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.     

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.     

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.     

Inoculation and its effect on primary solidification structure of hypoeutectic grey cast iron

Abstract

The solidification of grey cast iron is controlled by the addition of inoculants. This is done in order to provide nucleation sites and hence facilitate the formation of eutectic cells and decrease the degree of undercooling. The number of eutectic cells and the graphite morphology affect the final properties of the casting. Preceding the nucleation of graphite and the eutectic cells is the nucleation of the primary austenite. It was found that the addition of inoculants also influences the primary solidification. The largest effect on the primary dendrites is obtained by inoculation using pure iron powder. It was also shown how the columnar to equiaxed transition (CET) depends on the number of equiaxed dendrites per unit volume. In addition, the primary structure was found to influence the eutectic solidification. The relationship between the secondary dendrite arm spacing and the eutectic cell size was found to correlate well with the work of others.     

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.     

Influence of Gd and B on solidification behaviour and weldability of Ni–Cr–Mo alloy

Abstract

The influence of Gd and B on the solidification behaviour and weldability of Ni–Cr–Mo alloy UNS N06455 has been investigated by Varestraint testing, differential thermal analysis and microstructural characterisation. These alloys are currently being developed as structural materials for nuclear criticality control in applications requiring transportation and disposition of spent nuclear fuel owned by the US Department of Energy. The Gd containing alloys were observed to solidify in a manner similar to a binary eutectic system. Solidification initiated with a primary L→y reaction and terminated at ~1258°C with a eutectic type L→y+Ni₅Gd reaction. The solidification cracking susceptibility of the Gd containing alloys reached a maximum at ~1 wt-%Gd and decreased with both higher and lower Gd additions. Low cracking susceptibility at Gd concentrations below ~1 wt-% was attributed to a relatively small amount of terminal liquid that existed over much of the crack susceptible solid+liquid zone. Low cracking susceptibility at Gd concentrations above ~1 wt-% was attributed to a reduced solidification temperature range and backfilling of solidification cracks. The addition of B above the 230 ppm level leads to the formation of an additional eutectic type reaction at ~1200°C and the secondary phase within the eutectic type constituent was tentatively identified as Mo₃B₂. The B containing alloys exhibited a three step solidification reaction sequence consisting of primary L→y solidification, followed by the eutectic type L→y+Ni₅Gd reaction, followed by the terminal eutectic type L→y+Mo₃B₂ reaction. Boron additions had a strong, deleterious influence on solidification cracking susceptibility. The high cracking susceptibility was attributed to extension of the crack susceptible solid+liquid zone induced by the additional eutectic type L→y+Mo₃B₂ reaction and extensive wetting of the grain boundaries by the solute rich liquid. Simple heat flow equations were combined with solidification theory to develop a relation between the fraction liquid f _L and distance x within the solid+liquid zone. Information on the phenomenology of crack formation in the Varestraint test were coupled with the calculated f _L–x curves and were shown to provide useful insight into composition–solidification–weldability relations.     

Role of Al, Ti, and Zr in Gray Iron Preconditioning/Inoculation

A research was done to investigate the effect of strong deoxidizing elements, such as Al, Zr, and Ti, in gray irons in laboratory experiments. The conclusions drawn were based mainly on thermal analysis, chill (carbides) sensitivity, graphite characteristics, and SEM analysis. Al and Zr have visible beneficial effects in preconditioning of gray irons, by favoring lower undercooling during solidification. Ti has an inconclusive role, with limited influence, but promotes undercooled graphite formation. Complex (Mn,X)S compounds, nucleated on the previously formed small oxide-based sites, were found as the major nucleation sites for graphite in gray irons, with specific distribution of Al, Ti, and Zr. Al,Zr-FeSi preconditioning of electrically melted and Sr-FeSi inoculated gray irons avoided type D graphite and carbides in 3 mm sections castings.     

Columnar to equiaxed transition during growth of a univariant eutectic

Abstract

In this work the transition from columnar to equiaxed growth is studied during the solidification of the univariant eutectic L→ α(Al) + θ-Al₂Cu in the ternary Al–Cu–Ag system. The experimental technique used is upwards solidification with a controlled cooling rate. In the lower part of the sample where the temperature gradient is high, a cellular eutectic is observed. When the temperature gradient decreases, the morphology first changes into two-phase dendrites and finally into an equiaxed eutectic. When the amount of Ag is increased, the formation of eutectic colonies becomes easier. Also, a higher cooling rate facilitates the formation of eutectic colonies. The addition of TiB₂ does not influence the nucleation of eutectic colonies. Although the composition is on the eutectic groove, primary α(Al)-dendrites are found in the samples with TiB₂. This leads to segregation effects due to the density difference between α(Al) and the liquid.     

