The present study investigated the influence of TiC particle addition on the viscosity of CaO–MgO–Al2O3–SiO2 melts by using the rotating cylinder method. It was found viscosity increased as increasing the volume fraction of TiC, but decreased as enhancing the rotation speed. The temperature dependence of viscosity for the same composition can be described by the Arrhenius law. However, temperature has little influence on the relative viscosity of TiC bearing slag. There is also no obvious difference of activation energies for different compositions with or without the addition of TiC solid particle. Therefore, the activation energy of solid-liquid mixture may be mainly determined by the composition of liquid phase. Two kinds of TiC with the particle sizes of 1.0 μm and 10 μm were used to study the influences of solid size on viscosity, and it was found the TiC particle with a small size can increase the viscosity much more greatly. The Einstein-Roscoe equation cannot be used directly to the TiC containing two-phase mixtures. By modifying the parameter, it can well describe the viscosity variation behavior. According to the calculated results, the maximum volume fraction increases as increasing the rotation speed or decreasing the particle size of TiC. The apparent volume of TiC is about 2.2–3.3 times of its real volume.
The pre-oxidation of Panzhihua ilmenite concentrate was investigated at 1073 K and 1273 K under air atmosphere. The products of the pre-oxidation at 1073 K were Fe2O3, TiO2 and Fe2Ti3O9, while Fe2O3, TiO2 and Fe2TiO5 at 1273 K. The influences of pre-oxidation on the carbothermic reduction were studied at 1473 K, 1573 K, 1673 K and 1773 K under argon atmosphere. It was found that the main phases during the reduction process were Ti3O5, Fe, Fe3C, Ti2O3 and TiCxOy. The pre-oxidation was beneficial to the formation of TiCxOy phase compared to the direct carbothermic reduction without pre-oxidation. And the pre-oxidation at 1273 K was better for the carbothermic reduction than pre-oxidation at 1073 K. The rate controlling steps for the carbothermic reduction obeyed the diffusion model. After being reduced for 2 h at 1773 K, the product was changed to TiC with only little O content.
Thermodynamic interaction between chromium and sulfur in liquid iron was studied using the slag/metal equilibration technique to measure the effect of chromium on the equilibrium sulfur distribution between the slag and carbon saturated Fe–Cr–S alloys containing chromium up to 30 mass% in the temperature range from 1823 K to 1923 K. The sulfide capacity of the CaO–Al2O3 slag was separately determined using carbon saturated liquid Fe–S alloys under CO atmosphere as a function of temperature. The sulfur distribution decreases with chromium content and increases with melt temperature significantly. The specific effect of chromium on the activity coefficient of sulfur in liquid Fe–Csat.–Cr–S alloys was determined by considering the effect of carbon on sulfur at carbon saturation. The interaction parameters determined in the present study were compared with the previous data determined by the gas-metal equilibration technique using a H2S–H2 gas mixture.
This study has been performed to understand the reaction behavior of wustite particles and the effect of existing state of CaO component in the iron ore sintering bed for their effective utilization as an agglomeration agent. Changes in the structure and pressure drops of a sintering bed were measured by using a laboratory-scale sintering simulator. When wustite particles were mixed with model pellet of raw materials, pressure drop of the sintering bed did not show significant change independent of average CaO content between 5 and 10 mass%CaO. This is because most of wustite particles and model pellets kept their initial shapes and therefore the structural change of the sintering bed did not occur. On the contrary, pressure drop of the bed changed drastically when wustite particles were mixed with CaO particles with 1.0–2.0 mm in particles size. In these cases, wustite and CaO particles were melted and agglomerated each other. Addition of CaO component at the vicinity of the wustite particles seems to decrease in liquidus temperature of local composition and promote the melt formation. In order to effectively utilize wustite containing materials as an agglomeration agent, it is essential to arrange sufficient amount of CaO component close to such particles for melt formation at lower temperature.
