A comprehensive study of the Ce addition effect on microstructure, inclusions, and primary carbides in H13 steel was carried out. 3D morphology of inclusions and primary carbides was assessed by the ...non-aqueous electrolytic method. The addition of 0.0038 mass% Ce had no obvious refinement effect on dendritic structures in H13 steel. With Ce content increase from 0.0038 to 0.019 mass%, dendritic structures of H13 steel were refined. Al2O3 and MnS inclusions in original H13 steel promoted heterogeneous nucleation of primary carbides. Al content increased, and S content decreased with increasing Ce content. When the latter was increased from 0.0038 to 0.019 mass%, the original inclusions were modified first to Al11O18Ce and CeAlO3, then to Ce2O3, and, finally, to Ce2O2S. Numerous small-sized Ce2O2S inclusions were found in steel with 0.019 mass% Ce, which promoted nucleation of γ-Fe during solidification and contributed to the refinement of as-cast dendritic microstructures of H13 steel. Besides, Ce2O2S inclusions suppressed heterogeneous nucleation of primary carbides. The size of primary carbides decreased, and their morphology became less developed due to the finer microstructure hindering their growth. Finally, banded structures in forged Ce-bearing H13 steel were improved.
A comprehensive study of the Ce addition effect on microstructure, inclusions, and primary carbides in H13 steel was carried out. 3D morphology of inclusions and primary carbides was assessed by the ...non-aqueous electrolytic method. The addition of 0.0038 mass% Ce had no obvious refinement effect on dendritic structures in H13 steel. With Ce content increase from 0.0038 to 0.019 mass%, dendritic structures of H13 steel were refined. Al2O3 and MnS inclusions in original H13 steel promoted heterogeneous nucleation of primary carbides. Al content increased, and S content decreased with increasing Ce content. When the latter was increased from 0.0038 to 0.019 mass%, the original inclusions were modified first to Al11O18Ce and CeAlO3, then to Ce2O3, and, finally, to Ce2O2S. Numerous small-sized Ce2O2S inclusions were found in steel with 0.019 mass% Ce, which promoted nucleation of γ-Fe during solidification and contributed to the refinement of as-cast dendritic microstructures of H13 steel. Besides, Ce2O2S inclusions suppressed heterogeneous nucleation of primary carbides. The size of primary carbides decreased, and their morphology became less developed due to the finer microstructure hindering their growth. Finally, banded structures in forged Ce-bearing H13 steel were improved.
The tensile crack initiation and propagation of M50 bearing steel were investigated. It is found that the primary carbides were fractured during the tensile test by in situ SEM, while decohesion of ...the carbide/matrix interface was hardly observed. In order to capture the initial tensile crack, quasi-in situ X-ray microtomography was performed, indicating that the earliest crack was formed from the broken interior carbides. This indicates that multiple cracks already existed in the matrix when they were observed on the sample surface. Electron backscattered diffraction (EBSD) and the finite element method (FEM) were applied to analyze the fracture behavior of carbide particles. The fracture of carbide particle is produced by the incompatible deformation between the matrix and carbides. The frequency of carbide fracture was found to be related with its fracture strength, size and shape. M2C carbide fracture was more easily broken than the MC carbide during tensile test. In addition, the long axis of most M2C carbides is approximately parallel to its (0001) plane and normal to tensile stress, leading to cracking more easily. Finally, a suggestion of adding pre-deformation is proposed to make M2C carbides broken sufficiently and further refine the primary carbides.
•X-ray microtomography is applied to analyze the evolution of primary carbides.•Incompatible deformation between the matrix and particles leads to carbide fracture.•M2C carbide is more easily broken than MC carbide.•Pre-deformation before diffusion annealing should be applied to refine carbides.
•Impact of sample preparation technique on visualized microstructure constituents.•Evolution of microstructure and primary carbides during heat treatment.•Characterization of sub-micron primary ...carbides with EBSD single-spot measurements.•Primary carbides MC, M2C and M6C in investigated dual hardening steel.
Dual hardening steels combine precipitation of both secondary hardening carbides and intermetallic phases in a martensitic matrix. Due to this combination, the carbon content necessary to achieve high hardness levels can be reduced, resulting in a decreased amount of large and embrittling carbides.
In this study, the influence of different heat treatments on microstructure evolution and secondary hardness is investigated. Different metallographic preparation methods were tested in order to visualize the microstructure. Carbides were characterized using spot-pattern electron backscatter diffraction.
For light optical investigations, preparation with V2A-pickle lead to the best results. Preparation with colloidal silica suspension achieved the best results for investigations by scanning electron microscopy and for carbide characterization using electron backscatter diffraction. It was found that a homogenization treatment prior to austenitization was unable to increase the amount of dissolved carbides, and thus had no effect on secondary hardness. By increasing the austenitization temperature, the amount of carbides and secondary hardness could be increased significantly.
The effect of cerium (Ce) on the solidification microstructure of Cr4Mo4V bearing steel was investigated via a combined experimental and theoretical method. With a trace amount (0.056 wt%) of Ce ...addition, the coarse columnar grains in as-cast microstructure transform into equiaxed ones, and the average diameter is reduced from 56 to 27 μm. The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined, and their volume percentage decreases from 4.15 vol% to 2.1 vol%. Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries, both contribute to the refinement of grains. Thermodynamic calculations reveal that primary carbides are precipitated after γ-austenite forms near the end of the solidification process. The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted, while the growth is restrained owing to the less segregation of alloying elements.
