HIGH STRENGTH DUCTILE IRON PRODUCED BY ENGINEERED COOLING:PROCESS CONCEPT

Simon N. LekakhMissouri 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.

Keywords: High-strength ductile iron, engineered cooling, thermal simulator techniques, Computational Fluid Dynamics (CFD), tensile strength

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 thermo­physical 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

International Journal ofMetalcasting/Volume 9, Issue 2, 2015

cooling parameters, such as rapid undercooling of aus­tenite combined with isothermal holding at 35(M-20°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.

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.

21

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.

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.

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).

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 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 (p) 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 p 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.

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).

An example of the final structure along with the p 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.

22 International Journal o f Metalcasting/Volume 9, Issue 2, 2015

Te

mp

era

ture

, °C

Table 1. Chemistry and Mechanical Properties o f Industrial Ductile Iron

Chemistry, wt. % Mechanical PropertiesC Mn Si Cu UTS, psi YS, psi Elong. % HB

3.77 0.47 2.33 0.39 110 000 63 000 8.8 212

0 .2 ( c )

Figure 2. As-cast microstructure o f industrial ductile iron: (a) pearlite/ferrite matrix, (b) lamelar pearlite structure, and (c) two- and three-dimensional graphite nodule diameter distributions.

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.

International Journal ofMetalcasting/Volume 9, Issue 2, 2015 23

p (r

elat

ive)

250

230

210

170

110

20C 300 400 900 1000500 600 700Temperature, °C

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.

Temperature, °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.

24 International Journal ofMetalcasting/Volume 9, Issue 2, 2015

Har

dn

ess

(HB

)

Cooling rates above 10°C/s (18°F/sec) suppresses the eu- tectoid 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 (M and Mf) an additional dilatom- etry test was performed (Fig. 5b). In this test, the specimen was reheated in a quartz fixture by high frequency induction power, while displacement was measured by a sub-micron precision laser triangulation sensor.

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.

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 ausfer- rite/fine pearlite developed (Fig. 7b). A change in electrical

600 (a)

Figure 6. (a) Effect of cooling rate on hardness and (b) the

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.

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.

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.

450 (b)

1500 5 10 15 20

C ooling rate, C/s

microhardness of individual phases.

International Journal o f Metalcasting/Volume 9, Issue 2, 2015 25

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).

— 0.3 Cls

— 1 C/s

— 2 C/s

— 5 C/s

— 10 C/s

— 20 C/s

10000

Figure 8. Achievable structures by applying continuous cooling to room temperature and to isothermal holding temperature (380°C/716°F) at different cooling rates.

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 comer of a 6 x 8 x 1” plate casting. The simulated “soft” cooling methods included cool­ing in still air and with forced air 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

26 International Journal ofMetalcasting/Volume 9, Issue 2, 2015

Table 2. Chemistry of Laboratory Ductile Iron, wt. %.

c Mn St Cu3 .6 5 0 .55 2 .3 6 0 .55

900 (b ) 5900 Forced air-water . Forced air (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.

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.

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 (Fc46Si6.1M glCalLaO.lAl) and inoculated by Superseed® (FelOSiOAAlOACalSr). 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).

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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 'A” 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).

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

27

Coo

ling

rate

, C

/sec

F ig u re 10. (a) E x p e rim e n ta l m o ld s w ith v e rtic a l 6 x 8 x 1 ” p la te s a n d top r is e rs a n d (b ) th e rm a l c u rve s c o lle c te d fro m m o ld c o o le d re fe re n c e c as tin g s (in s e rte d th e rm o c o u p le s TK1 a n d T K 2 -re d a n d b lu e ) a n d fro m e n g in e e re d c o o le d c as tin g s (in fra re d p y ro m e te r s u rfa ce te m p e ra tu re -b la c k ).

— E n g in e e re d c o o lin g — A s c a s t

F ig u re 11. T en s ile tes ts : true s tre s s -s tra in c u rv e s o f b a s e (a s -c a s t, m o ld c o o le d ), a n d e n g in e e re d co o led , la b o ra to ry p ro d u c e d d u c tile irons.

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.

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

F ig u re 12. M ic ro s tru c tu re a n d te n s ile s u rfa c e fra c tu re o f m o ld c o o le d (a) a n d e n g in e e re d c o o le d (b), la b o ra to ry p ro d u c e d d u ctile irons .

28 International Journal o f Metalcasting/Volume 9, Issue 2, 2015

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 to 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 Rum- mel, 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).

Technical Review & Discussion

High Strength Ductile Iron Produced by Engineered Cooling: Process ConceptSimon N. Lekakh, Missouri University of Science and Technology, Rolla, MO, USA

Reviewer: The “Thermal Simulator” introduced by the author is another way to obtain the transformation curves. However, the specimens and heating are unusual. The ex­periments in which real sand molds are used are really just another way of conducting quench experiments.

