Fabrication of SiC particle dispersed spheroidal graphite cast iron composite by vacuum assisted casting and its microstructure

Abstract

SiC particle preforms were infiltrated with spheroidal graphite cast iron melt by vacuum assisted casting in the sand mould, and spheroidal graphite cast iron composites in which the particles were dispersed in the surface region were fabricated. Although the melt infiltration was not accomplished when the melt was poured under atmospheric pressure, the infiltration was accomplished by the vacuum assisted casting when the SiC particle volume fraction and preform thickness were optimised. When the Si content of the cast iron was 2˙5 mass%, the phase consisting of mainly Fe₃Si was formed at the particle/matrix interface due to the reaction between the cast iron melt and the particles during the infiltration. The matrix of the composite consisted of fine spheroidal graphite particles, ferrite, pearite and chill crystal. Although the increase in the Si content suppressed the reaction and chill, no infiltrated area was observed in the composite.     

Mechanical characteristics of spheroidal graphite cast irons containing Ni and Mn with mixed ferrite and bainitic ferrite microstructure

Abstract

This study aims to clarify the influence of additive elements of Ni and Mn on tensile and impact properties of three kinds of spheroidal graphite cast irons (SG irons), which are as cast, annealed and austempered samples. Spheroidal graphite cast irons with Ni (0–4˙5 mass-%) and Mn (0–0˙5 mass-%) melted by a high frequency induction furnace and cast into a Y block CO₂ mould with 30 mm in thickness. From the viewpoint of heat treatment, tensile strength and hardness of SG irons become larger in the order of ferritised<as cast<austempered ones. Matrix structures of SG irons, which are conducted to austempering treatment from α and γ mixture range, consists of bainitic ferrite with high toughness. Austempered SG iron with 3%Ni in 0˙1%Mn series is found to become higher tensile strength compound with elongation and toughness of 901 MPa, 17% and 915 kJ m^?2.     

Microstructure and properties of austempered ductile cast iron with refined graphite nodules

Abstract

The present study examined the influence of refinement of graphite nodules on microstructure and tensile properties of austempered ductile cast iron (ADI). A casting technique using deoxidation treatment enabled manufacture of thin walled castings made of ductile cast iron without ledebulite. The thin walled casting (t=2 mm) was subjected to extreme refinement of graphite nodules, where the number of graphite nodules was 1750 mm^?2. Decrease in graphite nodule diameter resulted in refinement of ausferrite and γ-pool structures in ADI and rapid reaction of austempering. The significant increase in the number of graphite nodules resulted in a remarkable drop in the tensile strength and elongation of ADI. These results can be explained by the graphite nodule distance.     

Modelling the effect of graphite morphology on the modulus of elasticity in cast irons

Abstract

Nine grades of pearlitic cast iron containing different graphite morphologies (from flake, compacted and spheroidal) have been studied. The parameters investigated include the graphite aspect ratio, nodularity, graphite size and modulus of elasticity. These parameters have been correlated and compared with different existing bound and model equations. It has been found that the modulus of elasticity of the graphite phase increases as the aspect ratio and nodularity of the graphite increases, i.e.flake graphite gives a lower modulus of elasticity than spheroidal graphite. The experimental values of the modulus of elasticity show good agreement to bound and model equations, although flake graphite cast irons show higher deviation from the modelled values. An equation for the correlation between the graphite modulus of elasticity and the nodularity is presented. Introducing this linear correlation into an existing model for the determination of the effective modulus of elasticity gives a continuous function, including all grades of cast irons, with a very good agreement with experimental values. The modulus of elasticity of cast irons can be accurately predicted from both bound and especially model equations, using the aspect ratio and nodularity of the contained graphite particles. The fit is improved by using a modulus of elasticity of the graphite phase that is based on the graphite morphology, considering that the modulus of elasticity of the graphite is different in the basal and prismatic planes.     

Influence of heat treatment on matrix structures in cast iron

Abstract

Flake graphite cast iron (FC) samples containing 2%Si, in which the S contents changed from 0˙006 to 0˙076%, were heated at 1123 to 1323 K and then cooled to determine the pearlite morphological change, namely lamellar or granular. For the spheroidal graphite cast iron (FCD) samples, the P contents changed from 0 to 0˙2% and those of Sn were from 0 to 0˙06% with 0˙5%Cu using the 2˙5%Si and 0˙3%Mn iron.

The pearlite morphology of the FC samples cooled from 1123 K was granular. The pearlite structure of the Sn–FCD samples was also granular, when cooled from 973 and 1023 K, and that of P–FCD was granular only in the sample cooled from 1023 K, otherwise the morphology was lamellar.

