| Iron and steel materials occupy an important position in the mechanical engineeringindustry, because of their high strength and good toughness. In resent years, mechanical&engineering structures are developing in the direction of large-scale and high parameter,which put forward higher requirements on the performance of iron and steel materials.Strengthening of iron and steel material by added particles has a number of advantages. Inthe study, NbC particle was selected as strengthening phase, and were added to low carbonsteel and high chromium cast iron melt in the melting and casting process, respectively,achieving microstructure refinement and mechanical properties strengthening.The mechanical activation and thermal synthesis technology was developed. TheNb(C) homogeneous solid solution was prepared by the mechanical activation of ballmilling Nb-graphite powder, the subsequent heating initiated chemical reactions betweenNb-C in the solid solution, thus, NbC particles less than1μm with complete crystalstructure were prepared at low temperature and in short time. This method solvedproblems in mechanical alloying process for preparing NbC powder such as long cycle,incompleteness of product’s crystal structure.Al was added into the milling Nb-graphite mixed powder and substituted in the Nbcrystal. The subsequent heating of the (Nb-Al)C solid solution resulted in the Nb(Al)Cformation. The true density of NbC particle was reduced to equal to the steel liquid densityvalue.The preparation of composite powder method was proposed to improve thedispersion of NbC particles in iron and steel melt. Fe powder was added into the Nb(C)homogeneous solid solution formed by ball milling, and the mechanical alloying wascontinued to form Nb(C)-Fe composite. After vacuum heating, in situ NbC nanoparticlesgenerated in the Fe powder matrix. The added Fe powder played the role of grinding agentand diluent. The prepared nano-NbC/Fe composite powder was added into20steel meltthrough laboratory vacuum smelting process. With the increase of melting temperature andholding time, the dissolved quantity of NbC particles increased, the number of NbC particles servicing as crystallization core declined, thus, the grain refinement effectreduced. The suitable smelting temperature is1550oC, residence time of NbC added intothe molten steel is2min. With the increase of the amount of NbC particles, thesolidification microstructure of20steel was refined gradually, the mean grain size offerrite reduced to26μm in the steel with0.5wt%NbC addition, equivalent to the1/4value of original steel. Strength, hardness and wear resistance of20steel increased withthe addition of NbC, the elongation rose to a maximum value when0.15wt%NbC wasadded and then decreased.The preparation of cementite powder by mechanical activation and thermal synthesistechnology was studied. The formation of Fe3C belongs to the dissolution precipitationmechanism, decomposition of cementite occurs at higher temperature, so it can’t get Fe3Cgeneration rate of100%. The process parameters of getting the maximum Fe3C productionrate (~80%) are: the milling time of11h, heating temperature of700℃, holding time of15min. According to the coincidence of cementite and NbC process parameters, the nano-NbC/(Fe3C-Fe) composite powder was prepared, which is suitable for adding in cast ironmelt. The composite powder was added into high chromium cast iron melt throughsuspension casting process, and the ingots was produced. It is found that the dissolutionrate of NbC nanoparticles in high chromium cast iron melt is greater than that of itsdissolution rate in liquid20steel. Therefore, in the melting experimental conditions mostof the NbC had dissolved. The NbC-austenite eutectic formed in the clearance timebetween primary austenite and eutectic Cr7C3, during solidification. Not completelydissolved NbC and precipitated NbC became the heterogeneous nucleation core of Cr7C3crystallization, and impeded the growth of Cr7C3. The Cr7C3eutectic carbide size wasrefined for nearly1/2. The hardess increased4HRC, the wear resistance and impacttoughness increased by83%and59%, after the high chromium cast iron modified by thecomposite powder. |
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