Investigation of Cast Iron Processing to Produce ...web.mst.edu/~lekakhs/webpage Lekakh/Articles/12-024.pdfInvestigation of Cast Iron Processing to Produce Controlled Dual Graphite

Investigation of Cast Iron Processing to Produce Controlled Dual Graphite Structure in Castings

S.N. Lekakh, J. Qing, V.L. Richards

Missouri University of Science and Technology, Rolla, MO

Copyright 2012 American Foundry Society

ABSTRACT

The change in graphite shape from flake to spheroidal

significantly increases the strength of cast iron while

simultaneously decreases thermal conductivity to about

half of the value in flake graphite iron. In many industrial

applications, such as the cylinder head of diesel engine, a

combination of high strength and thermal conductivity is

essential. A compromise would be to use compacted

graphite iron, while a more effective way would be to

develop controlled distribution of the dual graphite

structure that matches a specific region’s function of the

casting design. In this study, the process of casting such

functionally region-specific material was modeled and

experimentally investigated.

It was shown that four main conditions need to be

satisfied for achieving a controlled dual graphite

structure:

a distinct change in ductile/gray iron melt

composition during mold pouring by melt treatment;

sequential fill of the mold cavity by different melt

compositions using special gating systems;

minimum post filling momentum in the mold and

avoidance of natural convection in the melt before

casting solidification.

Different approaches were tested using an experimental

casting with flat internal cores. The experimental results

achieved thus far were compared to modeling predictions.

The different possible process routes are discussed.

Keywords: ductile iron, gray iron, solidification, dual

graphite structure, modeling.

INTRODUCTION

The shape of graphite in cast irons significantly affects

mechanical and thermo-physical properties of castings.

Table 1 provides the typical mechanical properties of cast

irons with ferritic and pearlitic metal matrix.1, 2

Changing

from the flake-shaped graphite in gray iron (GI) to

spheroidal graphite (SG) in ductile iron (DI) increases

tensile strength by nearly a factor of three without

affecting hardness and provides a higher value of “quality

factor,” which is commonly defined as the ratio of

UTS/HB.

Changing the graphite morphology from flake to

spheroidal also reduces the thermal conductivity at the

same time as strength is increased. In many industrial

applications, such as the cylinder head of a diesel engine,

a combination of high strength and thermal conductivity

is essential. A compromise could be achieved by using

compacted graphite iron. However, for a specific design

application such as the diesel engine head there are

regions of the casting, whose function requires high

strength, and other regions, whose function is primarily

heat conduction between the combustion zone and the

cooling system. Typically, the required metal matrix

consists of pearlite with minimum ferrite and absence of

free carbides.

In many unintentional experiments, different shapes of

graphite have been observed in different parts of the same

casting, while the achievement of controlled,

appropriately located and stable dual graphite structure is

a serious challenge. Stefanescu and coauthors3

investigated the surface layer, so called casting skin, in DI

and compacted graphite (CG) iron. Some metallographic

features were identified and their formation mechanisms

were proposed: graphite degradation due to the fade of

magnesium and graphite depletion due to decarburization

near surface by reaction with oxygen diffused from mold.

Therefore, spheroidal graphite degenerates to CG; CG

degenerates to flake graphite; type A flake graphite

degenerates to type D. For example, flake graphite—fine

flake and compacted to coarse compacted— coarse

compacted and some spheroidal and exploded graphite

layers were identified in the skin. These skin layers

diminish the mechanical properties of the CG iron

casting. These layers are undesirable in conventional

castings while they might be desirable for castings with a

controlled dual graphite structure.