The pearlite morphology of FC and FCD samples depends on the solubility of C in the austenite which can be varied by the heating temperature. This mechanism is identical to that of the lamellar to rod transition in eutectic alloys.     

Effect of antimony on the eutectic reaction of heavy section spheroidal graphite castings

Abstract

There is a strong demand for heavy section castings made of spheroidal graphite with a fully ferritic matrix, e.g. for manufacturing hubs for windmills. Such castings with slow solidification process are prone to graphite degeneration that leads to a dramatic decrease of the mechanical properties of the cast parts. Chunky graphite is certainly the most difficult case of graphite degeneracy, though it has long been known that the limited and controlled addition of antimony may help eliminate it. The drawback of this remedy is that too large Sb additions lead to other forms of degenerate graphite, and also that antimony is a pearlite promoter. As part of an investigation aimed at mastering low level additions to cast iron melts before casting, solidification of large blocks with or without Sb added was followed by thermal analysis. Comparison of the cooling curves and of the microstructures of these different castings gives suggestions to understand the controlling nucleation and growth mechanisms for chunky graphite cells.     

Studying elastic deformation behaviour of cast irons by acoustic emission

Abstract

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.     

Effects of nodularity on thermal conductivity of cast iron

Abstract

The thermal transport properties of five predominately pearlitic grades of grey, compacted graphite and spheroidal graphite iron have been investigated by the laser flash technique. Samples have been taken from cylinders cast in controlled thermal environments designed to produce three dissimilar cooling rates. Digital image analysis has been utilised in order to characterise the different graphite morphologies. The results indicated linear relationships between the thermal transport properties and the roundness of the graphite and the nodularity for compacted graphite and spheroidal graphite iron. A pronounced decrease in the thermal conductivity occurred when the lamellar graphite structure was transformed into compacted graphite. The thermal conductivity of compacted and spheroidal graphite iron has been recalculated with good accuracy over a temperature range of 25–500°C by means of regression analysis.     

Influence of graphite morphology on dry sliding wear of flake graphite cast irons

Abstract

Four flaky graphite cast irons of different graphite structures with a pearlitic matrix were prepared to clarify the graphite structure's influence on the dry sliding wear property. Two melts of cast iron with different carbon contents were solidified at two different cooling rates. The four resultant samples had type A flaky graphite or type D eutectic graphite structure with different graphite volume fractions and fully pearlitic matrixes. A pin on the disc type wear test evaluated the four samples' dry sliding wear properties. Results showed that the type D graphite structure wore down faster than the type A graphite structure did. The type of graphite morphology influenced the specimen wear rate as strongly as the graphite volume fraction did in flaky graphite cast irons of this experimental range.     

Control of graphite formation in solidification of white cast iron

Abstract

Graphite formation should be strictly suppressed for the most abrasion resistant white cast irons, since austenite (γ)+graphite eutectic structure shows lower hardness and selectively wears thus deteriorates the abrasion resistance even though the austenite transform to hard phase such as martensite. On the other hand, a small amount of fine graphite is desired to distribute in rolls for hot steel mills to suppress the scoring. However, strong carbide formers such as Cr, V, Nb have been increasingly added to rolls, in order to crystallise more harder carbides. As γ+carbide eutectic grows, the residual liquid among eutectic cells becomes poor in carbide formers and rich in elements which promote graphite formation. Therefore an appropriate alloy design is essential for the hot steel milling rolls. In this study, the graphite formation mechanisms are discussed for chromium cast iron, high speed steel type cast iron and Ni hard type cast iron.     

Nucleation of graphite in cast iron melts depending on manganese,sulphur and oxygen

Abstract

The form and distribution of graphite in grey iron influences the mechanical properties and depends on numerous factors, such as nucleation and cooling speed conditions. The main focus of the present work is the influence of manganese, sulphur and oxygen on the nucleation of graphite in lamellar cast iron melts. Previous studies showed that the nucleation in GJL melts is initiated by a MnS particle. For investigations in the field of nucleation in grey iron the authors examined several EN-GJL-200 specimens. The specimens were cast with and without inoculant. The studies of the specimens were realised using light microscopy, SEM-EDS (energy dispersive X-ray analysis), WDS (wavelength dispersive X-ray analysis). The experimental results were compared with the software Thermo-Calc calculations. From the experimental results and the Thermo-Calc calculations it can be concluded, that Mn and S contents and the Mn/S ratio respectively plays a very important role for the nucleation. A direct influence of oxygen on the nucleation of graphite could not be observed.     

