Superplastic behaviour of fine-grained white cast irons

Superplastic behaviour of three hypo-eutectic white cast irons with different carbon contents was investigated at elevated temperatures. Hot and warm working, and rapid solidification technology (RST) were used to refine the coarse cementite structure. The resulting microstructure consisted of a mixture of small carbides and fine ferrite grains. Hot and warm working was found to be a successful method to refine the structure of white cast irons having a carbon content of less than 2.6%. Rapid solidification technology was the most promising process to refine high carbon (> 3.0%) cast irons. The refined white cast irons exhibited low flow stress and high strain-rate sensitivity in both tensile and compression in the temperature range 650–770 °C. Tensile elongation to failure of 300% was found for 3.0% C RST iron, 220%, 150% and 80% for 2.2%, 2.6% and 3.0% C hot and warm rolled cast irons, respectively.     

Low‐temperature annealing and graphitizing of white‐solidified low‐alloy cast irons

Hypoeutectic iron‐carbon and iron‐carbon‐silicon model alloys as well as conventional cast irons GJL‐250_mod and EN‐GJS‐600‐3 have been produced and processed by different solidification techniques, i. e. mold casting, electron beam surface remelting and melt spinning. The white‐solidified alloys exhibit different degrees of microstructural refinement indicated by a secondary dendrite arm spacing of 0.3 μm–12 μm. The effects of microstructural refinement and silicon content on the hardness as well as on coarsening of cementite and graphitizing at temperatures of 540 °C to 670 °C have been investigated. The hardness of the as‐solidified alloys increases with decreasing secondary dendrite arm spacing and increasing silicon content. High silicon content effectively retards coarsening of pearlitic cementite, and thus is beneficial to retain the hardness at small thermal load. On the downside, high degree of microstructural refinement and high silicon content promote and accelerate graphitizing at temperatures > 600 °C. The results are discussed in terms of the applicability of a recently developed two‐step surface treatment for cast irons, i. e. electron beam remelting followed by nitriding.     

Identification of Cu-rich precipitates in pearlitic spheroidal graphite cast irons

This investigation studies the partitioning of Mn and Cu – and Si– between ferrite and cementite in the pearlite of two alloys containing similar Mn and Si concentrations, but different Cu contents, keeping the Cu concentration within the usual industrial limits. The results confirm the partitioning of all the elements between ferrite and cementite during pearlite growth. Furthermore, the presence of nanometric Cu-rich precipitates was detected for an alloy with typical Cu contents for achieving pearlitic structures in spheroidal graphite cast irons. This might be linked to the microsegregation of Cu developed during the solidification step. These precipitates have not been reported before and cannot be ignored as they can affect the transformation kinetics and the mechanical properties of the alloy.     

Microstructural properties and hot pressing of rapidly solidified hypoeutectic low alloy white cast iron powders

Abstract

Hypoeutectic low alloy white cast iron powders were produced using a rapid solidification technique. The morphology and microstructural properties of these powders were investigated with respect to cooling rate and particle size. The density of hot pressed compacts as a function of parameters such as hot pressing time and pressure is described. It was found that retained austenite in the form of cells or dendrites was the main constituent of the powders. At 720°C the powder particles can be hot pressed into high density compacts that have a fine cementite–ferrite microstructure. These ultrafine grained compacts exhibited good superplasticity at elevated temperatures. An elongation to failure of 300% was observed.

MST/1682     

Laser Surface Melting on cast iron containing undercooled graphite

Laser surface melting experiments were carried out on an alloy cast iron with 3% C, 2% Si and 0.25% Ti containing undercooled graphite. Different heating times and energy densities were used to achieve different penetration depths and cooling rates. The microstructures obtained after surface melting and after various heat treatments are presented and discussed. It is shown how the cooling rate and concentration influence the solidification behaviour of high carbon cast irons. The composition chosen for the experiments can be considered as representative of many cast irons.     

