High temperature oxidation of inoculated high Si/SiMo ductile cast irons in air and combustion atmospheres

The present work aims to investigate the effect of heating temperature(400,600 and 800°C)and inoculating elements(Ca,Ca-Ba,Ca-RE)on oxidation behavior of ductile irons containing 5.25%Si and 4.8%Si-2.3%Mo in dry air and combustion gas containing water vapour(natural gas burning).The oxidation is influenced by the gas atmosphere type,the iron alloying system,and the inoculating elements depending on the heating temperature.The weight gain increases from 0.001%-0.1%(400°C)to 0.05%-0.70%(600°C)and up to 0.10%-2.15%(800°C).No particular effects of the considered influencing factors are found when heating at 400°C,while at 600°C,mainly the oxidation gas atmosphere type shows a visible influence.At the highest heating temperature of 800°C,a limited increase of the weight gain is found for dry air atmosphere(up to 0.25%),but it drastically increases for combustion atmospheres(0.65%-2.15%).The water vapour presence in the combustion atmosphere is an important oxidising factor at 600-800°C.The alloying system appears to influence the oxidation behavior mainly at a heating temperature of 800°C in the combustion atmosphere,as evidenced by the lower weight gain in 5.25%silicon cast iron.Positive effects of inoculating elements increase with the heating temperature,with Ca and Ba-FeSi inoculation generally showing better performance.Irons inoculated with CaRE-FeSi exhibit a higher degree of oxidation.These results are in good relationship with the previous reported data:Ca-Ba-inoculation system appears to be better than simple Ca for improving the graphite parameters,while RE-bearing inoculant negatively affects the compactness degree of graphite particles in high-Si ductile irons.As the lower compactness degree is typical for graphite nodules in high-Si ductile irons,which negatively affects the oxidation resistance,it is necessary to employ specific metallurgical treatments to improve nodule quality.Inoculation,in particular,is a potential method to achieve this improvement.

New developments in high quality grey cast irons

The paper reviews original data obtained by the present authors,revealed in recent separate publications,describing specific procedures for high quality grey irons,and reflecting the forecast needs of the worldwide iron foundry industry.High power,medium frequency coreless induction furnaces are commonly used in electric melting grey iron foundries.This has resulted in low sulphur(<0.05wt.%)and aluminium(<0.005wt.%)contents in the iron,with a potential for higher superheating(>1,500°C),contributing to unfavourable conditions for graphite nucleation.Thin wall castings are increasingly produced by these electric melt shops with a risk of greater eutectic undercooling during solidification.The paper focused on two groups of grey cast irons and their specific problems:carbides and graphite morphology control in lower carbon equivalent high strength irons(CE=3.4%-3.8%),and austenite dendrite promotion in eutectic and slightly hypereutectic irons(CE=4.1%-4.5%),in order to increase their strength characteristics.There are 3 stages and 3 steps involving graphite formation,iron chemistry and iron processing that appear to be important.The concept in the present paper sustains a threestage model for nucleating flake graphite[(Mn,X)S type nuclei].There are three important groups of elements(deoxidizer,Mn/S,and inoculant)and three technological stages in electric melting of iron(superheat,pre-conditioning of base iron,final inoculation).Attention is drawn to a control factor(%Mn)x(%S)ensuring it equals to 0.03–0.06,accompanied by 0.005wt.%–0.010wt.%Al and/or Zr content in inoculated irons.It was found that iron powder addition promotes austenite dendrite formation in eutectic and slightly eutectic,acting as reinforcement for the eutectic cells.But,there is an accompanying possible negative influence on the characteristics of the(Mn,X)S type graphite nuclei(change the morphology of nuclei from polygonal compact to irregular polygonal,and therefore promote chill tendency in treated irons).A double addition(iron powder+inoculant)appears to be an effective treatment to benefit both austenite and graphite nucleation,with positive effects on the final structure and chill tendency.

Performance of heavy ductile iron castings for windmills

The main objective of the present paper is to review the specific characteristics and performance obtaining conditions of heavy ductile iron(DI) castings,typically applied in windmills industry,such as hubs and rotor housings.The requirements for high impact properties in DI at low temperatures are part of the ENGJS-400-18U-LT(SRN 1563) commonly referred to as GGG 40.3(DIN 1693).Pearlitic in-uence factor(Px) and antinodularising action factor(K1) were found to have an important in-uence on the structure and mechanical properties,as did Mn and P content,rare earth(RE) addition and inoculation power.The presence of high purity pig iron in the charge is extremely beneficial,not only to control the complex factors Px and K1,but also to improve the 'metallurgical quality' of the iron melt.A correlation of C and Si limits with section modulus is very important to limit graphite nodule flotation.Chunky and surface-degenerated graphite are the most controlled graphite morphologies in windmills castings.The paper concluded on the optimum iron chemistry and melting procedure,Mg-alloys and inoculants peculiar systems,as well as on the practical solutions to limit graphite degeneration and to ensure castings of the highest integrity,typically for this field.

