Equi-axed growth and related segregations in cast metallic alloys

Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation.

Numerical simulations of directional solidification of binary Al-Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.     

Equi-axed growth and related segregations in cast metallic alloys

Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation.Numerical simulations of directional solidification of binary Al–Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.     

Reaction products and their solidification process of the plasma sprayed Fe2O3–Al composite powders

Fe₂O₃–Al composite powders were deposited onto steel substrate by plasma spraying. The reaction products of the Fe₂O₃–Al composite powders in the plasma flame and their solidification process were investigated. The results showed that the reaction products of the Fe₂O₃–Al composite powders in the plasma flame were Fe–Al–O ceramic melt and Fe melt. Fe was not always formed in the reaction products of each composite particle, and the formation of Fe was dependent on the composition distribution and the reaction kinetics process of each composite particle. The composition inhomogeneity, discontinuity and porousness of the composite particles resulted in the difference of the reaction kinetics of each composite particle and the composition difference of the droplets. Hercynite solid solution, Fe, Al₂O₃ and FeAl phases were formed by non-equilibrium solidification of the Fe–Al–O ceramic melt. For the smaller size droplet, when it spread on the substrate, the spreading droplet was quickly chilled to form equiaxed grains with size of 100–200 nm. Nano-sized or submicron equiaxed grains, nano-sized columnar grains and cellular structure were formed in the larger spreading droplet. Fe rich hercynite solid solution nucleated and grew preferentially and then Al rich hercynite solid solution grew, which form composition segregation in the columnar grains. The main phases in the composite coating were hercynite solid solution.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

Effect of interfacial segregation of magnesium on high carbon (18%Cr) cast steel

Abstract

The effect of Mg on the microstructure and properties of a high carbon cast steel (nominal composition, wt-%: 18Cr–2Ni–0·75Mo–Mn–Si–Fe_bal.) is investigated. It is shown that the microaddition of Mg refines the primary carbide (Cr_0·51Fe_0·49)₇C₃ and promotes an equiaxial dendritic structure, and the resulting structure refinement improves significantly the impact toughness of the alloy. Using Auger electron spectroscopy and electron probe microanalysis, it is shown that there is an unusually high Mg segregation (~70 at.-%) at the carbide interface region. It is proposed that the Mg enrichment at the carbide interface is primarily a result of the liquid phase separation that occurs in liquid Fe–Mg alloys when the Mg level attains a minimum value (~1–2 at.-%), and that this initial enrichment is a result of a combination of Mg rejection by the carbide and uphill diffusion of Mg to the carbide interface region. A thermodynamic analysis is presented to show the degree of component (Cr, Ni) segregation required for uphill diffusion of Mg to occur, which is in qualitative agreement with that observed.

MST/1046     

Simulation of temperature field,flow field and solidification structure for Ag–28Cu–2Ge–0.4Co alloy

ABSTRACT

The temperature field, flow field and solidification structure of Ag–28Cu–2Ge–0.4Co alloy during solidification under water cooling, air cooling and slow cooling conditions were investigated, respectively. The results indicate that the temperature distribution is the most uniform and the solid–liquid phase region is the widest under slow cooling condition. The temperature gradient at the solidification front is the largest under water cooling condition. The solidification rate increases with the distance away from the sidewall under three cooling conditions. Under water-cooled conditions, the liquid flow at the front of the liquidus is much smaller than that in the other two conditions. The ratio of equiaxed crystals and columnar crystals in the solidified structure is different under different cooling conditions.     

Numerical Simulation of Morphology and Microsegregation Evolution during Solidification of Al-Si Alloy

A stochastic model coupled with transient calculations for the distributions of temperature, solute and velocity during the solidification of binary alloy is presented. The model can directly describe the evolution of both morphology and segregation during dendritic crystal growth. The model takes into account the curvature and growth anisotropy of dendritic crystals. Finite difference method is used to explicitly track the sharp solid liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations are performed to simulate the evolution of columnar and equiaxed dendritic morphologies of an AI-7 wt pct Si alloy. The effects of heat transfer coefficient on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys are analyzed. This model is applied to the solidification of small casting. Columnar-to-equiaxed transition is analyzed in detail. The effects of heat transfer coefficient on final casting structures are also studi     

Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation – Part I: Model description

