high strength ductile iron produced by the …
TRANSCRIPT
IJMC14-2440-2
HIGH STRENGTH DUCTILE IRON PRODUCED BY THE ENGINEERED COOLING: PROCESS CONCEPT
Simon N. Lekakh
Missouri University of Science and Technology, Rolla, MO, USA
Copyright © 2015 American Foundry Society
Abstract
Traditionally, high strength ductile irons are produced by a combination of alloying and heat treatment, both
operations substantially increase the cost and carbon footprint of casting production. In this study, the concept of a
process for the production of high strength ductile iron using engineered cooling is discussed. The process includes
early shakeout of the casting from the mold and application of a specially designed cooling schedule (engineered
cooling) to develop the desired structure. The high extraction rate of internal heat is achieved by controlling the
thermal gradient in the casting wall and the surface temperature. Experimental “Thermal Simulator” techniques and
Computational Fluid Dynamic (CFD) simulations were used to design the cooling parameters. The concept was
experimentally verified by pouring plate castings with 1” wall thickness and applying the engineered cooling
techniques. The tensile strengths of ductile iron increased from 550‒600 MPa for castings solidified in the mold to
1000‒1050 MPa after engineered cooling.
Introduction
The majority of industrially produced ductile iron castings have an as-cast microstructure consisting of graphite
nodules distributed in a ferrite/pearlite metal matrix. This microstructure is formed during solidification (primary
structure) and the subsequent eutectoid reactions, which control the metal matrix structure. The current state-of-the-
art cast iron industrial processes control the mechanical and thermo-physical properties through the primary
solidification structure by:
- variation of carbon equivalent for controlling the primary austenite/graphite eutectic ratio
- inoculation for promoting graphite nucleation and decreasing chill tendency
- using a magnesium treatment for modifying graphite shape (flake in GI [graphite iron], vermicular in CGI
[compacted graphite iron] and spherical in SGI [spheroidal graphite iron])
- melt refining to remove dissolved impurities (S, O, N)
- melt filtration for improving casting cleanliness
Only one method, alloying with additions of Cu, Mo, Ni and other elements, is practical for the direct control of the
metal matrix structure formed during the eutectoid reaction. The disadvantages of additional alloying include: (i) the
high cost of additions and (ii) a limited ability to increase strength in the as-cast condition. Acceleration of cooling
during the eutectoid reaction can produce a similar effect on the metal matrix structure. Furthermore, special cooling
parameters, such as rapid undercooling of austenite combined with isothermal holding at 350‒420°C (662‒788°F)
can produce an ausferrite, or bainite structure with increased strength and toughness. Currently, an additional
austempering heat treatment is used to produce such austempered ductile iron (ADI) castings.
Several different ideas involving integrating rapid cooling into the metal casting process in order to increase strength
without requiring an additional heat treatment have been discussed during the last few decades in the metalcasting
community. Recently, ductile iron with an ausferrite structure was produced in the as-cast condition by a
combination of alloying by 3‒5% Ni, early shakeout, and air cooling to the isothermal bainitic transformation
temperature.1 This process produced material strengths in the as-cast condition similar to an additionally heat treated
ADI; however, such a high level of alloying substantially increases casting cost.
The objective of this study was the development of a process for the production of high-strength ductile iron in the
as-cast condition, eliminating both alloying and additional heat treatment.
IJMC14-2440-2
Process Concept
The final microstructure of cast iron is very sensitive to the cooling profile during eutectoid transformation because
this solid state reaction is controlled by the carbon diffusion rate. Generally, sand (green and no-bake) mold
processes have limited ability to control the cooling rate during the eutectoid reaction due to restricted heat flux
from the casting into the low thermal conductivity mold. In this case, the formation of fine products during the
eutectoid reaction and/or stabilization of retained austenite by undercooling, allowing bainitic transformations, are
restricted by a slow cooling rate. In Figure 1, the blue line on the continuous cooling transformation diagram
schematically represents a cooling pass resulting in the ferrite/pearlite structure formed in sand mold castings. If the
higher strength of ductile iron castings produced in sand molds is required, alloying is used to stabilize the
undercooled austenite. Alloying moves the transformation curves further to the right allowing a fully pearlitic
structure to be formed at the lower cooling rate.
