| Hardfacing technology is one of the key technologies on the green remanufacturing technology field. By using hardfacing technology, the failed workpieces can be remanufactured. Not only their dimension and shape can be restored, but also the excellent wear resistance, thermal fatigue resistance and corrosion resistance can be obtained on the surface of the remanufactured workpieces. Compared with others remanufacturing technologies, hardfacing technology has the apparent advantages, such as high bonding strength between coating and base metal, wide applicability and low price. Therefore, it has been applied in modern industry widely.In this work, the traditional Cr5 hot-rolled supporting roller steel was taken as the research material. The novel flux cored wire used for hardfacing hot-rolled supporting roller was developed, and the microstructure of the hardfacing alloy was characterized. Then, the welding technology was optimized. Meanwhile, the tempering technologies of the hardfacing alloys were determined by microstructure observation and hardness measurement with different tempering conditions. Especially, the work hardening phenomenon, which often appears on the medium-high carbon alloy steel also been studied. Moreover, the effects of La2O3 addition on the microstructure and mechanical properties of the hardfacing alloy were investigated systematically. Subsequently, using a first-principles density functional plane-wave ultrasoft pseudopotential method, the mechanisms of La Al O3 as the heterogeneous nucleus of γ-Fe and α-Fe were calculated and the regular of La solid solute in γ-Fe and α-Fe were predicted.The phase diagram calculated results show that, the elements Ni and Mn can expand the austenite phase region of the hardfacing alloy, the elements Mo and V can promote the precipitation of the M7C3 and MC carbides, respectively. The microstructure of the hardfacing alloy prepared by novel flux cored wire used for hardfacing hot-rolled supporting roller consists of martensite, retained austenite and a small amount of carbides. Meanwhile, the high-alloy martensite with white reticular morphology is distributed on the crystal boundary. The high-alloy martensite, whose average nanoindentation is 14.33 GPa, shows bulk morphology in field emission scanning electron microscope. The contents of alloy elements in the high-alloy martensite are higher than those in the normal one, while the former decrease 30% rapidly after be tempered. As same as the normal martensite, the high-alloy martensite shows the body-centered tetragonal crystal structure, and the c-axis of the high-alloy martensite is longer than that of the normal one. The welding heat input is selected as 23.5~27.0 J/mm to ensure the hardfaing alloy with the excellent performance.The optimum tempering technology of the novel hardfacing alloy is 480 oC for 8h.When the amount of the retained austenite in the hardfacing alloy is high, it will transform to martensite spontaneously, which leads to work hardening phenomenon, until the amount of the retained austenite is decreased to 4.9 vt.%. During the work hardening process, the austenite grain transforms to the [001] and [101] oriented martensite grain preferentially.With the La2O3 addition increases from 0 wt.% to 4.98 wt.%, the primary austenite grain dimension decreases firstly and then increases. When the La2O3 addition is 3.32 wt.%, the dimension of the primary austenite grain is the smallest, which is 18 μm. Moreover, the amount of retained austenite increases from 18.4 vt.%to 25.4 vt.% firstly, then remain constant, while that of high-alloy martensite decreases from 19.6 vt.%to 7.7 vt.% gradually. With the increase of the La2O3 addition, the hardness, wear resistance, strength and toughness of the hardfacing alloy all increase firstly and then decrease. When La2O3 addition is 3.32 wt.%, all of the properties above mentioned are the most excellent, in which, the harness is HRC 50.4, the weight loss is 0.13 g/h, the tensile strength and yield strength are 1502 MPa å'Œ 1334 MPa, and the impact energy is 7.10 J/cm2, respectively.For the two terminated structure La Al O3(100)/lack C γ-Fe(100) interfaces,when △μLa is in the range of(-14.46 e V,-14.08 e V) and(-1.402 e V,-1.025 e V), the interfacial energy of Al O2 terminated structure and La O terminated structure can satisfy the requirement that La Al O3/lack C γ-Fe interface as the heterogeneous nucleation interface. For the La Al O3(100)/doped C γ-Fe interface, when △μLa is small, part of the Al O2 terminated structure(Al O2-doped C and Al O2-lack C) interfacial energy can satisfy the requirement that La Al O3/ doped C γ-Fe interface as the heterogeneous nucleation interface. However, when △μLa is large, Al O2 terminated structure interface cannot as the heterogeneous nucleation interface, while, part of the La O terminated structure interface can be the heterogeneous nucleation interface. For the La Al O3(100)/α-Fe(100) interface and La Al O3(100)/α-Fe(110) interface with La O terminated structure, when △μLa is in the range of(-2.447 e V,-2.064 e V) and(-1.009 e V,-0.722 e V), the interfacial energies of La Al O3(100)/α-Fe(100) interface and La Al O3(100)/α-Fe(110) interface are both in the range of 0 J/m2~0.204 J/m2, which means that the two interfaces can be the La Al O3/α-Fe heterogeneous nucleation interfaces.For the La atom solid solute γ-Fe(α-Fe) supercell, the stability, degree of ionization, deformation resistance and stiffness of La atom substitutional solid solute supercell are better than those of La atom interstitial solid solute supercell, while the plasticity, metallicity and anisotropy of La atom substitutional solid solute supercell are worse than those of La atom interstitial solid solute supercell. |
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