Part I of this two-part investigation presents a volume-averaging multiphase solidification model that accounts for mixed columnar-equiaxed solidification, non-dendritic and dendritic crystal growth, nucleation of equiaxed grains, columnar primary dendrite tip tracking, melt flow, sedimentation of equiaxed crystals, and their influence on macrostructure and macrosegregation. Five distinct thermodynamic phases (phase regions) are defined: solid dendrites in equiaxed grains, the interdendritic melt between equiaxed dendrites, solid dendrites in columnar trunks, the interdendritic melt between trunk dendrites, and the extradendritic melt. These five phase regions are quantified by their volume fractions and characterized by their solute concentrations. The five phase regions are grouped into three hydrodynamic phases: equiaxed grains consisting of solid dendrites and interdendritic melt, columnar trunks consisting of solid dendrites and interdendritic melt, and extradendritic melt. The extradendritic melt is separated from the interdendritic melt with a grain envelope, whose profile connects the primary, secondary or tertiary dendrite tips to form a ‘natural’ enclosure of the equiaxed grains or columnar trunks. The envelope is further simplified as a volume-equivalent sphere for equiaxed grains, or as volume-equivalent cylinder for columnar trunks by use of morphological shape factors. Expansion of the envelopes during solidification is determined by dendrite growth kinetics, using the Kurz–Giovanola–Trivedi model for growth of columnar primary dendrite tips and the Lipton–Glicksman–Kurz model for growth of columnar secondary dendrite tips (radial growth of the columnar trunk) and equiaxed primary dendrite tips. The solidification of the interdendritic melt is driven by the supersaturation of the interdendritic melt and governed by the diffusion in the interdendritic melt region. Illustrative process simulations and model verifications are presented in Part II.     

Effect of electromagnetic centrifugal casting on solidification microstructure of cast high speed steel roll

Using self‐made electromagnetic centrifugal casting machine, optical microscopy (OM) and D/max2200pc X‐ray diffraction, the solidification microstructure and phases of as‐cast high speed steel(HSS) roll made by sand casting, centrifugal casting and electromagnetic centrifugal casting were investigated. The experiment results show that the phases of as‐cast high speed steel (HSS) roll are alloy carbide (such as W₂C, VC, Cr₇C₃), martensite and austenite. The centrifugal casting and electromagnetic centrifugal casting can apparently improve the solidification structure of HSS roll. With the increase of electromagnetic field intensity (B), the volume fraction of austenite in the HSS solidification structure increased obviously and eutectic ledeburite decreased, the secondary carbide precipitated from the austenite is more fine and distribution of secondary carbide is more even.     

On the structure and properties of wear- and corrosion-resistant nickel-chromium- tungsten-carbon-(silicon) alloys

In some applications, for chemical and physical reasons hard nickel-based alloys have to be used instead of cobalt-based alloys but boron must be avoided. The nickel-chromium-tungsten-carbon system with and without silicon was therefore studied in several concentration ranges at 1050°C with respect to structure, phase, hardness and corrosion and wear resistance. Alloys containing 2% carbon, 10% tungsten and more than 10% chromium are composed of a nickel solid solution and an M₇C₃ carbide in both cast and homogenized (1050°C, 180 h) conditions. On increasing the tungsten content up to 20% the M₂C carbide becomes dominant, and this is associated with a remarkable increase in the hardness of the alloys. Additions of 2% silicon do not change the M₇C₃ and M₂C carbides present. In some cases a carbon-stabilized silicide M₅Si(C) was observed. Silicon additions decrease the liquidus temperature range relatively little, but they affect particle shape and size and the grain size distribution. The relation of various chromium, tungsten and silicon contents to corrosion and wear resistance was studied. The corrosion resistance depends on the chromium content of the nickel solid solution but also on carbide formation (tungsten and carbon content). The silicon content of the nickel solid solution is important too.Because their liquidus temperature is close to 1300°C the alloys cannot be used as self-fluxing and fusing powders for flame spraying but they can be sprayed by plasma torches and they can, of course, be welded.     

Phase and microstructure evolution in Cu-In-Cr alloys

Abstract

Cu-In-Cr ternary alloy specimens were prepared with a metal mould and analysed by OP, EPMA, SEM, etc. The results show that the phase composition of Cu-11In-10Cr (wt-%) ternary alloy in casting microstructure includes Cu solid solution, Cr solid solution and Cu₉In₄ (δ) phase. Cu solid solution has dendritic morphology while Cr solid solution is star or petal shaped and is dispersed. In solidification, the solid/liquid interfaces of Cu solid solution and Cr solid solution are nonfaceted and they form by continuous growth, Cu solid solution forms dendrites under the influence of constitutional supercooling, while Cr solid solution forms equiaxed dendrites owing to the restricted chromium content. Chromium content has an influence on the morphology of Cu solid solution dendrites and Cr solid solution.     