Cast Iron Inoculation Enhanced by Supplementary Oxy-sulfides Forming Elements

Inoculation is one of the most important metallurgical treatments applied to the molten cast iron immediately prior to casting, to promote solidification without excessive eutectic undercooling, which favors carbides formation usually with undesirable graphite morphologies. The paper focused on the separate addition of an inoculant enhancer alloy [S, O, oxy-sulfides forming elements] with a conventional Ca-FeSi alloy, in the production of gray and ductile cast irons. Carbides formation tendency decreased with improved graphite characteristics as an effect of the [Ca-FeSi + Enhancer] inoculation combination, when compared to other Ca/Ca, Ba/Ca, RE-FeSi alloy treatments. Adding an inoculant enhancer greatly enhances inoculation, lowers inoculant consumption up to 50% or more and avoids the need to use more costly inoculants, such as a rare earth bearing alloy. The Inoculation Specific Factor [ISF] was developed as a means to more realistically measure inoculant treatment efficiency. It compares the ratio between the improved characteristic level and total inoculant consumption for this effect. Addition of any of the commercial inoculants plus the inoculant enhancer offered outstanding inoculation power [increased ISF] even at higher solidification cooling rates, even though the total enhancer addition was at a small fraction of the amount of commercial inoculant used.     

Relation between SDAS and eutectic cell size in grey iron

Abstract

The solidification of cast components is a complex and important process as this is the moment when the final properties are established. For hypoeutectic grey iron, solidification starts with nucleation and growth of the primary austenite followed by the eutectic reaction forming eutectic cells. In this work, the microstructure and significance of the different constituents formed during solidification has been examined. It was found that the size of the eutectic cells is a function of secondary dendrite arm spacing (SDAS). The SDAS, on the other hand, was found to depend on the solidification time and hence the growth rate of the dendrites. The effect of chemical composition on SDAS and eutectic cell size was found to depend on cooling rate. It is suggested that the relationship between the eutectic cells and dendrite arm spacing is based on segregation effects and the nucleating capacity of the melt.     

Iron Powder Treated Gray Irons: Critical Shape Characteristics for Graphite Nuclei

A program was conducted to research how to characterize the size and shape of micro-particles. These can act as graphite nuclei, but are altered by adding a commercial iron powder, or after a similar treatment combined with inoculation. Resin sand mold (RSM) and metal mold (MM) solidified sample structures were subjected to automatic image analysis. In general, a higher cooling rate, typical for MM solidification, favors smaller size and more compact particles, even in RSM media. Iron powder treatment led to the largest particles with unusual morphologies, better defined by complex shape factors, which employ actual perimeters, rather than the simpler median size and aspect ratio method. Conventional inoculation employed after an iron powder treatment altered the particles (smaller and more compact), which benefited their effectiveness to act as graphite nuclei, especially at slower solidification rates in RSMs. The results confirm that promoting more compact micro-inclusions, at smaller sizes, involved in graphite nucleation, reduces the sensitivity to chill and improves the eutectic cell characteristics in gray cast iron.     

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.     

Improving chill control in iron powder treated slightly hypereutectic grey cast irons

Recent studies revealed that in eutectic to slightly hypereutectic grey irons (CE = 4.3%-4.5%) the presence of austenite dendrites provides an opportunity to improve the cast iron properties, as a high number of eutectic cells are "reinforced" by austenite dendrites. An iron powder addition proved to be important by promoting dendritic austenite in hypereutectic irons, but was accompanied by adverse effect on the characteristics of potential nuclei for graphite. The purpose of the present paper is to investigate the solidification pattern of these irons. Chill wedges with different cooling moduli (CM = 0.11 - 0.43 cm) were poured in resin bonded sand and metal moulds.Relative clear / mottled / total chill measurement criteria were applied. Iron powder additions led to a higher chill tendency, while single inoculation showed the strongest graphitizing effect. The various double treatments show an intermediate position, but the inoculant added after iron powder appears to be the most effective in reducing base iron chill tendency, for all cooling moduli and chill evaluation parameters. This performance reflects the improved properties of (Mn,X)S polygonal compounds as nucleation sites for graphite, especially in resin bonded sand mould castings. Both austenite and graphite nucleation benefit from a double addition of iron powder + inoculant,with positive effect on the final structure and chill tendency.     

A multi-phase micro-segregation model

Abstract

A multi-component multi-phase micro-segregation model was developed to cope with eutectic transformations of ductile iron, assuming mixed dendritic–globular solidification morphology. Solute partitioning is calculated by an equilibrium assumption at the interfaces using commercial CALPHAD software. Time-sensitive micro-segregation patterns and phase fractions are solved by the micro-segregation model. The development of fraction liquid over temperature with time was compared to Gulliver–Scheil simulations for a model with and without cross-diffusion. The micro-segregation model was coupled to a commercial process simulations tool to deal with interaction effects between material and process scale during solidification of a benchmark test casting made of EN-GJSA-X NiCr 20–2. The precipitation kinetics of phases, especially of graphite, is of particular interest during solidification of ductile iron, due to feeding effects. The coupling procedure as well as important aspects during solidification of ductile iron will be explained.     

