【作者】 周野飞；

【导师】 杨庆祥；

【作者基本信息】 燕山大学 ， 材料学， 2013， 博士

【摘要】 堆焊技术具有高效、廉价等优点，是绿色再制造的核心技术之一。采用堆焊技术对尺寸大、附加值高以及具有耐磨、耐热或耐腐蚀等特殊性能需求的零部件进行修复与再制造，可以有效的延长这些零部件的使用寿命，具有重要的应用价值。Fe-Cr-C合金含有大量的M （M=Cr, Fe）7C3型碳化物，具有较好的耐磨性。然而，传统Fe-Cr-C合金中碳化物较为粗大，在服役过程中碳化物易剥落，从而限制了该合金在堆焊领域的广泛应用。本文自行制备了自保护Fe-Cr-C堆焊药芯焊丝，在系统分析了该堆焊合金组织和性能演变规律的基础上，进行合金成分设计和焊丝配方改进，通过加入强碳化物形成元素Ti、Nb和V以及稀土氧化物La2O3和CeO2，研究了Fe-Cr-C堆焊合金中M₇C₃型碳化物的形状、尺寸和分布，制备出耐磨性优异的Fe-Cr-C堆焊层。同时，通过Bramfitt二维点阵错配度理论和第一性原理计算，对M₇C₃异质核心的非均质形核问题进行了理论分析，探讨了掺杂相诱发M₇C₃碳化物异质形核的可能性，解释了M₇C₃碳化物的细化机理。Fe-Cr-C堆焊合金主要由M₇C₃碳化物、马氏体（α-Fe）和奥氏体（γ-Fe）组成。随着C含量的升高，堆焊合金显微组织由亚共晶组织向近共晶组织，进而向过共晶组织过渡。Fe-27Cr-[1.5-5.5]C （wt.%）合金的共晶反应发生在3.1wt.%C处。当合金中C含量大于3.1wt.%时，凝固初期发生过共晶反应，从液相中首先析出M₇C₃碳化物。随着合金中C含量增加，M₇C₃碳化物数量增加，尺寸变大，且堆焊层的硬度、抗粘着磨损能力均有所提高，同时，犁皱区材料塑性变形能力下降，易造成M₇C₃碳化物的剥落。在堆焊过程中，M₇C₃碳化物体现出择优取向的特征，沿堆焊热流密度方向生长。初生M₇C₃碳化物为多边形棒状结构，共晶M₇C₃碳化物为条状或针状形貌。M₇C₃碳化物在择优生长面的硬度为21.2±0.3GPa，杨氏模量为291±3GPa；在非择优生长面的硬度为20.1±0.3GPa，杨氏模量为267±3GPa。Fe-Cr-C堆焊合金中加入M（M=Ti, Nb, V）元素，可以生成初生TiC、NbC以及二次VC碳化物。MC碳化物的形成细化了Fe-Cr-C合金的组织。根据Bramfitt二维点阵错配度理论，（110）TiC与（010）Cr₇C₃的错配度δ=9.3%，TiC作为Cr₇C₃的非均质形核核心的有效性是中等的，可以作为Cr₇C₃的非均质形核核心，对其起到了细化作用。此外，MC的生成在一定程度上消耗了熔池中C原子的浓度，抑制了初生碳化物的长大，从另一方面促进了Cr₇C₃的细化。但是，当M元素过量加入时，堆焊合金由过共晶组织向亚共晶组织转移，合金的耐磨性下降。Fe-16Cr-3.8C堆焊合金中合适的加入量为0.63wt.%Ti。稀土氧化物La2O₃或CeO₂加入Fe-Cr-C堆焊合金后参与冶金反应，生成的稀土化合物部分留存于M₇C₃碳化物中或M₇C₃碳化物与奥氏体的边界，对熔池起到了脱氧和脱硫的作用。稀土氧化物可以细化堆焊合金组织中初生M₇C₃碳化物，增强合金的耐磨性；但稀土氧化物过量加入时，其对堆焊合金组织的细化程度明显减弱，甚至粗化。Fe-25Cr-5C堆焊合金中合适的加入量为4.0wt.%La2O3和2.0wt.%CeO₂。在相同外界条件下，Cr的掺杂使Fe_7-xCr_xC₃多组元碳化物趋向稳定。与六方结构碳化物相比，正交结构的Cr₇C₃碳化物形成能更低，在合金凝固初期优先形成。随着Cr含量的升高，Fe_7-xCr_xC₃碳化物的硬度升高，Fe₄Cr₃C₃的硬度值达到最大。TiC/Cr₇C₃两种构型界面处均存在Cr-C-Ti共价金属链。TiC（100）型界面构型可以促进Cr₇C₃在其表面异质形核。Fe₃Cr₄C₃/LaAlO₃的界面存在LaO和AlO₂两种终止面。LaO终止型界面理想结合功较大，界面间距较小且界面能较小，有利于初生碳化物在LaAlO3粒子表面上异质形核。更多还原

