Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.     

Analysis of zonal shrinkage in alloy steels

Abstract

Thermal behaviour of the solidifying steel structure is important for understanding of the defects during ingot solidification. During solidification and cooling, most metals shrink. As a consequence in upper part of solid ingot, pores and pipes of typical shapes and size are formed. Forming of pipes is closely related to the casting and solidification processing parameters. In the present paper the influence of liquid temperature, chemical composition and temperature gradient on the shrinkage intensity are investigated. The ratio of the pipe depth to total ingot height as a criterion of the pipe size is used. The values of the temperature gradients on the base of the numerical model solidification are obtained. The experimental measurements of temperature change have been carried out on laboratory steel ingot. The results by numerical model are compared with the experimental ones and showed a good agreement.     

Modelling of the thermo-physical and physical properties for solidification of Ni-based superalloys

The thermo-physical and physical properties of the liquid and solid phases are critical components in the modelling of casting simulations. Such properties include the fraction solid transformed, enthalpy release, thermal conductivity, volume and density all as a function of temperature. Due to the difficulty in experimentally determining such properties at solidification temperatures, little information exists for multi-component alloys. As part of the development of a new computer programme for modelling of materials properties (JMatPro), extensive work has been carried out on the development of sound, physically based models for these properties. Wide ranging results will presented for Ni-based alloys, which will include more detailed information concerning the phases formed during solidification and their composition and the density change of the liquid that intrinsically occurs during solidification due to its change in composition.     

Mechanism of formation of solute bands in resistance seam welds of mild steel

Abstract

Metallographic examination of resistance seam welds in mild steel sheet made at welding speeds in the range 2–16 m min^?1 indicated that solute bands were formed as a direct result of differences in growth rate of the solid/liquid interface during the solidification process. The frequency of the bands in the longitudinal direction was equal to that of the waveform of the welding current used. It is considered that bands seen in the transverse direction represent discrete sections of the solidification front in the longitudinal section. The associated bands of solute segregation in the substructure were formed as a result of the sinusoidal phase shifted current waveform which was shunted back through the solidifying weld seam, thereby arresting the movement of the solid/liquid interface. The extent of these bands can be related to welding current, welding speed, and sheet thickness.

MST/3371     

Effects of solidification rate on segregation and fluid flow tendency in mushy zone of directionally solidified superalloy Inconel 718

Abstract

The microstructure and composition of the interdendritic liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates between 2 and 100 μm s^?1 have been investigated by SEM and EDAX techniques. The interdendritic liquid segregation profiles along the mushy zone are presented. The liquid density difference and Rayleigh number in the interdendritic liquid were calculated and analysed as well. It was found that when the solidification rates increased in the range 10–70 μm s^?1, segregation of Nb decreased, but segregation of Mo was most serious at 20 μm s^?1. The liquid density difference increased the most for rates from 20 to 40 μm s^?1 as temperature decreased. The maximum relative Rayleigh number was highest at 10°C below the liquidus temperature at 20 μm s^?1, which indicated the conditions where fluid flow most easily occurred for Inconel 718. The relative Rayleigh number synthetically considers the factors affecting fluid flow and can give a reasonable prediction for fluid flow tendency.     

Revisiting dynamics and models of microsegregation during polycrystalline solidification of binary alloy

Microsegregation formed during solidification is of great importance to material properties.The conventional Lever rule and Scheil equation are widely used to predict solute segregation.However,these models always fail to predict the exact solute concentration at a high solid fraction because of theoretical assumptions.Here,the dynamics of microsegregation during polycrystalline solidification of refined Al-Cu alloy is studied via two-and three-dimensional quantitative phase-field simulations.Simulations with different grain refinement level,cooling rate,and solid diffusion coefficient demonstrate that solute segregation at the end of solidification (i.e.when the solid fraction is close to unit) is not strongly correlated to the grain morphology and back diffusion.These independences are in accordance with the Scheil equation which only relates to the solid fraction,but the model predicts a much higher liquid concentration than simulations.Accordingly,based on the quantitative phase-field simulations,a new analytical microsegregation model is derived.Unlike the Scheil equation or the Lever rule that respectively overestimates or underestimates the liquid concentration,the present model predicts the liquid concentration in a pretty good agreement with phase-field simulations,particularly at the late solidification stage.     

