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Page 1: Solidification Behavior, Microstructure, Mechanical Properties

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SAE TECHNICALPAPER SERIES 2004-01-0792

Solidification Behavior, Microstructure,Mechanical Properties, Hot Oxidation

and Thermal Fatigue Resistance ofHigh Silicon SiMo Nodular Cast Irons

D. Li, R. Perrin, G. Burger, D. McFarlan,B. Black, R. Logan and R. Williams

Wescast Industries Inc.

Reprinted From: Advances in Lightweight Automotive Castingsand Wrought Aluminum Alloys

(SP-1838)

2004 SAE World CongressDetroit, MichiganMarch 8-11, 2004

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Page 3: Solidification Behavior, Microstructure, Mechanical Properties

2004-01-0792

Solidification Behavior, Microstructure, Mechanical Properties, Hot Oxidation and Thermal Fatigue Resistance of

High Silicon SiMo Nodular Cast Irons

D. Li, R. Perrin, G. Burger, D. McFarlan, B. Black, R. Logan and R. Williams Wescast Industries Inc.

Copyright © 2004 SAE International

ABSTRACT

It is well known that 4 to 6% silicon spheroidal irons are suitable for use at high temperature. This paper describes solidification behavior, microstructure, mechanical properties, high temperature oxidation, and thermal fatigue of high silicon SiMo cast irons. Cooling curves of cast irons were recorded using a thermal analysis apparatus to correlate with the solidified microstructures. Uniaxial constrained thermal fatigue testing was conducted in which the cycling temperatures were between 500oC and 950oC. Oxidation behavior was studied by measuring the specimen weight and the penetration depth of oxides from laboratory cyclic oxidation testing. The coefficient of thermal expansion and critical temperature of the phase transformation A1 during heating were determined through dilatometry testing.

INTRODUCTION

The high-silicon (high-Si) and silicon-molybdenum (SiMo) ductile irons are currently used to produce exhaust manifolds and turbocharger housings for automobiles. With increasing Si content, the high temperature oxidation/spallation resistance, the critical temperature A1, and microstructural stability are enhanced [1-4]. Studies [5] have also shown that, to a significant extent, Si increases uniaxial thermal fatigue life and can, to a certain extent, replace Mo. The drawbacks to increased Si levels include a lower impact toughness and higher Brinell hardness, which may possibly cause some casting handling and machining problems during mass-production. Also certain manifold designs can be susceptible to thermal cracking when utilizing this material with too high Si contents. This may be due to lower overall ductility and thermal conductivity of this material compared to some other manifold irons. Typical SiMo ductile irons containing 4% Si and 0.5 to 2% Mo [6] can be used up to the material temperatures of 840 to 850oC [7] and in some cases with beneficial designs, even a little higher. The primary objective of this work is to examine if SiMo irons with a higher Si content ranging from 4.4 to 5.0% can be used for large-scale production of exhaust manifolds. Increased Si additions will change the freezing behavior, shrinkage tendency, elongation, impact toughness, oxidation resistance, thermal fatigue life, casting handling, and machinability. This work was

undertaken to investigate the suitability of high-Si SiMo for exhaust manifold applications, and to document the properties mentioned above through both experiments and modeling. EXPERIMENTAL PROCEDURES

A number of regression trials and DOE (design of experiments) on alloy chemistry and processing were performed at the Richard W. Levan Technical Centre of Wescast Industries Inc. Heats weighing approximately 100 kg were made using an induction furnace of 350 kW and 1 kHz. Conventional treatment and inoculation were used. Pour temperatures varied from 1390oC to 1460oC. Test bars and prototype manifolds were cast in chemically bonded sand and greensand molds respectively. Cooling curves of cast iron melts were measured using a thermal analysis apparatus. Approximately 240 manifolds with high-Si SiMo were cast and machined during the plant trials to evaluate handling and machinability. Optical microscopy and digital image analysis software were used for completing the microstructural analysis of the trial samples. In addition to casting experiments, solidification modeling was conducted for different chemistries and pour temperatures. The solid phase transformation temperature and coefficient of thermal expansion were determined by dilatometry testing at the heating rate of 1 and 5oC/min separately. Muffle furnaces were utilized for elevated temperature oxidation testing. The oxidation specimens were 10×10×30 mm rectangular bars. The total surface area (1400 mm2) is larger than the minimum value (400 mm2) recommended by ASTM G54-84. Before testing, samples were cleaned ultrasonically and heat treated at 210oC for 2 hours. After obtaining the initial weight of each sample, the oxidation testing started at 900oC. During testing the specimens were withdrawn from the furnace, cooled, and replaced at intervals of 24 to 48 hours. Room and elevated temperature tensile and charpy impact testing were performed at both internal and external laboratories. Y-blocks were cast at Wescast Industries Inc. and provided to Climax Research Services (CRS) in Michigan for uniaxial and constrained thermal fatigue testing between 500oC and 950oC. The specimens were heated by a radio-frequency (RF) furnace (450 kHz) within a rigid test frame. Further descriptions of