High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract

Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. After tempering for 2 h at 570°C all austempered microstructures consisted offerrite and cementite, but cementite was most finely distributed in the material that had been austempered at 300°C, and coarsest in that austempered at 440°C. It is concluded that if SG cast iron is to be nitrided conventionally at temperatures >500°C, then prior austempering to obtain controlled microstructures is of limited value.

MST/3106     

Coefficients for equilibrium partition of a third element between solid and liquid in iron-carbon base ternary alloys and their relation to graphitization during iron-carbon eutectic solidification

During iron-carbon eutectic solidification, the coefficients for partition of a third element between the eutectic liquid and its solid were evaluated thermodynamically. The coefficientk _M ^A/L for the equilibrium partition of the third element (M) between austenite and liquid iron largely depended on the interaction between carbon and the third element and a simplified method for the evaluation ofk _M ^A/L was introduced. The coefficients,K ^S andK ^M, for the partition of the element between the eutectic liquid and its solid in the stable and metastable eutectic solidification, respectively, were also calculated fromk _M ^A/L and the coefficientk _M ^C/A for the equilibrium partition of the element between cementite and austenite. It was indicated by the thermodynamics of the free energy for the co-existing phases that the effect of a third element on graphitization occurring during eutectic solidification was related quantitatively to the value of K which was represented byK ^S-K ^M. The effect of a third element on the difference between the stable and metastable eutectic temperatures and on the carbon activity of liquid iron was closely related to K or the equilibrium partition coefficient,k _M ^C/A .     

Breakup of eutectic carbide network of white cast irons at high temperatures

The fracture toughness of white cast irons is related to the morphology of eutectic carbides,being better when isolated than when network-like. In this paper observations on the breakup of eutectic cementite network during annealing treatment of white cast irons are reported using a high temperature microscope and scanning electron microscopy (SEM). Dissolution-induced breakup and capillarity-induced breakup are identified. The former occurs through growth of holes and pre-existing fissures as well as through fragmentation at narrow necks and narrow roots of branches. The latter is observed through growth of perturbations. Dissolution-induced breakup is closely associated with the morphology of the as-cast eutectic cementite. A combination of solidification processing and heat treatment thus produces a more positive breakup effect.     

Hot deformation behaviour and ledeburite refinement mechanism for hypoeutectoid low alloy white cast irons

Abstract

Extensive hot deformation was performed on three white cast irons having different carbon contents. The mechanism of formation of ledeburite eutectic during plastic working was investigated. It wasfound that white irons can undergo hot forging, hot rolling, and warm rolling at 700°C–1100°C. The refinement of massive carbide occurs by the combined contributions from plastic working and diffusion of carbon atoms. The structure of worked cast iron consists of small cementite particles distributed in a fine ferrite matrix.

MST/1374     

Alloy design of ductile phosphoric iron: Ideas from archaeometallurgy

Alloy design criteria to produce ductile phosphoric irons have been proposed based on a detailed microstructural study of ancient Indian irons. The alloy design aims at avoiding phosphorus segregation to the grain boundaries by (a) soaking the phosphoric iron at high temperatures within the ferrite + austenite region to precipitate austenite allotriomorphs, (b) utilizing a critical amount of carbon to segregate to grain boundaries, and (c) precipitation of some of the phosphorus in solid solution in the ferrite matrix as fine coherent phosphide precipitates.     

