The precise control of nitrogen content in high nitrogen martensitic stainless steel is the guarantee of its excellent performance, and is a crucial issue for its manufacturing process using pressurized gas nitriding. In this paper, the effects of nitrogen pressure, temperature and alloy composition on nitrogen solubility and pressurized gas nitriding kinetics of Fe–Cr–Mo–C alloy melts were investigated. In order to correct the deviation of nitrogen solubility from Sieverts’ law under high nitrogen pressure, the first- and second-order interaction parameters of nitrogen on itself were obtained; accordingly, the nitrogen solubility model under pressure was established and well verified. The nitrogen solubility decreased with increasing temperature and C content, and increased with increasing Cr and Mo contents. The nitrogen mass transfer in liquid phase was the nitriding rate-determining step under different nitrogen pressures. The apparent mass transfer coefficient of nitrogen was approximately 0.0218 to 0.0230 cm·s−1, and showed a weak dependence with pressure. The apparent mass transfer coefficient of nitrogen increased significantly with the increase of temperature and electromagnetic stirring, and was rarely affected by the content of Cr, Mo and C. Considering the influence of pressure change on nitriding rate in pressurization stage, a novel kinetic model for gas nitriding under high pressure was established and exhibited effectiveness in practice.
A 1/5 scale water model simulation was used to study the kinetic mechanism for 300 t mechanical stirring desulfurization station, factors including rotating speed, immersion depth and size of impellers were investigated. The results show the dispersion behavior of desulfurizer can be divided into three stages, (1) the entrainment depth of desulfurizer increases with the rotating speed increasing, (2) the entrainment depth of the desulfurizer remains unchanged, (3) the desulfurizer is thrown out into the water from impeller blades. For the reason that the desulfurizer is sticky to the top surface and the middle of impeller blades, which will cause the shortening of the impeller life, the optimum rotating speed is the minimum rotating rate that the desulfurizer reaches the upper surface of impellers, which is related with immersion depth of impellers compared with the bath level and the length of impellers compared with the diameter of the ladle. After the application of a larger impeller, with 14.0% impeller life longer than before and the sulfur element is removed to 0.0006% on average.
The viscosity of the slag constituted by 55%CaF2–20%CaO–3%MgO–22%Al2O3–x%B2O3 (mass fraction: x ≤ 3) was measured by rotating cylinder method during the electroslag remelting (ESR) of 9CrMoCoB steel to analyze the effects of the B2O3 content on slag viscosity, break point temperature, and activation energy for viscous flow. Meanwhile, the slag’s structural characteristics were studied by Raman spectrometry. Results indicated that when T ≥ 1663 K, the viscosity changed gently with temperature, showing the value of about 0.055 Pa·s, which was rarely affected by w(B2O3). However, with the increase of w(B2O3) in the slag, the Ca3B2O6 with a low melting point was formed, reducing the slag’s break point temperature from 1628 K to 1563 K. According to the NBO/T value and Raman spectra, B2O3, as a network generating oxide, improved the slag’s degree of polymerization, thus strengthening and complicating its network structure at high temperatures. In addition, as the w(B2O3) increased from 0 to 3%, the activation energy for viscous flow rose from 64 kJ·mol−1 to 94 kJ·mol−1 in the slag.
In the present work, the dephosphorization experiments by using new double slag converter steelmaking process (NDSP) has been carried out in a 180 ton top-bottom combined blowing converter under the low temperature range of 1345–1450°C and low basicity of about 1.50. With increasing dephosphorization endpoint temperature, the dephosphorization ratio, the P2O5 content and phosphorus distribution ratio logLP first increase and then decrease. The apparent equilibrium constant of dephosphorization reaction first increases and then decreases, and the viscosity of dephosphorization slag decreases. The average area fraction of the P-rich phase first increases and then decreases, and those of the matrix phase and the Fe-rich phase increase and decrease, respectively. The morphologies of P-rich phase change from small oval shapes to long strips, then to massive shape, and further to irregular small blocks. The value of the coefficient n in nC2S–C3P of P-rich phase first decreases from 4.8–6 to 2–4.8, then increases to 6–20. Based on ion-molecule coexistence theory (IMCT), C2S and C3P have the strongest affinities in the calcium silicate and calcium phosphate in the dephosphorization slag, respectively. Increasing the enrichment degree of C2S–C3P in the dephosphorization slag is conducive to improving the phosphorus enrichment capacity of the dephosphorization slag and dephosphorization ratio of hot metal. The changing trends of phosphorus enrichment degree in dephosphorization slag characterized by the measurement results of slag phases and the calculation results of IMCT are well consistent, indicating that the IMCT calculation results can correctly express the phosphorus enrichment degree of dephosphorization slag.
