Formation Of Chunky Graphite Research Articles

Examples of Chunky Graphite Formation in Production of Ductile Iron Castings and Effective Countermeasures

Examples of Chunky Graphite Formation in Production of Ductile Iron Castings and Effective Countermeasures

Influence of Chemical Composition and Cooling Rate on Chunky Graphite Formation in Thick-Walled Ductile Iron Castings

Influence of Chemical Composition and Cooling Rate on Chunky Graphite Formation in Thick-Walled Ductile Iron Castings

Influence of Carbide-Promoting Elements on the Microstructure of High-Silicon Ductile Iron

Because of its low cost, steel scrap is one of the most important raw materials for the production of ductile iron (DI). The amount of carbide-promoting elements in steel scrap, such as chromium, manganese, molybdenum, niobium and vanadium, is expected to increase in the future. Most of these elements have a negative impact on the microstructure and mechanical properties of DI. The solubility of carbide-promoting elements in solid solution-strengthened DI materials, standardized in DIN EN 1563:2011, is modified by the high silicon content. For these new materials, the tolerance limits for carbide-promoting elements and their mutual influence must be known to ensure a sustainable production process. To investigate the individual and combined impact of carbide-promoting elements on the carbide content in high-silicon ductile iron EN-GJS-500-14, experimental investigations and thermodynamic–kinetic microstructure simulations were carried out. Microstructure was characterized using metallographic analysis, and quantitative relations between chemical composition and microstructure were developed by means of regression analysis. Besides this quantitative analysis, it was found that the formation of grain boundary carbides can be detected via thermal analysis. Furthermore, experiments and simulations showed that vanadium promotes the formation of chunky graphite in high-silicon DI castings.

Effect of Cooling Rate on Graphite Morphology and Mechanical Properties in High-Silicon Ductile Iron Castings

This paper investigates the effects of silicon content above 3 wt% and different cooling rates on the graphite morphology and the mechanical properties of ductile cast iron. To research and determine these effects, three grades of high-silicon ductile iron were cast with silicon content of 3 wt%, 3.5 wt% and 4.5 wt%. In order to investigate the effect of different cooling rates during solidification, four sections with different thicknesses were cast in the same mold, i.e., cube with wall thickness 180 mm and standard Y2, Y3 and Y4 keel blocks. Microstructure analysis and tensile test were performed on the as-cast samples. In the second step of analysis, detailed metallographic observations on specially prepared samples were performed in order to investigate the risk of graphite degeneracy appearance, especially chunky graphite formation due to high silicon content and its effect on mechanical properties. The results showed that graphite content, ferrite fraction, yield strength and tensile strength were increased by increasing the silicon content. On the other hand, elongation decreases with increasing silicon content. Also, based on the metallographic observations of the present work, the risk of chunky graphite formation increases with increase in silicon content and with decrease in the cooling rate. Chunky graphite formation affects negatively mechanical properties, especially elongation in ductile cast irons with silicon content higher than 3.5 wt%.

Effect of Bismuth on Preventing Chunky Graphite in High-Silicon Ductile Iron Castings

The effect of Bi level on the macro- and microstructure of thick-walled high-silicon ductile iron castings has been investigated, as well as the influence of chunky graphite on the mechanical properties. Influence of cooling rate on graphite morphology was also investigated. High silicon level has caused the formation of chunky graphite, in very high amounts, along the entire cross section of the casting. Microstructure reveals that the addition of Bi affects not only the reduction of chunky graphite amount, but also the distribution along the casting cross section which is not uniform any more as in the casting without Bi. When Bi is added, the zone with chunky graphite appears independently of the cooling rate. Positive effect of Bi on the reduction of chunky graphite has been shifted towards higher levels, from 0.006 to 0.01 wt% Bi. Higher levels of Bi are more efficient also for the improvement in spheroidal morphology of graphite. The addition of 0.01 wt% Bi gave the best results, while the Ce/Bi ratio amounted to 0.6. This paper has proven that Bi acts on the increase in the strength values also in the high-silicon ductile iron casting. According to microstructure observations, it can be concluded that eutectic cells of chunky graphite are formed during bulk eutectic reaction. First, the primary graphite nodules are precipitated, followed by the initial phase of eutectic reaction during which the austenite dendrites precipitate and grow encapsulating at the same time primary graphite nodules.