The tar separated out from the lump coal has an important effect on COREX process. In this paper, a designed experimental method is used to study the kinetics of tar’s separation from lump coal. The weight percent gain of the tar was defined as the tar’s separation conversion using the weight-loss ratio in TGA for reference, and then the isothermal and non-isothermal kinetic models were used to analyze the dynamics mechanism of tar’s generation. Based on the characteristic of tar’s generation, we suppose that the shrinking core model is suited to the early heating process of lump coal, and the mid-term and later stage of tar’s generation are respectively controlled by the coal pyrolysis and the spread of the tar molecule. Thus, the equation of non-isothermal unreacted core model, the first order reaction model and the Anti-Jander diffusion equation are respectively established to describe the tar’s generation. The fitting results of these three models showed a good linear dependence relation illustrating that the experimental method and the hypothetical models in this paper are reliable. What’s more, a way is provided to study the tar’s or other substances’ dynamic separation through the experiment method.
HIsarna is a new coal based smelting reduction process, which has the excellent features of using coal and fine hematite ore directly as raw materials instead of coke and pellet. In this context, the reduction kinetics of hematite ore fines in the smelting cyclone was studied in the laboratory scale. The gas-solid and gas-molten particle reduction behaviour were studied with a High-temperature Drop Tube Furnace (HDTF) and a combination of various characterization methods was used to track the kinetic behaviour such as chemical titration, optical microscope and Scanning Electron Microscope (SEM). A series of experiments with different reaction time (210–2020 ms) has been conducted at different temperatures from 1550 K to 1750 K, thus enabling the kinetic study of the partially reduced hematite ore particles. It was found that a quantity of micro pores was formed during the reduction process mainly due to the loss of oxygen. The un-reacted shrinking core model could be used to describe both gas-solid particle reaction and gas-molten particle reaction.
A computational fluid dynamics–simultaneous reaction model (CFD–SRM) coupled model has been proposed to investigate the effects of different contents of aluminum, manganese and silicon in slag and liquid steel, the arrangement of bottom blowing tuyeres and the height ratio of slag and metal on the slag-metal reactions and desulfurization efficiency in gas-stirred ladles. The results show that as the aluminum is added into liquid steel, both the desilication rate and demanganization rate decrease, and the desulfurization rate increases rapidly. Meanwhile as the initial aluminum content [%YAl]t=0 exceeds 0.01 mass%, the desulfurization efficiency increases slowly. With the increase of initial silicon content [%YSi]t=0, the demanganization rate decreases, while the desulfurization rate increases. When the [%YSi]t=0 exceeds 0.5 mass%, the effect of [%YSi]t=0 becomes weak. As well, the variations of manganese content in liquid steel has little effect on the desulfurization efficiency. For the initial content (%Yi)t=0 in slag, with the decreasing of (%YAl2O3)t=0, the desulfurization rate increases rapidly, but as the (%YAl2O3)t=0 is less than 23.0 mass%, the desulfurization efficiency increases slightly. With the increasing of the (%YMnO)t=0, the desulfurization efficiency decreases rapidly. Furthermore, the dual blowing gives higher desulfurization efficiency in comparison with the center blowing or eccentric blowing with one tuyere. With the increasing of height ratio of slag and metal (h/H), the desulfurization efficiency increases, but when the h/H exceeds 0.4, the desulfurization efficiency changes weakly.
The effect of rare earth metal addition on the non-metallic inclusions in spring steel used in fastener of high speed railway was investigated by metallographic examination; SEM-EDS and component analysis, aiming at deform those harmful inclusions to improve service life of spring steel. MgO·Al2O3 inclusions were found in present experimental steel, which is also confirmed by the stability diagram of MgO/MgO·Al2O3/Al2O3 from thermodynamic consideration. After Ce addition, the evolution process of Al2O3·MgO inclusions was determined through the surface and line scanning. The effects of time and Ce content on the evolution of Al2O3·MgO inclusions were examined. It was indicated that Al2O3·MgO inclusions were wrapped by rare earth inclusions to form a ring like shape Ce-riched band around the inclusion, which would be useful to improve fatigue and corrosion resistance of spring steel. It was found that diffusion of Ce3+, Al3+ and Mg2+ in inclusions core and intermediate layer would be the limited step during evolutions of inclusions.
