Influence of inoculation on solidification in cast iron

Abstract

Inoculation of cast iron has been mainly studied for the nucleus materials and the development of new inoculants. The effect is widely recognised; nevertheless, the mechanism was not fully understood. Inoculation leads to a decrease in chill depth, depressed the formation of D type graphite to form A type graphite and increase in the eutectic cell number. The transition from A type to the D type graphite is also accelerated by Ti addition; therefore we used Ti added samples for the study of D type graphite formation. The increase in the eutectic cell number produces a decrease in the cell size, namely the radius; nevertheless, the solidification time of the castings does not change by inoculation. Therefore, the solidification rate of the cell, namely the linear solidification rate, should decrease with inoculation and the decrease produces a depression in the chill and D type graphite due to the smaller undercooling. Therefore, we have to consider the three-dimensional solidification rate for discussing the cooling curve.     

Scanning Auger Microprobe Study of Interfacial Adsorption Behaviour of some Minor Elements in Spheroidal-graphite Cast Iron

The interfacial adsorpton behaviour of some minor elements contained in inoculants used in the manufacture of spheroidal-graphite cast iron have been studied by using a scanning auger microprobe at the fracture surfaces of specially-prepared specimens. It is shown that bismuth, boron and sometimes aluminium are adsorbed at the graphite/ melt interfaces, while neither calcium, zirconium, strontium, barium nor the nodularising element magnesium are detected at the interfaces during solidification of s.-g. iron. It is evident that the inoculation mechanism of Bi and B is mainly related to the interfacial adsorption behaviour during solidification, while Ca, Zr, Sr and Ba mainly act as heterogeneous substrates for graphite nucleation. Based on the results, some more powerful Bi or B containing inoculants for s.-g. cast iron have been developed.     

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.     

Inoculated Slightly Hypereutectic Gray Cast Irons

The current experimental investigation in this article was designed to characterize the structure of mold (M) and ladle (L) inoculated, low-S (0.025 wt.% S), low-Al (0.003 wt.% Al), slightly hypereutectic (CE = 4.4-4.5 wt.%) electric melted gray irons, typical for high performance thin-wall castings. It describes the effect of a Ca, Al, Zr-FeSi inoculant addition of 0-0.25 wt.% on structure characteristics, and compares to similar treatments with hypoeutectic irons (3.5-3.6 wt.% CE, 0.025 wt.% S, and 0.003 wt.% Al). A complex structure including primary graphite, austenite dendrites, and eutectic cells is obtained in hypereutectic irons, as the result of nonequilibrium solidification following the concept of a coexisting region. Dendrites appear to be distributed between eutectic cells at higher eutectic undercooling, while in inoculated irons and for lower undercooling, the eutectic cells are “reinforced” by eutectic austenite dendrites. A Zr, Ca, Al-FeSi alloy appears to be an effective inoculant in low S, low Al, gray cast irons, especially for a late inoculation technique, with beneficial effects on both graphite and austenite phases. First, inoculation influenced the nucleation of graphite/eutectic cell, and then their characteristics. A further role of these active elements directly contributed to form nucleation sites for austenite, as complex (Mn,X)S particles.     

球铁件孕育块瞬时孕育工艺试验

简要叙述孕育的基本原理，介绍砖块式孕育剂的化学成分、规格及使用方法。用该孕育块分别放在直浇道底部和浇口杯底部并进行了型内孕育和浇口杯孕育工艺试验，浇注质量为6t的厚大断面球铁件，结果石墨数量增加，尺寸变小，消除了碳化物，提高伸长率。     

反石墨化变质剂在高碳当量灰铸铁中的扩散行为及其对组织的影响

采用变质剂固体植入铸铁熔体进行扩散的方法，研究了单一变质剂和复合变质剂在铁水中浓度的连续变化过程对高碳当量灰铸铁石墨组织细化程度及初生奥氏体枝晶数量的影响。试验结果表明，在细化石墨、增加初生奥氏体枝晶数量方面，复合变质剂比单一变质剂效果显著，复合变质剂的加入量可在较大范围内调整，而单一变质剂的调整范围较狭窄，复合变质剂各成分之间存在一个合适的比例。     

高Ba孕育剂在厚大灰铸铁件中的应用

比较了2种含Ba量不同的孕育剂用于厚大灰铸铁机床底座生产的效果,结果显示:采用高Ba孕育剂的单铸试棒力学性能和金相组织优于采用低Ba孕育剂的单铸试棒,而且所生产铸件的本体硬度也明显高于采用低Ba孕育剂的铸件。在不加Cu、Mo等贵重金属合金的情况下,采用w(Ba)量为10%的孕育剂进行包内孕育和浇注随流孕育,可以使铸件力学性能和金相组织均达到HT300的要求,铸件本体硬度为190~220 HB,硬度均匀性好,很好地解决了铸件表面硬度与内部硬度差大的问题。     

含砷、锡、铅、钛球墨铸铁孕育剂的研究

在７５硅铁单一复合铋、锑、钙、锶、钡、锆、铝、锰孕育的基础上,选择含砷、锡、铅、钛球墨铸铁多元复合孕育剂的组元。利用正交设计对多元复合孕育剂的组成进行了优化组合。通过断面均匀性试验和孕育衰退试验检验了多元复合孕育的效果。     

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.     

