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.     

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.     

Precipitation and evolution of nodular graphite during solidification process of ductile iron

The quantity and morphology of spheroidal graphite have an important effect on the properties of ductile iron,and the characteristics of spheroidal graphite are determined by the solidification process.The aim of this work is to explore the precipitation and evolution of graphite nodules in hypoeutectic,eutectic,and hypereutectic ductile irons by thermal analysis,liquid quenching and metallographic technique.Results show that hypoeutectic ductile iron has the longest solidification time and the lowest eutectic temperature;eutectic ductile iron has the shortest solidification time;hypereutectic ductile iron has the highest eutectic temperature.After solidification is completed,hypoeutectic ductile iron has the lowest nodule count,nodularity and graphite fraction;eutectic ductile iron has the highest nodule count,nodularity and the smallest nodule diameter;hypereutectic has the highest nodule diameter and graphite fraction.The nucleation and growth of graphite nodules in hypereutectic ductile iron starts before bulk eutectic crystallization stage,however,the precipitation and evolution of graphite nodules of hypoeutectic and eutectic ductile irons mainly occur in the eutectic crystallization stage.The graphite precipitated in eutectic crystallization of hypoeutectic,eutectic,and hypereutectic ductile irons,are 61%,68% and 43% of total graphite volume fraction,respectively.Simultaneously,there are plenty of austenite dendrites in hypoeutectic and hypereutectic ductile irons,which are prone to shrinkage defects.Therefore,the eutectic ductile iron has the smallest shrinkage tendency.     

Thermal analysis control of in-mould and ladle inoculated grey cast irons

The effect of addition of 0.05wt.% to 0.25 wt.% Ca, Zr, Al-FeSi alloy on in-ladle and in-mould inoculation of grey cast irons was investigated. In the present paper, the conclusions drawn are based on thermal analysis. For the solidification pattern, some specific cooling curves characteristics, such as the degree of undercooling at the beginning of eutectic solidification and at the end of solidification, as well as the recalescence level, are identified to be more influenced by the inoculation technique. The degree of eutectic undercooling of the electrically melted base iron having 0.025% S, 0.003% AI and 3.5% Ce is excessively high (39 - 40℃), generating a relatively high need for inoculation. Under these conditions, the in-mould inoculation has a more significant effect compared to ladle inoculation, especially at lower inoculant usage (less than 0.20 wt.%). Generally, the efficiency of 0.05wt.% - 0.15wt.% of alloy for in-mould inoculation is comparable to, or better than, that of 0.15wt.% - 0.25wt.% addition in ladle inoculation procedures. In order to secure stable and controlled processes, representative thermal analysis parameters could be used, especially in thin wall grey iron castings production.     

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.     

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.     

过共晶球铁凝固过程中奥氏体的生长方式与形貌特征

采用着色腐蚀技术显示出过共晶球铁中的高温凝固组织，观察分析过共晶球铁凝固过程中奥氏体的生长方式与形持征。结果表明：在非平衡凝固条件下，过共晶球铁凝固时通常会析出初生枝晶和晕圈枝晶，其形成条件主要与冷却速率和熔体的过冷有关。随铸件模数Mc增大，枝晶数量减少，二次臂间距显著增大，形态趋于不发达，由初生枝晶向晕圈枝晶过渡。当Mc≤0.3cm时，二次臂间距的实测值与理论预测值基本吻合。初生石墨球周围往往形成环状封闭奥氏体壳；共晶前期石墨球周围形成封闭或不封闭的框架奥氏体壳；共晶后期石墨球往往被周边生长着的共晶奥氏体所包覆，最终成为共晶奥氏体的一部分。     

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.     

过共晶球墨铸铁中石墨球周围奥氏体壳的形成机制

有用着色腐蚀技术，可清晰地显示出过共晶球墨铸铁中的高温凝固组织，观察分析了石墨球周围奥氏体壳（奥氏体壳是奥氏体枝晶的一部分）的形成机制。结果表明，初生枝晶和晕圈枝晶交替生长，促成石墨球周围奥氏体壳的形成；奥氏体以石墨生长面（0001）为衬底形核、生长，在初生石墨球周围形成环状封闭奥氏体壳；共晶前期石墨球在枝晶臂间或框架中形核、生长、尔后为枝晶高次臂所包围，形成封闭或不封闭的框架奥氏体壳。共晶后期石墨球不形成奥氏体壳。     

Shrinkage evaluation in ductile iron as influenced by mould media and inoculant type

Abstract

The present work was undertaken in hypereutectic ductile irons, to simultaneously study cooling curves, specimen contraction curves, microstructures and shrinkage tendencies as a function of inoculant selection and mould rigidity. Specific equipment was developed for simultaneous recording of cooling and contraction curves. The highest level of both concentrated and total shrinkage was recorded on the green sand mould system, where a much higher level of the initial eutectic expansion was observed, as compared to furan resin moulds. It was found that the Ca,Ce,S,O–FeSi inoculation gives the highest nodule count and a unique wide distribution of nodules sizes (large and small size nodules, in a peculiar ratio), as well as lower tendency to shrinkage formation. Also, the strongest graphitising effect before the start of eutectic solidification and more prolonged graphitising throughout the end of eutectic freezing characterise were observed with this complex inoculant system in addition to its peculiar action on minimising the shrinkage tendency of ductile irons.     