Chunky graphite in spheroidal graphite iron: review of recent results and definition of an predicting index

Graphite degeneracy in heavy-section spheroidal graphite cast irons is mostly associated with the formation of chunky graphite which consists of large eutectic cells with interconnected graphite strings. At low level, appearance of chunky graphite is limited to its non-aesthetic effect on machined surfaces, while at higher level it is detrimental for mechanical properties of the components. Chunky graphite is often related to high silicon levels and too high cerium additions during the spheroidization treatment. The appearance of this defect may be limited by controlled additions of antimony that is thought to tight the excess of cerium, but other impurities and low level elements may have to be considered during melt preparation. This contribution proposes a review of recent results and approaches on chunky graphite appearance, primarily but not exclusively in the case of heavy-section cast irons. Based on this literature review and series of experimental data, a predictive index for evaluating the risk of chunky graphite appearance is proposed. Lines for further research work aimed at a better understanding of graphite degeneracy are finally suggested.

Some paradoxical observations about spheroidal graphite degeneracy

Differential thermal analysis experiments have been performed on samples machined from Y2-blocks cast with different high-silicon spheroidal graphite irons. Depending on magnesium, silicon, cerium and antimony content, the as-cast microstructure showed various levels of chunky graphite in the central part of the blocks. In contrast, the microstructure of the materials after remelting and resolidification during differential thermal analysis consisted of lamellar or compacted graphite. The formation of chunky graphite in the as-cast microstructure is rationalized using an index or silicon equivalent that has been recently suggested. The differences in the microstructures after differential thermal analysis are discussed in terms of available free magnesium. Emphasis is finally put on the striking differences in characteristic size of the microstructures made of compacted graphite as compared to lamellar graphite and chunky graphite. This leads to tentative conclusions about growth of compacted and chunky graphite which would be worthy of further experimental investigations.

Effect of Si and Ni Addition on Graphite Morphology in Heavy-Section Spheroidal Graphite Iron Parts

Metallographic analysis is applied to the study of the chunky graphite morphology in heavy-section castings of spheroidal graphite cast irons. Three castings with different Si and Ni content were prepared. Three positions in casting from the edge to the centre, with different cooling rates, were chosen for microstructure observation. The effect of the Si and Ni content on the graphite morphology and mechanical properties of heavy-section spheroidal graphite cast iron parts were investigated. Cerium containing commercial inoculant was used for in-stream inoculation. Chunky graphite area was estimated in micro-and macrostructure. Mechanical properties were determined on tensile test bars taken from the centre of the casting. Macro-and microstructure examination showed that the castings with high Si-content and Ni addition had chunky graphite present, while the castings produced by use of low Si and Ni containing charge had no chunky graphite. High Si-content is strong chunky graphite promoter, especially in castings with slow cooling rate. Ni addition also promotes chunky graphite formation, but only in thermal centre of the casting (where the cooling rate is the lowest). The elongation is severely lowered when chunky graphite appears in the microstructure.

Influence of solidification defects on the fatigue behaviour of heavy‐section silicon solution–strengthened ferritic ductile cast irons

AbstractMechanical and fatigue properties of heavy section solution–strengthened ferritic ductile iron (SSF‐DI) castings have been examined in this contribution. Amounts of silicon varying from 3.2 to 3.55 wt% have been used. The effect of antimony on counteracting the formation of chunky graphite has been evaluated. In order to estimate the influence of solidification defects (microshrinkage porosities or degenerated graphite particles) on the fatigue strength of the investigated materials, scanning electron microscope observations of the fracture surfaces and extreme value analysis of the crack initiating defect dimension, √area, have been performed. The propagation of the fatigue crack through the material has been observed by means of microstructural analysis of the cross sections of the fracture surfaces. It has been found that the treatment with Sb is useful to avoid the formation of degenerated graphite particles. Moreover, it has been observed that the most of the fatigue life is spent in the crack initiation stage, where the microshrinkage porosities play a major role. The presence of chunky graphite affects mostly the propagation stage. Finally, mechanical and fatigue properties of SSF‐DI have been compared with those obtained from traditional ductile cast iron specimens taken from the same casting geometry.