In continuous casting process, solidification should evenly proceed to have as good steel quality as possible. Molten steel starts to solidify in a water-cooled mold to create solidified shell followed by shell growth and termination in a secondary water cooling zone. Visualization of the flow pattern of spray water greatly helps to analyze how to have even shell. Computational fluid dynamics is useful represented by the grid based methods of FVM, FDM, and FEM. However, they are not appropriate for simulation of spray water flow because of complex free surfaces. So, the particle based method of MPS has been applied. A typical roll arrangement was modeled where spray water flow was particularly focused on. As a result, standing water on rolls overflows according to the water flow rate of spray accounted for in this study. Accuracy of the numerical model has been verified by water model experiments equipped by acrylic plates, rolls and spray nozzles. The computational results with a practical condition agreed well with the experimental results. Heat transfer coefficients between water and slab surface were estimated by the calculated results to simulate how solidification proceeded in practice. It was found that uneven water flow significantly affected unevenness in temperature distribution of a slab.
Control melt flow in tundish is very important for clean steel production. To explore the fluid flow mechanism, the RTD curves and its data, velocity vector fields and streamlines of the molten steel, and the relationship between the RTD curves and flow pattern in a single-strand slab caster tundish with a capacity of 30 tons have been investigated by both hydrodynamic and mathematical simulations. The RTD data and flow field of the original tundish have been studied in a 1:3 reduced scale hydrodynamic model. Meanwhile, the streamlines, velocity vector fields and the RTD data of the ratio of width to length (W/L) in tundish are mathematically simulated. In order to descript the flow pattern better, a new method is proposed to calculate the data of RTD curves with double peaks. The results showed that the RTD curves changed from double peaks to single peak with the increasing of the W/L in tundish. Both hydrodynamic and mathematical simulations results suggest that the W/L in tundish is the most important factor to change the flow pattern actually, that is, the short-circuiting flow disappeared with the increase of the W/H in tundish gradually. Furthermore, we have elaborated the mechanism the RTD curves change from double peaks to single peak. With increasing W/L, the wide-side walls play an important role to retard the short-circuiting flow on the inlet-outlet plane straight towards the outlet. Meanwhile, the dead region and its volume fraction are also the objects of our attention and exploration.
Abnormal sticky behavior in continuous casting of steel refers to an event when in an instrumented mould, temperatures of all layers of thermocouple drop to a near single temperature level and thus doesn’t help in identifying any phenomena happening in the mould. No signal is generated to capture any sticker or any other abnormal event occurring in the mould. In this situation the risk of sticker breakouts increases significantly. The knocks on effect are casting delays and slab downgrades. This study was carried out by collecting top slag samples and mould slag films from the mould for both normal and abnormal heats. Optical microscopy and SEM images revealed presence of pores in the slag film complemented with large crystals in abnormal heats. Porous structure in the slag film creates high thermal resistance to the heat flow from the slab to the mould, which retards the solidification of the slab. Also crystalline structure of slag film hinder heat transfer between solidifying shell and water cooled mould and lead to abnormal sticky behavior. Measurement of diffusible hydrogen content of liquid steel in tundish was measured to find out if hydrogen is accelerating the slag crystallization. It was found out that, in case of abnormal heats hydrogen content was 30–40% higher than the normal heats. The primary and secondary steelmaking parameters were looked into and it was found out that the choice of material added and addition pattern along with treatment time has major contribution in increasing hydrogen content of steel.
Intense reaction between silica in mold fluxes and aluminium in liquid steel during casting of high-Al non-magnetic steel 20Mn23AlV (1.5–2.5 Al in mass percent) would significantly alter both chemical compositions and properties of mold fluxes. This would subsequently lead to severe casting problems such as lots of slag rims, breakout and poor surface quality. Investigation carried out in this paper started with plant sampling, followed by a look at how the variation of Al2O3/SiO2 ratio with reaction time can affect the casting process and product quality. Thus, this work focuses on the study of increasing Al2O3/SiO2 and partial substitution of CaO with BaO in CaO–SiO2 system mold fluxes in terms of heat transfer and crystallization behavior. The techniques implemented are heat flux simulator and single hot thermocouple technique (SHTT). The results showed that an increase in Al2O3/SiO2 inhibits heat transfer, increases crystallization temperature and critical cooling rate while shortens incubation time, additionally, accelerates precipitation of phase with high melting temperature. However, greater substitution of CaO with BaO accelerates heat transfer, reduces crystallization temperature and critical cooling rate at the cost of longer incubation time even at elevated Al2O3/SiO2. Eventually, partial substitution of CaO with BaO, to some extent, counteracts the effect of increasing Al2O3/SiO2 on heat transfer and crystallization properties of mold fluxes for casting of high-Al steels.