The low-reactive tundish flux is urgently required for the smelting of rare-earth steels. The effect of Ce2O3 content on the properties (melting point and viscosity) and structure of CaO–Al2O3–Ce2O3–MgO–SiO2 slag were analyzed in the present investigation. The research results indicated that the polymerized QAl4 and QAl3 units were modified to QAl2 units in aluminate, and the depolymerization of SiO4-tetrahedrons from QSi1 to QSi0 units with Ce2O3 addition increasing from 5 wt% to 20 wt%. In addition, when the Ce2O3 addition increased from 15 wt% to 20 wt%, the increasing area fraction in AlO6-octahedral units further led to a decrease in the degree of polymerization (DOP) in the melts. When the Ce2O3 addition increased from 5 wt% to 20 wt%, the melting temperature decreased from 1320°C to 1301°C. Additionally, the softening temperature and the fluidity temperature also decreased 20°C and 24°C, respectively. The viscosity decreased as the Ce2O3 addition increased from 5 wt% to 20 wt%, which was attributed to the decreased DOP and the increased superheat. A reasonable content for Ce2O3 should not be excess 10 wt% in the current slag.
The viscosity of 30%CaO-30%SiO2-15%Al2O3-5%MgO-10%Na2O-10%CaF2-xCe2O3 (mass%, x = 0, 5, 10, 15) were measured by the rotating column method, and then the viscosity and the Tbr (break temperature) of the mold flux were analyzed based on the results. Meanwhile, the structural characteristics of the mold flux were investigated using Raman spectroscopy and XRD (X-ray diffraction). The results show that Ce2O3 predominantly destroys the silicate network structure at high temperature, reduces the polymerization degree of the mold flux, simplifies the high-temperature structure of the mold flux, and reduces the friction resistance of viscous flow. From the apparent phenomenon, the viscosity of the mold flux decreases at high temperature. In addition, with the increase of Ce2O3 content in the mold flux, Ca4Si2O2F changes to Ce9.33(SiO4)6O2, which enhances the crystallization ability of the mold flux and increases the Tbr of the mold flux.
This study examines the evaporation of Al from a molten commercial Ti alloy with different Al concentrations after partial melting in a small electron beam furnace. Electron probe micro analysis confirmed that the Al concentration in the molten region was uniform. The movement of Al in the molten region was not found to be the rate determining step. The Al concentration in the molten region consistently decreased with an increasing melting time but in a non-linear manner. The activity of Al in the molten alloy was calculated using thermodynamic data, which indicated that it increased with an increasing Al concentration in the ingot. However, the activity of Al in the molten alloy did not increase linearly under the influence of the other alloying elements. The overall mass transfer coefficient of Al from molten alloy during evaporation increased with an increasing initial Al concentration with the mass transfer coefficient of Al depending on the activity of Al. An evaporation model was constructed by considering the mutual interaction between Al and the other alloying elements. This evaporation model was able to predict the amount of Al evaporation from the multi-component Ti alloy melt.