Author: There are many methods which can be used to study phase transformations during ductile iron cooling, including cooling castings with different wall thicknesses in sand molds. In this case, the structure o f the metal ma­trix will be affected by the cooling rate during the eutectoid reaction (wall thickness, mold properties) and will also be dependent on the prior solidification structure. The kinet­ics o f solid!solid reactions is primarily controlled by car­bon diffusion. There are two parameters which will have an effect on perlite/ferrite ratio: diffusion time, related to the cooling rate, and diffusion distance, related to near­est neighboring distance between graphite nodules formed during solidification.

When the “Thermal Simulator" method is applied, these factors, which are important to understanding reaction ki­netics in actual castings, can be easy differentiated. Addi­tionally, the electrical resistivity supplied to the specimen electrical power provide important information about trans­formation rate. It is agreed that other methods, for example interrupted quenching, can also be used for reaction kinetic study. However, interrupted quenching is significantly more

International Journal ofMetalcasting/Volume 9, Issue 2, 2015

time consuming. Another concern is that specimens used in the “Thermal Simulator” are subjected to a second reheat­ing, which is “metallurgically" different than just after ear­ly shake out. The potential for possible discrepancies was checked by testing a variety o f reheating temperature!time cycle parameters.

Reviewer: The results of water being sprayed on the cast­ings after early shakeout (Engineered Cooling), were just as expected with the tensile strength being higher and elongation being half that of plates that were cooled in the mold.

Author: The term “Engineered Cooling” was introduced to emphasize that previously studied methods, including “sim­ple water spray” and “direct quench o f casting in water” af­ter early shakeout are not working well. These processes can introduce large thermal stress and, more importantly, will result in undesirable hard and brittle martensitic structure (Ms point o f ductile iron above 200°C, Fig. 8). Engineered cooling allows us to control three important parameters: (i) an overall cooling rate favorable for developing a desirable microstructure, (ii) the avoidance o f rapid chilling below Ms temperature and martensite formation, and finally (Hi) the temperature gradient in the casting wall. The combination of these parameters (referred to as the “process window”) is very important to get the best possible combination of strength and ductility. These parameters will be optimized in the future.

Reviewer: Warpage of the plates due to the early shakeout and water quench is a common problem with ductile iron castings that are shaken out red hot such that they distort significantly depending on the part geometry and variations in section thickness.

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Author: To some extent, any accelerated cooling will always introduce the additional thermal stresses. The AFS steering committee and Research Board were also concerned about these problems and suggested that the induced thermal stress should be verified in Phase 2 o f this project. Special methods (spray system design, optimal droplet size, comput­erized flow control, and others) will allow extraction o f a specific amount o f heat at a rate required to move solid! solid reaction kinetics in a desirable direction and subsequently, changes can be made in cooling regimes to decrease ther­mal stress. The engineered cooling method provides an ad­ditional opportunity for control o f the produced structure at minimal resulting thermal stresses.

Reviewer: The author makes some comparisons to austem- pered ductile iron in that tensile strength can be matched. However, with austempered ductile irons, elongation is much better than that observed with the engineered cooling parts.

Author: “Thermal simulator" tests revealed the process­ing window parameters needed to achieve fully ausferritic or various mixed structures having different combinations of fine pearlite with ausferrite. The question is: “What can we achieve by applying the Engineered Cooling process to real castings?" Recently published results, cited in the in­troduction, showed a possibility to achieve a fully ausfer­ritic structure by early shakeout and air cooling to a par­ticular temperature. However, these results were achieved

30

in 3% Ni ductile iron. In this study, a mixed structure with some amount o f ausferrite was developed in a plain ductile iron casting with 1 ” wall thickness subjected to Engineered Cooling. In Phase 2, exploration o f the structures which can be developed through applying Engineered Cooling to unal­loyed and low alloy compositions will take place for cast­ings with different wall thicknesses. The combination o f the “Thermal Simulator”, CFD modelling, and the computer controlled cooling system will be used in that study.

Reviewer: The practicality of shaking out parts early in most foundry operations and then spraying the parts with water is difficult to accept. This will require some special plant engineering to overcome.

Author: This article presents the results o f Phase 1 of the AFS funded project, which was aimed to prove a process concept. It is agreed, the actual implementation will need serious plant engineering to overcome practical difficul­ties, including design o f tailored equipment for each specific metal casting practice. However, if successful, the proposed process can be used for the production o f high strength duc­tile iron in an as-cast condition without a need for alloying. The process could shorten processing time, eliminate the need to re-heat castings, and potentially have many other technical and economic advantages. In Phase 2, there will be an investigation o f the important issues pointed out in this review.

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