球墨铸铁表面电弧增材制造Fe‒Cr合金结合区组织和性能

目的 在球墨铸铁基体上电弧增材制造Fe-Cr合金,研究结合区组织和性能,以期获得具有良好冶金结合、满足冲裁模具性能要求的双金属构件。方法 采用GMAW工艺增材制造,用金相显微镜和扫描电子显微镜表征结合区的显微组织,并分析其形成机制。结果 Fe-Cr合金与球墨铸铁结合区无明显裂纹和气孔,其凝固组织为柱状晶和等轴晶,冷却后转变为马氏体和残余奥氏体,但其分布不均匀,在界面处有一富奥氏体层。结合区内球墨铸铁受热影响发生奥氏体化和部分熔化,熔化发生在临近结合界面的石墨球周围,其冷却后形成一层马氏体和一层莱氏体的双层壳型组织结构,未熔化部位的组织为马氏体和铁素体,珠光体球墨铸铁比铁素体球墨铸铁形成的马氏体多。结合区内硬度分布不均匀,球墨铸铁的硬度从基材到结合界面逐渐升高,最高达630HV,Fe-Cr合金平均硬度为510HV。结论 电弧增材制造Fe-Cr合金与球墨铸铁基体冶金结合良好,Fe-Cr合金组织为马氏体和残余奥氏体,有较高的硬度,能满足冲裁模具的性能要求。     

Advanced P/M aluminium alloy: rapidly quenched structure and decomposition behaviour during annealing

An Al-Fe-Cr-Zr alloy is currently being developed for elevated temperature service utilizing rapid solidification technology. The inert gas atomized powder has been assessed microstructurally and the solidification behaviour interpreted as a function of particle size. It was significant that both fine and coarse powders exhibited structures created by solute trapping during the initial stages of solidification. An unidentified phase (or possibly phases) was detected by X-ray and electron diffraction and shown to form at cell boundaries. Zirconium was retained in solid solution during solidification of all powder structures and subsequently precipitated as fine particles of the metastable Al₃Zr phase during heat treatment. This precipitation was extremely fine and deemed suitable for elevated temperature strengthening. In contrast, the iron and chromium additions partition to the cell boundaries and precipitate during heating to their respective equilibrium compounds.     

A transformation toughening white cast iron

An experimental white cast iron with the unprecedented fracture toughness of 40 MPa m1/2 is currently being studied to determine the mechanisms of toughening. This paper reports the investigation of the role of strain-induced martensitic (SIM) transformation. The dendritic microconstituent in the toughened alloy consists primarily of retained austenite, with precipitated M7C3 carbides and some martensite. Refrigeration experiments and differential scanning calorimetry (DSC) were used to demonstrate, firstly, that this retained austenite has an "effective" sub-ambient MS temperature and, secondly, that SIM transformation can occur at ambient temperatures. Comparison between room temperature and elevated temperature KIc tests showed that the observed SIM produces a transformation toughening response in the alloy, contributing to, but not fully accounting for, its high toughness. SIM as a mechanism for transformation toughening has not previously been reported for white cast irons. Microhardness traverses on crack paths and X-ray diffraction (XRD) on fracture surfaces confirmed the interpretation of the KIc experiments. Further DSC and quantitative XRD showed that, as heat-treatment temperature is varied, there is a correlation between fracture toughness and the volume fraction of unstable retained austenite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Kinetics of granulation of discontinuous phase in eutectic structures

Abstract

Useful properties of eutectic alloys have been found to relate to the presence of granulated discontinuous phase, and its formation is encouraged during solidification; although difficulties are encountered in some alloy systems. In the present paper it is suggested that discontinuous phase granulation can be achieved through either solidification processing or heat treatment. The kinetics of granulation during heat treatment and the factors affecting it have been analysed and examined for eutectic silicon in Al–Si alloys and for graphite in cast irons during heat treatment, using the techniques of high-temperature and quantitative metallography. The concept of combined control of the size and morphology of the discontinuous phase through solidification and heat treatment was developed.

MST/254     

Effect of microsegregation on carbide-free bainitic transformation in a high-silicon cast steel

ABSTRACT

A high silicon cast steel was studied in the as-cast condition in order to characterise its solidification macrostructure and microsegregation. The steel, poured into 32?mm-keel-block-shaped moulds, has a coarse solidification structure and marked microsegregation, containing low-alloyed areas with a total alloy content (Cr?+?Mn?+?Si) of 2.3 wt-% and high-alloyed zones of 5.3 wt-%. The bainitic transformation behaviour at 300°C was studied at different austempering times. The bainitic reaction occurs at different rates within the specimen volume, because of its chemical heterogeneity. An austempering heat treatment leads to an inhomogeneous carbide-free bainitic microstructure with different phase amounts, morphologies and sizes. The heterogeneous distribution of sizes and chemical compositions of retained austenite is speculated to benefit mechanical properties.     