Many researchers have proven the role of sulfur as a

modifier of the graphite morphology. It has been proven

that sulfur is more desirable as denodulizing element than

titanium. Moreover, late sulfur addition favors the

graphite nucleation in ductile iron. Lekakh and Loper8

demonstrated a significant increase in nodule count after

late S-O-additions in magnesium treatment melt. Riposan4

demonstrated that larger sulfur additions facilitated the

transition: SG→ CG → coral graphite→ type B

graphite→ type A graphite, in this order as the sulfur

content increased. Denodulizing magnesium (Mg) treated

Paper 12-024.pdf, Page 1 of 10AFS Proceedings 2012 © American Foundry Society, Schaumburg, IL USA

Table 1. Typical Mechanical Properties of Gray (GI), Compacted Graphite (CGI) and Ductile Iron (DI) with Spheroidal Graphite

1, 2

Material Matrix UTS, MPa

Hardness, HB

Elongation, %

Quality factor (YST/HB)

Heat conductivity,

W/mK

GI Pearlitic 220-270 190-230 Less than 1 1.0-1.2 45-55

CGI Ferritic Pearlitic

330-410 420-580

130-190 200-250

5-10 2-5

2.3-2.5 2.0-2.3

35-45

DI Ferritic Pearlitic

420-600 600-800

140-200 240-300

15-25 3-10

2.7-3.0 2.5-2.7

30-35

iron with sulfur additives has been adopted to produce CG

iron of various SG/CG values by adjusting Mg/S ratio. All

studies mentioned previously were done for casting with

mono-shape graphite. However, the achievement of

controlled dual graphite structure in one casting is a

significant challenge.

Melt mixing during mold pouring; natural convection

after the fill and other possible physical phenomena,

which occur during casting filling and solidification

(shrinkage, elemental segregation), could prevent the

formation of the desired dual structure. One of most

important factors preventing the formation of dual

graphite structure is natural convection in the melt, which

results from changes in melt density during cooling and a

significant thermal gradient in the filled casting. In fluid

mechanics, the Rayleigh number (Ra) includes the main

parameters influencing natural convection:

( ) Equation 1

where: Gr is Grashof number which describes the

relationship between buoyancy and viscosity, Pr is

Prandtl number, which describes relationship between

momentum and thermal diffusivities, g is the acceleration

due to gravity, β is thermal expansion coefficient, ϑ is

kinematic viscosity, α is thermal diffusivity, ∆T is

temperature difference and X is characteristic length.

When the Ra number is below a critical value, heat

transfer is primarily controlled by conduction, while heat

transfer is primarily due to convection at higher Ra

values. Factors favoring increased convection are high

thermal expansion, β, and large casting dimensions, X.

The thermal differential, ΔT, is the main driving factor for

convection rate while the total mass transferred by

convection also depends on processing time. High cooling

rate, T,’ increases thermal differential, ΔT, and

simultaneously decreases duration for cooling the poured

melt to near the solidification temperature at which a

dramatic increase of viscosity, ϑ, decreases Ra value.

Therefore, the real situation is very complicated and is an

appropriate application for Computational Fluid

Dynamics (CFD) modeling.

The mixed effect of the natural convection flow and

residual flow from fluid momentum after the completion

of mold filling were numerically investigated5 and the

results clearly showed the necessity to carry out a coupled

filling and solidification analysis. Mampaey6 studied the

influence of mold filling and natural convection on cast

iron solidification using a combination of experimental

tracer technique and CFD modeling. These experiments

indicated that solute as well as precipitated graphite

nodules could be transported by the convection flow in

casting.

The objective of the study reported in this paper was

evaluation of the different possible processing routes for

production of iron castings with a controlled dual graphite

structure. The experimental methods were combined with

CFD modeling with the goal to control the location of

each of the desired structures.

PROCEDURES EXPERIMENTAL The four experimental heats, designated as A, B, C, and

D, were prepared using base pearlitic ductile iron melted

in 200-lb induction furnace from charge materials, which

included industrial ductile iron returns (Table 2). The melt

was heated to 1500C (2732F) and treated in a pocket ladle

by Fe-Si-Mg followed by FeSi base inoculants.

Chemistries of these heats in the ladle after treatment are

given in Table 3.