A transient three-dimensional (3D) comprehensive model is established to understand the macrosegregation in the electroslag remelting (ESR) process. The electromagnetism, two-phase flow and heat transfer are included. The volume of fluid (VOF) approach is employed to trace the metal droplet. The solidification is modeled by an enthalpy-based technique. The solute transport is analyzed by the continuum mixture model. A reasonable agreement is obtained between the experiment and simulation. The results indicate that the liquid composition in mushy zone becomes enriched in Ni. The interdendritic metal with a higher Ni is replaced by the Ni poor metal carried by the downward flow in the pool. The Ni composition accumulates at the pool bottom and the concentration increases with time. The species movement is dominated by the thermal buoyancy because of the forced cooling. A negative segregation in the lower part and a positive segregation in the upper part are formed in the ingot. Thanks to the rapid solidification, the Ni is immobilized before moving to the other place resulting in a lower segregation level. The segregation becomes severer with the increasing current. The maximal positive and negative segregation indexes increase from 0.02742 to 0.03226 and from −0.01346 to −0.01561 with the current ranging from 1000 A to 2000 A.
Due to the inherent difficulty in making direct observations of the behavior of the dynamic flow of liquid steel and the inclusions in the continuous casting of the steel, mathematical and physical modeling, having great popularity and acceptance, has been an invaluable aid to the understanding of the fluid flow phenomena. Up to date, there is little information available in the literature regarding the behavior of the inclusions, especially inside the funnel type mold. It has been found that the accelerometer is a transducer capable of relating the vibration with the behavior of the inclusions in the continuous casting mold. It was indicated that higher levels of vibration in the thin slab mold is greater than the removal of inclusions therein. As a starting point of the results a mathematical modeling, previously carried on the work group, were used,1) where an analysis of the fluid flow process of the continuous casting thin slab was carried out. Two nozzle designs, two depths of 22 and 34 cm, and three casting speed of 4, 5 and 6 m/min were simulated. In all cases, just 100 particles were simulated within the flowing liquid metal, because once the mathematical calculations or the processing time increases as the quantity of particles grow. These have previously been treated with a liquid, sensitive to the black light, and then under this type of light the inclusions are luminescent. All cases were also solved in the simulation software Fluent® where a slag layer, in which all the inclusions that reach it are trapped, is generated at the meniscus of liquid metal. The area near the nozzle has a greater concentration of particles, which is due to low speed or flow pattern change in said zone. These inclusions are floating in this area eventually become stripped and trapped in the slag layer.
The shroud nozzle is used to transport the molten steel from the ladle to the tundish in the continuous casting process. A hydraulic manipulator transfers the shroud nozzle and connects it to the collector nozzle installed on the bottom of the ladle. During the nozzle connection, however, the misalignment between the shroud nozzle and the collector nozzle frequently occurs due to the initial position error of the ladle, unexpected external forces on the nozzle, etc. In case the nozzle is not aligned properly, there is air-inflow into the molten steel through the crevice, and this results in the defect of steel products by oxidation. In addition, if the nozzle tightening force is not enough to hold the shroud nozzle while the molten steel flows into the nozzle, the nozzle vibration occurs by the irregular flow of the molten steel and it gives rise to another air-inflow to the molten steel. POSCO currently uses the remote operated manipulator for the shroud nozzle manipulation, but this cannot measure and control a fine nozzle alignment appropriately. In this paper, we designed and controlled a novel 5-DOF hydraulic manipulator to solve the nozzle misalignment problem. Each joint of the manipulator contains a torque sensor which enables to detect the external forces exerted to the shroud nozzle, and this is utilized to measure the misalignment. Moreover, the nozzle misalignment can be adjusted in real-time by the proposed force integral controller using joint torque sensors.
