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Chunky graphite in ductile iron castings

Chunky graphite in ductile iron castings
Chunky graphite in ductile iron castings

Chunky graphite in ductile iron castings

R K?llbom *, K Hamberg ** and L-E Bj?rkegren *.

* Swedish Foundry Association, Sweden, ** Chalmers University of Technology, Sweden.

Abstract Non-spherical graphite morphology is detrimental on the mechanical properties of ductile iron castings. This includes the branched and interconnected chunky graphite that occasionally occurs in the thermal center of castings. In this work the graphite morphology in ferritic ductile iron that contained chunky graphite was studied. Chunky graphite was shown to be a progressively degenerated morphology of spherical graphite. Attempts to investigate the presence and segregation patterns of elements that might play a role in the still concealed formation and growth mechanism of chunky graphite were made. No macro segregation was detected. The possible role of micro segregation of trace elements was discussed but could not be determined. The graphite nucleation potential seemed to be low in the chunky graphite areas due to the lack of available oxygen and/or sulphur.

Key words ductile iron, graphite morphology, chunky graphite,segregation, nucleation

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Introduction

Spherical graphite morphology is an important factor to attain high quality ductile iron castings. Every other type of morphology is detrimental on the mechanical properties. This includes the branched and interconnected chunky graphite that occasionally occurs in the thermal center of ductile iron castings. The presence of chunky graphite decreases the ultimate tensile strength (R m) and especially the fracture elongation (A5). The possible decrease in a ferritic ductile iron has been shown to reach 25% and 50% respectively depending on the amount of chunky graphite [1]. However, the hardness (HBW) and the yield strength (R p02) are hardly affected at all by chunky graphite. Regarding the dynamic properties of almost 100% chunky graphite containing material the crack propagation rate is not significantly affected but the fracture toughness of the material will be lower [2].

The risk of chunky graphite formation is increasing with long solidification time. Consequently, the amount of chunky graphite tends to increase with increased wall thicknesses [1]. The call for further research work to determine the cause and growth mechanism of chunky graphite is escalating with increased used of heavy section ductile iron castings in demanding applications within the heavy automotive and the windmill industries among others.

According to Gagné and Argo chunky graphite shows a spiral crystal growth pattern caused by carbon supersaturation and constitutional supercooling as the driving forces [3]. Fast diffusion rate due to the lack of, or partially disrupted, austenite shell around the graphite nodule has also been discussed as one possible cause for chunky graphite formation [3]. According to Itofuji et al the chunky graphite forms as a result of the lack of magnesium gas bubbles in the melt and, further on, the growing graphite is in contact with residual liquid iron through thin liquid channels in the austenite [4]. The liquid channels are formed when segregated elements lower the solidus temperature. Liu et al regards the chunky graphite to be a deteriorated form of nodular graphite [5]. It has in fact been observed that the transition between the graphite structures type A flake, type B flake, type D undercooled flake, coral, compacted, chunky and spherical graphite is continuous and not intermittent [6]. The different morphologies were stated, by Liu et al to occur as a result of change in solidification rate and as a function of alloy addition or segregation [6].

In this work the graphite morphology in ferritic ductile iron that contains chunky graphite has been studied and attempts have been made to investigate the presence and segregation patterns of elements that might play a role of the concealed formation mechanism of chunky graphite. Experimental

High silicon alloyed ductile iron were prepared in a 250 kg induction furnace. The charge material consisted of pig iron 41 %, returns 17 % and

steel scrap 42 %. Furan bonded moulds were used to cast a pattern consisting of five 200 mm x 200 mm blocks with thicknesses ranging from 10 to 200 mm. The temperature change of the melts was logged during solidification using thermocouples placed in the center of each block. The solidification time ranged from 90 seconds to 90 minutes for each block respectively. Besides the blocks a component cast in the same silicon alloyed ductile iron has been investigated. The component is a front axle housing aimed for a dumper. Specimens were cut out from an area with a hot spot with fairly long solidification time (approx. 30 min).