Table 2. Charge for Melting Ductile Iron

In charge Weight (lbs)

Plain-C steel disks 50

DI return 80

Induction Iron 20

Desulco 9001 3

Cu 0.8

Total induction furnace charge 153.8

Ladle treatment

FSM (46% Si, 5.7% Mg, 1% Ca, 0.4% La, 1% max Al,), 1-10 mm 2.4

Inoculants (75% Si, 4% Al, 1% Ca), 1-0.2 mm 0.8

Part of the magnesium-treated ductile iron melt was

substantially treated with sulfur for the development of a

flake graphite structure. Powder pyrite (FeS2) of size


Table 3. Chemistry (wt. %) of Experimental DI Metals in Ladle

Heat C Si Mn Cu S P Ni Mg

A 3.62 2.32 0.34 0.63 0.003 0.010 0.08 0.030

B 3.72 2.44 0.40 0.65 0.007 0.014 0.05 0.035

C 3.74 2.60 0.42 0.64 0.003 0.015 0.04 0.043

D 3.66 2.50 0.33 0.64 0.004 0.010 0.04 0.047

Fraction—100 mesh was used for sulfur post—

nodulization-treatment. A list of different experimental

techniques used in these heats is summarized in Table 4,

the reaction tundish design for Heat B, the casting designs

for Heat C with bottom gating system and the casting

design for Heat D with side gating system were illustrated

in Figs.1, 2 and. 3, respectively. The objective and

description of each heat will be discussed in detail in the

following portions of this paper.

Table 4. Experimental Heats Description

Heat Melt treatment

Mold, Casting Gating system

A In ladle 5 stationary molds with 1in. vertical plates

-

B In stream One stationary reaction tundish and 5 moving molds

with 1in. vertical plates

-

C

In stream

Mold with two 1in. horizontal plates and one 11/4in.

internal flat core

Bottom

D In ladle (double pour)

Mold with three 1in. horizontal plates and two 11/4in. internal flat core

Side

a)

b)

Fig. 1. This is the experimental setup of Heat B, used in-stream FeS2 feeding into stationary tundish with a bottom hole (a) and moving molds (b) for collection of treated iron.

Fig. 2. A casting design with a gating system having multiple bottom ingates (Heat C) is illustrated.

Fig. 3. A casting design with a gating system having two side ingates (Heat D) is illustrated.

Thermal analysis of ductile iron treated in the ladle

employed the adaptive thermal analysis system (ATAS).

The thermal cooling curves from castings were obtained

with K-type thermocouples protected by quartz tubes and

connected to a 24-bit National Instrument DAQ. No-bake

molds were placed on an electronic platform scale to

monitor pouring weight on a continuous basis. All heats

were also recorded as video. Chemistry of chilled buttons

was characterized using an arc spectrometer and a Leco

C-S determinator. Castings were cut and microstructure

was evaluated at different locations. Non-metallic

inclusion analysis was done using an Aspex automated

Scanning Electron Microscopy/Energy Dispersive X-ray

(SEM/EDX system.

COMPUTATIONAL Fluent CFD software was used for analysis of the effects

of mold pouring, post-filling momentum, melt natural

convection and casting solidification on sulfur and

magnesium distribution in the casting. The experimental

castings consist of two or three in. thick 10 in.x10 in.

plates with one or two 1-1/4in. thick 8 in.x8 in. internal

flat cores. Taking into consideration the small thickness to

length ratio (1:10), a 2-d computational domain was used

100

50

200

160

50

Ø18


to simplify the model and economize on computational

requirements. The transient, pressure-based solver with

absolute velocity formulation was used. Heat transfer,

volume of fluid (VOF) with turbulent flow modules were

coupled for modeling of mold pouring. Heat transfer,

laminar flow and solidification modules were coupled for

solving melt cooling with casting solidification. An

additional species transportation module with two

volumetric species including cast iron melt and melt with

magnesium in solution was also coupled for both, mold

filling and casting cooling models. This module was used

for tracing convection and diffusion transport of

magnesium in the melt during processing.