Fatigue and fracture resistance of ferritic ductile cast iron: the effect of Sb and solidification time

In this paper, we explore the effect of inoculants and solidification time on the mechanical properties of an EN-GJS-400-type ferritic ductile cast iron (DCI). For this purpose, static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests are carried out on three different material conditions. They are produced under fast cooling (solidification time 2h45min), representative of thin walled castings, and very slow cooling (solidification time 10—13h), representative of thick walled castings, this latter with and without the addition of Sb. It has been found that the long solidification time leads to an overgrowth and degeneration of the spheroidal graphite nodules. The addition of Sb avoids the formation of chunky graphite observed in the slowly cooled condition but results in large exploded graphite nodules. These effects impact negatively on tensile strength, total elongation and fatigue strength. Conversely, the resistance to fatigue crack growth is even superior and the fracture toughness comparable to that of the fast cooled condition. Metallurgical and fractographic analyses are carried out to explain this behaviour.

Effects of high silicon contents on graphite morphology and room temperature mechanical properties of as-cast ferritic ductile cast irons. Part I – Microstructure

Studying a series of near eutectic spheroidal graphite cast irons with various amounts of silicon up to 9.12wt% confirmed that: 1. silicon stabilizes ferrite to such an extent that ferrite is the Fe-rich phase appearing during solidification at the highest silicon contents; 2. silicon triggers graphite degeneracy such as chunky graphite. As well-known, cerium and magnesium do also increase the risk of chunky graphite formation while antimony counteracts cerium. Based on the metallographic observations of the present work, an index is proposed to evaluate the risk of chunky graphite appearance from the silicon, magnesium, cerium and antimony contents. Above a critical value of this index, the risk for chunky graphite formation increases steadily. Using data from previous studies, it is further demonstrated that the critical value decreases with increase in casting modulus as expected. The evolution of mechanical properties of the prepared cast irons will be presented in a second part of this study.

Vpliv modifikacije na nastanek grudastega grafita v ulitkih iz gnetljivega železa

Vpliv modifikacije na nastanek grudastega grafita v ulitkih iz gnetljivega železa

Chunky graphite formation in ductile cast irons: effect of silicon, carbon and rare earths

AbstractUse of rare earths, high silicon and carbon contents, and low cooling rates are reported as possible reasons for formation of chunky graphite in ductile iron castings. The understanding of this graphite degeneration is however limited, and the above conclusions are still controversial. To get further insight into this topic, ductile cast iron melts have been prepared with various carbon and silicon contents and using nodularizing alloys with various rare earth levels. These melts have been cast in blocks with different sizes to include also the effect of cooling rate on graphite degeneration. Metallographic investigation revealed that the chunky-affected areas enlarge when decreasing the cooling rate, when increasing the silicon content and without low-level addition of rare earth. In the discussion, a schematic based on change of liquid/graphite interfacial energy with alloying elements is proposed which describes the conditions for the formation of chunky graphite instead of nodular graphite.