Tensile test bars were machined, perpendicular to gravitational direction, from the center of the blocks and from the components. The graphite morphology was studied in the fracture surfaces of the bars using SEM. The microstructure was studied in different positions within the test bars using conventional optical microscope. The deep etched technique used consisted of 40 minutes etching in a mixture of HCl and HNO3(3:1) followed by a cleaning step in Vogel′s etchant and thereafter well rinsed in ethanol. Some specimens were color etched in boiling sodium hydroxide (10g) + picric acid (10g) + potassium pyrosulfite (10g).

Using GD-OES (Glow Discharge Optical Emission Spectroscopy) investigation of macro segregation was made, comparing nodular and chunky graphite areas within the same 200 mm cubic block. Measurements were made in up to six positions located 10, 20, 30, 45, 60 and 90 mm from the cast surface. The analyzed positions are indicated in Figure 1.

SEM with EDS as well as EPMA were used as analyzing tools in orders to investigate micro segregation tendencies.

Results

Graphite morphology

Chunky graphite is mainly located in the thermal center. Nevertheless, the chunky graphite zone can represent a reasonably large volume of the casting. This is exemplified by Figure 1 where the presence of chunky graphite appears as a shaded area in a sawed cross section. Some areas within the dark zone consist of nodules but the main graphite morphology is chunky, Figure 2. From Figure 1 it appears as the transition from nodular to chunky graphite growth happens very sudden in an interrupted manner. However, optical microscopy and SEM studies of the graphite morphology put forward a gradual change.

The graphite morphology in the vicinity of chunky graphite areas in this 200 mm cubic casting is classified to be a mixture of form IV to VI according to the standard EN ISO 945:1994. Large irregular graphite lumps, which cannot be classified by the standard, as well as very small islands of chunky graphite, are also found in those areas, Figure 3.

The examined fracture surfaces of the tensile test bars are located within the chunky graphite zone shown in Figure 1.SEM investigations of graphite in the fracture surfaces indicate gradual degeneration from spherical to chunky morphology. Figure 4 shows well-shaped nodules. Approaching chunky graphite areas different graphite morphologies as in Figure 5 and 6can be observed. These observations, that chunky graphite is a progressively degenerated morphology of nodules, are in line with the theory of Liu et al [5]. The degenerated graphite shape in Figure 7and the pyramidal growth of chunky graphite branches in Figure 8 confirm the observations of Liu et al [5].

All specimens from the front axle housing showed somewhat different graphite morphology compared to the 200 mm thick block. In most locations normal spherical graphite morphology emerged. Roughly 15% degenerated chunky graphite appeared at the most in the hot spots. Figure 9 shows a typical area, here the cell boarders appear in a brown to white color. The blue etching parts in the microstructure contain degenerated graphite and do appear before the brownish cell boarders. Here two variants of degenerated graphite can be seen. One type of graphite that appears as normal chunky graphite (see Figure 10 in deep etched condition) and one more like a stringer of graphite (Figure 11). These graphite stringers lie between the secondary dendrite arms. The classical chunky graphite seems to be placed in the center of the dendrite arms. In all cases the degenerated graphite can co-exist with spherical graphite.

Segregation of elements

Bulk analyzes did not show any significant difference in chemical composition between the different positions indicated in Figure 1. Some variations could be seen in Si content, for example, between different positions but no coupling to chunky graphite could be confirmed. Macro segregation between the nodular areas outside the chunky graphite zone as well as inside the zone was hence not detected, Table 1.

Attempts to investigate micro segregation tendencies of low content elements such as Ce, Ca and S in the blocks by using EPMA turned out to be unsuccessful since the concentrations were below the detection limit of the instrument. This was a fact close to spherical graphite as well as nearby chunky graphite. The average Si content was somewhat higher near the chunky graphite compared to that near a nodule. Nevertheless, the difference was not greater than the Si fluctuation between two nodules.

Closer investigations of the graphite in the front axle housing show some differences. The stringer like graphite had in most cases been nucleated on oxides. Spot analysis of the oxide particles revealed normal oxides containing Si and or Mg. The melts that produced the components had a

rather high amount of residual magnesium content, 0.060-0.065 (%), this might explain the amount of particles with high magnesium content. The stringer graphite is not considered to be a chunky graphite variant.