二次孕育对大断面球铁组织及性能的影响

介绍了4种不同类型的二次孕育剂对模拟尺寸为φ590 mm×800 mm的大断面球墨铸铁的组织及性能的影响。结果表明,二次孕育工艺对球铁尤其是大断面球铁的石墨及力学性能均有良好的改善作用;利用Si-Ba-Ca孕育剂进行孕育处理后再添加0.15%的该孕育剂作为二次孕育处理,可改善铸件组织及提高铸件力学性能;S-O孕育剂和Si-Ba-Ca孕育剂的效果相对较好,模拟铸件中碎块状石墨区域明显减少、石墨球形态圆整、球化率较高,还可以有效避免孕育衰退,使铸件获得较好的微观组织和最佳的综合力学性能。     

球化处理后w(C)量降低原因分析

采用冲入法在500 kg的堤坝包中进行球化处理,用RESiFeMg合金作球化剂,球化反应时间>40 s;用SiBa合金作孕育剂,在180 kg的浇包内进行孕育处理;用直读光谱仪分析球化处理前后铁液的激冷试样。发现:(1)球化处理后w(C)量比原铁液的w(C)量低;(2)石墨漂浮是导致球化处理后w(C)量降低的主要原因。     

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.     

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.     

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.     

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.     

铸铁中石墨的形核问题

ＳｉＯ２可以作为石墨的形核基底．纯Ｓｉ和Ｆｅ—Ｓｉ合金对铸铁都没有任何孕育效果，当含有少量Ｃａ、Ｓｒ、Ａｌ等元素时才会产生明显的孕育效果．孕育铸铁的共晶团的数目与石英棒晶化层的厚度有密切的关系，晶化层越厚，共晶团数目越多．用Ｃａ—Ｓｉ孕育时，石英玻璃棒的晶化速度相当于在空气中的１０００倍．当用含Ｃａ的Ｓｉ或Ｆｅ—Ｓｉ孕育剂孕育时，吸附于玻璃态ＳｉＯ２的表面，促使非晶态ＳｉＯ２发生晶化，晶化的ＳｉＯ２作为石墨的基底，产生明显的孕育作用．     

机车用灰铸铁件组织中E型石墨的消除

在灰铸铁产品金相组织中经常会出现E型石墨,E型石墨的存在严重影响了灰铸铁的性能,遗传因素是E型石墨产生的主要原因。在铁水孕育处理过程中加入增碳剂和加入硅钡复合孕育剂,能够有效抑制E型石墨的产生。     

MICRODISTRIBUTION OF NODULIZER IN VARIOUS PHASES OF CAST IRON

应用有机电解液低温电解及放射性测量方法,测定了普通铸铁及高纯Fe-C-Si合金中的合金化Ce量及石墨中的Ce量,石墨中的Ce量大都略高于或相近于基体中的Ce量,在Ce含量相同的情况下,铸铁中的合金化Ce量比钢中多。 涂层法自射线照相结果表明:Ce在片状石墨中呈均匀分布,而在球状石墨中的分布却是多样的。开花状石墨中的Ce多于片状石墨及球状石墨中的Ce.球状石墨周围的铁素体及珠光体是贫Ce.远离球状石墨的珠光体及莱氏体是富Ce.初生奥氏体枝晶转变形成的珠光体和蜂窝状莱氏体都含有较少的Ce,而板状渗碳体型莱氏体含Ce较多.Ce明显富集于共晶时最后凝固的区域。球状石墨的形成主要取决于原铁水中的反球化剂(硫)含量,它的影响比球化剂的残留量或合金化量的作用更大。     

Casting of Iron Aluminides

Iron aluminides form an interesting class of materials which combine excellent corrosion and oxidation resistance with good mechanical properties at moderate to high temperature (up to 500 °C). These materials, however, suffer from low room-temperature ductility (under 5% elongation in tension), which is mostly due to environmental effects. Casting is a processing route traditionally applied to brittle alloys (e.g., gray cast irons), but to cast a part without defects, several thermochemical properties are needed, as well as information on the tendency of the alloy to form foundry defects (e.g., shrinkage voids, pores). The present work aims to provide this information using parts produced on laboratory scale. In particular, the solidification contraction and the efficiency of TiB₂ as inoculant are investigated. Three alloys with nominal composition (in at.%) Fe28Al, Fe28Al6Cr, and Fe28Al6Cr1Ti (about 1.5 kg for each melt) were melted in an induction furnace under argon flux protection using conventional raw materials (carbon steel, commercial aluminum, metallic chromium, and commercial ferrotitanium). The resulting melts were treated by adding Al-TiB₂ master alloy used in the aluminum industry and poured into “staircase” molds, designed to investigate feeding distance effects in complex parts. Characterization of the microstructure of the alloys revealed that alloys Fe28Al and Fe28Al6Cr showed κ-carbide precipitation, while alloy Fe28Al6Cr additionally showed chromium carbides at dendritic boundaries. Addition of 1 wt.% Ti in alloy Fe28Al6Cr1Ti changed the solidification microstructure, refining the dendrite morphology and forming TiC-containing eutectic in interdendritic spaces.     

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.