In this study, the rheological and crystallization behavior of CaO–BaO–Al2O3-based mold slags was investigated through the measurement of the viscosity-temperature relationship and the record of crystallization behavior during the continuous cooling process from 1300 to 600°C applying a modified confocal scanning laser microscopy. Variations of the viscosity, break temperature, initial crystallization temperature and crystallization phases of mold slags with the substitution of SiO2 by Al2O3 and Al2O3 by BaO at a gradient interval of 4 mass% were discussed, and crystallization parameters of average crystallization rate, Ozawa index and effective crystallization activation energy were calculated to explain the crystallization performance. The slag viscosity at 1300°C, the melting and break temperature increased with the substitution of SiO2 (16 to 0 mass%) by Al2O3 (20 to 36 mass%), while those decreased with the substitution of Al2O3 (32 to 16 mass%) by BaO (4 to 20 mass%). With the gradual substitution of SiO2 by Al2O3, the initial crystallization temperature increased from 820 to 1273°C at the cooling rate of 1°C/s, major precipitated phases gradually changed from CaF2 to CaF2, LiAlO2 and BaAl2O4, the average crystallization rate and the Ozawa index fluctuated but had the same tendency. With Al2O3 gradually replaced by BaO, the initial crystallization temperature decreased from 970 to 775°C at the cooling rate of 1°C/s, major precipitated phases changed from CaF2 to CaF2 and BaAl2O4, the crystallization rate of slags was affected by the difference of the nucleation and growth rate of different crystals.
The complex inclusions of Al2O3–SiO2 is a common inclusion in the steel, which leads to the stress concentration in the steel products. The formation of MnS on the surface of Al2O3–SiO2 could reduce the stress concentration due to the high plasticity of the MnS. In this study, the formation of MnS–Al2SiO5 complex inclusions is investigated at the atomic level by using first principles calculation based on the density functional theory (DFT). The adsorption energy of the atoms of Mn and S on the Al2SiO5 (110) surface was calculated with various initial positions and sequence. The interaction among the atoms was calculated to analyze the stable structures after the adsorption of the Mn and S on the Al2SiO5 (110) surface. The formation of the MnS on the surface of Al2SiO5 was proved by analyze the structure formed by the adsorbed atoms that shows the similar tetracyclic structure of the MnS crystal.
Lowering the reducing agent rate (RAR) of blast furnaces (BFs) is an effective way to mitigate CO2 emissions in the steel industry. It is well known that low-slag sintered ores can be effective at lowering the RAR. Therefore, in this study, we focused on the manufacturing technologies of low-slag high-FeO sintered ores. High-temperature sintering was used to maintain enough strength before and during the reduction, resulting in the chemical composition of sintered ores with a T.Fe above 60 mass% and an FeO content above 8 mass%. Improvement in the permeability of the sintering bed and the melt properties were the main challenges. Stand-support sintering, selective granulation, and binder use are essential for improving the permeability. The use of brucite and reducing the Al2O3 content of ores while designing their penetration length are essential for improving the melt properties as well. We implemented all of these measures in plant trials, and low-slag high-FeO sintered ores were successfully manufactured. The sintered ores exhibited a low slag volume in the cohesive zone, resulting in a lower RAR of the BF both in the offline simulator and actual plant operation. The sintered ores differed from other low-slag sintered ores manufactured previously in two aspects; large numbers of both small-sized magnetite grains and voids between grains. The reducibility of magnetite was superior to that of hematite with CaO doping, whereas MgO doping had the opposite effect. These behaviors of magnetite may explain the high reducibility of the manufactured low-slag high-FeO sintered ores.
In this work, large-scale simulations of the blast furnace hearth are presented, conducted using a model combining Computational Fluid Dynamics, the Volume of Fluid method, and the Discrete Element Method. Using a 5 m diameter, full-3D geometry, the influence of burden weight, bi-disperse packing, and blocked tuyeres on the liquid and solids flow within the hearth are investigated. Horizontal and vertical porosity profiles are presented, and the influence of the dynamic liquid level on the state of the deadman is evaluated. The liquid iron flow during tapping is visualised, and the influence of a coke-free space on the flow pattern is analysed. The magnitude of the circumferential flow through the corner of the hearth is analysed, and found to decrease with increasing burden weight pressure and coke diameter in the bed centre. A significant influence of the dynamic deadman on the liquid flow pattern is found, especially in case of a floating deadman. In addition to the liquid flow, the solid coke flow towards the raceways is analysed. Two pathways for coke particles towards the raceway are uncovered, one path through the actively flowing layer above the deadman, and a second path moving through the deadman and entering the raceways from below. The balance between these two mechanisms was found to change during the tapping cycle. Lastly, implementations for heat and dissolved carbon mass transfer are presented, and demonstrated using a full-scale 10 m hearth simulation. Additional closures for heat and mass transfer rates are required, but the current model is found in good shape for future work.
