Thin Wall Ductile Iron Castings Research Articles

Influence of rare earths on shrinkage porosity in thin walled ductile cast iron

Ductile cast iron has been cast in test bars with thickness from 2 to 10 mm. The rare earth elements La and Ce have been added to some of the castings to evaluate their influence on microstructure and shrinkage tendency. Both La and Ce increased the graphite nodule count, especially for thickness of 6 mm and below. La gave the best nodule size distribution with many small nodules. La had less shrinkage tendency than Ce in the 10 mm test bars. This tendency was less pronounced for the 6 and 4 mm test bars and other factors may have a large influence at these thicknesses. Increasing the temperature T1, which is controlled by the growth of off-eutectic austenite dendrites, increased the shrinkage tendency.

The Effects of Fracture Origin Size on Fatigue Properties of Ductile Cast Iron with Small Chill Structures

Reducing the weight of a machine structure is an increasingly important consideration both for the conservation of resources during production and for the energy saving during operation. With these objectives in mind, thin-walled ductile cast iron has recently been developed. Because rapid cooling could result in brittle microstructure of cementite (chill) in this cast iron, it is necessary to investigate the effect of cementite on the fatigue properties. Therefore, fatigue tests were carried out on a ductile cast iron of block castings which contained a relatively small amount of cementite. Fracture surface observation indicated that the fracture origins were located at graphite clusters and cast shrinkage porosity, not at cementite. It appears that when the size of the cementite is smaller than that of the graphite, the cementite does not affect the fatigue properties of ductile cast iron. Not surprisingly, the fatigue lives were found to increase with decrease in the size of the fatigue fracture origin. The threshold initial stress intensity factor range ΔKini,th for fatigue failure was found to be about 3-4MPa√m, independent of microstructure.

Graphite nodule count and size distribution in thin-walled ductile cast iron

Graphite nodule count and size distribution have been analysed in thin-walled ductile cast iron. The 2-D nodule counts have been converted into 3-D nodule counts by using a finite difference method (FDM). Particles having a diameter smaller than 5 μm should be neglected in the nodule count as these are inclusions and microporosities that do not influence the solidification morphology. If there are many small graphite nodules, as in thin-walled castings, only 3-D nodule counts calculated by FDM will give reliable results. 2-D nodule counts and 3-D nodule counts calculated by simple equations will give low results. The 3-D size distribution showed the presence of primary graphite nodules in hypereutectic castings. In thin plates the nodule count is similar in both eutectic and hypereutectic plates. In thicker plates the hypereutectic casting has the higher nodule count.

Undercooling and nodule count in thin walled ductile iron castings

Casting experiments have been performed with eutectic and hypereutectic castings with plate thicknesses from 2 to 8 mm involving both temperature measurements during solidification and microstructural examination afterwards. The nodule count was the same for the eutectic and hypereutectic castings in the thin plates (≤4·3 mm) while in the 8 mm plate the nodule count was higher in the hypereutectic than in the eutectic castings. The minimum temperature before the eutectic recalescence (Tmin) was 15 to 20°C lower for the eutectic than for the hypereutectic castings. This is due to nucleation of graphite nodules which begins at a lower temperature in the eutectic than in the hypereutectic castings. The recalescence ΔTrec was however also larger for the eutectic casting and in the thin plates the maximum temperature after recalescence (Tmax) was the same in the eutectic and hypereutectic plates. This is because higher undercooling gives a larger driving force for the solidification process and the equal nodule counts therefore give the same Tmax. However, the higher undercooling before recalescence increases the risk for formation of carbides during the solidification. In the 8 mm plates, the lower nodule count in the eutectic plates also gave a lower Tmax than in the hypereutectic castings.

Temperature measurement during solidification of thin wall ductile cast iron. Part 1: Theory and experiment

Temperature measurement using thermocouples (TCs) influences solidification of the casting, especially in thin wall castings. The problems regarding acquisition of detailed cooling curves from thin walled castings are discussed. Experiments were conducted where custom made TCs were used to acquire cooling curves in thin wall ductile iron castings. The experiments show how TCs of different designs interact with the melt and how TC design and surface quality affect the results of the data acquisition. It is discussed as to what precautions should be taken to ensure reliable acquisition of cooling curves. Measurement error depending on TC design and cooling conditions is shown. A method is presented that allows acquisition of cooling curves in thin walled ductile iron castings down to a thickness of at least 2.8 mm. The obtained cooling curves can be used to compare nucleation and growth during solidification of castings with different plate thicknesses.

