亚共晶高铬铸铁二次加热组织演变及触变挤压充型性能研究

【作者】 蒋文明；

【导师】 周荣；

【作者基本信息】 昆明理工大学 ， 材料加工工程， 2008， 硕士

【摘要】 本文采用倾斜板冷却体法制备亚共晶高铬铸铁半固态坯料,并对半固态坯料进行二次加热,然后进行触变挤压充型。考察了保温时间和重熔温度对坯料在二次加热过程中组织演变的影响规律。研究了保温时间和重熔温度对坯料充型性能的影响,充型件的质量、组织,并对充型件力学性能进行了初步分析。半固态坯料在半固态区域保温过程中,先共晶奥氏体晶粒经历了由球化到合并长大的循环演变过程,且保温前期循环时间要短于后期,保温后期晶粒形貌恶化程度较前期严重。随着重熔温度的升高,奥氏体晶粒球化和合并长大的速度由快转慢,晶粒恶化程度逐渐变小。当重熔温度过高时,晶粒粗化严重,试样氧化严重,并发生流淌现象。铸态试样组织为发达的枝晶网络结构,当重熔温度较低时,即使延长保温时间,枝晶组织也难以球化。提高重熔温度及延长保温时间,枝晶组织才能够发生熔断,并开始球化,但球化效果不理想,且晶粒粗化严重。半固态坯料较铸态坯料具有较好的充型性能,铸态坯料难以充型。当重熔温度较低时,半固态坯料难以完全充型;提高重熔温度,当保温时间较短时,坯料难以充满整个模具型腔;延长保温时间,坯料能够充满整个型腔,且充型件表面质量较好。当重熔温度过高时,坯料充型性能很好,但充型件表面质量较差。流变充型件内部质量不致密,充型件中部存在较多的集中缩孔;触变充型件内部质量较好,但有些工艺参数下的试样上部存在一些裂纹和缩松。触变充型过程中,晶粒被压碎,使得晶粒较充型前得到显著细化。试样轴向由上部到下部,晶粒尺寸不断减小,其中上部存在大量粗枝晶,中部晶粒圆整度较高,且大小分布均匀。试样横向从边部到中部,晶粒略有长大,但变化不明显。随着保温时间的延长,充型件组织中的晶粒尺寸有所增大。触变充型件较致密,且充型件组织细小,较流变充型件具有较高的力学性能。更多还原

【Abstract】 Semi-solid billets of hypoeutectic high chromium cast iron prepared by inclined cooling plate method were reheated, then thixotropic extruding filled. The influences of holding time and reheating temperature on microstructure of semi-solid billets and as-cast billets during reheating were investigated. The influences of holding time and reheating temperature on the filling property and microstructure and mechanical property of filled parts were studied.Semi-solid billets during the semi-solid interval, pro-eutectic austenite grains circulated from the spheroidizing to the merge growing process, and the cycle time was prolonged gradually, and the final period of exacerbation was more prominent than prior period. With the reheating temperature up, the speed of the spheroidizing and the merge growing of grains changed from quick to slow, and exacerbation changed small gradually. When the reheating temperature is enough excessive, the coarsing of grains was serious, and sample oxidized serious and flowed.As-cast billets possess dendritic structure, when the reheating temperature was lower, even if prolonged the holding time, the dendritic structure was also hard to spheroidized. Increasing the reheating temperature and prolonging the holding time, the dendritic structure just began to burn out and spheroidized, but the spheroidized effect was not ideal, meanwhile the coarsing of grains was serious.Semi-solid billets have better filling property than as-cast billets, and as-cast billets are difficult to filling. When the reheating temperature is low, billets can not filling wholly. Increasing the reheating temperature, when the holding time is short, and billets are difficult to filling wholly. Prolonging the holding time, reheating of billets is more homogeneous, billets can filled wholly with better surface quality. When the reheating temperature is too higher, although billets can filling wholly, the surface quality of filling parts is lower.The internal soundness of rheologic filled parts is unsound, and has much central shrinkage cavity in the middle of filled parts. The thixotropic filled parts have better internal soundness, but some filled parts have shrinkage porosity and crack upside the parts.The microstructure of thixotropic filled parts has much refinement than before filled parts, because grains were crushed. Axial direction of parts from top to down, the size of grains become fine gradually, and the grains in the upper position of sample has much dendritic, and the roundness of grains in the middle of parts was high, and grains distributed evenly. Cross direction of parts from margin to center, grains grow in some sort, but the change was not obvious. Prolonging the holding time, the microstructure of filled parts was a little increase.The thixotropic filled parts have high mechanical property than rheologic filled parts, and filled parts have higher density and fine grains.更多还原