Reduction behavior of various multi-component calcium ferrites at 900°C were investigated by using in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy. Reaction products (intermediate components) were determined by XRD and change in X-ray absorption spectra of Fe and Ca K-edges were analyzed to determine reaction rate constants. SFCA-I (Ca3(Ca,Fe)(Fe,Al)16O28) and SFCA (Ca2(Fe,Ca)6(Fe,Al,Si)6O20) consist of layered structure of spinel and pyroxene. Early stage of reduction reaction, diffraction peaks of spinel structure were observed which indicating SFCA-I and SFCA decomposed into these units at the first step of the reduction reactions. The spinel was reduced sequentially into FeOx then Fe. Intermediate component, Ca2(Fe,Al)2O5 originated in pyroxene module was hard to reduce and reaction was controlled by decomposition of this phase. Reduction of SFCA-I started later than SFCA (with 5.7 mol% Al2O3) under hydrogen gas reduction condition at 900°C. SFCA with a high aluminum content indicated lower reducibility than that with a low one.
Present day, the production of hot metal (HM) via the blast furnace route remains an integral part of the global steel industry. With global pressure to curb greenhouse gas emissions, injection of hydrogen is considered a promising solution while ironmaking transitions to alternate technologies. A comprehensive heat and mass balance model calibrated to an operating blast furnace was used to assess the operational limits of hydrogen injection through the tuyeres, replacing Pulverised Coal Injection (PCI). Constrained by a minimum top gas temperature and minimum Raceway Adiabatic Flame Temperature (RAFT), the maximum injection rate was determined to be 19.5 kg-H2/t-HM when replacing 37.4 kg-PCI/t-HM (i.e. a replacement ratio of 1.9 kg-PCI/kg-H2 or 1.54 kg-C/kg-H2). At the maximum hydrogen injection rate, the specific CO2,eq emissions were seen to decrease by 8% in the top gas. In the case where the increased level of hydrogen increases stack reduction efficiency, the maximum hydrogen injection rate is decreased, while the replacement ratio is increased significantly. A maximum hydrogen injection rate of 14.3 kg-H2/t-HM with a replacement ratio of 4.5 kg-PCI/kg-H2 was achieved when the stack reduction efficiency was 100%, with a CO2,eq emission decrease of 14%. The optimal scenario for injection of hydrogen was determined to be maintaining a constant production rate, allowing the RAFT to decrease, and replacing PCI.
Current global demand for the carbon emission reduction and the depletion progress of high-grade iron ore deposits require the integrated solution toward sinter and ironmaking process. Several countermeasures have been proposed regarding the development of highly reducible burden materials for the blast furnace and the use of beneficiated fine iron ores in the sintering machine. However, their application to actual sintering and ironmaking equipment have been limited because the sinter productivity is deteriorated. Here the present study investigated co-production of the carbon composite pellet and sinter with existing sintering machines. Even if fired in an oxidizing atmosphere, carbon-core-pellets can be produced under conditions that simulate the conventional sintering process. This is because a protective shell over the carbon core inhibits the carbon from combustion. Importantly, the structure and composition of the pellets are properly designed so that this protective shell does not melt overly or break during firing. The design parameters govern the strength and reducibility of pellets after firing, and those influence is analyzed quantitatively. Overall, a pot test trial in which designed pellets have been blended exhibited the viability of the pellet-sinter co-production.