A piece-wise-linear approximation of cast iron density

was used for considering shrinkage in liquid and solid

conditions, as well as the volume increase during graphite

eutectic solidification.7 A simultaneous assumption of

rapid increase in melt viscosity at the moment of graphite

precipitation was applied in the cast iron property data set.

These coupled models allowed considering joint effects of

mold/gating system geometry, heat transfer and cast iron

solidification on transport of dissolved species. Therefore,

it was possible to evaluate the influence of melt mixing in

the mold on formation of dual graphite structure in the

casting.

EXPERIMENTAL RESULTS HEAT A The objective of this heat was to evaluate the effect of

post-treatment sulfur additions to an Mg-treated ductile

iron melt on the shape of graphite in a one-inch wall

thickness casting, solidified in a no-bake mold. Ductile

iron was produced by magnesium treatment and

inoculated in 200-lb ladle. The iron was poured into six

15-lb hand ladles with different amounts of FeS2. The

chemistry and the results of ATAS thermal analysis are

given in Table 5 and Fig.4. Transformation of

microstructure in magnesium treated iron by post-

treatment sulfur additions is shown in Fig. 5. Progressive

sulfur additions produced the expected transformation of

cooling curves in a series typical for iron with spheroidal

graphite, CGI and finally for gray iron with flake

graphite. The specific parameters, which increase, are

recalescence (R) and temperatures of eutectic reaction

(TELOW and TEHIGH). A small amount of post-treatment

sulfur addition (0.02-0.05% FeS2) increased the number

of spheroidal graphite nodules, counted by automated

SEM/EDX analyzer (Table 6). The mechanism of this

effect has been described in literature.8 An addition of

oxygen (CuO will be reduced by C and/or Si in the melt8)

to a large amount of FeS2 had no significant effect on

cooling curve parameters.

A graph of residual sulfur in the casting versus sulfur

additions to the ladle has two distinct regions (Fig. 6a).

First portion of the added sulfur reacted with dissolved

magnesium and part of the reaction product (MgS) floated

to the surface of the melt decreasing the amount of

remaining sulfur in the melt. After full neutralization of

magnesium, the slope of this graph increased because all

thebadded sulfur went into solution in the melt. The

experimental FeS2 additives for forming flake graphite

were close to calculated stoichiometry of MgS reaction. It

was also shown, that post-treatment sulfur addition

produced a relatively wider CGI stability window (from a

full vermicular structure without flake graphite to a

mixture of vermicular with 50% spherical graphite) when

compared to the typical narrow window (Mgresidual =0.015-

0.020) for magnesium treatment (Fig. 6c).

The non-metallic inclusion characteristics are given in

Table 7. In Mg-treated ductile iron, the main inclusions

were different types of complex Si-Al-Mg-Ca oxides with

a limited amount of Mg-Ca-sulfides. Post-treatment sulfur

addition significantly increased the amount of sulfide

inclusions, some of which were attached to flake graphite

(Fig. 7b). It is interesting to observe that sulfides were

mainly present as MgS in the base Mg-treated ductile iron

while complex Mg-Mn-S inclusions were present with

flake graphite in iron produced with post-treatment sulfur

additions (Fig. 7a).

Table 5. Chemistry of Metals and Critical Points on

Thermal Curves from Six Ladles

S Mg TL TELOW TEHIGH R

Base 0.002 0.03 1148.0 1140.1 1141.0 0.9

0.02% FeS2 0.008 0.034 1148.0 1139.8 1141.5 1.7

0.05% FeS2 0.012 0.029 1149.9 1142.9 1147.0 4.1

0.1% FeS2 0.022 0.028 1150.7 1142.9 1149.2 6.3

0.1% FeS2

+0.1%CuO 0.022 0.029 1151.8 1142.4 1148.6 6.2

0.14% FeS2 0.042 0.029 1149.9 1149.9 1156.8 6.9

a)

b)

Fig. 4. Effect of ladle post-treatment sulfur additions into magnesium treated iron (a) on the cooling curves) and (b) on the solidification parameters is graphed.