【Abstract】 With high efficiency and low price, hardfacing technology is one of the coretechnology of Green Remanufacturing. By hardfacing technology, the work-pieces withlarge size, high additional value and special performance requirements, such as wearresistance, heat resistance and corrosion resistance, can be restored and remanufactured,which in turn effectively prolong the service life of the work-pieces. Therefore, it issignificant for the wide application of hardfacing technology. Fe-Cr-C alloy, with excellentwear resistance, contains a large number of M （M=Cr, Fe）7C3carbides. However, fortraditional Fe-Cr-C alloy, it is the large and block M₇C₃carbide, which causes the carbidesdesquamated. Therefore, the application of the alloy has been limited widely in hardfacingfield.Flux-cored wires of Fe-Cr-C alloy with the self-shield ability were prepared in thiswork. On the basis of analyzing the microstructure-properties of the Fe-Cr-C hardfacingalloy, the composition of the alloy was designed and the recipe of the flux-cored wires wasoptimized. By adding strong carbide forming elements Ti, Nb and V as well as rare earth（RE） oxides La2O3and CeO2, the shape, size and distribution of M₇C₃carbide in Fe-Cr-Calloy were investigated. Meanwhile, by the aid of Bramfitt two-dimensional lattice misfitand first-principles calculation, the heterogeneous nucleation of M₇C₃carbide wastheoretical analyzed, the doped phase inducing heterogeneous nucleation of M₇C₃carbidewas discussed and the refinement mechanism of M₇C₃carbide was explained.The microstructure of Fe-Cr-C hardfacing alloy consists of M₇C₃carbide, martensite（α-Fe） and austenite （γ-Fe）. With increased C content, the microstructure can be changedfrom hypoeutecitc to eutectic, and even hypereutectic ones. The eutectic reaction ofFe-27Cr-[1.5-5.5]C （wt.%） occurs at3.1wt.%C. When C content in the alloy is larger than3.1wt.%, the hypereutectic reaction occurrs at the initial stage of solidification. Withincreased C content, the amount and dimension of M₇C₃carbide are increased, meanwhile,the hardness and adhesive wear resistance of the hardfacing alloy can be improved.However, the plastic deformation of the hardfacing alloy in plough area of the wear scratch becomes weaker, which causes that the M₇C₃carbides easily desquamate from thematrix.During hardfacing process, M₇C₃carbides show the preferred orientationcharacteristic, and grow along the direction of welding heat flux density. The shape ofprimary M₇C₃carbide is polygonal rod, while that of eutectic M₇C₃carbide is strip orneedle. The hardness and Young modulus of M₇C₃carbide in preferred orientation sectionare21.2±0.3GPa and291±3GPa, and those in non-preferred orientation section are20.1±0.3GPa and267±3GPa.When alloy elements M（M=Ti, Nb, V） were added into Fe-Cr-C hardfacing alloy, theprimary TiC, NbC and secondary VC carbides can be formed, which in turn refine themicrostructure of Fe-Cr-C hardfacing alloy. According to Bramfitt’s two-dimensionallattice misfit theory, the misfit of （110）TiCand （010）Cr₇C₃is δ=9.3%, so the effectiveness ismiddle. Therefore, the TiC carbide can be as heterogeneous nuclei of the Cr₇C₃, and refinethe primary Cr₇C₃carbide. Moreover, the concentration of C atom in the molten pool isconsumed by the formed MC carbide, which accelerates the refinement of Cr₇C₃carbidein turn. However, when the excessive alloy elements M were added, the microstructure ofthe hardfacing alloy changes from hypereutectic one to hypoeutectic one, and the wearresistance of the alloy can be decreased. The optimum amount of Ti in Fe-16Cr-3.8Chardfacing alloy is0.63wt.%.When RE oxides La2O3and CeO2were added, they participate in the metallurgicalreaction of Fe-Cr-C hardfacing alloy. The formed RE compounds remain in M₇C₃carbideor at the boundary of M₇C₃carbide and austenite, which in turn play a role in deoxidationand desulfurization to the molten pool. Moreover, the primary M₇C₃carbide can be refinedand the wear resistance of the hardfacing alloy can be improved by the added RE oxides.However, when the excessive RE oxides were added, the refinement of M₇C₃carbide issignificantly weakened, and even coarsened. The optimum amounts of RE oxides inFe-25Cr-5C hardfacing alloy are4.0wt.%La2O3, and2.0wt.%CeO2.With the same condition, the doped Cr atoms contribute to the stability of Fe7-xCrxC3multiple carbides. The orthorhombic Cr₇C₃carbide can be preferentially formed at theinitial stage of solidification, for its formation energy is lower than that of hexagonal one. With increased Cr content, the hardness of Fe_7-xCr_xC₃multiple carbides increases, and thatof Fe₄Cr₃C₃is the maximum. Cr-C-Ti covalent chains can be observed at the two kind ofTiC/Cr₇C₃interface. TiC（100） interface promotes the heterogeneous nucleation of Cr₇C₃carbide, which shows a stronger grain refinement ability than that of TiC（110） interface.The interface of Fe₃Cr₄C₃/LaAlO₃can be divided into LaO and AlO₂terminations.LaO-terminated interface, with larger work of adhesion, smaller interfacial separation andinterfacial energy, is favorable for primary carbide to carry out heterogeneous nucleationon LaAlO3particle surface.更多还原