Employing a specially designed cooling schedule (engineered cooling) during solid state transformations allows
control of the structure without needing to alter the alloy chemistry. The various high strength products of the solid
state reaction could be formed in lean ductile iron during the decomposition of undercooled austenite. The
combination of high carbon concentration in austenite and the suppression of carbon diffusion by high cooling rate
stabilizes the undercooled austenite. Under these conditions, carbon has a major role as an alloying element. The
possible structures, achievable by engineered cooling, are shown schematically by the red lines in Fig. 1.
Figure 1. Illustration of phase transformations in ductile iron castings in sand mold (blue line) and in the studied process of engineered cooling (red lines).
The key feature of the studied engineered cooling process is a seamless integration of the desired cooling profile into the
casting process, combining early shakeout (at a temperature above eutectoid transformation) and controlled cooling after
to maximize strengthening. This paper addresses: (i) optimization of engineered cooling process parameters for
creating high strength ductile iron and (ii) an experimental test to prove the process concept.
Experimental Simulations Using Engineered Cooling
In order to experimentally simulate the different engineered cooling scenarios, a special device called a “Thermal
Simulator” was developed. Small test specimens (2” x 0.25” x 0.15”), machined from the ductile iron castings
received from the casting industry, were subjected to a heating/cooling cycle. The specimen heating was performed
by a computer controlled high ampere DC current power supply. Temperature measurement was done by a
thermocouple welded on to the hot zone and a high-precision infrared pyrometer with a 1 mm spot size. The
compressed air used in the cooling loop was controlled by a proportional electromagnetic valve. These two
controlling loops (heating and cooling) in combination with the small thermal inertia of the test specimen allowed
IJMC14-2440-2
for reproduction of any cycle with up to 80°C/sec (144
° F/sec) heating and cooling rates. The “Thermal Simulator”
measured the electrical resistivity (ρ) of the specimen (a structure sensitive physical property) and the supplied
electrical power (W) at constant heating or cooling rate (a parameter sensitive to the heat of phase transformation,
similar to scanning calorimetry test). A combination of ρ and W measurements was used to determine the phase
transformation temperatures and kinetics.
The test specimens were machined from industrially produced 6” x 8” x 1” plate castings, with the
chemical composition of the major elements shown in Table 1. Mechanical properties obtained from the
round standard bars are also given in this table. The as-cast pearlite/ferrite microstructure and two- and
three- dimensional2 graphite nodule diameter distributions are shown in Fig. 2.
Table 1. Chemistry and Mechanical Properties of Industrial Ductile Iron
Chemistry, wt. % Mechanical Properties
C Mn Si Cu UTS, psi YS, psi Elong. % HB
3.77 0.47 2.33 0.39 110 000 63 000 8.8 212
a) b) c)
Figure 2. As-cast microstructure of industrial ductile iron: (a) pearlite/ferrite matrix, (b) lamelar pearlite structure, and (c) two- and three-dimensional graphite nodule diameter distributions.
These ductile iron specimens were subjected to heating and cooling cycles designed to simulate engineered cooling.
The original as-cast structure was restored by heating in order to saturate the austenite with carbon and prevent the
homogenization of substitutional elements. It is well known that negative segregation of Si and positive segregation
of Mn occur during solidification and influences the metal matrix structure formed during eutectoid reaction. Two
types of heating cycles were studied: (a) heating to austenization temperature (920°C/1688
°F), 5‒30 minutes holding
for saturation of austenite by carbon, and continuous cooling with 0.3‒20°C/sec (0.54‒36
° F/sec) cooling rate to
room temperature (Fig. 3a) and (b) isothermal treatment, including the same austenization heating schedule followed
by 2‒20°C/sec (3.6‒36
° F/sec) cooling to 60 minutes isothermal hold at 380
°C (716
°F) and fast cooling to room
temperature (Fig. 3b).
a) b)
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1000
0 500 1000 1500 2000 2500 3000 3500
Tem
pera
ture
, 0C
Time, sec
20 C/s
10 C/s
5 C/s
2 C/s
1 C/s
0.3 C/s
0
100
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1000
0 1000 2000 3000 4000 5000 6000
Te
mp
era
ture
, 0C
Time, sec
20 C/s
10 C/s
5 C/s
5 C/s (a)
2 C/s
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Figure 3. Heating and cooling cycles used to simulate engineered cooling: (a) continuous cooling and (b) isothermal heat treatment after different cooling rates from austenization temperature.