Effect of Dy on the microstructures of directionally solidified NiAl–Cr(Mo) hypereutectic alloy at different withdrawal rates

The effect of various Dy content on the microstructure of Ni–31Al–32Cr–6Mo hypereutectic alloy was studied at the withdrawal rates of 6, 30 and 90 μm/s. The results show that the solid–liquid interface morphology has an evolutionary process of planar → cellular → dendritic interface with the increasing withdrawal rate. The primary Cr(Mo) dendrites are gradually weeded out through competitive growth between the primary phase and the eutectic phase. The volume fraction of primary Cr(Mo) dendrites decreases with the modest addition of Dy (0.05 wt.%) at 6 μm/s. When the withdrawal rate increases to 30 μm/s, the appropriate addition of Dy (0.1 wt.%) refines the microstructure, such as the width of intercellular zone and the lamellar thickness in the intercellular zone. With the increase of withdrawal rate to 90 μm/s, the addition of Dy has no significant effect on the microstructure. In addition, the white Dy-containing phase can occur in the boundary of eutectic cells when the Dy content is no less than 0.1 wt.%.     

不同孕育剂处理的高强度灰铸铁加工性能研究

为研究不同孕育剂对灰铸铁切削加工性能的影响,制备了高强度HT350材质的试样.对比研究了孕育剂75SiFe、SrSi、BaSi、SiSr(80％)+75FeSi(20％)处理的高强度灰铸铁的力学性能和切削加工性能.结果表明,SiSr(80％)+75FeSi(20％)复合孕育处理的灰铸铁较单一孕育处理的灰铸铁石墨细小弯曲,基体组织均匀性好,具有较高的硬度以及较小的切削抗力,但其断面敏感性较大.单一孕育剂中75SiFe孕育处理的灰铸铁具有较高的抗拉强度、较小的硬度以及较低的断面敏感性.灰铸铁中A型石墨越细小,基体组织越均匀,灰铸铁的加工性能越好,说明显微组织的均匀性对灰铸铁的加工性能具有较大影响.     

Complex（Mn, X）S compounds - major sites for graphite nucleation in grey cast iron

Despite the cubic system, the ability of sulphides to nucleate graphite can be enhanced by inoculating elements which transform them in complex compounds with a better lattice matching to graphite, a low coagulation capacity, good stability and adequate interracial energy. (Mn,X)S compounds, usually less than 5.0 μm in size, with an average 0.4-2.0 μm well defined core (nucleus), were found to be important sites for graphite nucleation in grey irons. A three-stage model for the nucleation of graphite in grey irons is proposed: (1) Very small micro-inclusions based on strong deoxidizing elements (Mn, Si, Al, Ti, Zr) are formed in the melt; (2) Nucleation of complex (Mn,X)S compounds at these previously formed micro-inclusions; (3) Graphite nucleates on the sides of the (Mn,X)S compounds with lower crystallographic misfit. Al appears to have a key role in this process, as Al contributes to the formation of oxides in the first stage and favors the presence of Sr and Ca in the sulphides, in the second stage. The 0.005-0.010% Al range was found to be beneficial for lower undercooling solidification, type-A graphite formation and carbides avoidance.     

低钙钡的硅钡孕育剂

介绍一种低钙钡的硅钡孕育剂，其Si、Ba、Ca、Al组元配比合理，既具有强的石墨化作用，又具有良好的脱氧性能，因此能延缓孕育衰退时间，防止铸件局部白口，使铸件硬度均匀。用于铸态球铁，可以获得足量的铁素体；用于灰铸铁，可以稳定获得A型石墨，石墨长度达4～6级，力学性能提高。     

Volume change during the solidification of grey cast iron: its relation with the microstructural variation,comparison between experimental and theoretical analysis

The expansion/contraction during the solidification of grey cast iron was studied using Linear Variable Differential Transformer (LVDT). The experiments were conducted with and without melt treatment. Two types of inoculant used for melt treatment: ASSC (Si, Ca, Sr & Al) and MBZCAS (Si, Ca, Zr, Ba, Mn & Al). Microstructural investigations carried out to quantify the eutectic cells, undercooled graphite, primary austenite and secondary dendritic arm spacing (SDAS). It was found that the casting shows hardly any shrinkage during early solidification but in the eutectic region, the casting expands until the end of solidification. The measured and the calculated volume changes are close to one another, but the former shows more expansion. The addition of MBZCAS promotes more flake graphite, and ASSC does not increase eutectic cells much. In addition to that, it lowers the primary austenite fraction, promotes more eutectic growth, decreases undercooled graphite and SDAS. As a result, the volume expansion changes in the eutectic region.     

