核率的理论关系式,表明脉冲电流密度增加,渗碳体的石墨化进程加快。脉冲电流通过增加渗碳体的不稳定性系数和时间指数、降低碳原子在奥氏体中的扩散激活能来提高渗碳体的分解速率,通过降低石墨的形核势垒、缩短形核孕育期、增加石墨的形核位置来提高石墨的形核率,通过增加扩散前置系数、降低扩散激活能来提高碳原子在奥氏体中的扩散系数。石墨数量的增加缩短了碳在奥氏体内的扩散距离,促使碳原子快速转变成石墨,从而加速渗碳体的石墨化过程。实验发现,与未经脉冲电流处理相比,在920℃保温3min的同时施加300V、15Hz、3min的脉冲电流处理后,组织中渗碳体的含量降低了11%,新生石墨的数量增加了2%,新生石墨主要在奥氏体晶界形核并长成球状。室温下施加j₍（max）≈1.67kA·mm^-2的脉冲电流处理后,组织中渗碳体的含量比未加脉冲电流处理时降低了12%,新生石墨的数量比未加脉冲电流处理时增加了3%,新生石墨主要在渗碳体附近形核并长成球状。这一结果表明,无论是在高温还是在室温对球墨铸铁施加脉冲电流,都能加速渗碳体的石墨化过程。实验结果验证了脉冲电流可加速球墨铸铁固态石墨化过程的理论分析。采用电子探针分析表明,脉冲电流处理后奥氏体基体中碳的分布变得更加均匀,证实了脉冲电流可加强碳原子在奥氏体内的扩散能力,与理论分析结果一致。本文系统研究了不同脉冲电流处理参数下球墨铸铁的固态石墨化过程。结果表明,脉冲电流作用下渗碳体的石墨化过程必须有温度做保证才能发挥电流的辅助作用。在高温施加低密度脉冲电流或在室温进行高密度脉冲电流处理,随着电流密度的增大、脉冲电流频率的加快和脉冲处理时间的延长,渗碳体的石墨化进程加快。在电压为3000V、电容为800μF的高密度脉冲电流处理下,球墨铸铁在13s内温度达到1100℃,渗碳体实现了快速石墨化。当球墨铸铁在920℃保温3min的同时施加参数为800V、20Hz、3min的脉冲电流时,渗碳体的石墨化过程基本完成。与完全消除渗碳体的正常高温石墨化相比,脉冲电流处理下,渗碳体石墨化的时间缩短了7min,温度降低了60℃。本文研究了脉冲电流作用下球墨铸铁的基体组织转变行为。结果发现,在球墨铸铁的高温石墨化中施加脉冲电流使空冷时铁素体的转变量增多,珠光体的量减少,珠光体的片层间距减小。本文的研究工作为应用脉冲电流促进球墨铸铁固态石墨化提供了理论和实验依据。更多还原

【Abstract】 To the spherical graphite iron pipe made by hydrocooling metallic centrifugal casting, the high-temperature annealing is employed to remove the cemenite of the as-cast pipes, which leads to high energy consumption and the pipe distortion. In the paper, the pulse current treating technique was designed to accelerate the solid-state graphitization of spherical graphite iron. Firstly, the solid-state graphitization of spherical graphite iron by electropulsing was studied by the way of theory. The vality of theoretical research results was verified by the way of experimental technique. The mechanism of solid-state graphitization of spherical graphite iron under the effect of pulse current was formulated systematically.The theoretical research results show that the free energy of cementite increases by the pulse current due to the dislocation pile-up around the cementite, which makes the stability of the cementite reduced. The driving force for the graphitization of cementite by electropulsing increase, which create favorable thermodynamic condition for the quick graphitization of cementite. In the paper, the theoretical relationship between the dissolution rate of cementite, nucleation rate of graphite and the pulse current density was established. With the increase of pulse current density, the graphitization of cementite was accelerated. The dissolution rate of cementite was increased by increasing the unstability coefficient and time exponent, decreasing the diffusion activation energy. The nucleation rate of the graphite is increased by reducing the potential barrier of the graphite nucleation, shortening the incubation time of the graphite nucleation and increasing the nucleation site of the graphite. The diffusion coefficient of carbon increases by increasing diffusion preposition coefficient and reducing diffusion activation energy by the pulse current. The increased graphites shorten the diffusion distance of carbon in austenite and the carbon atom transform to the graphite quickly. So the graphitization of cementite is accelerated by the pulse current.The experimental results show that the content of cementite is decreased by 11% and the content of neonatal graphite is increased by 2% when the pulse current with the parameter of 300V、15Hz、3min was employed at 920°C compared with the unelectropulsing treating. The neonatal graphite nucleates at the austenite grain boundary. The content of cementite is decreased by 12% and the content of neonatal graphite is increased by 3% when the pulse current with j_max≈1.67kAmm^-2 was employed at room-temperature compared with the unelectropulsing treating. The neonatal graphite nucleates near the cementite. These results showed that the graphitization of cementite is accelerated no matter the pulse current is employed at high temperature or at room temperature, which proves the theoretical analysis result that the pulse current can accelerate the solid state graphitization in spherical graphite iron. The electron probe analysis showed that the distribution of carbon becomes more homogeneous in the austenite after electropulsing treating, which declares that the pulse current can accelerate the carbon diffusion ability in the austenite. The experimental results agree well with the theoretical analysis results.The solid-state graphitization by the pulse current with different parameter was analysed theoretically. The results showed that secondary action of the pulse current to accelerate the graphitization of cementite can be displayed when the temperature is high enough. When the high density pulse current was applied at room-temperature and low density pulse current was employed at high temperature, with the increase of pulse current density, pulse current frequency and pulse current treating time, the graphitization of cementite is accelerated. The quick graphitization of cementite can be realized when the sample was heated to 1100℃in 13s by the pulse current with 3000V、800μF. The graphitization of cementite was finished basically when the pulse current with 800V、20Hz was employed at 920℃for 3 min. Compared with the normal graphitization for removing cementite completely, the time of high temperature graphitization was shortened about 7min, the temperature was decreased about 60℃.In the paper, the matrix structure transition of spherical graphite iron by the pulse current was studied. The results showed that the content of ferrite was increased, the content of perlite was decreased and the lamellar distance of perlite was decreased when the pulse current was employed during high temperature graphitization.The research work creates theoretical and experimental evidence for using electropulsing to accelerate solid state phase graphitization of spherical graphite iron.更多还原