An example of the final structure along with the ρ and W curves obtained during the heating and 2
°C/s (3.6
°F/sec)
continuous cooling of industrial ductile iron is shown in Fig. 4. Both the change of slope on the electrical resistivity
curve and the positions of the peaks on the power curve indicate the transformation temperature. The electrical
resistivity was also used to validate transformation times during the isothermal holds.
a) b)
Figure 4. (a) Electrical resistivity and power curves during heating and 2°C/s (3.6° F/s) continuous cooling
cycle (shown by arrows) and (b) fine pearlite microstructure after test.
Cooling rates above 10
°C/s (18
° F/sec) suppresses the eutectoid reaction controlled by carbon diffusion (Fig. 5a)
which allows undercooled austenite to transform directly into martensite by displacement or diffusionless
mechanism (Fig. 5c). In order to verify martensitic transformation start and finish temperatures (Ms and Mf) an
additional dilatometry test was performed (Fig. 5b). In this test, the specimen was re-heated in a quartz fixture by
high frequency induction power, while displacement was measured by a sub-micron precision laser triangulation
sensor.
a) b)
-10
-5
0
5
10
15
20
25
30
35
100 150 200 250 300
ΔL
/L x
10
4
Temperature, 0C
MF
MS
IJMC14-2440-2
c)
Figure 5. (a) Electrical resistivity and power curves, (b) dilatometric curve showing martensitic
transformation, and (c) quenched martensitic microstructure after 10°C/s (18°F/sec) continuous cooling to
room temperature.
Continuous cooling. Cooling rate had a significant effect on the macro-hardness of the ductile iron (HB, black lines
in Fig. 6a) by changing the volumes of phases and the phases' internal structure and microhardness (HV, black lines
in Fig. 6b). Cooling rates up to 2°C/s (3.6
° F/sec) increase the volume and microhardness of pearlite. Cooling rates
from 2‒10°C/s (3.6‒18
° F/sec) exhibit a sharp increase of hardness, mainly because of the formation of quenched
martensite with 550‒600 HV microhardness. These continuous cooling experiments showed the limitations, as too
high of a cooling rate resulted in undesirable martensitic transformation.
a) b)
Figure 6. (a) Effect of cooling rate on hardness and (b) the microhardness of individual phases.
Isothermal treatment. In order to develop microstructures that would provide a combination of high strength and
toughness, isothermal heat treatments were investigated using the “Thermal Simulator.” The industrial ductile iron
specimens were heated to 920°C (1688
°F), cooled to 380
°C (716
°F) at different cooling rates, and isothermally held
for 60 minutes. The effect of austenite carbon saturation was verified by increasing the holding time at 920°C
(1688°F) from 10 to 30 minutes for one experiment. In the studied ductile iron, a cooling rate above 2
°C/s (3.6
°
F/sec) promoted the localized formation of ausferrite in interdendritic regions. At 5°C/s (9
° F/sec) cooling rate a
mixture of ausferrite/fine pearlite developed (Fig. 7b). A change in electrical resistivity indicated that ausferrite
formation was complete at 25 minutes during 380°C (716
°F) isothermal holding (Fig. 7a). At 10
°C/s (18
° F/sec)
cooling rate an ausferrite structure with small local pearlite spots around graphite nodules developed (Fig. 7c);
however, the increased austenite carbon saturation time at 920°C (1688
°F) promoted the stability of undercooled
austenite at a lower cooling rate and resulted in larger ausferrite volume.
200
250
300
350
400
450
500
550
600
0 5 10 15 20
Hard
ne
ss (
HB
)
Cooling rate, C/s
Continuous cooling
Isothermal
As cast150
200
250
300
350
400
450
0 5 10 15 20
Mic
roh
ard
ne
ss
(H
V)
Cooling rate, C/s
Upper bainite
Pearlite
Ferrite
As cast
IJMC14-2440-2
a) b) c)
Figure 7. Microstructures of ductile iron at 920°C (1688°F)for 10 minutes, followed by cooling at (a) 5
°C/s (9
°
F/sec) and (b) 10°C/s (18
° F/sec) to 380
°C (716°F) and 60 minutes isothermal hold; (c) electrical resistivity
during isothermal holding at 380°C (716°F) after 5
°C/s (9
° F/sec) cooling rate from 920
°C (1688°F).
