In this paper, the effect of solidification pressure on the dendrite structure and characteristics of carbides in H13 die steel ingot was investigated by experimental and calculational methods. Based ...on the effect of pressure on the cooling rate, a formula is proposed to calculate the secondary dendrite arm spacing: λ2 = 71.45 × R−0.37. It is applicable when the maximum value of pressure is around 2 MPa and the cooling rate is between 0.5 and 3 K/s. With increasing pressure from 0.1 to 2 MPa, the effects of pressure on the segregation ratio of V, Mo, Cr and C are little and can be neglected, which caused by the combined effect of equilibrium partition coefficient, diffusion coefficient and cooling rate. Therefore, the characteristics of carbides are determined by the decreasing the secondary dendrite arm spacing and increasing cooling rate with the increment of pressure. With increasing pressure from 0.1 to 2 MPa, the types of carbides are not change, which are MC and M2C in H13 die steel ingot. Meanwhile, the mean area of carbides decreases obviously with increasing pressure, and the decrement in mean area at the edge is larger than that at the center of ingot.
The carbide characteristics of high vanadium high-speed steel (W3Mo4Cr5V6) manufactured by electroslag remelting (ESR) are analyzed by field emission scanning electron microscopy (FESEM), electron ...probe microanalysis (EPMA) and micro-Vickers. The results illustrate that three types of carbides are mainly formed in W3Mo4Cr5V6 which includes the lumpy and strip-like MC in the intracrystalline, lamellar M2C at grain boundary with discontinuous network and globular secondary carbides dispersed in matrix, among these, the eutectic strip-like MC occupies the main part. The more uniform distribution and finer size for both MC and M2C are obtained with high solidification rate (SR). Both 2D and 3D morphologies of MC carbides show obvious symmetry with clustered distribution and various shapes. The MC and M2C are rich in V and Mo, W respectively, the compositional characteristics are not only related to the carbide type and morphology, but also the precipitation order and distribution density. The Vickers hardness of primary MC, eutectic MC and M2C are measured and the average values are 2205 HV, 850 HV and 896 HV respectively which includes the adverse effects of soft matrix on the test results of hard carbides. A new and exact test method for pure carbides should be developed in the future.
Ultrahigh-strength (UHS) steels have shown great potential in the field of high-end equipment manufacturing in demand of lightweight engineering and performance upgrade. A significant research effort ...has been directed toward the development of advanced UHS steels with excellent combination of strength and toughness. In the course of development, tailoring precipitates by means of composition design and process optimization is absolutely a critical module. In this mini review, typical UHS steels strengthened by carbides and intermetallics are systematically summarized and discussed. With the increase of strength, the toughness losses of UHS steels strengthened by carbides and intermetallics have been compared in detail. In particular, the in-depth mechanisms leading to various strength/toughness variation trends have been discussed, extracting the bottleneck in developing new-generation UHS steels containing merely one type of precipitate. Meanwhile, prospects on designing advanced UHS steels strengthened by coexisting dispersive precipitates have been proposed to achieve better performance.
The effects of cooling rates on solidification behaviors, segregation characteristics and tensile property of GH4151 alloy were investigated using microstructure characterization and tensile test. ...Firstly, a relationship between the secondary dendrite arm spacing and cooling rate was determined and it was confirmed to be valid. Secondly, it can be found from microstructure observations that the morphology of (Nb, Ti)C carbides transits from blocky and script type to fine script type and spotty type, and the refined
γ
′ phase was observed due to decrease of segregation with increasing cooling rates. Thirdly, the solidification microstructures of the industrial-scale samples were analyzed. The morphology of
η
phase changes from indistinguishable shape, fine needle-like shape to large block-like shape with increasing ingot diameter. As a result, the mechanical properties of alloy decrease due to increase of brittle precipitations. The experimental results show that the precipitation behavior of GH4151 is affected by segregation degree of elements, and the segregation degree is determined by solute distribution process and solid back-diffusion process.
