PDF(Pubmed)
Abstract：
SiMo ductile cast iron combines ease of part fabrication with good mechanical properties, including a usable plasticity range. Its poor corrosion resistance inherited from grey cast iron could be alleviated through alloying with Al or Cr additions capable of forming a dense oxide scale protecting the substrate. However, the presence of Al and Cr in cast iron tends to make the material brittle, and their optimum alloying additions need to be studied further. The present work was aimed at investigating the effect of crystallization rates on microstructure changes during directional crystallization of SiMo-type alloys with up to 3.5% Al and 2.4% Cr. The experiment was performed using the Bridgman-Stockbarger method. The tubular crucible was transferred from the hot section to cold section at rates ranging from 5 mm/h to 30 mm/h with a 4/5 crucible length and then quenched. The introduced Al promoted graphitization up to a point, wherein, at the highest applied addition, the graphite precipitation preceded crystallization of the rest of the melt. A rising level of Cr in these alloys from 1% to 2.4% resulted in the formation of low and high contents of pearlite, respectively. The higher crystallization rates proved effective in increasing the ferrite content at the expense of pearlite. In the investigated cast iron samples with smaller applied alloying additions, Widmanstätten ferrite or ausferrite, i.e., fine acircular phase, were often found. The switch from directional crystallization to quenching caused a transition from a liquid to solid state, which started with nucleation of islands of fine austenite dendrites with chunky graphite eutectic separating them. As these islands expanded, they pushed alloying additions to their sides, promoting carbide or pearlite formation in these places and forming a super-cell-like structure. The performed experiments helped gather information concerning the sensitivity of the microstructure of SiMo cast iron modified with Al and Cr to crystallization rates prevailing in heavy cast structures.
摘要：
SiMo球墨铸铁结合了易于制造的零件和良好的机械性能，包括可用的可塑性范围。可以通过与能够形成保护基材的致密氧化皮的Al或Cr添加物合金化来减轻其从灰铸铁继承的不良耐腐蚀性。然而,铸铁中Al和Cr的存在会使材料变脆，它们的最佳合金化添加量需要进一步研究。本工作旨在研究结晶速率对SiMo型合金在高达3.5％Al和2.4％Cr的定向结晶过程中微观结构变化的影响。使用Bridgman-Stockbarger方法进行实验。以4/5坩埚长度,以5mm/h至30mm/h的速率将管状坩埚从热段转移至冷段,然后淬火。引入的Al促进了石墨化，其中，在最高应用的加法下，石墨沉淀先于熔体的其余部分结晶。这些合金中的Cr含量从1％上升到2.4％，导致形成低含量和高含量的珠光体，分别。更高的结晶速率证明在以珠光体为代价增加铁素体含量方面是有效的。在所研究的铸铁样品中，合金添加量较小，Widmanstätten铁氧体或奥氏体铁氧体，即，细圆形相，经常被发现。从定向结晶到猝灭的转变导致了从液态到固态的转变，首先是细奥氏体枝晶岛的成核，块状石墨共晶将它们分开。随着这些岛屿的扩张，他们把合金添加剂推到了他们的一边，促进碳化物或珠光体在这些地方的形成和形成超细胞样结构。进行的实验有助于收集有关用Al和Cr改性的SiMo铸铁的微观结构对重型铸造结构中普遍存在的结晶速率的敏感性的信息。