Technical note Relationship between degeneration of M7 C3 and precipitation of M23 C6 in a cobalt base superalloy

Abstract

The relationship between degeneration of the primary M₇ C₃ carbide and precipitation of the secondary M₂₃ C₆ carbide has been investigated in a cobalt base superalloy after 100 h aging at 850°C. The results indicated that the primary M₇ C₃ carbide could not transform in situ into the secondary M₂₃ C₆ carbide. The M₂₃ C₆ formed by a direct reaction between chromium and carbon atoms. The dissolution of M₇ C₃ provided the precipitation of M₂₃ C₆ with essential carbon, which acted as a carbon reservoir. Near M₇ C₃ , the chromium content is a controlling factor of M₂₃ C₆ precipitation, while away from it, the carbon content dominated the reaction. The precipitate free zone around M₇ C₃ is attributed to chromium depletion which is inherited from the as cast condition.     

Influence of liquid metal feeding on the flow and macrosegregation in DC casting

ABSTRACT

The influence of liquid metal feeding on the flow and macrosegregation during direct chill (DC) casting of Al-4.5wt-%Cu billets is investigated. An analytical study for constricted inlets predicts that the inflow only reaches the bottom of the sump for narrow inlets, a result confirmed by steady-state axisymmetric simulations of equiaxed solidification. For wide inlets, and inlets using a combo bag, the inflow is swiftly entrained towards the chill and the final macrosegregation predictions are all similar. For narrow inlets without a combo bag, penetration of the thermal jet to the bottom of the sump locally delays solidification producing an inclination in the packed solid interface and the possible growth of columnar dendrites in this region.     

对流扩散-多相相变体系内柱状晶/等轴晶形成过程的数值模拟

采用扩散支配相变动力学方法对Fe-Bi-Mn系易切削合金侧向快速凝固过程进行数值研究。建立对流扩散-多相相变体系三维凝固模型,考虑固、液、气三相扩散流动相变对合金凝固的影响,模拟研究合金中MnS和Bi(易切削相)的柱状晶/等轴晶形成过程。结果表明:合金凝固过程中MnS和Bi的柱状晶/等轴晶形成模式强烈受对流扩散和多相相变影响;对流扩散为正值处,溶质的多相质量相变速率较大且富集程度较低,流动稳定易形成柱状晶;对流扩散为负值处,溶质的多相质量相变速率较小且富集程度较高,当晶尖处溶质富集到一定程度,对流扩散与多相相变产生的紊流使柱状晶尖端断裂,成为等轴晶形核中心,此处为等轴晶稳定形成区域。     

Chromium carbide laser-beam surface-alloying treatment on stainless steel

In order to improve the resistance to wear, oxidation and corrosion of a stainless steel die, chromium carbide surface-alloying treatment was carried out on a 12 % Cr stainless steel using a CO₂ laser. Cr₃C₂ powder slurry was coated on the stainless steel and then a 3 kW CO₂ laser beam was used to irradiate the specimen. The thickness of surface-alloyed layer was about 0.3 mm and the chromium concentration was about 40 % throughout the alloyed-region. Large amounts of Cr₃C₂ and Cr₇C₃ were also distributed in this alloyed layer. From the results of the isothermal oxidation test at 960 °C for 100 h, it was found that the surface-alloying treatment improved the oxidation resistance by about 100 times due to the distribution of chromium carbides and the increase in the chromium concentration. The results of the cyclic oxidation test revealed that the oxidation layer was very stable on the chromium carbide surface-alloyed region, while it scaled off very easily from the substrate region due to porous oxidation products. The microhardness was about 1100 H_v due to the dispersion and precipitation of chromium carbides in addition to the martensitic structure in the surface-alloyed region. The microhardness did not decrease much, despite heating at 960 °C for 100 h. The chromium carbide surface-alloying treatment improved the wear-resistance greatly, and the results of the wear-resistance test were very consistent with those of the microhardness test.     

