Rapid Solidification Microstructure and Carbide Precipitation Behavior in Electron Beam Melted High-Speed Steel
Rapid Solidification Microstructure and Carbide Precipitation Behavior in Electron Beam Melted High-Speed Steel
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DOI：
10.1007/s11661-020-05661-z
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发表时间：
2020-02-11
影响因子：
2.8
通讯作者：
Chen, B.
中科院分区：
文献类型：
--
作者：
Jin, J.;Gao, R.;Chen, B.
研究了电子束熔炼S390高速钢的凝固组织和碳化物析出行为。EBM组织由分散在自回火马氏体基体中的非常细的一次碳化物的不连续网络以及有限量的残余奥氏体组成。碳化物网络由M2 C/M6 C和MC碳化物组成。在as-EBM组织中发现柱状晶和近等轴晶组织，枝晶间共晶碳化物的存在有助于揭示枝晶凝固本质。顶层显微组织观察证实，柱状枝晶结构的晶粒位于附近的微熔池边界，表明外延生长的平均生长方向平行于最大的热梯度。在微熔池中心，平行于束流行进方向的枝晶生长形成了近等轴晶。碳化物分解通过扫描透射电子显微镜揭示，并通过透射菊池衍射证实。MC碳化物（富V后富W）在M2 C（W、Fe、Mo和Co按重要性顺序排列）与基体之间的界面处成核，然后从外向内生长，但它们的成核可能发生在M2 C碳化物本身。由相邻扫描线引起的热效应似乎触发MC -> M2 C + γ-Fe的固态相变。对元素迁移进行了理论计算，并与实验结果进行了比较。在as-EBM S390中，类似于65 HRC的高硬度和类似于2500 MPa的良好的横向断裂强度意味着EBM处理可以用于制造高合金化工具钢。通过后加工热处理，可获得最佳洛氏硬度（73.1 ± 0.2 HRC）和最佳横向断裂强度（3012 ± 34 MPa）。
The solidified microstructure and carbide precipitation behavior in an S390 high-speed steel processed by electron beam melting (EBM) have been fully characterized. The as-EBM microstructure consists of discontinuous network of very fine primary carbides dispersed in auto-tempered martensite matrix together with a limited amount of retained austenite. The carbide network consists of M2C/M6C and MC carbides. Both the columnar and near-equiaxed grain structures were found in as-EBM microstructure and the presence of inter-dendritic eutectic carbides assisted in revealing the dendritic solidification nature. The top-layer microstructure observation confirmed that the columnar dendritic structured grains were located adjacent to the micro-melt pool boundary, indicating an epitaxial growth with the average growth direction parallel to the maximum thermal gradient. At the center of the micro-melt pool, the near-equiaxed grains were developed by dendritic growth parallel to the beam traveling direction. The carbide decomposition was revealed by scanning transmission electron microscopy and confirmed by transmission Kikuchi diffraction. The MC carbides (rich in V followed by W) nucleated at the interface between M2C (W, Fe, Mo, and Co in the order of significance) and the matrix and then grew from the outside inward, but their nucleation might occur from the M2C carbide itself. The thermal effect induced by the adjacent scan lines seems to trigger a solid-state phase transformation of MC -> M2C + gamma-Fe. The elemental migration was theoretically calculated and compared with the experimental results. The high hardness of similar to 65 HRC and good transverse rupture strength of similar to 2500 MPa in as-EBM S390 means that EBM processing can be used to fabricate highly alloyed tool steels. With the help of the post-processing heat treatment, the best Rockwell hardness of 73.1 +/- 0.2 HRC and transverse rupture strength of 3012 +/- 34 MPa can be obtained.