Thermodynamic modeling of the Si-P and Si-Fe-P systems was performed using the CALculation of PHAse Diagram (CALPHAD) method based on critical evaluation of available experimental data in the ...literature. The liquid and solid solutions were described using the Modified Quasichemical Model accounting for the short-range ordering and Compound Energy Formalism considering the crystallographic structure, respectively. In the present study, the phase boundaries for the liquidus and solid Si phases of the Si-P system were reoptimized. Furthermore, the Gibbs energies of the liquid solution, (Fe)
(P,Si)
, (Fe)
(P,Si)
, and (Fe)
(P,Si)
solid solutions and FeSi
P
compound were carefully determined to resolve the discrepancies in previously assessed vertical sections, isothermal sections of phase diagrams, and liquid surface projection of the Si-Fe-P system. These thermodynamic data are of great necessity for a sound description of the entire Si-Fe-P system. The optimized model parameters from the present study can be used to predict any unexplored phase diagrams and thermodynamic properties within the Si-Fe-P alloys.
Rolling deformation is the fundamental process path in steel production, which not only changes the shape and size of the billet, but also improves the internal structure and mechanical properties. ...In the deformation process of steel matrix, the inclusion also evolves. In this paper, the research on inclusion evolution in solid steel rolling deformation process is reviewed, and the corresponding control strategies are pointed out from the perspective of inclusion deformation mechanism. The control methods for MnS focuses on reducing the difference in the hardness between MnS and steel matrix and on selecting the suitable rolling deformation process. The control method for silicate inclusions primarily focuses on altering the composition. The control direction of well-modified calcium aluminate inclusions involves removing the large size particles completely. Moreover, the strategy for inclusions with high hardness is to control their size. Finally, avenues for future research on inclusion deformation are proposed.
The N-containing Fe–Cr–Ni–Nb austenitic heat-resistant steels have become the research focus of high-temperature material. Nitrogen plays an important role on the strength and structural stability of ...the steels, and thus the accurate control of nitrogen content is of great significance to the smelting process. In this paper, the nitrogen solubility in liquid Fe, Fe–Nb, Fe–Cr–Nb, Fe–Ni–Nb and Fe–Cr–Ni–Nb systems from 1823 to 1873 K were investigated by gas-liquid metal equilibrium experiments. In liquid Fe–Nb system with a niobium content of 5 to 20%, the solubility of nitrogen increased with niobium content. The first-order interaction parameter of niobium on nitrogen at 1873 K and its relationship with temperature were determined as follows: , . In the liquid Fe–Cr–Nb and Fe–Ni–Nb systems, the second-order cross-interaction parameters of chromium or nickel with niobium on nitrogen were determined as follows: , . Furthermore, a more accurate nitrogen solubility prediction model for the liquid Fe–Cr–Ni–Nb system was established based on the existing thermodynamic parameters and the interaction parameters obtained in this study.
This study systematically investigated the influence of high nitrogen (N) addition (0.205 wt.%) on microstructure and mechanical properties of as-cast M42 high speed steel. The results demonstrate ...that the conventional and high-nitrogen M42 cast ingots are mainly composed of martensite, retained austenite and various precipitates (M2C, M6C as well as MC in M42 cast ingot or M(C, N) in M42N cast ingot). The addition of N could increase the retained austenite content, trigger the transformation of MC to M(C, N), favor the formation of M2C at the expense of M6C, and improve the distribution uniformity of M6C at the macroscopic scale. Moreover, the addition of N could lead to the reduction of the secondary dendrite arm spacing as well as the decrease of the thickness and area fraction of eutectic carbides, and improve the distribution uniformity of eutectic carbides at the microscopic scale. The M(C, N) particles form directly from the liquid phase prior to the formation of primary austenite, which could act as the heterogeneous nuclei of primary austenite and thus promote the refinement of the as-cast microstructure. The addition of N slightly decreases the macro-hardness and ultimate compression strength of the cast ingot but increases its ductility, which could be ascribed to the increase of retained austenite content and the reduction in the amount of eutectic carbides. Therefore, high N addition can significantly improve the as-cast microstructure of M42 high speed steel, which is promising for the further enhancement of the mechanical property and service life of the final product.
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