•Erosion behaviors of five high chromium cast iron alloys were investigated.•Erodent particles were ferric oxide specks with the hardness of 657 HV.•Microstructure consists of primary Cr7C3 and ...eutectic.•Erosion resistance is dominated by bulk hardness of hardfaced alloys.•Erosion rate for a sample containing Mo, Nb, V, and W is the lowes.
Depositing hardfacing material onto a surface with the welding method is a common way of protecting surfaces against wear damage. In this study, five high chromium hardfacing alloys were used to prepare samples for the gas blasting erosion test. Erodent particles were ferric oxide specks with a hardness of 657 HV and an average size of 260 μm. Impinging stream eroded the surface of samples with the gas flow of 100 m/s at an angle of 30° while the particle feed was increased from 140 to 630 g. The results indicated that erosion resistance is dominated by bulk hardness of hardfaced alloys. In addition, the erosion rate for a sample containing Mo, Nb, V, and W was the lowest since it had the highest bulk hardness among all other samples.
The coarse primary carbides in high-carbon alloy steels can significantly impair their overall performance. Reducing and refining the primary carbides efficiently are always of great significance in ...material processing. Here, an innovative approach ― the selective microzone remelting (SMR) method, is proposed to dramatically reduce and refine the coarse primary carbides. The method is achieved by utilizing an electric current and is facilitated with the substantial difference in electrical resistivity between carbides and the matrix phase. When a sample contains massive coarse primary carbides is subjected to an electric current, the high electrical resistivity of carbides leads to the selective remelting of local carbides before the sample reaches solidus temperature during heating while during cooling the big difference in electrical resistivity suppresses the carbides formation but promotes the austenite nucleation and growth. This mechanism has been validated through the successful treatment of M50 bearing steel, in which the large-sized blocky primary carbides were eliminated completely, leaving only small-sized eutectics with extremely thin lamellas. Besides, the microporosity in the as-cast sample has also been reduced significantly via SMR treatment. These studies demonstrated that the proposed SMR treatment can be a promising method to reduce and refine coarse precipitates formed during solidification in various alloys.
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•Selectively remelting of local carbides is realized via electrical heating.•The different electrical resistivity between carbides and matrix is utilized.•The proposed method can efficiently refine the primary carbides in steels.•The method is applicable for other alloys with coarse precipitates.
Hypereutectic high chromium cast iron (HHCCI) coatings were prepared by flux-cored arc welding (FCAW) under 230 °C, 80 °C and 30 °C interlayer temperatures, and an extra liquid nitrogen cooling was ...used to obtain −125 °C terminal cooling temperature. The microstructure evolution mainly primary carbide (P–C) and matrix, and abrasive wear behavior were evaluated. For natural air cooling, the amount of primary carbide increased, and the size and the mean free path between P–C decreased, with the decrease of the interlayer temperature. Due to the anisotropy in the P–C shape, the volume fraction of P–C, expressed by the area fraction, was significantly affected by the growth orientation. Liquid nitrogen cooling further optimized the morphology of P–C, in terms of the amount, size and distribution. Due to the −125 °C terminal cooling temperature, the martensitic transformation developed in the matrix, showing a body-centered tetragonal structure and exhibiting a Kurjumov-Sachs (K–S) orientation relationship with parent austenite. The linear relationship between wear resistance and P–C was investigated by Pearson's correlation coefficient, and the amount, size and mean free path expressed a strong correlation with wear resistance. Finally, the coating cooled by liquid nitrogen showed the best wear resistance, owing to the excellent morphology of P–C, and the strengthened matrix.
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•Detailed SEM, CT and 3D characterisation of spallings formed after RCF has been performed.•Rare earth addition can prolong the RCF life of M50 bearing steel.•For M50 bearing steel, a ...criterion on RCF failure types is given for the first time.•The M50 bearing steel with rare earth addition exhibits lower density butterflies and shorter accompanied cracks in the subsurface.•The RCF is the result of the competition between surface-initiated failure and subsurface-initiated failure.