1120

1130

1140

1150

1160

1170

1180

0 50 100 150 200

Tem

pe

ratu

re,

0C

Time, sec

Base

0.02%FeS2

0.05%FeS2

0.1%FeS2

0.1%feS2+0.1%CuO

0.14%FeS2

0

1

2

3

4

5

6

7

8

1140

1145

1150

1155

1160

0 0.02 0.04 0.06 0.08

Re

ca

lesce

nce

, 0C

Te

mp

era

ture

, 0C

S addition, wt. %

TL

TELOW

TEHIGH

R


Fig. 5. These photomicrographs show the microstructures of iron castings from six ladles with post-treatment sulfur additions in the ladles.

Table 6. Quantitative Metallography of Graphite Nodules in Mg-Treated Irons with Post-Treatment

Sulfur Addition

Additions N, 1/mm2 D aver, µm

Base 145 24.0

0.02%FeS2 169 22.5

0.05%FeS2 199 20.3

Table 7. Non-Metallic Inclusions in the Castings

Additions Total #/mm2 Average D, µm

sulfides oxides sulfides oxides

Base 83 175 1.37 1.48

0.14 % FeS2

552 56 1.47 1.74

a)

b)

c)

Fig. 6. Effect of post-treatment sulfur addition in the ladle on (a) remaining sulfur and magnesium concentrations in iron, (b) shape of graphite in the castings and (c) typical graphite morphology in magnesium treated iron

10 are graphed.

a) b)

Fig. 7. (a) Composition (b) and backscattered SEM image of sulfide inclusions attached to graphite flake in iron with post-treatment sulfur addition are shown.

HEAT B The objective of this heat was to produce a sharp change

in graphite shape during pouring of Mg-treated melt by

post-treatment with in-stream sulfur injection during mold

0

0.01

0.02

0.03

0.04

0.05

0 0.02 0.04 0.06 0.08

Sin

ca

sti

ng

(w

t. %

)

S added in ladle (wt. %)

Sad [S][S] + [Mg] = {MgS}

Sad [S]

0

0.01

0.02

0.03

0.04

0.05

0 0.02 0.04 0.06 0.08

[S] a

nd

[Mg

] in

iro

n, w

eig

ht %

S addition in melt, weight %

[S][Mg]

FlakeCompactedNodule

Stoichiometry level


pouring. The percentage of FeS2 additions (> 0.1 wt.%)

for transformation of spheroidal graphite to flake graphite

was defined in the previous Heat A with post-treatment

with sulfur ladle additions. The experimental procedure

for heat B included melt pouring into a stationary tundish

with a bottom nozzle and melt collection into moving

molds as shown in Fig. 1. A graphite nozzle insert in the

stationary tundish with an 18 mm diameter hole was used

for controlling the fill rate near a constant—2 kg/sec. The

first three molds were poured using the base Mg-treated

ductile iron and after that the powder feeder was turned

on for in-stream post-treatment sulfur injection. A powder

feeder injected 2.8 - 3.0 g/s of FeS2 in-stream. In this

experiment, post filling mixing of treated and untreated

melts was prevented and in-stream post-treatment sulfur

injection, provided the desired sharp change in shape of

graphite from spheroidal to flake in sequentially cast

plates (Fig. 8).

Fig. 8. These are the microstructures of casted 1 in. thick plates in different molds: base and in-stream FeS2 injected melt.

HEAT C The objective of this heat was post-treatment in-stream

sulfur addition by injection during pouring the

experimental casting with a bottom gating system having

multiple bottom ingates (Fig. 2). Optimal post-treatment

in-stream sulfur injection parameters were verified in the

previous Heat B. The experimental casting consisted of

two horizontal plates with an internal flat core. The goal

was to produce graphite structure modification in the

bottom plate of the casting. According to this

experimental plan, the last portion of the melt filling the

bottom plate needed to be sulfur injected. Table 8 and Fig.