Variation in as cast and aged structures of centrifugally cast heat resistant steel solidified in an electromagnetic field and under different cooling conditions

Abstract

The influences of an applied electromagnetic field and cooling conditions on the as cast and aged structures of centrifugally cast heat resistant HK40 steel tubes were investigated. The results reveal that both the electromagnetic field and the cooling ability of the casting mould have significant effects on the macrostructure morphology and the volume fraction and distribution of eutectic carbides, as well as on the homogeneity of secondary precipitation caused by high temperature aging treatment. The differences in the structures are attributed to the different solidification processes of the cast tubes. It is indicated that a reasonable combination of electromagnetic field and cooling conditions during solidification is greatly beneficial, leading to an improvement of creep resistance of cast tubes.     

Numerical Simulation of Solidification of Work Roll in Centrifugal Casting Process

A program on the solidification process of horizontal centrifugal casting coupled with eutectic carbides segregation has been developed in this paper.Due to the geometrical features of work roll,a cylindrical coordinate system was used.The temperature field of the outer layer at the end of filling process was imported as the initial temperature condition for the solidification process.The model of eutectic carbides segregation caused by different densities between eutectic MC and the molten steel was coupled in the program.The temperature field of the outer layer of work roll during horizontal centrifugal casting process was investigated.Results show that the outer layer has a "sandwich shape" solid fraction manner.Results also indicate that the segregation of eutectic MC is quite severe during centrifugal casting process.It forms four zones of different content of carbides in radial direction.The simulated results of MC carbides segregation phenomenon agree with the experimental observations.     

Effects of boron and silicon on microstructure and isothermal solidification during TLP bonding of a duplex stainless steel using two Ni–Si–B insert alloys

Abstract

The influence of B and Si on microstructure and isothermal solidification during transient liquid phase (TLP) bonding of a duplex stainless steel using MBF-30 (Ni–4·5Si–3·2B, wt-%) and MBF-35 (Ni–7·3Si–2·2B, wt-%), was investigated. Based on experimental studies, the formation of Ni₃B within the joint centreline is dependent on B content; the morphology of Ni₃Si is dominated by Si concentration. There was a deviation between the times for complete isothermal solidification obtained by the experiment and the conventional TLP bonding model. Isothermal solidification of the liquid dependent on different solidification regimes is suggested to be a controlling factor contributing to the change in the rate of isothermal solidification observed using the two filler metals.     

Microstructural development during consolidation of inert gas atomised Al–7Mg–1Zr powder alloy

Abstract

The microstructural development of a rapidly solidified Al–7Mg–1Zr powder alloy during consolidation via hot extrusion is reported. This non-heat-treatable alloy is a proposed candidate for the production of net shape extruded components, which require a combination of high strength, low density, and good damage tolerance. By employing a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry a detailed understanding of the microstructural transformations that occur during consolidation has been achieved. Additionally, the above techniques have been employed to assess the effect of extrusion temperature on the final microstructure of the alloy. It is shown that rapid solidification techniques can retard the nucleation and growth of Zr containing intermetallics even at the relatively high concentrations found in the present alloy: however, segregation of the Mg component during solidification is found to be severe. A detailed analysis of the dissolution kinetics of the Mg bearing phase on heating shows that although the segregation is drastic, its refined nature permits the omission of a homogenisation treatment. This analysis also shows that for solidification at lower cooling rates homogenisation of the present alloy composition would be required. It is found that during consolidation the Zr rich solid solution decomposes to produce a very fine distribution of metastable ZrAl₃ dispersoids. Examination using TEM reveals that these dispersoids are formed in a discontinuous manner during the hot deformation associated with the extrusion process. Employing quantitative XRD techniques the variation in quantity and size of these intermetallics with consolidation temperature is assessed and the optimum distribution is observed when the alloy is consolidated at 500°C.

MST/1658     

Solute redistribution and segregation in solidification processes

In this review paper, research on solute redistribution coefficients for equilibrium, near equilibrium and non-equilibrium solidification processes were first analyzed. Then, different models advanced since the 1950s for solute redistribution during the directional solidification process with a planar interface were summarized. The so-called second-order opposite diffusion compensation method (SODCM) presented by the author was described in detail. Thirdly, solute redistribution in the dendritic solidiication process and the segregation were discussed. It is concluded that the problem can be described resulting Ψ_S –W_L functions. Among the different models, that obtained by the present author in 1994 was explained. Fourthly, the formation o macro–segregation mainly caused by liquid flow in the mushy zone is analyzed. A parameter about the driving force for the liquid flow is described.The parameter is considered to be the controlling parameter for the formation of macro-segregation. The relationship between the parameter and the compositions in steel is also given in the paper as an example.     

