Development of Bainitic Coral Graphite Cast Iron Dies for Plastics Injection

本课题旨在研制适用于注塑模具的贝氏体珊瑚石墨铸铁。为了弄清细小过冷石墨即珊瑚石墨形成的条件，首先测定了含钛铸铁和真空处理铸铁单向凝固时石墨-石墨共晶凝固前沿的温度。研究了氧、氮对石墨组织的影响，并对含镍、铜的珊瑚石墨二次铸铁进行了等温淬火，测试了其力学性能。结果表明：1）随着生长速率的提高，石墨的形貌由粗大型向细小过冷型转变。2）珊瑚石墨的形成并不一定需要大的过冷度; 3）细珊瑚石墨结构试样的溶质氧和溶质氮含量低于粗石墨结构试样。因此，钛的加入或真空处理促进了石墨组织的细化，其原因是由于熔体中氧和氮的还原。4）含镍、铜的珊瑚石墨铸铁经等温淬火后获得贝氏体组织。试验结果表明，该贝氏体珊瑚石墨铸铁具有良好的力学性能，可用于塑料注射模具。

The aim of this project is to develop bainitic coral graphite cast iron which is suitable for the plastics injection dies. In order to make clear the condition forming fine undercooled graphite, i.e. coral graphite, firstly the temperature of the solidification front of austenite-graphite eutectic was measured in Ti-bearing cast iron or vacuum-treated cast iron solidified unidirectionally. And the effect of oxygen and nitrogen on the graphite structure was investigated.Secondary, coral graphite cast irons containing nickel and copper were austempered and the mechani-cal properties of the irons were examined. The results obtained are summarized as follows.1) Increasing the growth rate changed the graphite morphology from flaky to fine undercooled type. The growth rate where the change took place decreased by the addition of titanium or vacuum treatment of melts.2) The formation of the coral graphite did not necessarily need large undercooling.3) The content of solute oxygen and solute nitrogen of the specimen with fine coral graphite structure was lower than those of the specimen with flaky graphite structure. Accordingly, it was concluded that the promotion of fine graphite structure by means of Ti addition or vacuum treatment was caused by the reduction of oxygen and nitrogen from the melt.4) Bainitic structure were obtained by austempering treatement of the coral graphite cast iron contain-ing nickel and copper. The mechanical properties of this bainitic coral graphite cast iron was judged to be suitable for the plastics injection dies.