9 provide experimentally achieved pouring data obtained

from electronic scale and video recording.

Table 8. Pouring Process Data in Heat C

Pouring time, sec

Time to start FeS2, sec

FeS2 to melt, %

Total weight,

kg

Weight of treated part, kg

40.5 21 0.15-0.20 43.8 21

Fig. 9. Pouring weight and pouring rate in Heat C (blue arrow) indicate the starting time of FeS2 injection.

Samples taken from the ladle of the base Mg-treated iron

had spheroidal graphite structure. The last portion poured

into the mold was post-treatment iron with in-stream

sulfur injection. This latter iron was sampled from the

gating and had flake graphite. Thus, in this experiment, a

sharp change of melt chemistry was achieved by using

post-treatment in-stream sulfur injection. However,

entirely mixed CGI microstructures were observed at

multiple locations throughout the casting (Fig. 10). This

result indicated that mold filling mixing, post filling

momentum, natural convection in the mold before casting

solidification or some combination of these could be

responsible for producing mainly CGI structure in casting.

(a) (b)

(c) (d)

(e)

Fig. 10. These are micrographs of the microstructure in sections of experimental casting in Heat C: (a) in ladle, (b) gating system, (c) bottom plate, (d) middle region and (e) casting top.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60

Po

uri

ng

ra

te,

kg

/se

c

We

igh

t, k

g

Time, sec

Weight


HEAT D The objective of this heat was experimental verification of

formation of dual graphite structure in the casting when

all of the projected effects, which promoted melt mixing

in the mold were prevented. The procedure for this heat

was designed as an ideal hypothetical case in which

different layers of casting were poured sequentially: first

with gray iron then with ductile iron with an intermediate

time delay for partial cooling of the first layer to avoid

melt flow by natural convection and mixing. The time

delay was designed to allow development of a mushy

zone in the bottom gray iron layer before pouring ductile

iron on the top of gray iron. The casting for this heat had

three 1in. thick, 10 in. x 10 in. horizontal plates with two

1-1/4 in. x 8 in. x 8 in. internal cores each having two core

prints. To use two poured melts, the mold had two side-

type gating systems with gates placed in the parting plane

for one system and above parting plane for the other

gating system (Fig. 3).

Two melts, gray iron and Mg-treated ductile iron, were

prepared simultaneously and sequentially poured into the

mold positioned on electronic platform scale. ATAS

thermal analysis results for these two irons, sampled from

the ladles, are shown in Fig. 11 and chemistries are given

in Table 9. Temperature in the casting was monitored by

four thermocouples, installed on different heights. Gray

iron was poured first into the bottom plate to

approximately one inch thick and the ductile iron melt

which followed was poured with approximately 150 sec

delay to the point that eutectic solidification had started in

the bottom plate (Fig. 12a). Calculation using a computer

assisted thermal analysis technique,9 estimated the

fraction of solid as 0.4-0.5 in the first layer at that time.

Macro observations from multiple vertical cuts (Fig. 13 a)

clearly showed two layers in the casting with a seamless

boundary. The geometry of this boundary had some

shallow areas beneath the gate locations and showed

penetration of the ductile iron from the second pour into

the semisolid gray iron. Micro-observation showed a

straight local boundary layer with a narrow reaction zone.

Vermicular type of graphite particles occur as

protuberances into ductile iron (Fig. 13c), which originate

from the flake graphite. This boundary structure was

formed between the Mg treated iron and the base iron

without Mg and indicated that some amount of residual

Mg is needed to promote growth of vermicular shape

graphite. There were no observations of oxidized films or

indications of cold shut/cold lap. This observation showed

that the narrow boundary layer will have no negative

effect on thermal and mechanical properties on casting

with dual graphite structure.

a)

b)

Fig. 11. These graphs show the ATAS thermal analysis of cooling curves from (a) GI and (b) DI that are used in double poured Heat D.

