Yefei Zhou

Influence of La2O3 addition on microstructure and wear resistance of Fe-Cr-C cladding formed by arc surface welding

Abstract The Fe-Cr-C claddings formed by arc surface welding with different La2O3 additions were investigated. The microstructures were observed by optical microscopy (OM), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The phase structures were measured by X-ray diffraction (XRD). The wear resistances of the claddings were tested by friction and wear experiment. On this basis, the carbide refinement mechanism by inclusion enriched with La was discussed theoretically. The results showed that, the microstructure of the Fe-Cr-C cladding consisted of primary (Cr,Fe)7C3 carbides and eutectic (γ-Fe+(Cr,Fe)7C3) structure. With La2O3 addition increasing, the primary carbides were refined, and the mass loss of the cladding decreased gradually. The Fe-Cr-C cladding with 4 wt.% La2O3 addition had a best wear resistance behaviour. The RE inclusion LaAlO3 as heterogeneous nuclei of the primary M7C3 was medium effective, and could refine the M7C3 carbides. Besides, the wear resistance could be improved by adding La2O3 in the claddings.

Effect of RE oxide on growth dynamics of primary austenite grain in hardfacing layer of medium-high carbon steel

Abstract The flux cored wires with different rare earth oxide additions for hardfacing the workpieces of medium-high carbon steel were developed. The microstructure of the hardfacing layer was observed using the optical microscopy. The average dimension of primary austenite grains in hardfacing layer was measured by image analyzer. The primary austenite grain growth activation energy and index were calculated according to Sellarss mode and Beck formula, respectively. Moreover, the effect of rare earth oxide on the growth dynamics of primary austenite grain was analyzed, and then discussed with the misfit theory. The experimental results showed that, by adding rare earth oxide, the average dimension of primary austenite grains in hardfacing layer of medium-high carbon steel decreased, and it was the smallest when the addition of rare earth oxide was 5.17 wt.%. Meanwhile, at this rare earth oxide addition, the primary austenite grain growth activating energy in hardfacing layer was the largest, while its index was the smallest. The calculated results indicated that the primary austenite grain could be refined because LaAlO 3 as heterogeneous nuclei of γ-Fe was the most effective.

Refinement of nano-Y2O3 on microstructure of hypereutectic Fe-Cr-C hardfacing coatings

Abstract The Fe-Cr-C flux-cored wires with 0 wt.%, 0.63 wt.%, 2.54 wt.% and 5.08 wt.% additions of nano-Y 2 O 3 were developed in this work. And the different hypereutectic Fe-Cr-C hardfacing coatings were prepared. The phase structures of the coatings were determined by X-ray diffraction. The microstructures were observed by optical microscopy. The morphologies of the hypereutectic Fe-Cr-C hardfacing coatings were observed by a field emission scanning electron microscope equipped with an X-ray energy dispersive spectrometer. The effectiveness of Y 2 O 3 as heterogeneous nuclei of primary M 7 C 3 was calculated with the misfit theory. The experiment results showed that, the microstructures of the hypereutectic Fe-Cr-C hardfacing coatings consisted of M 7 C 3 , γ-Fe and α-Fe phases. With the increase of nano-Y 2 O 3 additives, primary M 7 C 3 in hypereutectic Fe-Cr-C coatings could be refined gradually. The average size of the primary M 7 C 3 was the minimum, which was 22 μm, when nano-Y 2 O 3 additive was 2.54 wt.%. The calculated results showed that, the two-dimensional lattice misfit between the face (001) of Y 2 O 3 and face (100) of orthorhombic M 7 C 3 was 4.911%, which indicated that Y 2 O 3 as heterogeneous nuclei of M 7 C 3 was middle effective to refine the primary M 7 C 3 .

Wear resistance of hypereutectic Fe-Cr-C hardfacing coatings with in situ formed TiC

Abstract With excellent wear resistance, hypereutectic Fe–Cr–C coatings have caused great attention in the field of surface engineering. In this study, the hypereutectic Fe–Cr–C hardfacing coatings with certain amounts of titanium (Ti) additives were further developed. The precipitation of the carbides in the coatings was calculated. The microstructure of the coatings was observed and the phase structure of the coatings was determined. Besides, the wear resistance of the coatings was also measured. The experimental results show that the microstructure of the coatings mainly consists of primary (Cr,Fe)7C3 carbide and eutectic matrix. With increasing Ti additive, the primary carbide is refined, which in turn improves the wear resistance of the coatings.

Growth characteristics of primary M7C3 carbide in hypereutectic Fe-Cr-C alloy

The microstructure of the hypereutectic Fe-Cr-C alloy is observed by optical microscopy (OM). The initial growth morphology, the crystallographic structure, the semi-molten morphology and the stacking faults of the primary M7C3 carbide are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The in-suit growth process of the primary M7C3 carbide was observed by confocal laser microscope (CLM). It is found that the primary M7C3 carbide in hypereutectic Fe-Cr-C alloy is irregular polygonal shape with several hollows in the center and gaps on the edge. Some primary M7C3 carbides are formed by layers of shell or/and consist of multiple parts. In the initial growth period, the primary M7C3 carbide forms protrusion parallel to {} crystal planes. The extending and revolving protrusion forms the carbide shell. The electron backscattered diffraction (EBSD) maps show that the primary M7C3 carbide consists of multiple parts. The semi-molten M7C3 carbide contains unmelted shell and several small-scale carbides inside, which further proves that the primary M7C3 carbide is not an overall block. It is believed that the coalescence of the primary M7C3 carbides is ascribed to the growing condition of the protrusion and the gap filling process.

Fe-15 wt-%Cr-X wt-%C hardfacing surface layer: wear resistance and its enhanced mechanism with C additive

Abstract An electrode with different C additives was developed. The microstructure of the hardfacing surface layer was observed by optical microscopy. The phase structure was determined by X-ray diffraction. The hardness and wear resistance of the hardfacing surface layer were measured respectively. The worn-out surface and three-dimensional morphology were observed by field emission scanning electron microscope equipped with energy dispersive X-ray spectrometry. The relation curve between mass fraction of M7C3 carbide and temperature was calculated according to thermodynamics software Thermo-Calc. The results show that, with the increase in C additive, the hardfacing surface layer changes from a hypoeutectic structure to a hypereutectic one. Meanwhile, the primary phase changes from austenite to carbide. The hardness and wear resistance of the hardfacing surface layer increase gradually with the increase in C additive, and when the C additive is 25 wt-%, they are the largest. With the increase in C content, the precipitation temperature of M7C3 decreases from 1280 to 1260°C, while the maximum amount of M7C3 increases instead, which is from 0·154 to 0·313. The reason of the improvement for wear resistance of the hardfacing surface layer is that the carbide initiates and the amount of M7C3 increases.