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the CRS facility and testing procedures have been given elsewhere [5]. RESULTS AND DISCUSSION

SOLIDIFICATION AND SHRINKAGE - Understanding of the solidification process is essential to control microstructure, which in turn, determines the properties of materials. SiMo cast irons can be considered as a quarternary Fe-C-Si-Mo system. As the silicon content is increased, the carbon content of the eutectic and eutectoid is decreased. Also the eutectic and eutectoid temperatures change from a single value to a temperature range. Thus a simple linear relation CE = %C+1/3%Si is defined to represent the combined effect of silicon and carbon, namely the carbon equivalent. A chemistry map was drawn regarding C, Si, and CE with a microstructure prediction, as presented in Figure 1. High-Si SiMo discussed in this paper contains 4.4 to 5.0% Si and 0.5 to 0.9% Mo in iron, which is further divided into two groups according to Si content: high-Si I (4.4 to 4.7% Si) and high-Si II (4.7 to 5% Si). It is widely accepted that the CE values should be controlled around 4.7 for castings of wall thickness less than 25 mm in order to avoid solidification of austenite dendrites and primary carbides. Too low or too high CE will give rise to casting defects. This map can be further elucidated by experimental results as depicted in Figure 2. For a strongly hypereutectic chemistry such as CE = 4.89, the primary liquidus thermal arrest is clearly visible, which was referred to as the graphite liquidus [8], as illustrated in the cooling curve (a) of Figure 2. This corresponds to the formation of coarse graphite nodules prior to the eutectic reaction, as shown in micrograph (b) of Figure 2. Referring to curve (c) of Figure 2, the sample with CE = 4.7 shows one solidification event, namely that the liquidus temperature and the starting eutectic temperature are identical. The curve exhibits fairly smooth cooling to the low eutectic temperature where the bulk growth started followed by a small recalescence of less than 10oC. This curve led to a uniform nodule distribution, as seen in micrograph (d) of Figure 2. The Si content was 4.95% in Figure 2. However, if the CE is too low, both shrinkage and chilling tendency will increase. Furthermore thermal conductivity of cast iron will decrease if the C content is too low or/and the Si content is too high, which may reduce the thermal fatigue resistance of castings [1]. With regard to the alloying element Mo, part of it segregated and froze into intercellular regions to promote eutectic (lamellar shape) or primary (faceted shape) carbides. Furthermore, during the solid state transformation, fine Mo-rich particles of less than 1 µm in diameter were precipitated around the grain boundaries, as observed by Black et al. [9]. The degree of undercooling and the solidius temperature determined from cooling curves may be useful to predict formation of the primary carbides. Solidification behavior and nodule distribution significantly influences the shrinkage tendency. Figure 3 demonstrates the relationship between the carbon equivalent and shrinkage tendency through a series of studies including manifold castings (the two pictures in the left side), AFS (American Foundry Society) blind risers (the two pictures in the middle), and a solidification software modeling (the right side column). The CE value was 4.85 for the top row and 4.65 for the bottom