In order to find evidence of micro segregation further investigations were made with SEM – EDS. Mapping, spot and line analysis were tried without much success. The only evidence of segregation was found in the Si and Mn content between graphite particles. No evidence of tramp elements like the elements mentioned in the literature[1], were found. A possible reason is a relatively small concentration of the mentioned elements and an insensitive analyzing method. The method gave the response from a too great material volume that disturbed the analysis.

Further, spot analyses in two types of areas, chunky and nodular, as depicted in Figure 12were carried out. The focus was on systematic analysis of particles found during the EDS-mapping. A majority of the small particles (<10 μm) were found in the eutectic cell borders between nodular and chunky graphite areas, typical location is shown in Figure 12. However, particles were also found in the nodular area as well as in the chunky area close to the borderline. The amount of particles was greater in the cell border areas than in the chunky graphite areas. The chemical composition of the particles was not the same in all areas. In the cell borders and in areas with nodular graphite, the particles contained Mg, O and Si (see Figure 13). Some particles contained Ti and C. Frequently the particles in those areas also contained P and S. On the other hand, in the chunky graphite areas the number of particles was small. Most of the particles in those areas contained Mg and S but no oxygen. Trace of Al and Ca were found as well.

Discussion

During solidification the condition in the melt is gradually, but rapidly, changing to be more favorable for the chunky graphite growth manner. Different authors have debated the change in melt condition that promotes the chunky graphite growth. Several theories indicate, as mentioned in the introduction, that the cause of chunky graphite is related to the chemical composition of the melt. Heavy section castings with long solidification times are more prone to develop chunky graphite. The graphite precipitation and growth start out to be nodular and then changes towards chunky. Consequently, it can be assumed that variations in concentration due to segregation of certain elements might be a possible reason for the transition of graphite morphology growth. The elements Ca, Si, Al, Ni, Ce and other RE are said to promote chunky graphite, especially in absence of the elements Sn, As, Bi, B, Sb and Pb [1].

However, in this work no macro segregation of elements was found. Further, the role of micro segregation could not be determined since the chemical concentrations were too low to be detected by conventional analyzing methods.

Nevertheless, the evaluation of the results of this work renders a hypothesis that the collaboration between Mg, S and O is important for the chunky graphite formation. Skaland has depicted the nucleation sites for spherical graphite [7]. The substrate contains a MgS core circumscribed by a shell of magnesium silicate, normally MgO·SiO2. Skaland denote this substrate type A. Active elements introduced to the melt by inoculation, such as Ca, Ba, Sr and Al, will react with the magnesium silicate and form a hexagonal substrate that is a favorable site for graphite precipitation. If oxygen (or sulphur) is not present the needed hexagonal nucleus will not form.

The irregular graphite often found in microstructures of castings that contain chunky graphite (as Figure 3) indicates in fact low oxygen content in the melt. The Mg-treatment was experimentally well performed and the Mg content is high enough to produce nodular graphite. No vermicular graphite can be found at all. Therefore the irregular graphite consequently indicates insufficient inoculation. However, since the inoculation procedure was good it can be assumed that the inoculation has not worked properly due to low oxygen content.

In this work, an excess of Mg/O/Si containing particles was found in the areas containing graphite nodules as well as in the borderline between nodular and chunky areas. Therefore, one can assume that the nucleation requirements for spheroidal graphite can be fulfilled in those areas. A consequence, however, is that the areas containing nodules consume most of the oxygen. A strong indication for this is that only MgS particles are found in the chunky graphite areas demonstrating that the oxygen level has been too low to form the hexagonal structure that are needed to favor spherical graphite growth. The graphite nucleation is disturbed and chunky graphite will form between the nodular areas.

Besides Mg, elements as Ca, Al, Si, Ce consume oxygen by forming stable oxides. This further strengthens the hypothesis, that low available oxygen content might be a reason for chunky graphite formation, since these elements also are said to promote chunky graphite. Conclusions

1. The branched chunky graphite is a progressively degenerated

morphology of spherical graphite.

2. Unstable or changing melt condition during the solidification leads to

chunky graphite formation, a change that is still not fully defined.

3. In chunky graphite areas MgS particles were found while the amount of

magnesium oxides was limited compared to the areas that contained spherical graphite.