Effect of aluminum on the primary carbides of a hypereutectic high chromium cast iron

The effect of aluminum on the primary M₇C₃ carbides of a hypereutectic high chromium cast iron containing 4.0 wt% carbon and 20.0 wt% chromium was studied by means of optical microscopy, scanning electron microscope (SEM), energy dispersive X‐ray spectrometry (EDX), water quenching, and differential thermal analysis (DTA). Compared with specimen without aluminum addition, the primary carbides were all refined when different amount of aluminum was added into the melts, but the primary carbides in specimen with 0.3 wt% aluminum were the finest. With the addition of aluminum, aluminum element enriched at the boundary of primary carbides during solidification and was beneficial for the refinement of primary carbides. However, the increase of primary carbide growth time with the increase aluminum content had adverse effect on the refinement of the primary carbides. The comprehensive influence of those two factors leaded to the result that the primary carbides in specimen 1 with 0.3 wt% aluminum were the finest.     

等轴晶移动对宏观偏析影响的数值模拟

建立了自由等轴晶移动对宏观偏析影响的数学模型,对铸锭凝固过程中的对流和溶质分布进行了数值模拟.在模型中按照临界固相分数将糊状区分为紧密树枝晶和自由等轴晶两个不同的区域.对带冒口铸钢锭的宏观偏析进行了数值模拟,并同实验结果进行了比较.与假设糊状区内固相静止的模型相比,考虑等轴晶移动的模型得到的溶质分布结果与实验结果更接近.在凝固过程中,等轴晶随液体流动并在铸锭的底部中心聚集,在凝固后的铸锭中形成锥形的负偏析.还发现,在铸锭的中心靠上的区域形成正偏析,在铸锭的外部区域形成负偏析.     

Failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting. Part I: Material and fractographic characterization

The failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting and used in cast glass rolling was carried out by means of traditional characterization techniques (optical metallography, SEM, EDX microanalysis, tensile testing and XRD). The roll was in the as-cast condition and its microstructure featured large proportion of δ ferrite (between 20% and 27%) in a martensitic (α′) matrix, with the δ/α′ interfaces presenting an intergranular network of M₂₃C₆ carbides. The crack propagation began in the internal surface of the roll, with δ/α′ intergranular and transgranular cleavage in the “equiaxed region” of the casting, progressing to δ/α′ intergranular ductile fracture in the “columnar” and “chilled regions”. Tensile thermal stresses in the internal surface of the roll associated with microstructural embrittlement (network of interfacial carbide and microporosities) are thought to be the main causes for the premature failure of the roll. Finally, materials selection was performed to replace the CA-15M stainless steel with another class of stainless steel for centrifugal casting.     

Synthesis and characterization of nanostructured Cr3C2NiCr

Pre-alloyed Cr₃C₂-25 (Ni20Cr) powder was synthesized by mechanical ball milling in Hexane [H₃(CH₂)₄CH₃]and the variation of powder characteristics with milling time was investigated using SEM, X-ray and TEM. The average powder size drastically decreased with time during the first four hours of milling; then decreased slightly as milling continued up to 20 hours. For milling times in excess of four hours, the particle size approached 5 microns. X-ray diffraction analysis revealed a larger structural change in the NiCr solid solution powder relative to that experienced by the chromium carbide phases. This result indicated that the NiCr solid solution powder was subjected to heavier deformation than the chromium carbide powder. During the initial stages of milling, the brittle chromium carbide powders are fractured into sharp fragments and embedded into the NiCr solid solution powder. As milling continued a NiCr chromium carbide polycrystal composite powder was formed for times up to 20 hours of milling, transforming the sharp carbide fragments into spherical carbide particles. Conventional cold welding and fracturing processes primarily occurred only among the NiCr powder and composite powders. Milling times of up to 20 hours led to the formation of a poly crystal nanocomposite powder system in which chromium carbides, with average size of 15 nm, were uniformly distributed in NiCr matrix.     

Characterization of the solidification structure and texture development of ferritic stainless steel produced by twin-roll strip casting

In this paper, twin-roll cast strips of 17%Cr ferritic stainless steel were produced with a pilot caster. The effects of melt superheat on solidification structure and texture of cast strips were investigated, and the relationship between the percentage of equiaxed grains in the solidification structure and melt superheat was established. It was observed that the equiaxed grained strip exhibited a weak and random texture, and the columnar grained strip possessed a strong through-thickness <001> fiber texture tilted by about 15° from the normal direction (ND) of the strip.