row of pictures; while the other conditions are basically the same including the Si content (4.50%), mold materials, magnesium treatment, inoculation, and pour temperature. Simply speaking, the top samples produced much more severe shrinkage than the bottom row. When the CE is too high, solidification started with a higher liquidus temperature, thus forming the mushy liquid earlier and macro shrinkage, as shown in the top pictures of Figure 3. One can also say that high shrinkage level melts were always formed with a large number of coarser nodules [10]. For the bottom row with a CE = 4.65, much less shrinkage was observed within the cross sections of casting manifolds, AFS blind risers, and solidification modeling pictures. Generally speaking, shrinkage is also influenced by other many factors. MECHANICAL PROPERTIES – The silicon effects on mechanical properties have been well studied and understood [1-4]. When the Si content is lower than approximately 3.5% [11], the tensile strength decreases and elongation increases with increasing Si content, since more ferritic phase was formed within the matrix. If Si content exceeds that value, the tensile strength and hardness increase, while the elongation and impact toughness decrease with increasing Si. In this work, the Si content was higher than the critical value, and Mo content varied from 0.5 to 0.9%. Y-blocks with different Si contents were cast for various types of testing. Figure 4 presents the room temperature tensile and hardness data. It is seen that with increasing Si content, the ultimate tensile strength (UTS), 0.2% offset yield strength (YS), and the Brinell hardness went up, due to the solid solution strengthening and hardening, while the tensile elongation dropped. However, elongation of up 10% can be steadily achieved for 4.95% Si samples through controlling melt chemistry and inoculation practice. Standard short-time tensile testing at elevated temperatures was conducted. Figure 5 presents some UTS data obtained from our trials. It was observed that the Si strengthening effect diminished when the testing temperature was beyond 500oC. On the contrary, the UTS at 700oC and 900oC slightly decreased with increasing Si, perhaps because silicon reduced the solubility of carbon in cast irons and tended to dissociate iron carbides. More data are needed to examine whether silicon really lowers the elevated-temperature strength or not. It is well known that Mo can effectively increase the elevated temperature strength, both short-time tensile strength and long-term creep strength [1, 6]. The ductility is indeed reduced with increasing Si contents. A natural question arises: Can heat treatment improve the ductility of high-Si SiMo? Both sub-critical (785oC) and full (950oC) annealing were conducted for high-Si SiMo test bars of 16 mm in thickness. The samples were held at the mentioned temperatures for 4 hours and then slowly cooled (approximately 100oC/hr). Samples were removed from the furnace for air cooling after reaching 600oC to avoid possible elevated temperature embrittlement. As expected, the UTS and YS slightly decreased and elongation increased after annealing, as illustrated in Figure 6. Regarding the microstructure of Mo containing ductile irons, there exists the so-called Mo-rich phase around the cell-boundary regions [9]. The percentage of Mo-rich phase (Mp) can be determined

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from image analysis of the microstructure. It was found that the Mp content was reduced from 7.3% (as-cast), to 6.4% (subcritical annealing), and to 3.2% (full annealing), for the same chemistry (4.65% Si and 0.75% Mo). Less Mp percentage increased the elongation. However, there was no major change in the impact toughness before and after heat treatment, as seen in Figure 7. The absorbed energy was determined from the charpy impact tests which were performed on the standard unnotched rectangular (10×10×55 mm) bars. The samples with 4.65% Si show higher impact toughness than those with 5.01% Si even with a large scatter in the data. The unnotched charpy data of regular SiMo (4% Si) ranged from 30 to 45 J which is much higher than high-Si SiMo. The impact toughness is a measure of the amount of energy a material can absorb before fracturing. The charpy impact data indicate the handling severity of casting manifolds in the foundry process including shakeout, media drum cycling, and rotary shot-blaster. Plant trials with high-Si SiMo chemistry were conducted in order to evaluate the possible issues caused by rough handling and machining in large-scale production. HIGH TEMPERATURE OXIDATION - There are numerous publications on high-temperature oxidation and scaling with different alloy chemistries, testing temperatures and exposure times [1-4, 12-14]. Oxidation and spallation may possibly have detrimental effects such as reducing the stress-bearing ability and causing particles to travel into downstream component. In this paper, we report the results of SiMo with 4.0% Si and high-Si SiMo with 4.8% Si obtained from two conditions respectively. No airflow was introduced into the furnace under condition I, while under condition II fresh air was passed through the furnace throughout the run at the rate of 2.5 liters/min monitored by a flow meter. Under both conditions, the furnace temperature was calibrated to 900oC, and the samples were removed from the furnace and cooled down to room temperature every 1 or 2 days (cyclic oxidation testing). More than 20 cycles were run. Figure 8 shows the accumulative weight change, but the weight change for each period can be determined too from the curves. Under condition I (empty symbols) the total weight gain was almost nil for high-Si SiMo, while it was up 7 mg/cm2 for SiMo. Also SiMo shows much higher oxidation rate initially. However, after 100 hours, the two curves (empty symbols) have essentially the same slope. Under condition II (solid symbols), the superiority of high-Si SiMo versus SiMo becomes more evident. After 695 hours, the weight gain was 20.5 mg/cm2 for SiMo, while it was only 6.1 mg/cm2 for high-Si SiMo. Furthermore, a steeper slope of the SiMo curve (solid squares) indicates the degree of rapid oxidation. The solid penetration depth can be considered as a more meaningful measure of oxidation, because the weight change in Figure 8 represents the combined effects of weight gained by the formation of oxides and the weight lost by decarburization. The weight loss was dominant during the initial stage of the reaction. As oxidation proceeds, the contact between scale and metal is lost at the edges and corners. Hence the non-uniform oxidation occurred over the surface. It was observed from the metallographic cross-sections that the oxide layer was thicker and more uniform in the middle than at the corners. Therefore, the oxide depth was