Surface reactivity of thin wall ferritic ductile iron. The effect of nodule count and grinding variables

Thin wall ductile iron (TWDI) castings constitute an attractive alternative to several applications in which the strength to weight ratio becomes a key design variable. In TWDI, the nodule count for a given chemical composition is highly dependent on cooling rate during solidification, and hence on thickness. For mechanical parts, where accurate dimensional tolerance is mandatory, the most common machining process applied is grinding. This process induces significant temperature gradients and surface plastic deformations which could affect service performance, particularly in corrosion environments. In TWDI, surface properties become more relevant due to the high surface area to volume ratio. Therefore, the aim of this paper is to determine the effect of nodular count and grinding conditions on surface reactivity of ferritic TWDI. Electrochemical assays were carried out in a three-electrode cell in a 3.5 wt.% NaCl solution on both polished and ground samples. The results obtained indicate that surface reactivity increases with higher nodular counts and residual plastic strain.

Experimental validation of error in temperature measurements in thin walled ductile iron castings

An experimental analysis has been performed to validate the measurement error of cooling curves measured in thin walled ductile cast iron. Specially designed thermocouples (TCs) with Ø0·2 mm TC wire in Ø1·6 mm ceramic tube were used for the experiments. Temperatures were measured in plates with thicknesses between 2 and 4·3 mm. The TCs were accurately placed at the same distance from the surface of the casting for different plate thicknesses. It is shown that when measuring the temperature in plates with thickness between 2 and 4·3 mm the measured temperature will be parallel shifted to a level ∼20°C lower than the actual temperature in the casting. Factors affecting the measurement error (oxide layer on the TC wire, penetration into the ceramic tube and variation in placement of TC) are discussed. Finally, it is shown how useful cooling curve may be obtained in thin walled castings.

Numerical modelling of thin-walled hypereutectic ductile cast iron parts

Solidification of hypereutectic thin-walled ductile cast iron has been modelled in one dimension taking into account the precipitation of off-eutectic austenite dendrites during solidification. The simulations have been compared with casting experiments on plate geometries with plate thicknesses from 2.8 to 8.0 mm with good agreement for both cooling curves and nodule counts. The experimental results revealed that the eutectic solidification of plates with thicknesses less than 4.3 mm was characterised by two stages: (1) an initial arrest and (2) the main eutectic reaction. The initial arrest was associated with the discovered austenite dendrites in these plates. Moreover, the experimental results confirmed the numerical model’s prediction of off-eutectic austenite dendrites. The numerical model showed that the off-eutectic austenite dendrites mainly precipitate in the first stage before onset of the main eutectic solidification.

Rolling contact fatigue resistance of ductile iron with different nodule counts and matrix microstructures

Thin wall ductile iron castings are being used in the industry as a way to improve the strength to weight ratio of machine parts. The high cooling rate, suffered by thin wall parts during the solidification process, promotes several microstructural changes, such as, carbide precipitation and a noticeable nodule count increment. The present work, studies the effect that the increase in nodule count has on the rolling contact fatigue resistance of ductile iron with different matrix microstructures. Ductile cast iron test samples, with nodule counts ranging between 150 and 1400 nod/mm 2 were obtained. The samples were then heat treated in order to obtain three sets of different ADI grades and also a quenched and tempered set. The rolling contact fatigue properties were evaluated by using a flat washer type test rig. Different relations of the contact area versus the nodule size were obtained by using three different counterparts. The results show that an increase in the nodule count promotes a noticeable increase in the rolling contact fatigue life, being more important for the quenched and tempered samples. The relation between contact area surface and nodule size was the main variable influencing the rolling contact fatigue life.

Nucleation and solidification of thin walled ductile iron—Experiments and numerical simulation

Investigation of solidification of thin walled ductile cast iron has been performed based on experiments and numerical simulation. The experiments were based on temperature measurement and microstructure examination. Both hyper- and near-eutectic alloys have been investigated with plate thicknesses of 2.8, 4.3 and 8 mm. Temperature measurement revealed that eutectic solidification in the thinner plates (≤4.3 mm) was divided into two stages. The first stage was relatively short with little or no recalescence. The second stage was the main stage with a larger recalescence. In 8 mm plates the eutectic solidification had only one main stage. Color etching revealed the presence of austenite dendrites. Results of the experiments have been compared with a 1-D numerical solidification model that takes into account the precipitation of off-eutectic austenite during the eutectic stage. Simulations reveal that the first stage of eutectic solidification in the thin plates can be explained by growth of off-eutectic austenite.