Table 9. Chemistry of Two Metals Poured In Bottom and Top Parts of Casting (wt. %) in Heat D.

Sample C Si Mn Cu S P Ni Mg

GI melt 3.77 1.44 0.26 0.64 0.009 0.010 0.04 0.004

Casting bottom 3.59 1.46 0.22 0.54 0.007 0.010 0.03 0.006

DI melt 3.66 2.50 0.33 0.64 0.004 0.010 0.04 0.047

Casting top 3.00 2.30 0.16 0.30 0.004 0.010 0.02 0.040


a)

b)

Fig. 12. (a) The cooling curves are obtained from bottom and top parts of the double poured casting in Heat D and (b) the calculated solid fraction in bottom part of casting is indicated by arrow at the moment of second pouring.

a) b)

c) d)

Fig. 13. (a) This section of casting from Heat D indicates GI in bottom plate and DI in top part of casting. (b) This is a SEM image. (c) Not etched and (d) etched microstructures of the boundary show graphite shape transformation.

DISCUSSION

MODELING PREDICTION AND COMPARISON WITH EXPERIMENTS There are three possible melt mixing mechanisms. The

first mechanism involves melt mixing during mold filling.

Figure 14 illustrates the effect of the gating system on

predicted mixing during mold filling when the chemistry

of the iron was sharply changed during the pouring

process. In these figures, the red color represents the

highest percent Mg in the melt. In the bottom filled gating

system, ductile iron melt was delivered first and gray iron

(blue color) was poured last. The bottom filled casting

with multiple distributed gates provided better results as

compared to a two-gate bottom system (Figs. 14a, 14b).

However, the bottom gating system did not allow gray

iron melt to sequentially and fully replace the ductile iron

melt without intermixing, even at low inlet melt velocity.

The experimental results in Heat C (Fig. 10) confirmed

the intensive melt mixing in the casting when a bottom

gating system was used.

a) b)

c) d)

Fig. 14. These illustrations show the modeling of Mg distribution just after filling molds (a, b) through bottom and (c. d) and side gating systems with variations on ingate location (shown by black arrows).

Better results with less mixing during pouring were

predicted for a symmetrical side gating system (Fig. 14d).

In this case, gray iron was used first for pouring the

bottom part and ductile iron was poured second and

placed above the bottom layer. Experiments (Heat D)

confirmed that some penetration in the bottom layer could

take place by a falling stream from the side gates (Fig.

13a). Also, the gating system design must avoid any

“sloshing” effect from post filling momentum (the second

mixing mechanism) which could take place if the melt did

not enter the mold cavity symmetrically (Fig. 14c).

The third important mixing effect is natural convection

within the melt in the mold cavity. This effect is

illustrated in Fig. 15. It was assumed in this model that

the bottom layer of gray iron was poured ideally into the

lower part of the mold cavity and ductile iron was put on

the top without any post-filling momentum (Fig. 15a).

Two cases were modeled. In the first case, all of the melt

cooled and solidified in the no-bake mold. Modeling


predicted severe mixing by natural convection when the

casting solidified with complete destruction of the

initially layered structure (Fig. 15b). This result of severe

mixing was observed in Heat C. The mixing due to free

convection could be suppressed significantly if a bottom

chill was applied and the bottom layer of casting

consequently froze rapidly (Fig. 15c). Rapid melt cooling

shortens free convection flow time and concurrently and

quickly develops a mushy zone which increases melt

viscosity. These factors can significantly suppress mixing

due to free convection of the melt.

a)

b) c)

Fig.15. These illustrate the changing Mg-distribution by free melt convection and post filling momentum during melt cooling in mold before casting solidification: (a) after pouring, (b) before solidification in sand mold and (c) using bottom chilling.