Volume change during the solidification of SG iron: comparison between experimental results and simulation

In order to understand the shrinkage behaviour of spheroidal graphite (SG) iron during solidification, a volume change kinetic model was set up to simulate the volume change during the eutectic solidification, which was presented in an earlier paper. Furthermore in the present work experiments were carried out for comparison with theoretical prediction. The microstructure of the mushy zone during the solidification of SG cast iron was obtained by the quenching method and analysed by normal metallography and image analysis. The results show that the mushy zone exists in front of the interface between the liquid and the solid. The study by quantitative stereology shows that the graphite fraction in the mushy zone has the same trend as that of the theoretical prediction and the silicon content in cast iron strongly influences graphitization during the solidification. A heat-transfer model to stimulate the heat transfer of the experimental apparatus was developed. A modified Rappaz’s model was used to simulate the eutectic growth under fully equilibrium conditions. The theoretical prediction has been compared with the experimental results, and found to be in good agreement with each other. This revised version was published online in November 2006 with corrections to the Cover Date.     

Numerical model for microsegregation in ductile iron

Abstract

A numerical, non-steady state microsolute redistribution model is presented for ductile iron. The model takes into account solute diffusion in the solid and liquid phases, interface movement, a non-linear growth rate for the austenite phase and total solute conservation in the microvolume sphere. Preliminary calculations show that interface movement can be ignored and a linear austenite growth rate can be used for solidification conditions occurring during directional solidification experiments and keel block solidification. The numerical calculations of the solute distribution in the liquid and solid phases show reasonable agreement with the available experimental measurements.     

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.     

Modification of hypoeutectic low alloy white cast irons

In order to modify the continuous network of eutectic cementite normally found in low alloy white cast irons into a dispersive distribution, strategies of controlling the morphology of eutectic cementite by additives are discussed. Qualitative arguments are presented and applied to the development of a complex modifier REAINTi. With the addition of this modifier to low carbon white cast irons nearly one fifth of the original eutectic cementite network can be modified into blocky particles in the plane of polish, and one third into isolated irregular ones. The experimental data shows that Ti(CN) may act as an effective nucleant or a grain refiner for eutectic cementite. The modification effects of rare earth elements are attributed to their abilities to purify the melt, promote divorced eutectic growth and refine the microstructure. Aluminium contributes to the modification by its purification effect.     

Systematische Analyse der mikroplastischen Verformungsanteile bei einachsiger Zugbeanspruchung von ferritischen Gußeisen

Systematic Analysis of Microplastic Strain Components in Uniaxial Tensile Deformation of Ferritic Cast Irons In order to analyse the microplastical strain components of cast iron, four ferritic cast irons with similar chemical compositions but different graphite shapes as well as the graphite-free base alloy have been tested in uniaxial tension. Graphite shape changings from nodular over vermicular to lamellar shape reduce the proof stresses at very small strains much more than the 0,2% proof stresses. Also, the relationships between lateral contractions and longitudinal strains are strongly changed. As a consequence of three graphite-induced strain components at any given stress, the plastic strains of the cast irons are greater than the plastic strain of the graphite-free base alloy: i( plastic matrix-deformation, caused by the notch effect of graphite particles; ii) plastic matrix-deformation, caused by integral stress rise in the matrix due to the presence of graphite, and (iii) volume changes, caused by cracking in the graphite particles as well as decohesion at the graphite/matrix interface. In the case of coarse flake graphite cast irons, very pronounced volume changes occur, which strongly influence the stress, strain behaviour.