Figure 8 summarizes the achievable microstructures after continuous cooling to room temperature and cooling to
isothermal hold temperature (380°C/716
°F) at different cooling rates. A minimum cooling rate of 2
°C/s (3.6
° F/sec) is
required to achieve the fine pearlite structure in the ductile iron investigated. At higher cooling rates, a mixture of
fine pearlite and ausferrite can be formed by isothermal holding above the Ms temperature. Based on these
experimental studies, a range of process parameters for engineered cooling were suggested.
Figure 8. Achievable structures by applying continuous cooling to room temperature and to isothermal holding temperature (380°C/716°F) at different cooling rates.
Design Engineered Cooling
Computational fluid dynamic (CFD) simulations and experimental tests were used to design the engineered cooling
parameters. Thermal experiments involved reheating and cooling 1 x 6 x 8” industrial ductile iron plates. Three
parameters were considered: (i) cooling rate, (ii) temperature gradient in the casting wall, and (iii) surface
temperature. Based on a structure diagram (Fig. 8), the target cooling rate was above 2°C/s (3.6
° F/sec).
Minimization of the thermal gradient in the casting wall was also important to achieve a consistent structure and
prevention of thermally induced stress. Finally, to prevent a martensitic structure, the surface temperature needs
remain above the Ms point during cooling. Considering the real three-dimensional casting geometry, these
requirements substantially complicate an engineered cooling system design.
Computational fluid dynamic simulations (FLUENT software) was used to predict the effect of different cooling
methods on the temperature profiles in the center, on the surface in the middle of the large face, and at the corner of
a 6 x 8 x 1” plate casting. The simulated “soft” cooling methods included cooling in still air and with forced air
0
100
200
300
400
500
600
700
800
900
1000
1.5
1.55
1.6
1.65
1.7
1.75
1.8
0 500 1000 1500 2000 2500 3000 3500
Te
mp
era
ture
, 0C
p (
rela
tive
)
Time, sec
p
T
8 min25 min
100
200
300
400
500
600
700
800
1 10 100 1000 10000
Tem
pera
ture
, 0C
Time, sec
0.3 C/s
1 C/s
2 C/s
5 C/s
10 C/s
20 C/s
Ferrite
Pearlite
Martensite
Ausferrite
IJMC14-2440-2
convection. The heat transfer coefficients chosen for these cooling methods were 5 and 70 W/m2K, respectively.
Radiant heat transfer from the cast iron surface with 0.8 emissivity was also considered in these simulations. It was
seen (Fig. 9a and Fig. 9b) that these “soft” cooling methods do not provide the required cooling rate to achieve the
ausferrite structure in ductile iron castings with 1” wall thickness. On the contrary, an intensive water-spray cooling
method provides a high enough cooling rate, but significantly increases the temperature gradient in the casting and
quickly decreases the surface temperature below the Ms temperature. To optimize the cooling, a computer assisted
engineered cooling method was designed using wide angle water/compressed air atomizer nozzles with controllable
cooling intensity. An example of a simulated case is shown in Fig. 9c. Surface temperature feedback was used for
cooling control in these simulations. The simulated method provides the required cooling rate with a limited thermal
gradient and guarantees the surface temperature remains above the required level.
a) b) c)
Figure 9. Computational Fluid Dynamics (CFD) simulated cooling 1 x 6 x 8” ductile iron plate applying: (a) still air cooling, (b) compressed air cooling, and (c) engineered cooling with wide angle water/compressed air atomizer nozzles.
Experimental Verification of Engineered Cooling
A laboratory experimental heat conducted in a 100 lb. induction furnace with a charge consisting of
ductile iron foundry returns, pure induction iron ingots, and Desulco carbon. The melt was treated in the
ladle by Lamet 5854 (Fe46Si6.1Mg1Ca1La0.7Al) and inoculated by Superseed® (Fe70Si0.4Al0.1Ca1Sr).
The laboratory ductile iron chemistry (major elements) is given in Table 2 and is similar to industrial
ductile iron used for thermal simulation tests (Table 1).
