In the production of sound Thin Wall Ductile Iron (TWDI) castings, adequate melt treatment involving efficient inoculation and nodularization are imperative. There should be synergy between these two important treatments for the production of good nodularity, high nodule count and carbide free TWDI. Nodularization treatment is carried out first and followed by a two step inoculation treatment. Nodularization treatment is a modification treatment where the shape of the graphite is changed from flake to spheroid or nodules. This shape is responsible for the peculiar properties of ductile iron. Inoculation is followed immediately to facilitate heterogeneous nucleation for increase nodule count and suppression of carbide formation. Prior to the nodule treatment, it is important to ensure the melt is very low in sulphur (0.01% ≤ S ≤ 0.03%). The ferroalloys of ferrosilicon-magnesium and ferrosilicon used for these treatments have fading tendencies. Thus, the melt have to be poured into the mould within a maximum period of seven minutes after the nodularization treatment is done or the treatments would be ineffective. Temperature and treatment technique would affect nodularity and nodule count which in turn influences mechanical properties. The suppression of carbide formation is associated with the nucleating properties of the nodularizer and innoculant alloys. The nucleating property is understood as the number and potency of nuclei formed by the alloy addition. The nodularizer and innoculant additions also influence ductile iron solidification shrinkage [1] .

Ductile Iron application for lightweight components is being limited by inability to produce as-cast carbide free thin (≤3 mm) wall parts [2] . This inability results from the formation of large volumes of carbide precipitates, poor nodularity and nodule count, presence of non-nodular graphite structures and undesired matrix types in the cast TWDI part. Borrajo et al. [3] observed appreciable increase in nodule count when cooling rate increased, as a consequence of the thin walls. The observation is possible because of the under-cooling that takes place during solidification of the thinner sections, which activates a larger number of substrates for heterogeneous nucleation of graphite.

The fast cooling of thin section ductile iron castings requires special consideration to produce carbide-free castings [4] . The common feature that all ductile irons share is the spherical shape of the graphite nodules. These graphite nodules are nucleated on small inclusions during solidification [5] . The relative possibilities for nucleation and growth depend upon foreign particles in the melt, whether as trace impurities or as deliberate additions [6] . The amount and form of graphite in ductile iron are determined during solidification and cannot be altered by subsequent heat treatment. The principal factor in determining the different grades of ductile iron in specifications is the matrix structure [7] . In the as-cast condition the matrix will consist of varying proportions of pearlite and ferrite, these can be further altered by heat treatment.

Pouring very thin sections ductile iron presents danger of massive carbide precipitation [8] . The time after spheroidal treatment (holding time) has significant effect on the elongation, but negiligble effect on the tensile strength and hardness of TWDI castings. The graphite shape is also influenced by the holding time [7] . The wall thickness of spheroidal graphite cast iron has very strong effect on the graphite size and shape of the castings [9] .

Bockus and Dobrovolskis [10] studied production of ferritic ductile iron castings. The result shows that it is very difficult to obtain thin section of ferritic ductile iron as-cast, as it is associated with high solidification rate and formation of carbides. The investigation reveals that ductile iron castings will be ferritic (as-cast) only when large amounts (more than 50 percent) of pig iron are used in the charge.

In the production of quality cast irons the inoculation process is of vital importance. When comparing un-in- oculated and inoculated irons, differences in microstructure are easily revealed with strong effect on the final mechanical properties of the casting. Through inoculation the graphite nucleation and eutectic under-cooling of the iron can be controlled and this will be of crucial importance in giving the iron its required service properties [11] . Inoculation introduces additional nuclei which cause graphite precipitation and reduces the under-cooling and formation of carbides [12] .

Works on composition, [13] - [16] , treatment techniques [7] [12] and [17] and frequency [18] have been carried out to determine the best way to carry out these treatments. However, earlier studies lacked information on the appropriate temperature at which the nodularization treatment should be done for best yield and magnesium recovery.

This study presents a molten metal processing technique for treatment temperature that eliminates primary carbides and non-nodular graphite structures in pearlitic-ferritic ductile iron castings useful for automotive applications.

Seven treatment temperatures namely 1550˚C, 1530˚C, 1510˚C, 1490˚C, 1470˚C, 1450˚C and 1430˚C are studied to determine best properties and microstructure. The charge materials consist of mild steel scraps, ductile iron returns, ferrosilicon and graphite as shown in Table 1. The requirements of ASTM E2349 standard mould making procedures are employed using adequate moulding equipment to produce dense moulds. The moulding sand consists of silica sand, bentonite, additives (coal dust and starch) and water. The dimensions of the drag and cope are 410 mm × 375 mm with a height of 110 mm. The patterns for the cavity, runners and in-gates are placed in the drag section and rammed adequately while the sprue pattern of height 100 mm is placed in the cope. The pressurized casting technique is used with ratio of 3:2:1 which translates to cross sectional area at sprue: cross sectional area at runner: cross sectional area at ingate.

The molten metal is first tapped at 1555˚C into a preheated treatment ladle for the first treatment temperature of 1550˚C using the sandwich method. The treatment alloy used 12 - 16 mm ferrosilicon magnesium granules covered with steel scraps and placed at the bottom of the ladle. This same procedure is repeated for treatment temperatures of 1530˚C tapped at 1535˚C, 1510˚C tapped at 1515˚C, 1490˚C tapped at 1495˚C, 1470˚C tapped at 1475˚C, 1450˚C tapped at 1455˚C and 1430˚C tapped at 1435˚C. The treated molten metal are then transferred into the pouring ladle where the first inoculation treatment occurs in the melt stream. The second inoculation treatment is done on the melt stream during discharge into the moulds. The samples are labeled as shown in Table 2. These procedures are maintained for all the treatment temperatures and moulds were cast within six minutes after treatment to avoid fading of inoculants.

This study has shown that it is possible to obtain carbide free, good nodularity and nodule count in thin wall ductile iron castings through in ladle sandwich nodularization treatment method accompanied by two step inoculation treatment.

Lower treatment temperatures of 1450˚C and 1470˚C promote magnesium recovery, better microstructure and mechanical properties, but treatment temperature as low as 1430˚C is unfavourable as mechanical properties became significantly impaired. The higher temperatures of 1530˚C and 1550˚C produce unfavourable microstructure and poor mechanical properties.

Treatment temperature of 1450˚C results in carbide free matrix, good nodularity and nodule count structure with 123, 139 and 172 HBN, 389, 409 and 514 MPa tensile strengths, 2.6%, 2.9% and 3.0% elongation for 2, 3 and 4 mm thick samples respectively.

The authors gratefully thank the Management and Staff of Nigerian Foundries Ltd, Lagos, Nigeria, for their assistance in the use of their facilities. The contributions of Staff of Metallurgical Laboratory of the Department of Metallurgical and Materials Engineering, University of Lagos are recognized.