The study aims to investigate the influence of rare earth (RE) addition on the rolling contact fatigue (RCF) behaviour of M50 bearing steel. Ball-on-rod RCF experiments were conducted on specimens extracted from M50 bearing steels with and without RE addition. Results indicate that RE addition remarkably improves the RCF life, and the L10, L50 and LVS fatigue life is increased by 96.2%, 61.7% and 55.0%, respectively. For M50 bearing steel, a criterion on RCF failure types is given for the first time. According to the criterion, the failure is mainly surface-initiated for the M50 steel with RE addition, whereas the failure is mixed and subsurface initiated for the M50 steel without RE addition. As a corroboration, compared with steel with RE addition, steel without RE addition exhibits higher density butterflies and longer accompanied cracks in the subsurface. This result shows that the ability of M50 bearing steel to resist subsurface damage from RCF can be greatly improved by the refined primary carbides resulting from RE addition. The RCF life of bearing steel with RE addition can be much higher than that of bearing steel without RE addition if the RCF failure is mainly subsurface initiated.
The primary carbide precipitated during the solidification process will act as the crack source to reduce the performance of H13 steel. It is necessary to obtain the nature of the primary carbide in ...H13 steel to reduce its detriment. Therefore, the distribution characteristics and thermal stability of the primary carbide in cast Ce-H13 steel were analyzed in this paper. There is a huge difference in the shape of the primary carbide between the 2D observation and the 3D observation. The shape of the primary carbide is a dendritic structure, and the branch is rich-V carbide and the trunk is rich-Ti-V carbide. The primary carbide size in the 3D observation increases gradually from the margin of the Ce-H13 ingot to the center. The rapid growth of the branch leads to an increase in size, and the decrease in the cooling rate is the main reason for the increase in size. When the heating temperature is 1150°C, the rich-V carbide starts to dissolve and dissolved completely at 1250°C. However, the rich-Ti-V carbide just starts to dissolve when the heating temperature is 1250°C. The number density and size of primary carbide decrease gradually with the increase of the heating temperature. Elemental Ce can effectively decrease the size of the primary carbide, but not for the number density. The calculated results are in keeping with the experimental observations. High-temperature heating can effectively reduce the primary carbide size, but cannot eliminate it.
The microstructure, alloying elements segregation and characteristics of primary carbides in AISI H13 steel that solidified at different cooling rate were investigated by optical microscope (OM), ...field emission scanning electron microscope (FE-SEM), electro-probe microanalyzer (EPMA) and automated inclusion analyzer ASPEX. The microstructure of H13 steel samples become more refined with increased cooling rate. The equation of relationship between cooling rate (RC) and secondary dendrite arm spacing (λ) for H13 steel could be expressed as λ = 175.4RC-0.322. Primary carbides are located in interdendritic region, where existed obvious Cr, Mo, V and C segregation. Higher cooling rate promoted higher alloying elements segregation and facilitated earlier precipitation of primary carbides during solidification process. The number, size, amount and mean area of primary carbides decreased significantly with the increased cooling rate, however the shape of the primary carbides were insensitive to cooling rate. Thermodynamic calculation indicated that V-rich primary carbides precipitated at solid fraction larger than 0.94, Mo-rich primary carbides precipitated at solid fraction larger than 0.99 in the cooling rate range investigated. Lower cooling rate suppressed alloying elements segregation, but the precipitation of primary carbides could not be avoided in the cooling rate range.