Case-Based Reasoning (CBR) system is a kind of solving paradigm based on the past successful cases to get the solution for the current problem. When CBR is applied in complex industrial processes, solving efficiency is often not high due to too many influence factors involved. So it is necessary to reduce the number of attributes involved in CBR system for the fast modern industrial production, such as steel-making and continues casting process. A two-step CBR method is proposed for predicting the endpoint phosphorus content in BOF efficiently. First, the genetic algorithm is applied to find the optimal attributes subset based on the evaluation method of Correlation-based Feature Selection (CFS). Then, CBR system is applied for solving this problem with the reduced attributes. There are two kinds of similarity calculation method based on the euclidean distance and the gray distance, and two kinds of the weight decision method based on the even weight and the entropy weight for this CBR system. Four groups of experiment results show that the two-step CBR method has much more efficiency than the single CBR method, while maintaining almost the same prediction precision. The two-step CBR method can be used in the fast industrial process more efficiently.
Since the Cu content in steel causes hot shortness, it is important to understand the behavior of Cu during high-temperature oxidation in order to control the precipitated Cu. This study examined Cu distribution during the oxidation of steel. The oxidation tests revealed that precipitated Cu existing in the scale/steel interface was absorbed into the Fe3O4 layer or evaporated into the atmosphere as Cu. Then, a method proposed to suppress hot shortness was tested by oxidation-tensile tests at high temperature and the method was proven to be effective.
In this study, the bending formability of tubular pipes made of ferritic stainless steels during the rotary bending process was investigated. Three different types of ferritic stainless steel—STS 439, STS 429EM, and STS 441—were selected as the test materials. Finite element (FE) simulations were introduced to predict maximum bending angles, or equivalently the bending formability, for both as-received and annealed tubes. The results from experiments and FE simulations suggest the following main conclusions. First, the pipe materials used in the rotary bending process were subjected to prior work hardening during the tubing process, which resulted in reduced formability. However, proper heat treatment could enhance the bending formability. The optimum annealing conditions were determined from the microstructure analysis and mechanical assessments by uni-axial tensile tests for various heat-treated samples. An annealing temperature/holding time of 900°C/10–60 s resulted in enhanced formability without grain coarsening for the three tested ferritic stainless steels. Second, a FE model predicted maximum bending angles and thinning profiles at the extrados of pipes for both as-received and heat-treated tubes when the boundary conditions and friction coefficients were properly optimized.
A novel analytical approach for asymmetrical rolling of two unbonded clad sheet layers is creatively proposed to calculate the rolling force and torque. In this new approach, the vertical stresses which make calculation results be agree better with results measured in experiments are first applied in clad sheet asymmetrical rolling process. During varying of different roll speeds, roll radii and yield stress ratio of the two materials as well as different friction coefficients, two neutral points and one bonding point are obtained, then the plastic deformation region is divided into four zones. The neutral points, the bonding point, distribution of the rolling stress, the horizontal stress of the whole clad sheet and clad thickness ratio at the exit can be calculated by the present solution easily and rapidly. In order to verify the accurateness of the new model, the results received have been compared with those calculated by other traditional models as well as measured in the experiments conducted by previous scholars where the effects of vertical shear stress were ignored. As it can be seen, the maximum of errors is less than 9.8%.
The deformation behavior during unloading was examined under uniaxial tension in a mild steel sheet (body-centered cubic metal), an aluminum alloy sheet (face-centered cubic metal), and a magnesium alloy sheet (hexagonal close packed metal). A crystal plasticity finite-element method was also used to investigate the difference in the deformation behavior among on the materials. The nonlinearity during unloading was the largest in the magnesium alloy sheet, and the mild steel sheet showed a larger nonlinearity than the aluminum alloy sheet. On the other hand, the apparent elastic moduli determined from the linear approximation of unloading curves were not always consistent with the characteristics observed in the nonlinearity, and this inconsistency became pronounced as the degree of nonlinearity increased. It was found that the degree of nonlinearity would have a strong correlation with the strain rate sensitivity, suggesting that the apparent elastic modulus was not suitable to model the unloading behavior for materials with high strain rate sensitivity. The crystal plasticity analysis demonstrated that the nonlinearity was much larger in the magnesium alloy sheet than in the other two sheets as observed in the experimental results. The simulation results suggested that one of the reasons that gave rise to the nonlinearity during unloading would be the difference in the critical resolved shear stresses among the slip systems.