In this study, the influence of several operational variables of secondary steelmaking on the desulfurization process in a gas stirred ladle furnace was analyzed by mathematical simulation. The solution of the Navier-Stokes and energy equations were carried out using the multiphasic Eulerian model, while an User Defined Function, UDF, was developed and incorporated to numerical simulation to solve the kinetics of desulfurization. Drag and non-drag forces were also included, as well as interfacial exchange models to solve the interaction among phases. The fluidynamics results were validated by means of an experimental model by employing the Particle Image Velocimetry, PIV, technique. It was found that employed models accurately predicted slag layer behavior and steel velocity field. The results showed that the distribution of the sulfur contained into steel depends on the fluidynamics structure, initial steel temperature and injected argon gas rate. Also, the incorporation of the desulfurization kinetics through UDF’s coupled to the non-isothermal multiphase modeling, is more realistic in the prediction of the sulfur removal rate compared to the use of only kinetic models.
Energy consumption is a vital aspect in the production of high quality materials by using electro slag remelting as a refining method. The electrical conductivity and the melting point of the slag, determined by the slag composition, as well as the fill ratio, the amount of slag and the melt rate have as significant influence. Several of these factors as well as a new method to measure the slag surface temperature with a two-color pyrometer were investigated in laboratory scale remelting trials. Two different slags with an average and a low electrical conductivity were used. While there is only a small impact of the slag cap height and the melt rate, the slag composition showed a strong effect, both on the slag surface temperature as well as on the specific energy consumption. Additional investigations confirmed that the effect of the slag composition on the amount and compositions of non-metallic inclusions, respectively the cleanliness level of the steel after remelting is rather minor. The results suggest possibilities for easily applicable, improved process parameters, which can combine high product quality with significantly reduced energy consumption.
The effect of the nozzle radial position on both mixing time and slag eye due to bottom gas injection in metallurgical ladles was investigated using physical and mathematical modelling. Previous research has extensively investigated the effects of the nozzle radial position on mixing time, however, the few available results with respect to ladle eye indicate contradictory results. To identify the optimum nozzle radial position with respect to both mixing time and slag eye area a systematic investigation has been carried out. A water model with a geometric scale ratio 1:8 was employed to study the influence of different nozzle radial positions on both the slag eye area and mixing time. A mathematical model was developed using the Euler-Lagrangian approach considering multiphase flow. In previous investigations the optimum nozzle radial position has been defined considering only mixing time.
The experimental results from this work show a clear trend indicating a large influence of the nozzle radial position on the slag eye. Two new equations are proposed to describe the effect of the nozzle radial position on both mixing time and slag eye. An optimum nozzle radial position is suggested which yields high mixing conditions and also a lower slag eye area. Additionally, the best drag force model capable to predict large slag eye areas has been identified.
Surface defect classification plays an important role in the assessment of production status and analyzing possible defect causes of hot rolled strip steel. It is extremely challenging owing to the rare occurrence and various appearances of defects. In this work, an improved deep learning model is proposed to solve the problem of poor classification accuracy when only a few labeled samples can be available. Different from most inductive small-sample learning methods, a transductive learning algorithm is designed where a new classifier is trained in the test phase and therefore can fit in with the needs of unknown samples. In addition, a simple feature fusion technique is implemented to extract more sample information. Based on a real-world steel surface defect dataset NEU, the proposed method can achieve a high classification accuracy of 97.13% with only one labeled sample. The experimental results show that the improved model is superior to other existing few-shot learning methods for surface defects classification of hot-rolled steel strip.
To solve the problem of preparing the integrated batch planning for a steelmaking plant, this paper considers the continuous/quasi-continuous characteristics of the steel manufacturing process operation. On the premise of analysing the constraints of steelmaking procedure and continuous casting procedure, this paper firstly develops the charge plan model and casting plan model, and then forms and solves the integrated batch planning model. According to the actual of the production site, this paper introduces an improved elitist genetic algorithm (IEGA), defines the matching chromosome coding and decoding strategies with the actual, and suggests improving the genetic operators. Finally, we verify the proposed model and algorithm on the production data of a real steelmaking plant. We clarify the applicability of developing an integrated batch planning model based on model analysis and demonstrate the effectiveness of the IEGA through algorithm analysis.