Effect of rare earth element on microstructure formation and mechanical properties of thin wall ductile iron castings

Ductile iron castings with 2, 3, 4, 6, 8, and 25 mm thickness and various amount of rare earth elements (RE) (from 0 to 0.04%), were cast in sand molds to identify the effects of sample thickness and the content of RE% on microstructural formation and selected mechanical properties. The effects of RE content and sample thickness on microstructural formation, including on graphite nodule count, graphite nodule shape, spherodization, and ferrite amount, were observed. The yield strength of the samples with RE within the range investigated were lower than those of the specimens without RE. The elongation was improved with the addition of RE up to 0.03% in ductile iron castings. The additions of 0.02% RE caused a smaller graphite nodule size and a higher number of graphite nodules than those in the specimen without RE at all levels of RE addition; the nodule count decreased with increase in section size. The chill zones were observed in the 2 mm thick samples, but were absent in the samples from castings which were thicker than 2 mm, irrespective of the addition of RE. The nodularity of graphite nodules improved due to the addition of 0.02–0.04% RE. The specimens with RE content up to 0.03% had a lower tensile strength and hardness, higher elongation than that of the specimens without RE. The ferrite content in all castings increased with additions of 0.02% RE. The tensile strengths of the 2 and 3 mm thick samples were also estimated using the relationship between strength and hardness, obtained from the data on the tensile strength and hardness of the 25 mm thick samples.

Production and properties of thin-wall ductile iron castings

Fuel efficiency requirement for cars and trucks has conducted to an increased use of low-density materials such as aluminium in the automotive industry. However, such materials, which require tremendous amounts of energy for their production, have also significant drawbacks with respect to strength and cost. Therefore, they do not represent the panacea for weight reduction.Ductile iron remains a material of choice for automotive applications due to its castability, strength-to-weight ratio and cost. Indeed, the mechanical properties of Ductile Iron allow the reduction of the casting size, assuming that such parts could be produced carbide free in the as-cast condition. A comprehensive study was carried out to design a production route for thin wall Ductile Iron castings free of eutectic carbides. In this study, a test casting assembly with section thicknesses from 3 to 10 mm representative of actual castings was designed using computer modeling technique. Castings of various chemical compositions produced using different processing parameters were manufactured.The study showed that carbide free 3 mm thick castings can be manufactured when powerful inoculants, such as Bi-bearing ferro-silicon, are used. In such cases, carbide free castings with nodule count in excess of 1200 can be produced, while maintaining silicon content at a reasonable level (< 2.8%). Under these conditions, cooling rate and inoculation rather than silicon content control nodule count and matrix structure. The mechanical properties of the test castings were found slightly inferior to those of conventional section castings, but sufficiently high to compete with light metals in many applications. Routes to further improve the mechanical properties were investigated, including filtering which was found to improve the consistency of the mechanical properties.

Shape and Count of Free Graphite Particles in Thin Wall Ductile Iron Castings.

Recently, the scientific community has begun to study in detail the potential application of ductile iron in the production of thin wall components. Efforts are focused on the identification of the operative conditions necessary to obtain parts free of defects, with the desired microstructure. These aspects have been widely examined in the past for parts of conventional thickness (more than 5 mm), either experimentally or by using computational programs to model the solidification process. Nevertheless, modeling of thin walled parts is still unreliable, since specific databases are not available. The objective of this work is to study the evolution of the graphite nodule count and shape in ductile iron, as the section thickness diminishes down to 1.5 mm, using conventional casting procedures and resin bonded sand molds. A reasonably accurate correlation between solidification time and nodule count has been developed, based on experimental and modeled cooling curves. The morphology of graphite nodules has been characterized by image analysis, and the results correlated with the solidification time. The advantages of using solidification time as a parameter instead of thickness are also discussed.

Verification of Thin-Wall Ductile Iron Test Methodology

Abstract Modifications need to be made to standard test procedures to determine the actual properties of thin-walled ductile iron castings. A series of experiments was performed on test bars to examine the effects of surface finish, elongation measurement techniques, and the amount of material removed during grinding. A paired t-test was used to show that strength and ductility increased as a result of removing the as-received (as-cast) surface. This is attributed to inaccuracies in measuring the cross section area of the rough surface as well as an actual increase in strain through removal of surface effects. A nontraditional method of measuring elongation (to retain the fracture surface) was compared to the standard ASTM method. The nontraditional method was found to be a conservative measure of elongation that is highly correlated with plastic strain as measured by the stress-strain curve. The amount of surface removed during grinding was found not to affect the mechanical properties with the exception of hardness, a surface measurement. It is suggested that thin-walled castings be tested in the condition the casting will be used, i.e., if the casting will be used with the as-cast surface, testing with the as cast surface will accurately reproduce the decrease in strain caused by the surface effects. If the casting will be ground before use, any amount of surface can be removed to make the testing convenient.