measured from the middle of the cross-section for each sample. Figure 9 presents the oxide thickness from a series of samples tested at 900oC but for different times of up to 695 hrs for high-Si SiMo containing 4.8% Si, and 953 hrs for SiMo containing 4.0% Si respectively. Neglecting the initial period, it can be seen that the oxide penetration of both SiMo and high-Si SiMo basically follows the linear growth in terms of kinetics equations, which is typical for high-temperature oxidation of metals. However, high-Si SiMo shows a thinner oxide depth and smaller rate constant than SiMo. After 695 hours exposure the oxide depth of high-Si SiMo is 20 to 30% less than that of SiMo samples. The oxide nucleation and growth is illustrated as a function of the exposure time in Figure 10, by taking the SiMo chemistry as an example. After a short exposure time 2 hrs in Figure 10(a), the formed oxide consists of discrete, isolated particles with a diameter of approximately 80 µm. Each particle contains multilayered scales, as indicated by different colors. It appears that the oxide nuclei originated from the surface defects which may be impurities, grain boundaries, dislocations, or machining traces. Afterwards, the particles grew laterally to yield a continuous film on the surface as oxygen continued to dissolve and diffuse into the alloy phase, as shown in Figure 10 (b) after 8 hours at 900oC. Further increasing the testing time, continuous growth of the oxide scales took place, as seen in Figures 10 (c), (d) and (e). The micrographs show many voids and cracks in the scales. This can be explained as follows. At high temperatures, the iron ions and electrons migrated outwards and oxygen diffused inwards to burn out graphite in the parent metal, thereby resulting in some cavities. During cyclic oxidation testing, thermal stress and strain were generated due to the different thermal expansion and contraction between the scale and metal (The coefficient of thermal expansion of iron oxides is less than that of iron) and the volume ratio of oxide to metal. This possibly made the oxides crack, collapse, and detach from the metal matrix. Formation of porous zones and cracks in turn alters the oxide adherence and oxidation kinetics. Note that some cracks may be caused by sample preparation. During the testing, no scale was spontaneously spalled off from the samples. A stainless steel knife was used to manually scrape the surface of oxidized specimens to examine the oxide adherence. It was observed that many more oxide debris were descaled from the SiMo samples, indicating that the oxide layer of SiMo is not as adherent as that of high-Si SiMo. High-temperature oxidation of cast iron is more complex than that of pure metals. The following is a brief discussion based on our experimental results and theoretical descriptions [12, 13]. Cast irons are multi-component ferrous alloys, which contain major (Fe, C, and Si), minor (< 0.1% such as S, Mg, and Ce), and often alloying (> 0.1% such as Mo) elements. Although some minor active elements may exert a significant effect on the oxidation behavior [15], the influences of Fe, C, Si, and Mo are examined in this paper. Iron is the parent metal and forms with more than one oxide in the scale. The oxide was found to consist of Fe2O3 (haematite), Fe3O4, (magnetite), FeO (wustite), and (FeO)2SiO2 or Fe2SiO4 (fayalite) when containing a certain