Eutectic cell and nodule count in cast irons

Abstract

In the present work the predictions based on a theoretical analysis aimed at elucidating of eutectic cell count or nodule count N were experimentally verified. The experimental work was focused on processing flake graphite and ductile cast irons under various inoculation conditions in order to achieve various physicochemical states of the experimental melts. In addition, plates of various wall thicknesses s, were cast and the resultant eutectic cell or nodule counts were established. Moreover, thermal analysis was used to find out the degree of maximum undercooling for the graphite eutectic ΔT _m. A relationship was found between the eutectic cell or nodule count and the maximum undercooling ΔT _m. In addition, it was also found that N can be related to the wall thickness of plate shaped castings. Finally, the present work provides a rationale for the effect of technological factors such as the melt chemistry, inoculation practice, and holding temperature and time on the resultant cell count or nodule count of cast iron. In particular, good agreement was found between the predictions of the theoretical analysis and the experimental data.     

Effects of transition from lamellar to compacted graphite on thermal conductivity of cast iron

Abstract

Different levels of magnesium were added to a standard grey iron alloy in order to obtain a range of graphite morphologies from lamellar to compacted graphite. The thermal conductivity/diffusivity of samples, solidified at different cooling rates, was investigated by means of the laser flash technique. There is a significant decrease in the thermal conductivity as the morphology transits from lamellar to compacted graphite. The thermal conductivity of grey iron decreases considerably at elevated temperatures, whereas the thermal conductivity of compacted graphite iron is less sensitive to changes in temperature. At increased nodularities, compacted graphite irons exhibit a maximum thermal conductivity at ～400°C. The influence from the cooling conditions on the thermal conductivity decreases as the morphology alters from lamellar graphite to compacted graphite. The effective thermal conductivity of cast iron is modelled by means of existing models for composites.     

Development of boron white cast iron

Abstract

With boron substituting for carbon in cast iron composition and eutectic borides substituting for eutectic carbides in microstructure as the hard wear resistant phase, a new kind of wear resistant white cast iron has been developed. The microstructure and mechanical properties of this new white cast iron both in the as cast state and after appropriate heat treatments were studied. The results show that the as cast microstructure of the boron white cast iron comprises a dendritic matrix and interdendritic eutectics, and the eutectic compound is that of M₂B or M′_0˙9Cr_1˙1B_0˙9 type, where M represents Fe, Cr or Mn and M′ represents Fe or Mn. The morphology of the eutectic borides is much like that of carbide in high chromium white cast iron, but the hardness of boride is higher than that of carbide. The matrix in as cast microstructure comprises martensite and pearlite. After austenitising and quenching, the matrix mostly changes to lath type martensite and the eutectic borides remain unchanged. In addition, two different sizes of particles, with different forming processes during heat treatment, appear in the matrix. The boron white cast iron possesses higher hardness and toughness than conventional white cast iron and nickel hard white cast iron, and has a better balance between hardness and toughness than high chromium white cast iron.     

Mechanism of pinhole generation during welding of flake graphite cast iron and effects of composition of base metal

Abstract

The mechanism of pinhole generation during welding flake graphite cast iron is examined. As a result, the oxidisation progresses inwardly along the interface between matrix and flake graphite, and then oxides such as SiO₂ and MnO are generated. The oxidisation progresses through the graphite in the surface of the nearby eutectic cell. When oxidised cast iron is welded, oxides are deoxidised by the graphite and carbon monoxide gas is generated. The carbon monoxide gas comes up to the surface of the welding bead, but sometimes does not reach the surface and it becomes solidified from the top because the cooling rate at welding is very fast. This is how pinholes are generated.

Also the effect of components of flake graphite cast irons on the pinhole generation is investigated. The number of pinholes increases with the increase in C, Si, Mn, Ni or Cu. Conversely, when the elements of P, S, Cr, Mo and Mg increase, it becomes difficult to generate pinholes.     

Graphite nucleation control in grey cast iron

Abstract

A research programme has been undertaken to achieve a more detailed understanding of graphite nucleation control in grey cast irons, at different sulphur (0˙02–0˙1%), residual aluminium (0˙001–0˙010%) and zirconium (0˙001–0˙015%) levels in iron melts. It was found that three groups of elements are important to sustain a three stage model for the nucleation of graphite in grey irons:

(i) strong deoxidising elements (Al, Zr) to promote early formed very small microinclusions, oxide based, which will act as nucleation sites for later formed complex (Mn,X)S compounds

(ii) Mn and S to sustain MnS type sulphide formation

(iii) inoculating elements (Ca, Sr, etc.) which act in the first stage or/and in the second one of graphite formation, to improve the capability of (Mn,X)S compounds to nucleate graphite.