Niobium (Nb) microalloying can improve the material properties of H13 steel (0.4C-5Cr-1.2Mo-1V steel), but it also affects the natures of the primary carbides. Therefore, the effect of Nb content and ...cooling rate on the behavior of primary carbides in H13 steel was studied in this paper. The matrix structure was obtained by chemical etching, and then the formation location of primary carbides was identified by electron probe microanalysis (EPMA). The three-dimensional (3D) characteristics, including morphology, number density, and size, were obtained by a non-aqueous electrolysis method. The enrichment of alloying elements in the last-to-solidify region leaded to the formation of primary carbides during the solidification. The Ti4C2S2 phase precipitated first, and then the Mo-Cr-rich carbide was formed around the Ti4C2S2 phase. During the cooling process, the Ti4C2S2 phase partly transformed into Nb-rich carbide and then further partly transformed into V-rich carbide. There is a huge difference between the two-dimensional and three-dimensional morphologies of the primary carbides. As the Nb content increased, the size of last-to-solidify region decreased gradually and the size and number density of primary carbides in the 3D observation increasingly increased. However, as the decrease of the cooling rate, the size of primary carbides increased rapidly and the number density of primary carbides decreased markedly. The thermodynamic and kinetics calculation results agreed well with the experimental observations.
•Increasing ratio of DS/DL reduces segregation and primary carbide precipitation.•Fraction of residual liquid to precipitate carbide is reduced by enhancing DS/DL.•Driving force for phase transition ...of carbide is also reduced by increasing DS/DL.
This study provides a potentially viable approach to manipulate the precipitation of primary carbides in molten steel by modifying the diffusion coefficient of carbon. The solidification process of a Fe-C alloy is simulated using the multi-phase-field method, and we focus on the impact of diffusion coefficient of carbon on the solute segregation and cementite precipitation. Two benefits have been revealed as the ratio of the diffusivities of carbon in solid to that in liquid is increased. A potential advantage is the reduction in the volume fraction of the residual liquid enriched with carbon during the late solidification. Furthermore, the magnitude of the chemical driving force for the phase transition of cementite precipitation will be lower. The combined influence of both factors results in an exponential decrease in the volume fraction of cementite formed at the end of solidification.
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The characteristics and transformation of M2C and MC, the most common primary carbides in Cr4Mo4V bearing steel, have been investigated via scanning electron microscopy, energy dispersive ...spectrometry and X-ray diffraction at high austenitization temperatures. It was first found that the transformation of rod-like M2C and spherical MC primary carbides during heating occurs at approximately 1180 and 1200 °C, respectively. In the transformation process of M2C, the periphery first transforms into an MC-type carbide, and then the internal untransformed M2C and newly-formed MC-type melt, and re-solidify in a M2C” eutectic carbide structure with a composition change and a finer lamellar spacing. In contrast, MC carbides melt directly with the re-solidified product also being constituted of finer M2C” eutectic carbides. The transformation process is controlled by the diffusion of alloying elements Mo and V, while the matrix provides Fe for the formation of M2C”. In addition to elucidate the transformation mechanisms, this study also highlights the essential effect of the melting matrix on the modification of primary M2C and MC carbides in Cr4Mo4V steel.
•Transformation of primary M2C and MC carbides in Cr4Mo4V were investigated at high austenitization temperatures.•Primary M2C carbide is rod-like Mo − M2C, while M2C is spherical V − MC, and they both remain unchanged below 1180 ℃.•The transformation of Mo − M2C consists of a solid-solid phase transformation, melting, and re-solidification.•The transformation of V − MC consists of melting and re-solidification, and the product is finer M2C”.
The effect of Ta addition on characterization of primary MC carbides with regard to their morphology, composition, and size in Ti-Nb-Mo-W-alloyed Ni-based superalloy were investigated. Several tools ...including automated scanning electron microscopy as well as electrolytic etching technology were used for broad characterization of carbides including three-dimensional morphology. The addition of Ta significantly influences the composition and morphology of primary MC carbides. Large dendrite (Ti,Nb,W,Mo)C carbides were changed to smaller (Ta,Ti,Nb)C carbides. The hardness of both MC primary carbides and the matrix were increased. MC carbides in the studied superalloys were developed dendrite in three-dimensional morphology, which was much larger than that in two-dimensional morphology. The formation mechanism of carbides was clarified by thermodynamic calculation.
•Ta addition changed the composition and size of primary carbides significantly.•Ta addition increased the hardness of both MC primary carbides and the matrix.•Automated SEM was used to automatically analyze the characteristic of carbides.•Electrolytic etching technology was used for 3D characterization of carbides.