喷射成形H13钢的微观组织及形成特点

Microstructure and Forming Characteristics of Spray Formed H13 Tool Steel

采用常规铸造和喷射成形工艺分别制备了H13钢,应用OM、SEM、XRD等方法研究了铸态、喷射沉积态及过喷粉的组织。采用LMS-30粒度仪对过喷粉进行了粒度分析。结果表明:铸态H13钢为粗大的枝晶组织,合金元素与碳化物偏析严重,而喷射成形H13钢为细小的等轴晶组织,几乎没有碳化物,含有较多的残余奥氏体。过喷粉是枝晶组织,随粉末尺寸的减小,一次枝晶变小,二次枝晶臂已不明显,是欠发达枝晶组织。雾化压力的增加对小颗粒过喷粉的尺寸和数量影响甚微,但会明显减小大颗粒过喷粉末的尺寸,并增加其数量。喷射成形等轴晶的形成机制在气液比偏小时以未破碎枝晶的均质化与碎断枝晶的连接长大为主,而在气液比偏高时以大量破碎枝晶作为异质形核核心为主。

H13 tool steel was produced by traditional casting and spray forming,respectively.The microstructures of the as-cast H13 and as-spray formed H13 were investigated by OM,SEM and XRD methods.And the size distribution and microstructures of the over-sprayed powders were also investigated.The results show that the microstructures of as-cast H13 are coarse dendrites with serious carbides and alloying elements segregation.While the microstructures of as-deposited H13 are fine equiaxed grains without macro-segregation,containing no carbides and more retained austenite.The microstructures of over sprayed powders are still dendrites but with smaller size.As the size of the over-sprayed powders decreases,the primary dendrites become smaller and the secondary dendrite arms are not that obvious.The increase of the atomization pressure has little effect on the distribution of the over spray-formed powders with small sizes,but significantly reduces the size of the large particles and increases their number.When the gas-melt ratio is low,the forming of equiaxed grains in the as-deposited H13 is resulted from the process of the homogenization of the unbroken dendrites and the connection and growing of the broken dendrites.While the gas-melt ratio increases,the size of the equiaxed grains decreases due to that a large number of the broken dendrites remained and greatly increase the heterogeneous sites.