Iulian Riposan

Graphite nucleant (microinclusion) characterization in Ca/Sr inoculated grey irons

Complex (Mn, X)S compounds nucleated on Al2O3-based sites were found as the major nucleation sites for graphite flakes in both un- inoculated and inoculated grey irons. (Mn, X)S particles (1–8 fun) and their corresponding nuclei (0.1–2.7 μm) were characterized as size, morphological features and distribution patterns of different elements (Mn, S, Ca, Sr, Al, O, Fe), under the influence of inoculant type (HP-FeSi, Sr-FeSi, Ca-FeSi) and cooling rate (4.3 and 542°C/sec). These particles present so common features for all irons (distinct core; always Al and O presence in the core; Mn and S throughout the compound; usually high number, etc), but also the peculiar features (Ca presence throughout almost all compounds independent of inoculant type; Sr mainly in the core of Sr-FeSi treated iron; ovoid tendency of compounds in Ca-FeSi treated irons; lower nuclei size, lower Mn/S ratio in the compounds body and more complex (Mn,X)S compounds in inoculated irons, etc). More intricacy of (Mn, X)S compounds in inoculated irons leads to lower crystallographic misfit given the graphite (more favorable to graphite nucleation).

Three-stage model for nucleation of graphite in grey cast iron

Abstract A three-stage model for the nucleation of graphite in grey irons has been proposed. Stage 1 involves formation of small oxide based sites (usually <2·0 μm) in the melt. Stage 2 involved precipitation of complex (Mn,X)S compounds (usually <5·0 μm) nucleated by stage 1 microinclusions. Finally, in stage 3, graphite nucleates on the sides of the (Mn,X)S compounds, which have low crystallographic misfit with graphite. Three groups of elements are important to sustain this sequence for effective graphite nucleation. These are strong deoxidising elements, such as Al and Zr, to form a high count of very small stage 1 microinclusions, Mn and S to sustain MnS type sulphide formation, and inoculating elements, which act in the first stage and/or in the second stage of the graphite nucleation sequence. Inoculating elements improve the capability of (Mn,X)S compounds to nucleate graphite. In inoculated irons, the (Mn,X)S compounds are more complex. They have a lower Mn/S ratio and higher capability to nucleate graphite, especially when preconditioning/inoculating elements contribute with a high count of effective stage 1 particles.

Al,Zr–FeSi preconditioning of grey cast irons

Abstract Laboratory research has been conducted to investigate the effect of strong deoxidising elements such as Al, Zr or Ti in preconditioning of uninoculated and inoculated grey irons. Experimental heats were performed in an acid lined crucible induction furnace at 3˙7–3˙9% carbon equivalent, 5˙5–6˙5 Mn/S ratio and, <0˙005%Al and Ti. The content of 0˙027–0˙028%Al, 0˙011–0˙017%Ti or 0˙011–0˙013%Zr was obtained in preconditioned irons. Not only inoculation but also the preconditioning of the base iron has a strong effect on the solidification pattern. Preconditioning led to an improved level measuring the most significant temperatures of cooling curves and resulted in a positive trend of eutectic and of the end of solidification representative parameters, in both uninoculated and inoculated irons. A foundry case study is represented to illustrate the efficiency of Al and Zr bearing FeSi preconditioning of electrically melted, inoculated grey iron: decreasing the eutectic undercooling and recalescence, improved graphite characteristics, avoiding carbides, increased tensile strength, while heat treatment was eliminated, for 5 mm sections castings.

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.

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.

Control of Surface Graphite Degeneration in Ductile Iron for Windmill Applications

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 resin-acid moulding and coremaking systems and how to reduce this defect. In the resin mould technique the sulphur in the P-toluol sulphonic acid (PTSA), usually used as a hardener, has been identified as one factor causing graphite degeneration at the metal mould interface. Less than 0.15% S in the mould (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 mould, 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. The specific characteristics and optimum conditions for the manufacture of heavy sectioned ductile iron windmill castings were reviewed. The paper concludes on the optimum iron chemistry, melting procedure, Mg-alloys and specific inoculants systems, as well as the practical solutions to limit graphite degeneration to ensure castings with the highest integrity.