The present work develops a hollow axis motor driven swing arc process for narrow gap vertical GMA welding, and investigates the characteristics of swing arc force, heat and weld formation. This process uses an upwardly bending conductive rod through the established mathematical models to weave circularly the arc, and then enables arc force to resolve into three favorable components while regulating arc energy distribution in groove, finally yielding satisfied vertical welds even with concave surface. Experimental results show that the penetrated depths into groove sidewall and bottom plate grew with increasing groove gap and decreasing rod bending angle, and molten pool sagging and bottom twin peaks were suppressed by great bending angle and respectively in wide and narrow grooves.
The electrodeposition of Ni–W alloys was conducted from an unagitated sulfate solution containing citric acid at pH 5 and 60°C under coulostatic (3.44–6.22×105 C/m2) and galvanostatic (30–5000 A/m2) conditions. Before annealing, the lattice constant of Ni increased linearly with an increase in W content up to 40.7 mass% in accordance with Vegard’s law. This shows that the W formed a supersaturated solid solution in Ni. At W contents of <37.1 mass%, the deposit morphologies showed a field-oriented texture, with a preferred orientation of the specific plane towards the electric field during deposition, and platelet crystal edges exposed at the surface. At W contents of >40.7 mass% of the solid solubility limit, the cross section of the deposits showed a layered morphology, with a smooth surface and small granular crystals. After annealing, Ni4W precipitated with deposits of W contents of 32.6 and 37.1 mass%. Fine precipitates of Ni4W and NiW formed over the entire surface with W contents of 40.7–45.3 mass%. Before annealing, the hardness of the deposits increased with W content, and the increase was particularly large at a W content of 40.7 mass%. The hardness was almost constant regardless of the current density for W contents of >40.7 mass%. The alloy composition required to change the hardness of the deposits corresponded with that required to change the deposit structures. The hardness of the deposits increased for all W contents by annealing, with the increase being particularly large for W contents of >40.7 mass%.
Evaluation of the corrosion resistance of a new stainless steel, SUS443J1, in atmospheric environments is a significant issue. In this study, the growth behavior of pits on the surface of the ferritic stainless steel, SUS443J1, and an austenitic stainless steel, SUS304, were compared by field exposure tests and electrochemical measurements. It is known that the pit growth rate can be approximated as X = atn, where X is pit depth, t is time, a and n are constants. As a result of field exposure tests, pit growth rate values of approximately n = 0.2 were obtained for both 2B and HL surfaces of SUS443J1 and SUS304. From this, it could be predicted that the pit growth behaviors of SUS443J1 and SUS304 were mutually equivalent. The pitting potential values V’c10 of SUS443J1 and SUS304 were almost equivalent, and the repassivation potential showed the same tendency. The pitting potential decreased with increasing the maximum valley depth Rv of the surface. The repassivation potential was affected by the turning current density, where the sweep of potential was reversed. The turning current density represents the degree of pit growth. It was suggested that a deep pit would expand at a larger growth rate than that of a shallow pit because the deep pit grows easily and repassivation is difficult in deep pits.
Creep properties of modeled nickel-iron base superalloy with phosphorus (P) content of 8 to 450 ppm by mass have been investigated at 973 K under a stress of 333 MPa. The P-doped alloys exhibit many grain-boundary precipitates in which niobium and phosphorus are enriched after a certain heat treatment condition prior to the creep test. An alloy with P of 130 ppm shows a maximum value of grain-boundary coverage by the precipitates (designated as ρ) of 0.56. The time to rupture and minimum creep rate correspond well to the value of ρ. The rupture life was extended by up to 30 times and the minimum creep rate was decreased by more than two orders of magnitude in the alloy with P of 130 ppm, with respect to those in an alloy with P of 8 ppm where ρ=0. The grain-boundary precipitates remarkably delay the onset of accelerating stage, thereby leading to the longer rupture life. These results strongly suggest that optimization of the ρ value is a key to improve the creep properties of the alloys.