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amount of Si [12, 13]. They have different structures and formation mechanisms. The former two oxides were produced during the outward oxidation while the fayalite, wustite, and magnetite were formed during the inward oxidation, through a series of inward/outward transport of oxygen ionizes, iron ions and electrons. Due to the much greater mobility of defects in FeO, this layer will be thicker than others [12]. The thin layer Fe2O3 at the outer surface was not discerned under optical microscopy. In Figure 10, the two obvious layers should be Fe2O3 and FeO+Fe2SiO4 respectively according to the relative thickness and locations. Regarding the effect of carbon, decarburization happened through the reaction of graphite and oxygen, thus causing the weight loss (negative weight change), as seen in Figure 8. We conducted hot oxidation testing on SiMo irons with different silicon contents ranging from 2.8 to 5.0%. The specimens with higher Si contents seemingly had more weight loss at the initial stage, while the weight loss was not observed for the specimens with the Si content lower than 3.5%. This can be explained as follows. As mentioned in the preceding paragraph, the weight change determined in the testing displayed the difference between the weight gain and loss. For specimens with lower Si contents, the weight gain by oxidation prevails over the weight loss by decarburization. Therefore the total weight was monotonously increased for the low-Si samples (< 3.5% Si) a function of exposure time. On the other hand, less oxidation and thus more apparent weight loss appeared for high-Si samples. Similar to other properties of cast irons, oxidation resistance is significantly influenced by the graphite structure. Oxidation and growth take place in gray iron more rapidly than in compacted graphite iron (CGI) and more rapidly in CGI than ductile iron [1]. Silicon is a major and economic element to improve high-temperature oxidation resistance in cast iron. SiO2 reacts with FeO to form fayalite in close vicinity to the parent metal surface. When the fayalite particles become larger they are engulfed by the scale. These islands lie in the FeO layer as markers and prevent the inward movement of the outer scale [12]. The Si content can be increased to provide a denser fayalite broken stringer on the metal substrate, and even a continuous, protective SiO2 scale. However, the mechanical properties will be unacceptable when the Si content is too high. Oxide volatilization can occur in the Mo-O system at high temperature and oxygen pressure [12]. Because of much lower contents and oxygen affinities than other elements, and formation of Mo-rich precipitates [9], Mo is much less likely to be oxidized at 900oC than other elements in SiMo iron. COEFFICIENT OF THERMAL EXPANSION - Dilatometry testing was conducted to determine the critical temperature, A1, and the coefficient of thermal expansion alpha α. The phase transformation of cast iron occurs in a temperature range. Thus the critical temperature A1 can be defined as the onset of the phase transformation from ferrite to austenite upon heating, as indicated by inflections in the temperature-alpha curves (Figure 11). It is known that A1 increases with increasing heating rate because of the transformation kinetics. In this work, the samples were slowly heated at 1 or 5oC/min.

There is no significant difference in alpha between 4.9% Si SiMo and the regular SiMo with 4% Si when the temperatures are below the A1. Clearly, the A1 temperature was raised by approximately 60oC comparing the 4.9% Si with 4.0% Si samples. This may imply that the practical operating temperatures could be increased when replacing current SiMo with high-Si SiMo according to the Al difference. THERMAL FATIGUE RESISTANCE - In addition to high temperature oxidation resistance and microstructure stability, thermal fatigue life is also important for exhaust manifolds. There are a few types of thermal fatigue tests including bars [16], discs [17], and thin tubes [18] in terms of specimen shape. Our Y-blocks were machined into bars and tested under uniaxial conditions at CRS. The test method and procedures were described in Reference 5. It is noteworthy that the temperature range in the thermal fatigue testing was from 500 to 950oC, instead of cooling to ambient temperature. All data are listed in Table 1, which are fairly consistent for each alloy. The Mo content was approximately 0.6% for all samples. It is seen that the average number of cycles to failure increased with increasing Si content from 4.0% to 4.5%, and to 4.95%. It remains unclear why the high-Si SiMo displayed a higher thermal fatigue life in this temperature range. From the hot tensile data in Figure 5, silicon strengthening effect almost vanished at high temperatures. However, in the light of phase transformation, silicon raises A1 temperature and widens the phase transformation range [1-4]. Therefore, the effects of Si on the thermal fatigue resistance may be attributed to the different amount of phase transformation during the testing. Less amount of phase transformation induced less thermal stress during the testing. It was reported that some foundries produce manifolds and turbocharger housings with spheroidal and compacted graphite irons at a higher silicon content in order to improve the overall elevated-temperature performance including oxidation, structure stability, and thermal durability. Table1. The number of cycles to failure of SiMo nodular irons containing different amounts of Si in the uniaxial thermal fatigue testing between 500oC and 950oC. The Mo content was 0.6% in all samples.