Table 2. Chemistry of Laboratory Ductile Iron, wt. %.
C Mn Si Cu
3.65 0.55 2.36 0.55
Four no-bake sand molds with vertical 1 x 6 x 8” plates with top risers were poured (Fig. 10a). Two
reference plates had K-type thermocouples (protected by a quartz tube) and were mold cooled
(base process). The two other molds had an investment ceramic coated ½” rod in the riser sleeve for
transferring castings to the cooling device. These two castings had early shakeout and were subjected to
engineered cooling (Fig. 10b).
a)
0
1
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0 200 400 600 800
Co
oli
ng
rate
, C
/sec
Tem
pera
ture
, 0C
Time, sec
Center
Surface
Corner
Cooling rate (center)
25C
0
1
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Co
oli
ng
rate
, C
/sec
Tem
pera
ture
, 0C
Time, sec
Center
Surface
Corner
Cooling rate (center)
34C
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oli
ng
rate
, C
/sec
Te
mp
era
ture
, 0C
Time, sec
Center
Surface
Corner
Cooling rate (center)
35C
82C
Forced air-water Forced air
IJMC14-2440-2
b)
Figure 10. (a) Experimental molds with vertical 6 x 8 x 1” plates and top risers and (b) thermal curves collected from mold cooled reference castings (inserted thermocouples TK1 and TK2‒red and blue) and from engineered cooled castings (infrared pyrometer surface temperature‒black).
The achieved mechanical properties, microstructure, and SEM image of fractured tensile bar are shown in
Fig. 11 and Fig. 12 for the two cases: mold cooled casting (base) and engineered cooled casting. The base
casting had a structure of lamellar pearlite with 10‒15% ferrite. After engineered cooling, the structure
was a mixture of ausferrite and fine pearlite. Engineered cooling nearly doubled the tensile strength of
ductile iron from 550‒600 MPa at 8 % elongation to 1000‒1050 MPa at 4% elongation in the as-cast
condition. The tensile fracture surface of engineered cooled ductile iron had a smaller amount of exposed
graphite nodules, indicating the crack propagated through the matrix surrounding the graphite nodules
which created “domes” in the fracture surface.
Figure 11. Tensile tests: true stress-strain curves of base (as-cast, mold cooled), and engineered cooled, laboratory produced ductile irons.
a)
0
200
400
600
800
1000
1200
0 0.02 0.04 0.06 0.08 0.1
Str
es
s,
Mp
a
Strain, mm/mm
Engineered coolingAs cast
IJMC14-2440-2
b)
Figure 12. Microstructure and tensile surface fracture of mold cooled (a) and engineered cooled (b), laboratory produced ductile irons.
Conclusions
The concept of a process for the production of high strength ductile iron in the as-cast condition by
applying engineered cooling was discussed. The process includes the early shakeout and specially
engineered cooling control to develop the desired structure. The “Thermal Simulator” experimental
technique and CFD simulations were used to investigate potential process parameters for the investigated
industrial ductile iron. It was shown that a cooling rate above 2°C/s (9° F/sec) was needed to achieve
ausferrite formation. The process parameters were experimentally verified by pouring 1” thick plate
castings and subjecting them to engineered cooling after early shakeout. The tensile strength of ductile
iron was increased from 550‒600 MPa for mold cooled castings to 1000–1050 MPa for castings subjected
to engineered cooling. We will continue develop the process in Phase 2 of this project.
Acknowledgements
The author gratefully acknowledges the funding and support that has been received from: The American
Foundry Society; AFS Steering Committee: Mike Riabov (Elkem), Matt Meyer (Kohler Co.), Eric Nelson
(Dotson Iron Castings), and Don Craig (Selee); Missouri University of Science and Technology:
Professor Von Richards, Students: Seth Rummel, Antony Michailov, Jeremy Robinson, Mingzhi Xu,
Jingjing Quig, Joseph Kramer
References
1. de La Torre, U., Stefanescu, D.M., Hartmann, D., and Suarez, R., “As-cast Austenitic Ductile Iron,”
Keith Millis Symposium on Ductile Iron, AFS (2013).
2. Lekakh, S., Qing, J., Richards, V., Peaslee, K., “Graphite Nodule Size Distribution in Ductile Iron,”
AFS Transactions (2013).