This work deals with the feasibility of obtaining Austempered Ductile Iron with Dual Phase structures (DPADI) through heat treatment, starting from different as-cast microstructures. The mechanical properties on these microstructures were evaluated. DPADI microstructures were obtained by adding different tenors of silicon (2.4% to 4.2%) to the melts and keeping the other alloying elements constant. The study focused on the determination of the time required to achieve the percentages of equilibrium phases (ferrite and austenite) at different temperatures in the intercritical temperature interval as a function of the starting as cast microstructure. The results showed that, as the silicon content increases, higher amount of ferrite is present in the as cast structure, and the time required to reach the thermodynamic equilibrium phases in the intercritical temperature interval is markedly reduced. Similarly, for a constant chemical composition, as the intercritical austenitizing temperature increases, the time required to reach the quantities of the equilibrium phases decreases. Regarding mechanical properties, the tests revealed that, as expected, as intercritical austenitising temperature increases so do tensile strength and hardness due to the higher ausferrite content in the DPADI matrix. These results indicate that high silicon Ductile Iron (with Si content higher than 3%) with a mostly ferritic microstructure in as cast conditions yields DPADI microstructures able to dispense with prior annealing heat treatments since the time required to reach the phase equilibrium percentages is compatible with the industrial practice and the mechanical properties are similar as compared to DPADI structures deriving from fully ferritic matrices.
Although martensite is recognised as a very strong phase in carbon steels, its initial yielding commences at low stresses and the tensile stress-strain curve shows a smooth, rounded form. Evidence is presented from x-ray diffraction to show that this behaviour is due to the presence of intra-granular stresses that are residues after the shear transformation from austenite to martensite. These internal stresses are reduced in magnitude by plastic deformation and also by tempering. Reduction of internal stress due to plasticity is shown by a decrease in XRD line broadening after deformation. A simple model is presented in which the stress-strain behaviour is controlled by relaxation of the internal stresses almost up to the point of the ultimate tensile strength. It demonstrates that only a very small fraction of the material remaining in a purely elastic state provides a large stabilising effect resisting necking. A corollary of this is that the uniform elongation of martensitic steel actually increases with increase in the strength level. Effects of heat treatment are also reproduced in the model, including the increase in conventional yield stress (Rp0.2) that occurs after low temperature tempering.
The effect of grain size on solid particle erosion and cavitation erosion of a nitrogen alloyed austenitic stainless steel has been investigated. Heat treatment of the steel at elevated temperatures results in an increase in grain size and thus modification in mechanical properties. Particle erosion tests were performed using an air jet erosion tester. An ultrasonic processor with a stationary specimen was used to investigate the cavitation erosion performance. The cavitation erosion rates were found to increase with the increase in grain size. The particle erosion rate shows no significant change with increase in grain size. The worn surfaces were examined to study the characteristic damage features using scanning electron microscope (SEM). The nitrogen alloyed austenitic stainless steel exhibited superior resistance to cavitation erosion and particle erosion than a 316L stainless steel. The hardness, yield strength and ultimate tensile strength of the steels are related with the erosion resistance.
First-principles tensile tests were performed to investigate the effect of Cr segregation on the hydrogen-induced embrittlement in Fe grain boundaries. The Fe grain boundary with H and Cr segregation exhibited the higher maximum stress and strain to failure than those of the Fe grain boundary with only H segregation. Cr segregation suppressed the extension of the Fe-Fe bond weakened by H segregation. In the Fe grain boundary with H and Cr segregation, the decrease in the charge density in the bond with straining was suppressed because of charge transfer from the Cr atom to its neighboring Fe atoms. This charge accumulation in the grain boundary region is responsible for the reinforcement of bonds in the Fe grain boundary with H segregation, contributing to the suppression the hydrogen-induced embrittlement of Fe grain boundaries.
(0.2–0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and deformed by multi-pass caliber rolling (i.e. warm tempforming) with a rolling reduction of 78%, in order to obtain ultrafine elongated grain (UFEG) structures. The tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to determine the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with strong <110>//rolling direction (RD) fiber textures. The transverse grain size and aspect ratio in the UFEG structure tended to reduce as the carbon content increased, whilst the carbide particle size became slightly larger. The increase in the carbon content resulted in an increase in the yield strength from 1.68 to 1.95 GPa at room temperature; however, it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness. This was due to delaminations, where cracks were observed to branch in the longitudinal direction (//RD) of the impact test bars. The upper-shelf energy of the TF sample was enhanced as the carbon content decreased, and higher absorbed energy was also achieved as delamination occurred at lower temperatures. The delamination was found to be controlled not only by the transverse grain size, the grain shape, and the <110>//RD fiber texture but also by carbide particle distribution in the UFEG structure.