Test # 4% Si 4.5% Si 4.95% Si 1 51 78 92 2 52 87 95 3 52 88 88 4 56 80 80 5 56 87 6 47 93 7 49

Average 52 83 90 PRELIMINARY PLANT TRIALS - Approximately 240 manifolds containing 4.5% to 5.0% Si were cast and machined in the company plants to evaluate the rough handling and machinability and identify the capability with the current equipment. The SiMo production baseline was used, including gating, risering, molding, melting, treatment, pour, shake-out, cleaning, and machining parameters. It was

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observed that there were rough handling issues when the Si content is too high. For testing the durability and thermal fatigue resistance of castings, Wescast Industries Inc. has built a gas-fired engine exhaust simulator (EES). For comparison, EES testing will be further conducted on SiMo and high-Si SiMo irons for differently designed manifolds. CONCLUSIONS

Alloy chemistry, solidification, microstructure and properties of high-Si SiMo spheroidal cast irons have been investigated in this work. Plant trials were performed to assess the handling and machinability of high-Si SiMo manifolds. Conventional Mg-Fe-Si treatment and inoculation can be adopted for high-Si SiMo to produce satisfactory nodularity and uniform nodule distribution. Casting experiments and solidification modeling have revealed that the shrinkage tendency of high-Si SiMo can be controlled to be the same level as that of regular SiMo by using a suitable carbon equivalent from 4.35 to 4.85 mainly depending upon the critical thickness of castings. Tensile UTS and YS are significantly increased with increasing Si, while the strengthening effect of Si disappeared at the testing temperatures higher than 500oC. Tensile elongation at room temperature was decreased with increasing Si content. However elongation of up 10% has been achieved at room temperature when the silicon content approached 5% by controlling the freezing behavior and nodule distribution. Heat treatment can further increase tensile elongation. It was observed that the impact toughness (unnotched charpy testing) at room temperature of high-Si SiMo is much lower than that of regular SiMo. Further work is needed to improve the impact toughness of high-Si SiMo in order to alleviate potential problems of casting handling prior to engine assembly. It has been repeatedly shown that high-Si SiMo is superior to regular SiMo in high temperature oxidation and scaling resistance tests. Uniaxial thermal fatigue life was increased when silicon content was changed from 4.0 to 4.5 and to 4.95% when the testing temperatures were cycled between 500oC and 950oC. ACKNOWLEDGMENTS

The results reported here would not have been possible without the cooperative effort of the company leadership, corporate and plants. Special thanks are due to B. Plank, G. Burkhalter, J. Cassidy, A. Cormier, G. Liao, B. Kowal, F. Yu, P. Slater, D. Lanting, P. Pickard and A. Valentyn for technical support, and T. Thoma and C. Sloss for their helpful review and comments. The authors also thank P. Chan and R. Gundlach of CRS for the thermal fatigue testing and discussions. REFERENCES

1. J.R. Davis (editor), ASM Specialty Handbook: Cast Irons, ASM International, Materials Park, OH, 1996.

2. G.M. Goodrich (technical editor), Iron Castings Engineering Handbook, AFS (American Foundry Society), Des Plaines, IL, 2003.

3. R. Elliott, Cast Iron Technology, Butterworths, London, UK, 1988.

4. H.T. Angus, Cast Iron: Physical and Engineering Properties, Butterworths, London, UK, 1976.

5. R.B. Gundlach et al., “Thermal Fatigue Resistance of Silicon-Molybdenum Ductile Irons,” Presentation at DIS/AFS Millis Symposium, October 1998, Hilton Head, SC.

6. D.L. Sponseller, W.G. Scholz, and D.F. Rundle, “Development of Low-Alloy Ductile Irons for Service at 1200-1500 F’” AFS Trans., 76 (1968), 353-368.

7. K. Kampendonk, R. Williams, and R. Perrin, Private Communication, Wescast Industries Inc. July, 2003.

8. M.D. Chaudhari, R.W. Heine, and C.R. Loper, “Principles Involved in the Use of Cooling Curves in Ductile Iron Process Control,” AFS Trans., 82 (1974), 431-440.

9. B. Black et al., “Microstructure and Dimensional Stability in Si-Mo Ductile Irons for Elevated Temperature Applications,” Proc. of the 2002 SAE Inter. Body Eng. Conf. and Automotive & Transportation Tech. Conf., Paris, France (paper # 2002-01-2115).