Medium-Cr steel is considered as the most economical and effective pipeline material in a CO2 flooding environment; however, the effects of iron oxides on the corrosion resistance of medium-Cr steel are still not clear. Therefore, in the present study, a high-temperature, high-pressure reactor was used to accelerate the corrosion of a rolled oxidized sheet of 5Cr steel. The corrosion behavior of oxide scale leaching was studied by comparing the mass loss and gain rate with bare steel, through using XRD, Raman, XPS, SEM, EDS, EPMA and other methods. It was found that the 5Cr steel rolled scale was composed of three layers: the innermost layer contained Fe3O4, the middle layer consisted of Fe2O3, and the outer layer contained Fe3O4 and a small amount of Fe2O3. In the CO2 environment, the oxide scale mainly dissolved in the initial stage, and corrosion products were deposited in the later stage. The oxide scale delayed the occurrence of corrosion, and eventually, similar to the bare steel, a double-layer product film was formed. Also, the possible corrosion mechanism of the medium-Cr steel with the oxide scale was given.
Aluminized steel sheets are highly resistant to elevated temperature. When they are heated for several hours above 800°C, part of the coating layer may delaminate owing to the presence of continuous Kirkendall pores, which are formed between the bcc_A2 and bcc_B2 phases because of the differences in the diffusion coefficients of Al and Fe in each phase. In this study, a numerical simulation was performed to predict the occurrence of Kirkendall pores, and the simulation results were compared with experimental results. The Si in the aluminizing bath significantly increased the number of Kirkendall pores owing to its effect on the ratio of the intrinsic diffusion coefficients (DFe/DAl) in the bcc_B2 phase.
Effects of Zr addition on the microstructure formation of two intermetallic phases of Fe2Nb Laves (TCP) and Ni3Nb (GCP) in a carbon-free Fe-20Cr-35Ni-2.5Nb (at.%) novel austenitic heat-resistant steel at elevated temperatures were examined from the viewpoints of thermodynamics and kinetics. From a thermodynamic perspective, the maximum solubility of Zr in the matrix phase is about 0.1 at.% under homogenization treated states at around 1473 K. The dissolved Zr in the matrix eventually reduces the solubility limit of Nb, thereby playing an important role in promoting the formation of grain-boundary Fe2Nb phase as well as that of thermodynamically stable Ni3Nb-δ phase within grain interiors after long-term aging at 1073 K. The Zr dissolved in the matrix after the homogenization treatment is found to enrich in δ phase formed during the aging and raises the temperature limit of formation of δ phase by about 50 K. From a kinetic perspective, because of the increase in Nb supersaturation, the dissolved Zr not only enhances the precipitation of Laves phase at grain boundaries but also promotes the homogeneous nucleation of metastable Ni3Nb-γ″ phase within grain interiors from the beginning of aging and retards the phase transformation from γ″ to δ. These microstructure changes by Zr can be interpreted in terms of the phase equilibria and relative phase stability among different phases. The present results reveal that even a small amount of Zr in solution, not segregation to grain boundaries, has tremendous effects on the precipitation behavior and morphology of intermetallic phases.
Fe–Cr–Co alloys are becoming important as half-hard magnet which can be subjected to plastic deformation process for their novel applications including non-contact electromagnetic brake because of its large hysteresis loss. Its magnetic hardness depends on the modulated structure formed by spinodal decomposition. It is important to clarify the effect of plastic deformation on the spinodal decomposition for optimizing the heat treatment after plastic deformation process. In the present study, we examined the spinodal-decomposed structures in Fe–Cr–Co sheets cold-rolled to 25% reduction and that without rolling to clarify the influences of cold rolling. Also, spinodal decomposition under the presence of dislocation structure have been simulated by phase field method for the case with the presence of dislocation cell boundary with a high in-plane solute diffusivity at various migrating speed. It has been found that the spinodal decomposition is accelerated around dislocation owing to the elastic field and higher diffusivity, which results in inhomogeneous microstructure with various wave length of modulation. The existence of dislocation enhances the initiation of phase decomposition and the growth particles. The decomposed structure greatly depends on the in-plane solute diffusivity and migrating speed of the dislocation cell boundary.
