采用OM, TEM, SEM, EBSD, XRD等手段, 分析了不同Al加入量(0%, 0.6%, 1.2%)的M2高速钢的铸态组织, 研究了Al对高速钢凝固组织转变特别是共晶碳化物形貌和微观结构的影响规律. 结果表明, M2高速钢铸态组织主要由位错型马氏体和M₂C共晶莱氏体组成. 过量Al (1.2%)促进大量铁素体和针状碳化物形成. Al提高了共晶碳化物的分布均匀性, 促进M₂C形貌由纤维状转变为片状, 并使碳化物微观结构发生明显改变. 片状碳化物表面平行于(0002)晶面, 内部存在微孪晶、层错等缺陷, 片层之间具有不同的晶体取向, 而纤维状碳化物内部缺陷极少, 呈单晶取向. 与纤维状碳化物相比, 加Al后形成的片状碳化物高温加热时不易团球化, 对碳化物尺寸细化不利. 添加过量Al (1.2%)形成的铁素体无法通过常规热处理消除, 使高速钢淬火硬度显著降低.

The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M₂C eutectic ledeburite. Excessive amount of aluminum, 1.2%, favors the formation of ferrite and needle-like carbides. After the addition of aluminum, eutectic carbides are distributed more homogeneously. Additionally, the morphology of M₂C eutectic carbides transforms from the fibrous to the plate-like, and their microstructure also changes significantly. The plate-like M₂C has crystal defects, such as micro-twins and stacking faults, and different growing orientation between adjacent plates whereas the fibrous carbides have few defects and single crystal orientation. Compared to fibrous carbides, the plate-like carbides are much difficult to get spheroidized at high temperature, which is unfavorable for carbide refinement. The ferrite, formed by adding excessive amount of aluminum, cannot be eliminated by ordinary heat treatments, decreasing remarkably the hardness of high speed steel after quenching.