4. The lack of available oxygen (or possibly sulphur) to form nuclei for

spherical graphite precipitation might be a reason for chunky graphite formation.

5. Macro segregation was not found to be a reason for chunky graphite

formation. The role of micro segregation could not be determined.

References

1. K?llbom R, Hamberg K, Bj?rkegren L-E, Chunky graphite –

formation and influence on mechanical properties in ductile cast

iron, Gjutdesign 2005 Final seminar, Espoo, Finland, 13-14 June,

2005, VTT Technical Research Centre of Finland, Finland, 2005.

2. Bj?rkblad A, Conventional vs closure free crack growth in nodular

iron, Gjutdesign 2005 Final seminar, Espoo, Finland, 13-14 June,

2005, VTT Technical Research Centre of Finland, Finland, 2005.

3. Gagné M and Argo D, Heavy Section Ductile Iron Castings Part I

and Part II, International Conference on Advanced Casting

Technology, Kalamazoo, Michigan, USA 12-14 November 1986,

pp 231-256, ASM International.

4. Itofuji H and Uchikawa H,Formation Mechanism of Chunky

Graphite in Heavy-section Ductile Cast Irons,AFS Transactions

90-42, 1990, pp 429-448.

5. Liu P C, Li C L, Wu D H and Loper, Jr, SEM Study of Chunky

Graphite in Heavy Section Ductile Iron, AFS Transactions 83-51,

1983, pp 119-126.

6. Liu P C, Loper Jr C R, Kimura T and Park H K, Observations on

the graphite morphology in cast iron,AFS Transactions 80-41,

1980, pp 97-118.

7. Skaland T, A model for the graphite formation in ductile cast iron,

NTH Trondheim, 1992, ISBN 82-7119-384-8. Acknowledgements

The Nordic Innovation Centre and VINNOVA partly financially supported this work. The authors thank Volvo Construction Equipment for supplying the front axle housings (prototypes).

Tables

Table 1 GD-OES analyzes of chemical composition inside and outside the chunky graphite zone. The location of analyzed positions is shown in Figure 1.

MELT 1MELT 2

Pos123456246 Si 3.05 3.38 3.27 3.31 3.26 3.29 3.36 3.30 3.55 Mn0.190.160.170.180.190.180.180.210.19 P0.0710.0590.0550.0470.0550.0670.0460.0720.062 S0.0160.0130.0140.0160.0160.0150.0170.0180.014 Cu0.0200.0180.0190.0210.0210.0210.0180.0190.019 Al0.0120.0120.0130.0130.0130.0130.0140.0140.015 B0.00130.00060.00090.00110.00110.00110.00100.00160.0013 Sn0.0240.0250.0240.0240.0200.0220.0250.0200.020 Ca0.00010.00020.00010.00020.00020.00020.00010.00010.0001 Ce<0.001<0.001<0.001<0.001<0.001<0.001<0.001<0.001<0.001 Sb0.0050.0030.0030.0040.0030.0030.0030.0060.006

Figures

1 2 3 4 5 6

Figure 1 Chunky graphite Figure 2 Graphite morphology in zone located in the thermal the thermal center of a 200 mm center (200 mm cube).cube (105 X)

Numbers denote positions of

analyzes, see Table 1.

Figure 3 Mixture of graphite shapes Figure 4 Well-shaped nodules just outside the chunky zone (45 X)(750 X)

Figure 5 Approaching chunky Figure 6 Transition state in the graphite areas the graphite gradually border of a chunky graphite cell changes its morphology (350 X)(1000 X)

Figure 7 Degenerated graphite Figure 8 Pyramidal growth of (2000 X)chunky graphite branches

(deep etched 1500 X)

Figure 9 Color etched microstructure Figure 10 Deep etched graphite containing chunky graphite.that looks like classical chunky

graphite.

Figure 11 Stringer graphite that is not

considered to be chunky graphite.

Figure 12 Particles were detected in the borderline between chunky and nodular graphite areas as well as in each area respectively.

Figure 13 Particles in the borderline and in the nodular areas (see Figure

12) were composed of these elements. The peak to the left is not valid for

the analyze.

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