B微合金化对HK40合金铸造疏松的影响

EFFECT OF B MICRO-ALLOYING ON MICROPOROSITIES IN AS-CAST HK40 ALLOYS

利用SEM,OM和XRD等手段分析了HK40合金铸件铸造疏松形成原因,并研究了添加微量B对HK40合金的凝固组织及疏松形成的影响.结果表明:HK40合金铸件主要存在A和B 2种铸造疏松缺陷.A类疏松主要由于枝晶的快速生长并架桥联接导致架桥枝晶之间区域的补缩不足引起;B类疏松产生原因是相邻枝晶间区域生长的枝晶状M7C3型碳化物堵塞枝晶间补缩通道.B微合金化能降低HK40合金铸件较强的柱状晶生长趋势,细化枝晶,能抑制HK40合金A类铸造疏松缺陷的产生.同时,B微合金化增加了HK40合金枝晶间共晶相的体积分数,使枝晶间呈枝晶状M7C3型碳化物转变为层片状的M23C6型碳化物析出,避免碳化物堵塞相邻枝晶间的补缩通道,因而也减小了B类铸造疏松缺陷的形成倾向.

Casting microporosity defect is one of the important issues for as-cast HK40 alloys preparation, which is of great importance to application performance of the alloy castings. A comprehensive understanding of the mechanism on formation of the casting microporosity defect is still unclear for the alloys. In this work, the casting microporosity defect and influences of boron micro-alloying on the as-cast microstructures and microporosities in HK40 alloys castings were investigated by means of SEM, OM and XRD, etc.. The microstructures in the HK40 alloys with and without boron micro-alloying after quenching at high temperatures were also examined to check the solidification characteristic change attribute to boron addition. The results show that there are two types of casting microporosities in the castings. Type A is mainly caused by the rapid growth of dendrites and thus dendritic bridge connecting which lead to feeding shortages between the bridge dendrites. Type B is, however, resulted by the growth of M7C3 carbides in coarsened dendritic morphology which induce to the feeding channel blockage in adjacent interdendritic regions. Boron micro-alloying decreases the tendency of columnar grain formation and refines the dendrites in HK40 alloys which therefore suppresses the casting microporosity defect of type A. Additionally, boron micro-alloying not only increases the volume fraction of eutectic phases, but also changes the M7C3 carbides in dendritic morphology into the M23C6 carbides in lamellar morphology, which prevents the feeding channal blockage in adjacent interdendritic regions, thus reduces the casting microporosity defect of type B.