Microsturecture And Properties Of Tungsten, Niobium And Vanadium Indefinite Chill Cast Iron, And Mechanical Properties Of Their Carbides

Indefinite Chill Double-Poured （ICDP） rolls which are widely recognized as the firstchoice for the later stand of hot rolling mill （HSM） due to its superior thermostability,good resistance to adherence and thermocracking, and wear resistance. However, as theimprovement of the early stands’ materials for rough and finish rolling as well as theextended working period, the wear resistance of the later stand is becoming insufficient.Therefore, it is an urgent task to develop enhanced ICDP roller with enhanced wearresistance. In this paper, we have added W, Nb and V element into the conventionalindefinite chill cast iron （ICCI） with the aim to increase the wear resistance, and to shedlight on the effect of those alloying elements on the microstructure and solidificationprocess of ICCI. Furthermore, in view of the pivotal role played by the micro-structureand mechanical properties of carbides in defining the wear resistance behavior of cast iron,we also investigated the crystal structure and mechanical properties of the vanadium andniobium carbides. The aim of this work is to provide theoretical and experimental basicdata for the development of ICDP roller with high wear resistance.ICCI with12various contents of W, Nb, and V element ranges from1-5percent wereyielded by alloying. By the aid of Optical Microscopy （OM）, Scanning ElectronMicroscope （SEM）, X-ray Diffraction （XRD）, Differential Scanning Calorimetry （DSC）,wear resistance test, and Thermo-cal et al, the effect of alloying element on the solidifiedmicrostructure and properties of ICCI were investigated. Results show: the effect showsreliance on the type of alloying element. When the added W content is1-2%, there isalmost no effect on the solidified process; when the added W content is larger than3%,fishbone shape carbides （Fe₄W₂C） appears at grain boundary, but has trace influence onthe morphology of carbides that shows network shape at grain boundary. After adding Nbelement, carbides distribute in a discrete manner due to the prior precipitation of NbC.With the increase of added V alloying element, there is a gradual transformation for thecarbides from network at grain boundary （1%V）, to increased refinement （2-3%）, andfinally homogenously distributed in spherical and short rod-like shape. During the tempering of1-5%alloyed cast iron samples, the hardness showssecondary hardening with the increase of tempering temperature, and peaks at about400-500℃. The maximum hardness that corresponds to W, Nb, and V alloying is HRC60,HRC59and HRC60, respectively. The wear resistance of alloyed cast iron increases withthe content of alloying element. For the different allying element, there is a gradualdecreasing trend in the sequence of V, Nb, and W. The most significant results come to the5%V alloying by which the wear resistance is improved by13times.For the high-V alloyed cast iron, the solidified process changes from hypo-eutectic（4%V） to quasi-eutectic （6%V）, and further to hyper-eutectic transformation （8-10%V）.The distribution of carbides changes from inter-granular to chrysanthemum-likedistribution, and further to homogenous pattern. The wear resistance of heavy V alloyedcast iron depends not only on the hardness of matrix, but the distribution of the carbides.the8%V alloyed sample shows the highest wear resistance, followed by6%and10%Valloyed, and the4%alloyed sample shows the worst wear resistance enhancement.By extracting the V and Nb carbides from the solidified alloyed cast iron andmicrostructure analysis method including Field Emission Scanning Electron Microscopy（FE-SEM）, XRD, High Resolution Transmission Electron Microscopy （HR-TEM）, andNano-indentation, We found the dendritic carbides in heavy V alloyed chilled cast iron（8%V） are cubic NaCl-type vanadium carbides （VC） whose hardness and Young modulusis33.3and436GPa, respectively. The niobium carbides （NbC） in the heavy Nb alloyedcast iron （5%Nb） also share the same crystal structure with NaCl and shows irregular bulkshape, and its hardness and Young’s modulus is24.5and406GPa, respectively.By the aid of first-principle calculation, the mechanical properties of VC and NbCwith various stoichiometries were investigated. Results show, the shear modulus, Young’smodulus and Vickers hardness increase monotonously with carbon content, but the bulkmodulus shows irregular trend. The reasons were discussed from the standpoint ofelectronic structure, Mulliken Overlap Population （MOP）, and atomic configuration. In Nb-C system, pnma-Nb2C and P3₁-Nb₆C₅are found to be the ground state structures, andthe proposed phase: Fm-3m-Nb₂₃C₆, Pnma-Nb₃C, C₂/c-Nb₅C₂, and P6₃mc-Nb₇C₃areenergetically, mechanically, and dynamically stable.