Physico-chemical causes of the formation of chunky graphite in austenitic ductile iron

Chunky graphite (CHG) is a well-known phenomenon which occurs in heavy section spheroidal graphite iron (SGI) castings or SGI castings with higher alloying content. Chunky graphite results in a decrease of mechanical properties, in particular a decrease in elongation. Despite numerous studies, the root cause of the CHG formation remains unidentified. In publications analyzing the different stages of CHG formation based on quenched samples, it is assumed that carbon diffusion through the austenite halo is inhibited due to the size of the eutectic grain. Furthermore, due to the carbon supersaturation CHG nucleates near the liquid-austenite interface of the eutectic grain. Additionally, other studies show that partly incomplete graphite spheres can be interconnected with CHG where no nucleation is required. Using thermodynamic-kinetic material simulation two morphologies, the decoupled SGI growth and the coupled CHG growth, were modeled to simulate their impact on the solidification kinetics. In the present study, it was shown for the first time that with increasing Ni content conditions arise in the austenite halos which strongly reduce the diffusive flux of carbon towards the graphite spheroid and thereby decelerate the kinetics of SGI growth. With a content above 5 wt% Ni, the competing kinetics of CHG growth exceeds the SGI growth. Therefore, the preference in kinetics and the present thermodynamic constraints are critical to the morphology change from SGI to CHG.

Cast Iron Casing Cracking Due to Chunky Graphite Formation

Quality inspection of heavy turbomachinery casings made of ductile cast iron has revealed anomalies in the form of characteristic relief pattern on roughly machined surfaces. Detailed microstructural observations determined that this anomaly is related to degeneration of cast iron microstructure where undesired chunky graphite was formed instead of evenly dispersed graphite nodules. Mechanical tests showed that this microstructural alteration led to deterioration of mechanical properties, especially plasticity. Regularity was found as this problem affected mainly thick sections of casts, especially where the cooling rate was limited. This article discusses the consequences of chunky graphite formation, detection methods, acceptance criteria, and preventive actions.

Effect of inoculant containing rare earth metals and bismuth on microstructure and mechanical properties of heavy-section near-eutectic ductile iron castings

The effects of inoculation sequence and inoculant chemical composition on heavy-section castings microstructure, with particular attention to chunky graphite, were investigated. The combinations of inoculant types with inoculation sequences were chosen basing on foundry experience.Thermal and chemical analyses were used to control the experimental foundry processes. Metallurgical analyses were performed by means of optical and field-emission gun scanning electron microscope and important microstructural parameters were measured and correlated with the results coming from tensile tests. In-stream inoculation with inoculant containing Bi and rare earths was found to drastically reduce the formation of chunky graphite. This result was attributed to the major fading resistance of such inoculant compared to the standard ones, confirmed both by nodule fraction measurements and sub-micrometric aggregates of Bi detected at the spheroid centre in the central part of the casting.

Statistical Analysis of the Influence of Some Trace Elements on Chunky Graphite Formation in Heavy Section Nodular Iron Castings

To study the formation of chunky graphite (CHG) in heavy section castings, 68 nodular cast irons of both pearlitic and ferritic grades were cast in cubic blocks 30 cm in edge. The volume fraction of the blocks affected by this degenerate graphite was quantified and related to the chemical analysis of the materials by means of a multivariate analysis. For the composition domain investigated, the effects of Ce, Cu, La, P, Sb, and Sn were statistically established with a high R 2 correlation coefficient.

Effects of antimony addition and section size on formation of chunky graphite in ductile iron

The combined effects of section size together with antimony addition on formation of chunky graphite in ductile iron were investigated by characterising graphite morphology, nodule count and size. Results show that the increased casting section size leads to an increase in spheroidal graphite diameter and a decrease in graphite nodule count. Meanwhile, all the graphite is spheroidal except in 90 mm thickness casting, in which chunky graphite clearly emerges. Moreover, the same tendency concerning graphite nodule count and size is observed in castings with 0·012 mass-% antimony addition as cooling rate decreases. However, the graphite nodule is refined compared with the Sb free castings, and chunky graphite is absent in 90 mm thickness casting effectively. It is believed that high graphite nodule count suppresses chunky graphite formation in thicker casting because of antimony addition, which has no evident effect on graphite morphology when the casting thickness sizes are less than 90 mm.

Effects of Wall Thickness and Cerium on Formation of Chunky Graphite

Effects of Wall Thickness and Cerium on Formation of Chunky Graphite

Influence of Si and Cooling Rate on Chunky Graphite Formation

Influence of Si and Cooling Rate on Chunky Graphite Formation