The grains and primary carbides of Cr4Mo4V bearing steel was refined with the proper addition of Ce. We find that Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries, both contributed to the refinement of grains. The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refined γ-austenite grains in which the nucleation of carbides is promoted, while the growth is restrained owing to the less segregation of alloying elements. Display omitted
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The cooling process of liquid Fe-C and Fe-Cr-C systems from 2100 K to 1100 K was studied by ab initio molecular dynamics simulations. The evolution of local atomic structure was ...statistically analyzed by the pair correlation function, structure factor, coordination number, bond angle distribution, and the Honeycutt-Anderson (HA) index as well as the Voronoi index. We found that C atoms tend to be surrounded by metal atoms and the strong chemical bonds of Fe-C and Cr-C make the C-centered clusters more stable than Fe-centered clusters. The two alloy systems are dominated by icosahedral order in the liquid state but show different evolutionary trends of atomic structures during the cooling process. The Fe-C system undergoes a bcc-like crystalline phase transition at 1500 K, while the Fe-Cr-C system can be supercooled to an amorphous state with decreasing disclination density in the polytetrahedral packing. The analyses of clusters indicated that doping Cr can introduce new local topological and chemical orderings and thus renders a better glass-forming ability. Furthermore, to understand the mechanism of carbides precipitation from undercooled liquid of high-carbon chromium bearing steel, we simulated the local structures of several typical Fe and Cr carbides. We identified a close connection in the short-range order between the carbides and metallic liquids, which provides a theoretical basis for the formation of primary carbides.
The transformation of primary MC and M2C carbides, during high-temperature treatment, was investigated by quasi in-situ observation of Cr4Mo4V steel. It was found that the V-rich rod-like MC carbides ...have great thermal stability, and disintegrating and coarsening are their dominant transformation mechanisms. The M2C carbides mainly possess rod-like (M2C-R) and lamellar (M2C-L) morphologies. The M2C-R carbides are Mo-enriched, and their transformation follows the paths: the separate precipitation of MC and M2C-R + γ→MC + M6C. The V- and Fe-rich MC carbides nucleates randomly in the interior of M2C-R before the nucleation of Mo- and Fe-rich M6C carbides, which is controlled by the diffusion of V. Soaking for longer durations can cause spheroidization and coarsening of the MC carbides, along with the dissolution of M6C carbides. The evolution of Fe- and Mo-rich M2C-L carbides consists of two stages: the partial dissolution of M2C-L into the matrix and the transformation of M2C-L→MC + γ. These novel findings contribute towards the understanding of how primary carbides evolve during heat treatment and provide a potential method for carbides refinement in Cr4Mo4V steel.
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•The transformation behavior of primary carbides in Cr4Mo4V steel was investigated.•Disintegrating and coarsening are the dominant transformation mechanisms of V-rich rod-like MC carbides.•The M2C carbides mainly possess rod-like (M2C-R) and lamellar (M2C-L) morphologies.•The transformation process of M2C-R carbides follows the paths: the separate precipitation of MC and M2C-R + γ→MC + M6C.•The evolution of M2C-L carbides consists of the partial dissolution of M2C-L and the transformation of M2C-L→MC+γ.
This study aims to investigate the primary carbides precipitation in H13 steel solidified at relatively high cooling rates, ranging from 300 to 6,000 °Cmin
−1
, based on in situ observations with a ...high temperature confocal laser scanning microscope. In the cooling rate range investigated, the solidification microstructure becomes more refined as cooling rate increases and the relationship between the secondary dendrite arm spacing (SDAS),
λ
2
, and cooling rate,
T
˙
, can be expressed as
λ
2
=
128.45
T
˙
−
0.124
. Regardless of cooling rates, two kinds of primary carbides, i.e., the Mo-Cr-rich and V-rich carbides, are precipitated along the interdendritic region and most of them are the Mo-Cr-rich carbides. The morphology of Mo-Cr-rich carbide is not obviously influenced by the cooling rate, but that of V-rich carbide is obviously affected. The increasing cooling rate markedly refines the primary carbides and reduces their volume fractions, but their precipitations cannot be inhibited even when the cooling rate is increased to 6,000 °C·min
−1
. Besides, the segregation ratios (SRs) of the carbides forming elements are not obviously affected by the cooling rate. However, compared with the conventionally cast ingot, the SDAS and primary carbides in the steel solidified at the investigated cooling rates are much finer, morphologies of the carbides have changed significantly, and SRs of the carbides forming elements are markedly greater. The variation of primary carbide characteristics with cooling rate is mainly due to the change in SDAS.
The large primary carbides in H13 steel significantly affect its service life. In this study, pre-dispersion technology is used to disperse CeO2 nanoparticles, which are then introduced into H13 ...steel. The effects of CeO2 nanoparticles on the inclusions, primary carbides, microstructures, and properties of H13 steel are investigated. Ce-containing inclusions with size of 0.5–2 μm are generated and distributed uniformly in H13 steel owing to the reduction of CeO2 by Al and C. Both the dendrite structure and primary carbides in H13 steel are refined by CeO2 nanoparticles. The secondary dendrite arm spacing decreases from 13.51 ± 2.34 to 6.90 ± 1.23 μm. The size and number of primary carbides decrease from 1 to 5 to 1–3 μm, and from 1125 to 725/mm2, respectively. This is attributable to the disregistry of Ce-containing inclusions and primary carbides/austenite. The segregation of alloy elements in H13 steel is calculated using the Clyne–Kurz model. The properties of H13 steel are improved via the addition of CeO2; in particular, its tensile strength, total elongation rate, and hardness are 726.38 MPa, 23.43%, and 243.49, respectively, which corresponds to increases by approximately 5.58%, 8.27%, and 10.11%, respectively. The results can facilitate improvements to the performance of H13 steel and the development of rare earth steel.
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