HIGH STRENGTH DUCTILE IRON PRODUCED BY THE ENGINEERED COOLING: PROCESS CONCEPT

Transcription

1 IJMC HIGH STRENGTH DUCTILE IRON PRODUCED BY THE ENGINEERED COOLING: PROCESS CONCEPT Copyright 215 American Foundry Society Abstract Simon N. Lekakh Missouri University of Science and Technology, Rolla, MO, USA 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 55 MPa for castings solidified in the mold to 1 15 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-theart 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 C ( 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.

2 IJMC 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.25 x.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

3 Temperature, C Temperature, C IJMC for reproduction of any cycle with up to 8 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- dimensional 2 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 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 (92 C/1688 F), 5 3 minutes holding for saturation of austenite by carbon, and continuous cooling with.3 2 C/sec ( F/sec) cooling rate to room temperature (Fig. 3a) and (b) isothermal treatment, including the same austenization heating schedule followed by 2 2 C/sec ( F/sec) cooling to 6 minutes isothermal hold at 38 C (716 F) and fast cooling to room temperature (Fig. 3b) C/s 1 C/s 5 C/s 2 C/s 1 C/s.3 C/s a) b) C/s 1 C/s 5 C/s 5 C/s (a) 2 C/s 1 2

4 IJMC 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 1 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 (M s and M f ) 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 M F 2 ΔL/L x M S Temperature, C a) b)

5 Hardness (HB) Microhardness (HV) IJMC c) Figure 5. (a) Electrical resistivity and power curves, (b) dilatometric curve showing martensitic transformation, and (c) quenched martensitic microstructure after 1 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 1 C/s ( F/sec) exhibit a sharp increase of hardness, mainly because of the formation of quenched martensite with 55 HV microhardness. These continuous cooling experiments showed the limitations, as too high of a cooling rate resulted in undesirable martensitic transformation Continuous cooling Isothermal 35 Upper bainite Pearlite Ferrite As cast 25 As cast Cooling rate, C/s Cooling rate, C/s 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 92 C (1688 F), cooled to 38 C (716 F) at different cooling rates, and isothermally held for 6 minutes. The effect of austenite carbon saturation was verified by increasing the holding time at 92 C (1688 F) from 1 to 3 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 38 C (716 F) isothermal holding (Fig. 7a). At 1 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 92 C (1688 F) promoted the stability of undercooled austenite at a lower cooling rate and resulted in larger ausferrite volume. 2

6 Temperature, C p (relative) Temperature, C IJMC p T min 25 min a) b) c) Figure 7. Microstructures of ductile iron at 92 C (1688 F)for 1 minutes, followed by cooling at (a) 5 C/s (9 F/sec) and (b) 1 C/s (18 F/sec) to 38 C (716 F) and 6 minutes isothermal hold; (c) electrical resistivity during isothermal holding at 38 C (716 F) after 5 C/s (9 F/sec) cooling rate from 92 C (1688 F). Figure 8 summarizes the achievable microstructures after continuous cooling to room temperature and cooling to isothermal hold temperature (38 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 M s temperature. Based on these experimental studies, a range of process parameters for engineered cooling were suggested Ferrite Pearlite.3 C/s 1 C/s 2 C/s 5 C/s 1 C/s 2 C/s Martensite Ausferrite Figure 8. Achievable structures by applying continuous cooling to room temperature and to isothermal holding temperature (38 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 M s 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

7 Temperature, C Cooling rate, C/sec Temperature, C Cooling rate, C/sec Temperature, C Cooling rate, C/sec IJMC convection. The heat transfer coefficients chosen for these cooling methods were 5 and 7 W/m 2 K, respectively. Radiant heat transfer from the cast iron surface with.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 M s 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 Center Surface Corner Cooling rate (center) 25C 2 8 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 Center Surface Corner Cooling rate (center) 1 34C 1 2 A laboratory experimental heat conducted in a 1 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 (Fe46Si6.1Mg1Ca1La.7Al) and inoculated by Superseed (Fe7Si.4Al.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) Forced air-water Forced air Center Surface Corner Cooling rate (center) 82C 1 35C Table 2. Chemistry of Laboratory Ductile Iron, wt. %. C Mn Si Cu Four no-bake sand molds with vertical 1 x 6 x 8 plates with top risers were poured (Fig. 1a). 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. 1b). a)

8 Stress, Mpa IJMC b) Figure 1. (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 1 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 55 MPa at 8 % elongation to 1 15 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 Engineered cooling As cast Strain, mm/mm Figure 11. Tensile tests: true stress-strain curves of base (as-cast, mold cooled), and engineered cooled, laboratory produced ductile irons. a)

9 IJMC 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 55 MPa for mold cooled castings to 1 15 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 (213). 2. Lekakh, S., Qing, J., Richards, V., Peaslee, K., Graphite Nodule Size Distribution in Ductile Iron, AFS Transactions (213).

10 IJMC Q & A TO FOLLOW