Sensitivity study of IHTC on solidification simulation for automotive casting

Abstract

Interface heat transfer coefficient values between the mould/metal interfaces need to be precisely determined in order to accurately predict the thermal histories at different locations in automotive castings. Thermo-mechanical simulations are carried out for Al–Si alloy casting processes using a commercial code. The simulation results are verified with experimental data from the literature. Sensitivity studies show that the choice of the initial value of the interface heat transfer coefficient (IHTC) between chill/metal as well as the sand mould/metal interfaces has a marked effect on the cooling curves. In addition, having chosen an initial value of the IHTC, the analyses also show differences in the solidification rate of the casting alloy near the sand/metal and chill/metal interfaces, upon further cooling. The gap formation, which results in a change in IHTC from the initial value, does not affect the cooling curves in the vicinity of the sand/metal interface due to lower thermal conductivity of sand. However it is found to have a considerable effect in the chill/metal interfacial regions due to higher thermal conductivity of the chill. Based on these studies we recommend initial IHTC values of 3000 and 7000 W m^–2 K^–1 for sand/metal and chill (steel)/metal interfaces respectively, for application in casting simulations.     

Niobium alloying effect in high carbon equivalent grey cast iron

The effect of niobium on the formation of NbC phase and solidification structure in high carbon equivalent grey cast iron was investigated.The experimental results indicated that an increase in the niobium content is favorable to refining the graphite and eutectic cell;and the pearlite lamellar spacing is reduced.Based on the thermodynamic calculation the formation of NbC is prior to the eutectic reaction.The reduction in the pearlite lamellar spacing is mainly attributed to the decrease of eutectic tempera...     

Onset of rod eutectic morphology in directional solidification

Abstract

Initial dynamics of morphological selection in a succinonitrile-(D)camphor organic transparent rod eutectic system is investigated experimentally using directional solidification with specimen thicknesses of 20 and 200 μm. The shape of the solid/liquid interface, the specimen thickness, the initial single-phase boundary thickness, and the grain boundaries are all observed to influence the onset of the eutectic morphology in a geometrically constrained system. Additionally, initiation of eutectic growth within a thin liquid layer at the specimen slide wall and lateral propagation of the array is observed, as suggested in a previous study by the present authors.     

Effect of bismuth on refinement of graphite in thin wall spheroidal graphite cast iron

Abstract

Although it is known that the addition of bismuth refines the graphite nodule in spheroidal graphite cast iron, the refinement mechanism has not yet been clarified. In this research, the effect of bismuth on the refinement has been investigated by examining the microstructure of the spheroidal graphite cast iron containing a small amount of bismuth. Bismuth was added at 0˙01 mass% to the spheroidal graphite cast iron melt containing 3˙5–3˙7 mass% carbon and 2˙0– 2˙8 mass% silicon, then the melt was poured into the mould to obtain the stepped test bar with 2, 3, 5 and 10 mm thicknesses. The graphite nodule increased as the bismuth content increased. The diameter of the graphite nodule decreased as the thickness decreased, namely, as the cooling rate increased. The graphite nodule was further refined by the addition of bismuth. The increase in silicon content increased the graphite nodule count and the ferrite in the matrix. It postulated that bismuth exists as simple substance or a compound in the vicinity of the nucleus of the graphite.     

The Effect of Trace Elements on the Cooling Curves,Microstructure and Mechanical Properties of Eutectic Aluminium-Silicon Alloy

Abstract

The effect of trace elements, used for modification, on the cooling curves obtained during solidification, microstructure and mechanical properties of eutectic aluminium-silicon alloy was investigated. The results of this study indicate the following: 1 The addition of sodium or sodium plus strontium or antimony modifies the eutectic silicon while the addition of sulphur does not alter the microstructure.

2 Those elements which modify the eutectic-silicon, lower the eutectic solidification temperature, while those elements which do not bring about modification, do not alter the eutectic solidification temperature.

3 The addition of those elements which modify the eutectic aluminium-silicon alloy, viz., sodium or sodium plus strontium or antimony improves the UTS and percentage elongation. The addition of titanium to eutectic aluminium-silicon alloy containing these trace elements improves the UTS and percentage elongation to a further extent. Among the various trace elements added to eutectic aluminium-silicon alloy, the addition of sodium plus titanium improves the UTS and percentage elongation to the maximum extent.

     

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.