PROCESS CLASSIFICATION AND POSSIBLE PROCESS ROUTES Development of dual graphite structure in the same

casting is a serious challenge due to the requirement to

suppress “natural” melt mixing during mold pouring and

casting solidification. A process chart was developed

based on the experiments and the modeling study

discussed, previously (Fig. 16). The process includes

three stages which need to be specifically designed:

development of dual chemistry; sequential mold pouring

using different melt compositions and finally melt cooling

and solidification to prevent melt convection. The bottom

part of the chart indicates the three possible mixing

mechanisms which are deleterious to development of a

controlled dual graphite structure. These mechanisms

include: mixing during pouring, mixing by post-pouring

momentum and mixing by convection. The right side of

this chart contains different possible techniques which can

be exploited for these process stages. For example, dual

chemistries could be produced using two melts (Heat D),

in-steam treatment (Heat C) or so called in-situ techniques

which are the subject of ongoing research. Different

gating system designs included: a two stream system

(Heat D), a bottom-gating system (Heat C) where the

sulfur-treated last liquid metal poured would conceptually

push the first portion of poured metal up to the top; and a

side gating system which allows pouring one liquid metal

on the bottom of casting and placing other metal on the

top. Also, other different combinations can be designed

for specific casting geometry and location in the mold.

Finally, different methods are possible for suppressing

melt mixing in the mold. In Heat C, uncontrolled casting

solidification in no-bake mold created intensive melt

mixing. Freezing of the bottom layer of gray iron to a

mushy condition in Heat D allowed clearer separation of

the dual graphite structure. A more practical way to

control solidification could be by applying internal

or/external chill. A combination of these techniques can

be designed using the suggested process chart.

Fig. 16. Process chart for different routes for producing of dual graphite structure in cast iron casting is illustrated.

CONCLUSIONS

Development of dual graphite structure in the same

casting has potential for future application of cast iron.

Post-treatment of some proportion of the melt by in-

stream sulfur injection is sufficient to achieve controlled

dual graphite structure. This study showed that the main

technological problem of development dual graphite

structure is extensive liquid metal mixing during pouring,

and as a result of post filling momentum and natural

convection. Four different techniques were

experimentally tested and modeled. Both of the

microstructure and macrostructure examinations indicated

the effect of different mixing mechanism on formation of

dual graphite structure in casting.

From the results achieved by so far, mainly CGI structure

was observed throughout the casting poured through a

bottom gating system because of severe mixing. In this

case, pouring mixing, post pouring momentum and

natural convection are all possible mechanisms resulting

in undesired mixing structure. However, “double pour”

and partially freezing of the first poured liquid metal

prevented both pouring and post pouring mixing to some

extent and showed very sharp boundary between DI and

GI.


The classification of different process routes with the goal

of suppressing melt mixing was suggested in the process

chart. Based on the process chart, the different ways for

preventing mixing such as directional solidification by

chilling, in-situ sulfur treatment by localized reaction and

a side gating system, which impaired pouring mixing, can

be used for the development of dual graphite structure in

casting.

ACKNOWLEDGMENTS Research was sponsored by Benet Laboratories on behalf

of the US Army Contracting Command Joint Munitions

and Lethality Contracting Center—and was accomplished

under Cooperative Agreement Number W15QKN-11-2-

0001. The views and conclusions contained in this

document are those of the authors and should not be

interpreted as representing the official policies, either

expressed or implied, of AFS or Benet Laboratories or the

U.S. Government. The U.S. Government is authorized to

reproduce and distribute reprints for Government

purposes notwithstanding any copyright notation heron.

The authors would like to thank US Army ARDEC-Benet

laboratories for funding this research. The authors wish to

acknowledge technical support and discussion input of

George Kokos, James Barlow and Zhiping Lin from

Caterpillar. The authors wish to recognize the assistance

of Forrest Huebner, Clinton Ratliff, John Stanek, Jeremy

Robinson, Bradley Bromet, Ian Christian and James

Smoot for heat pouring and sample preparation.

REFERENCES

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