Microstructures and mechanical properties after spheroidizing annealing (SA) of GCr15 bearing steel with and without an alternating magnetic field (AMF) were investigated. It was found that the application of the AMF at the austenitizing stage promoted the dissolution of carbides, accelerated the austenitizing process and increased the hardness of the quenched sample. After isothermal annealing, the carbides in the sample with an AMF more uniformly distributed and became finer in comparison with those without an AMF. The average hardness of the sample treated in the AMF was lower than that without an AMF. The phenomena could be attributed to the enhanced diffusivity in the AMF.
Cast high-nickel austenitic ductile iron (CHNADI) is widely used in high temperature, low temperature and strong corrosive environments due to its good comprehensive mechanical properties. However, its low yield strength limits its application in nuclear power equipment. Herein, the influence of nodularizer and processing temperature on the graphite and mechanical properties of CHNADI was investigated to significantly promote its properties. With the increase of nodularizer dosage, the mechanical properties of tensile strength, yield strength, elongation, and impact toughness first increase and then decrease. When the dosage of nodularizer is 0.9 mass% and the processing temperature is 1480°C, CHNADI exhibits excellent mechanical properties with the hardness of 132 HB, tensile strength of 450 MPa, yield strength of 224 MPa, elongation rate of 46% and impact toughness of 91 J. In this work, a new strategy of preparing CHNADI without heat treatment is proposed to meet the strict requirements of this material, which is significant for the development of CHNADI.
The low mechanical properties of porous materials are a common problem. It is of great significance to produce new porous metal materials with higher impact properties by new preparation process. In this work, a new stainless steel metal powder and wire mesh composite porous plate was prepared by vertical rolling, folding, pressing and sintering. Most importantly, a device, with ultrasonic generator, could generate porous strip, was invented innovatively to feed powder and wire mesh automatically for applying in the first step of the preparation process. The porous strip that used to prepare porous plate had the unique characteristic of a powder-wire mesh-powder sandwich structure. The impact behavior and microstructure were comparatively investigated. The fracture morphologies were observed to determine the fracture mechanisms. The results manifested that sintering temperature had the outstanding influence on the impact properties and fracture morphologies, higher temperature got stronger impact properties. Wedge fracture, cone fracture and circular fracture were main type of fractured warp wire when porous plate was sintered at 1130°C. Quicker ultrasonic frequency, gave little change in fractographs, but brought lower porosity and excellent impact properties, which due to more powder participate in preparation. The gap between two rollers determined combined degree between powder and wire mesh then led to various impact strength, and diameter of the necking with roller gap of 0 mm was less than 0.1 mm.
If more than one fracture toughness test is performed for the same material, the representative fracture toughness value is obtained from the minimum of three equivalent (MOTE) in accordance with the related standards. MOTE is evaluated by referring to the lowest value of the test results when the number of results is 3 to 5, the second lowest when 6 to 10, and the third lowest when 11 to 15. With the conventional process, it is clear, however, that the value depends on the number of test results. When comparing the MOTE evaluation values with 3 and 5 test results, the MOTE based on 5 results is statistically lower than that with 3 results because both MOTEs refer to the lowest value, while increasing the number of test results from 5 to 6 will increase MOTE because the reference of MOTE changes from the lowest value to the second lowest. However, we are of the view that the number of test results should not affect the evaluation value.
Based on the above, we considered two new evaluation methods which are independent of the number of test results, and investigated the effectiveness of those methods by analytical and experimental approaches. As a result, the proposed methods continued to give a constant evaluation value regardless of the number of tests, and the variance was less than that of the conventional method in many cases. Therefore, we concluded that the proposed methods are superior to the conventional method.