The Importance of Rare Earth Contribution from Nodulizing Alloys and their Subsequent Effect on the Inoculation of Ductile Iron

The unique properties of rare earth elements (REE) have made them crucial to a number of emerging and growing technologies [medical diagnosis equipment, critical military-based technologies, catalysts, computers, magnets, hybrid cars, windmills etc.] increasing their demand, and strategic importance. Recently, increased demand has led to supply problems, with corresponding price fluctuations and difficulties in obtaining REE for use in the cast iron industry.The anti-nodularising action of deleterious residual elements up to a level corresponding to K = 2.0 (Thielman factor) can be counteracted by REE additions. In relatively high purity base iron in terms of anti-nodularising trace elements (K = 0.7 to 0.8) and 0.04 to 0.05% Mgres, higher REEres contents increased carbide tendency, for both un-inoculated and inoculated irons. A low content of REEres (0.005 to 0.01%) for K < 0.8 is practical and sufficient in a Ca-bearing FeSi alloy inoculated iron. The REE addition from a Ca,REE-FeSi alloy appears to be unnecessary, or even unacceptable, due to the risk of increased chill tendency.The use of a proprietary oxy-sulphide inoculant enhancer, (CaSi-based blend with S, O, Al, Mg) led to a substantial increase in the potency of the Ca-bearing inoculants. It was effective, when utilized in an in-mould inoculation of Mg-FeSi (0% REE or 0.26% REE) treatment, for K = 0.4–0.8. Avoidance of chill (free carbides), control of casting hardness and improved graphite quality parameters were observed with the proprietary oxy-sulphide inoculant.

Role of Lanthanum in Graphite Nucleation in Grey Cast Iron

Specific investigations concerning La-contributions in graphite nucleation process in Ca,Al,La-FeSi inoculated grey cast irons were performed by using a scanning electron microscopy (SEM, EDS). It was re-confirmed that complex (Mn,X)S compounds act as major nucleation sites for graphite flakes. La was mainly concentrated in the first formed oxide-based micro-inclusions (similarly to Al), but also at an important level in the shell of (Mn,X)S compounds (accompanying Ca). It is assumed that La forms micro-inclusions later than Al, as La-reached phase surrounded Al-reached phase. Complex Al-La small micro-inclusions, as possible better nucleation sites for (Mn,X)S compounds and La-Ca presence in the shell/body of these sulphides, possible better nucleation sites for flake graphite, appear to be the peculiar effects of ferrosilicon based inoculants, which include these active elements, promoting type-A graphite, inclusively in low S-grey iron.

Chilling properties of Ba/Ca/Sr inoculated grey cast irons

Abstract The effect on Ba, Ca and Sr ladle inoculation in grey iron of variations in S content (0·02–0·10%), Al content (0·001–0·010%) and carbon equivalent (CE) (3·5–4·0%) was studied. Chilling properties were measured in a large range of cooling modulus wedge samples (0·1–0·5 cm). Sulphur content has a particularly large influence on chilling behaviour. The difference between the inoculation performance of Ba, Ca and Sr alloy became more pronounced as cooling rate increased. As a general rule, the graphitising efficiency of the three inoculating elements is in the order Ba≤Ca<Sr. It was found that Ba and Ca show an appropriate effect only in the most favourable conditions (high CE, high S, high Al), whereas Sr is significantly more effective in all experimental conditions. For thin wall grey iron castings, 0·07–0·10%S, 0·005–0·010%Al, 3·8–4·0%CE and Sr–FeSi inoculation are recommended.

Identifying chill tendency of cast iron melts by thermal analysis

Abstract The effects of melting, superheating and holding in an acid lined coreless induction furnace were explored, as they affect the role of preconditioning and/or inoculation to restore low eutectic undercooling type solidification. Heating the iron melt in an acid lined coreless induction furnace will inevitably cause important changes to the liquid iron characteristics. Increased chill in the experimental irons correlates well with certain thermal analysis parameters, such as the degree of eutectic undercooling, referring to the both stable (graphitic) and metastable (white) eutectic temperature. Using the elements Al and Zr to precondition the molten base iron before tapping led to improved solidification parameters as measured by the most significant thermal analysis cooling curve events, in both untreated and inoculated irons. A double treatment incorporating preconditioning with inoculation improved the thermal analysis parameters, and consequently, the quality of the grey iron.