It was confirmed that 0˙07%S level is beneficial for graphite nucleation in grey irons with a lower incidence of carbides and undercooled graphite, compared to 0˙023%S cast irons. Low residual Al level (0˙001–0˙003%) results in higher chill and more undercooled graphite and lower eutectic cell count, in inoculated irons. A 0˙007–0˙010%Al content in the melt is important to sustain type A graphite nucleation and reduced chill. Not only inoculation but also the preconditioning (Al or/and Zr) of the base iron has a strong beneficial effect on the solidification pattern of cast irons. Both Al and Zr sustain the type A graphite formation with a lower degree of undercooling and free carbides. These elements were associated in a complex alloy (FeSi based), very efficient in preconditioning of grey irons for thin wall castings, at a low addition rate.     

The Effect of Phosphorus on the Microstructure and Properties of Compacted Graphite (CG) Irons

Abstract

The effect of up to 0.15% phosphorus on the microstructure and mechanical properties of step-block castings made from compacted- graphite iron has been examined. There was no major effect on the microstructure, but two minor effects were observed. First, the higher-phosphorus irons showed a small increase in the amount of carbides and pearlite within the microstructure, but there was no effect on the depth of surface chill. Second, at low magnesium contents the higher-phosphorus irons demonstrated a marked increase in the amount of flake graphite. Phosphorus had no adverse effect on tensile strength, and a reported lowering of tensile elongation at elevated phosphorus content was not found in this work. There was, however, a decrease in Charpy impact toughness at higher phosphorus levels.     

Effect of nodule count and cooling rate on As-Cast matrix of a Cu-Mo spheroidal graphite

The transformation of austenite to ferrite and graphite or to pearlite in spheroidal graphite cast iron depends on a number of factors, among which are the nodule count and the cooling rate. In this study, the pearlite fraction decreased as the nodule count increased for a given cooling rate. Furthermore, as the cooling rate increased, the fraction of pearlite increased. Both effects were more sensitive at low nodule count. The effects of altering these parameters on the relative amount of pearlite and ferrite in the matrix of a copper-molybdenum (Cu-Mo) spheroidal graphite cast iron were addressed using heats carried out in an induction furnace, and the melts were treated with magnesium ferrosilicon in a ladle. To vary the nodule count, the melts were inoculated with two different amounts of ferrosilicon. Pouring was performed into sand molds of cylindrical cavities with different section size in order to achieve various cooling rates. Both the nodule count and the cooling rate affected the relative amount of ferrite and pearlite in the matrix.     

Study of voids in chilled samples of magnesium treated irons

Abstract

Chilled samples of base iron and magnesium-treated iron were surveyed using optical microscopy, SEM and computer assisted microanalysis. Numerous voids, 0.5–1 m m in size, were observed in magnesiumcontaining chilled samples. Graphite spheroids at an early stage of growth, 3–5 m m in size, and inclusions were also observed. Voids were not found in the base iron samples but inclusions were observed. Magnesium enrichment was detected at the voids, graphite and inclusions of the magnesium treated samples. It is proposed that the voids represent the traces of magnesium vapour bubbles formed in the melt. Some graphite spheroids consist of a thin graphite layer at their surface and a hollow core, a morphology that supports nucleation on the void walls followed by growth towards the centre. Although inclusions were occasionally observed in the centres of graphite spheres, they do not appear to be essential for spheroidisation to occur. It is concluded that gas bubbles formed by free magnesium contribute to the formation of spheroidal graphite by supplying nucleation and growth sites.     

Cooling curve and graphite morphology in Ni–C alloys

Abstract

Ni–C alloys were used to study the solidification sequence of the graphite morphology based on a thermal analysis. Ninety grams of Ni–2˙2C alloys, with or without the addition of Mg, Ce and Ca, were melted at 1773 K and then cooled at 20 or 40 K min^?1. The graphite morphology is chunky in the Ce added specimen cooled at 20 K min^?1. Meanwhile, chunky graphite (CHG) and spheroidal graphite (SG) are observed in the pure Ni–C and Ca added Ni–C alloys. Spheroidal graphite forms in the Ni–C–Ce alloy cooled at 40 K ^?1. This cooling curve shows a continuous temperature decline during the eutectic reaction identical with that of the SG iron. Only flake-like graphite is formed in the Mg added specimen. From these experimental results, it was concluded that the formation of CHG occurs earlier than that of the SG in the Ni–C alloys. Thus, the solidification mechanism of the Ni–C system differs from that of the Fe–C one.