Sb微合金化对灰铸铁组织和性能的影响

研究了Sb微合金化对灰铸铁组织和性能的影响.在RESiFe+SiCa孕育条件下,Sb加入量小于0.04%,可降低铁液的白口倾向,提高灰铸铁的抗拉强度和硬度.Sb微合金化厌铸铁的组织为A型+少量D型石翠,基体为细片状珠光体.Sb略增加磷共晶数量,含RE和Ca的孕育剂可抑制这种作用.采用Sb微合金化生产法兰盘零件,本体R<,m>可达到Cr、Cu合金化的水平,而硬度却低17 HBS左右,有利于改善零件的加工性能.     

高碳当量灰铸铁用稳定化孕育剂的研究

采用微分热分析法、着色腐蚀技术及自动图象分析仪对比研究了１＃ＲＥ，６８Ｍｎ－Ｆｅ，６３Ｃｒ－Ｆｅ，３０Ｔｉ－Ｆｅ，纯Ｓｂ等稳定化孕育剂对高碳当量灰铸铁的过冷度，初生奥氏体枝晶粒量，共晶团数，石墨组织等的影响，研究了高碳当量灰铸铁的凝固组织及稳定化孕育剂的孕育机理，试验结果表明，稳定化孕育剂均能增大高碳当量灰铸铁的过冷度，增加初生奥氏体枝晶数量，共晶团数量，且随加入量的增加而增大，孕育作用的强弱按稳     

Grain count in castings: theoretical background and experimental verification

In this work, a theoretical model is proposed for heterogeneous nucleation on substrates whose size distributions can be described by the Weibull statistics. In particular, the proposed model suggests that the size distribution for the various nucleation sites is exponential in nature. Measurements of grain count were carried out on experimental Al–1·3Si and Al–5·0Cu single phase alloys inoculated using an Al–5Ti–1B master alloy. In addition, experimental nodular and flake graphite iron castings were processed under various metallurgical conditions. In single phase alloys, the area of the equiaxed dendritic grain count was estimated from the electron backscattering diffraction analysis, whereas the graphite nodule and graphite eutectic cell count were estimated on polished cast iron surface sections by stereological means. In addition, maximum undercoolings were determined by thermal analysis. The experimental outcome indicates that the grain count can be properly described by a proposed exponential function of the maximum undercooling at the onset of alloy solidification. Finally, the magnitudes of the nucleation parameters were experimentally determined in this work.     

Eutectic nucleation in Al-25wt.%Si alloy through DSC

Eutectic nucleation in hypereutectic Al-25wt.%Si alloy with the addition of P was studied by calculating the activation energy and nucleation work. The results indicate that P refinement, which can decrease the activation energy as well as nucleation work of eutectic solidification, can increase the eutectic nucleation frequency of Al-25wt.%Si alloy, so the addition of P can refine the eutectic structures of hypereutectic Al-Si alloys. According to the calculations, there is a certain relationship between the precipitation of primary silicon and eutectic solidifica-tion, and the eutectic nucleation mode is independent of morphology transition.     

Fourier Thermal Analysis of the Eutectic Formed in Pb-Sn Alloys

The effect of the presence of two different primary phases on the microstructural characteristics and solidification kinetics of Pb-Sn eutectic was analyzed using Fourier thermal analysis method (FTA) and microstructural characterization. Three Pb-Sn alloys, a hypoeutectic, an eutectic, and a hypereutectic alloy, were melted in an electric furnace under an argon atmosphere and poured into sand molds. Cooling curves were obtained and numerically processed using FTA. Microstructural observations of the probes indicate a lamellar morphology for the eutectic microconstituent of the hypereutectic alloy; the eutectic alloy shows the presence of both lamellar and anomalous eutectic and the hypoeutectic alloy shows only the presence of anomalous eutectic. FTA results indicate that in the case of the probes showing the presence of anomalous eutectic, there is a primary eutectic formed during recalescence at high undercooling and a secondary eutectic yielded at low undercooling at the eutectic plateau temperature. This result shows that the cause behind the observed differences in the eutectic morphologies of the experimental alloys lies on the nucleating ability of the primary phase available as a potential substrate for nucleation of the eutectic microconstituent.