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 interfacial 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 microinclusions 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. AI 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.

Simultaneous thermal and contraction/expansion curves analysis for solidification control of cast irons

The first part of the paper summarizes the performance of two mould devices,illustrating by representative shrinkage tendency results in ductile cast iron as affected by mould rigidity(green and furan resin sand moulds)and inoculant type(FeSi-based alloys).Less rigid green sand moulds encourage the formation of contraction defects,not only because of the high initial expansion values(ε(di))(max),but also because of the increased solidification undercooling.A high inoculation efficiency means not only lowering the carbides formation sensitivity and increasing the nodule count,but also a prolonged graphitization through to the end of the eutectic freezing,as observed by the high population of small late forming nodules,which leads to minimizing the tendency for shrinkage.The second part of the paper illustrates an application of this equipment to commercial foundry use.It conducts thermal analysis and volume change measurements in a single ceramic cup with cast iron quality as a variable.La-bearing FeSi inoculant appears to be more effective than RE(Rare Earth)-FeSi alloy in FeSiMgCa treated irons(no RE),in terms of reducing eutectic undercooling and(ε(di))(max),favourable for lower sensitivity to shrinkage formation.Experiments also compared solidification patterns for white[WI],grey[GI]and ductile[DI]irons,to correlate the most important events between the cooling curves and contraction curves,to evaluate the sensitivity to shrinkage formation.All of the irons have similar values for initial expansion up to the start of eutectic freezing,but,after that,the graphite formation promotes expansion(more than 5 times for nodular graphite),resulting in a difference in maximum expansion(2 times higher for DI).The graphitic expansion has two contrary effects.Increased graphitic expansion(force)leads to a higher shrinkage sensitivity during the first part of the eutectic reaction,but also to a decrease of shrinkage at the end of solidification,due to forcing the last liquid iron to occupy the previous formed cavities.Consequently,strong graphitization process promotion at the end of solidification favours the castings'soundness.

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 solidif ication and at the end of solidifi cation,as well as the recalescence level,are identif ied 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% Al 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 signif icant effect compared to ladle inoculation,especially at lower inoculant usage(less than 0.20 wt.%).Generally,the eff iciency 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.

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 graphiUzing 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.

Iron casting skin management in no-bake mould – Effects of magnesium residual level and mould coating

The relative performance of coatings for furan resin sand moulds [P-toluol sulphonic acid(PTSA) as hardener] [FRS-PTSA moulds], was compared by analyzing the surface layer for degenerated graphite in Mg treated iron with 0.020 wt.% to 0.054 wt.% Mgres. It was found that the iron nodularising potential(Mg, Ce, La content) and whether the mould coatings contained S, or were capable of desulphurizing were important factors. These moulds have S in the PTSA binder, which aggravates graphite degeneration in the surface layer, depending strongly on the Mgres with lower Mgres increasing the layer thickness. The application of a mould coating strongly influenced graphite deterioration in the surface layer of castings. It either promoted graphite degeneration to less compact morphologies when using S-bearing coatings, or conversely, limited the surface layer thickness using desulphurization type coatings. Independently of the S-source at the metal – mould interface, the presence of sulphur had an adverse effect on graphite quality at the surface of Mg-treated irons, but its negative effect could also reach the graphite phase within the casting section. If the coatings employed desulphurization materials, such as Mg O, or a mixture(Ca O + Mg O + Talc) or Mgbearing Fe Si, they protected the graphite shape, improving graphite nodularity, at the metal – mould interface, and so decreased the average layer thickness in FRS-PTSA moulds. Fe Si Mg was highly efficient in minimizing the casting skin by improving graphite nodularity. It is presumed that the Mg O or(Mg O + Ca O + Talc) based coatings acted to remove any S released by the mould media. The Mg-Fe Si coatings also reacted with S from the mould but additionally supplemented the Mg nodularising potential prior to solidification. This dual activity is achievable with coatings containing active magnesium derived from fine Mg-Fe Si materials.

Surface Graphite Degeneration in Ductile Iron Castings for Resin Molds

The objective of this paper is to review the factors influencing the formation of degenerated graphite layers on the surfaces of ductile iron castings for chemical resins-acid molding and core-making systems and how to reduce this defect. In the resin mold technique the sulphur in the P-toluol sulphonic acid （PTSA）, usually used as the hardener, has been identified as one factor causing graphite degeneration at the metalmold interface. Less than 0.15% S in the mold （or even less than 0.07% S） can reduce the surface layer depth. Oxygen may also have an effect, especially for sulphur containing systems with turbulent flows in the mold, water-bearing no-bake binder systems, Mg-Silica reactions, or dross formation conditions. Despite the lower level of nitrogen in the iron melt after magnesium treatment （less than 90 ppm）, nitrogen bearing resins have a profound effect on the frequency and severity of surface pinholes, but a limited influence on surface graphite degeneration.