Mechanisms of inclusion formation in low alloy steels deoxidised with titanium

Abstract

In the present investigation, the conditions for inclusion formation in two Ti deoxidised steels and one Al–Ca deoxidised steel have been examined by means of optical and electron microscopy, in combination with a thermo dynamic analysis of the phase relations involved. It is concluded that the Ti containing inclusions form as a result of a series of reactions occurring in the ladle, during solidification and in the solid state. The important solid state reaction products are MnS, TiN, and MnOTiO₂ . The presence of Mn rich compounds at the surface of the inclusions is consistent with the observation of a Mn depleted zone in the surrounding steel matrix. In contrast, the primary inclusions in the Al–Ca deoxidised steel are complex oxysulphides, which are thermodynamically more stable and can therefore form in the liquid state.     

Quantitative characterisation of last stage solidification in nickel base superalloy using enthalpy based method

Abstract

An enthalpy based method was used to determine the solidification characteristics in the Ni base superalloy IN713LC with the emphasis on the late stages of solidification. Solidification commences with freezing of γ-solid, which is followed by precipitation of carbides (MC) and subsequent divorced growth of MC and γ until solidification terminates. During solidification enthalpy change was measured using differential thermal analysis and latent heat was calculated using a multicomponent thermodynamic software database. The measured enthalpy and calculated latent heat were then used to determine liquid fraction evolution and local freezing rate. A quantitative comparison of calculated fraction liquid evolution and local freezing rate with those determined using equilibrium and Scheil approximations was carried out. The comparison reveals that the present method offers a more accurate approach for characterising the late stages of solidification than the equilibrium and Scheil models.     

Computer prediction of microsegregation in peritectic alloy systems

Abstract

A numerical model for the prediction of solidification and the accompanying microsegregation in peritectic alloys is described. Several finite difference schemes have been tested, based on one-dimensional computational cells for microsegregation calculations. The method selected as the most efficient is that of a fixed computational grid with both solid/solid and solid/liquid boundaries able to move freely between the nodal planes, using a Lagrangian interpolation procedure due to Crank in order to determine solute gradients at the interfaces. The solution method is essentially explicit, although the composition and position of the solid/solid interface have to be floated in an implicit manner to obtain consistent, unique, solutions in multicomponent alloys. Microsegregation and non­equilibrium solidus temperatures have been computed for a number of alloy steel systems using the proposed model and satisfactory agreement obtained with experimental results.     

Numerical modelling of permeability variation with composition in aluminium alloy systems and its relationship to hot tearing susceptibility

Abstract

A numerical micromodel has been developed to simulate the evolution of equiaxed primary phase grains during the solidification of alloys in the systems Al–Cu, Al–Si, Al–Mg, and Al–Zn. The microstructures generated have then been used to model liquid permeability as a function of composition for each system, for a given solid fraction and cooling rate. In all systems a marked minimum occurred in the permeability curve at a value < ～1 wt-% solute. The composition corresponding to the minimum permeability tended to increase with increasing equilibrium partition coefficient. It is argued, for each system, that the composition displaying minimum permeability would correspond to that composition exhibiting maximum susceptibility to hot tearing. Comparison of the permeability data with experimental hot tear test data for the same systems reveals the limitations of most hot tear tests.     

Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy

Abstract

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.     

Solute redistribution and segregation in solidification processes

In this review paper, research on solute redistribution coefficients for equilibrium, near equilibrium and non-equilibrium solidification processes were first analyzed. Then, different models advanced since the 1950s for solute redistribution during the directional solidification process with a planar interface were summarized. The so-called second-order opposite diffusion compensation method (SODCM) presented by the author was described in detail. Thirdly, solute redistribution in the dendritic solidification process and the segregation were discussed. It is concluded that the problem can be described resulting ϕ_S−w_L functions. Among the different models, that obtained by the present author in 1994 was explained. Fourthly, the formation of macro-segregation mainly caused by liquid flow in the mushy zone is analyzed. A parameter about the driving force for the liquid flow is described. The parameter is considered to be the controlling parameter for the formation of macro-segregation. The relationship between the parameter and the compositions in steel is also given in the paper as an example.     

InSb-InAs alloys prepared by rapid quenching (106–108 K/s)

Homogeneous InSb-InAs alloys are prepared by rapid quenching (10⁶-10⁸ K/s) from the liquid state. As evidenced by x-ray diffraction studies, rapid quenching prevents liquid-phase segregation, so that solidification proceeds by a diffusionless mechanism. The lattice parameters of the rapidly quenched alloys are determined. The effects of melt overheating and sample thickness on the diffusionless solidification process are analyzed. The nonlinear composition dependence of the lattice parameter in the InSb-InAs system is accounted for using the calculated degrees of dissociation of InSb and InAs along the liquidus line and at 1000‡C and the covalent radii of the constituent elements.