10. C.A. Bhaskaran and D.J. Wirth, “Ductile Iron Shrinkage Evaluation through Thermal Analysis,” AFS Trans., 110 (2002), 1-16 (paper # 02-003).

11. Foundry Handbooks, Vol. 1: Cast Irons, Mechanical Engineering Publisher, Beijing, 2002 (in Chinese).

12. N. Birks and G.H. Meier, Introduction to High Temperature Oxidation of Metals, Edward Arnold (Publishers) Ltd, London, UK, 1983.

13. P. Kofstad, High Temperature Corrosion, Elsevier Applied Science Publisher Ltd., Essex, England, 1988.

14. E.A. Loria, “Cyclic Oxidation of Chromized Steel and Competitive Materials at 650 to 815oC,” J. Mater. for Energy Systems, 8 (1986) 132-141.

15. E. Lang (editor), The Role of Active Elements in the Oxidation Behavior of High Temperature Metals and Alloys, Elsevier Applied Science, Essex, England, 1989.

16. K. Akiyama et al., “Analysis of Thermal Fatigue Resistance of Engine Exhaust Parts,” SAE Trans. 910430, 1991, 63-71

17. A. Weronski, Thermal Fatigue of Metals, Marcel Dekker Inc, New York, NY, 1974.

18. M.H. Aliabadi (editor), Thermomechanical Fatigue and Facture, WIT press, Boston, MA, 2002.

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2.9

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Trial DataC+Si/3 = 4.6C+Si/3 = 4.7C+Si/3 = 4.8

UniformNodule

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Figure 1: Silicon, carbon, and the carbon equivalent (CE) map for high-Si SiMo. The circles stand for some trial chemistries. Prediction of microstructure and shrinkage is presented.

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Figure 2: Measured cooling curves and the resulting microstructures. Curve (a) corresponds to micrograph (b), and curve (c) to micrograph (d).

(a)

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Figure 3: Casting experiments and modeling of shrinkage for high-Si SiMo of 4.5% Si. The C content is 3.35%, i.e. CE = 4.85 for the top row of pictures, while the C content is 3.15%, i.e. CE = 4.65 for the bottom pictures. The pictures in the left, middle, and right columns represent the flange sections of manifolds, sections of AFS blind risers, and solidification modeling respectively.

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Figure 4: Silicon content versus room temperatureproperties: (a) tensile UTS and 0.2% offset YS,(b) tensile elongation E%, and (c) Brinellhardness HBW. Mo content varied from 0.6% to0.9%. Each point represents three tests at least.

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0

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As-cast Sub-critical Full Annealing

Figure 5: Silicon content versus the hottensile UTS tested at differenttemperatures. Each point represents theaverage of three tests at least. The pointsare connected just for showing thetendency to change. Mo content is 0.75%.

Figure 6: Room temperature tensileresults of as-cast, subcriticallyannealed, and full annealedspecimens of high-Si SiMo (4.65%Si and 0.75% Mo).

Figure 7: Absorbed energy of non-notched charpy testing for 4.65% Siand 5.01% Si SiMo samples withdifferent treatment: as-cast,subcritical annealing, and fullannealing respectively. The sampledimension is 10×10 ×55 mm.

Page 11: Solidification Behavior, Microstructure, Mechanical Properties

-5

0

5

10

15

20

25

0 200 400 600 800

Time at Temperature of 900 C (hr.)

Wt.

Chan

ge (m

g/cm

^2)

Figure 8: Weight change rate versus time obtained from hot oxidation testing at 900oC. Squares and circles represent SiMo of 4% Si and high-Si SiMo of 4.8% Si respectively. Solid and empty symbols stand for airflow of 2.5 liters/min through the furnace and no airflow introduced respectively.

0

200

400

600

800

1000

1200

0 200 400 600 800 1000

4.0% Si4.8% Si

Time at 900oC (hr)

Oxi

de D

epth

(µm

)

Figure 9: Oxide depth measured from the cross-section of tested samples using optical microscopy. The vertical bars represent the measurement variations of the oxide thickness. The points are connected for showing the tendency to change.

Page 12: Solidification Behavior, Microstructure, Mechanical Properties

Figure 10: Cross-section micrographs of SiMo samples with 4% Si after cyclic oxidation testing at 900oC for different exposure times (hours) in total: (a) 2, (b) 8, (c) 24, (d) 415, and (e) 695. Airflow of 2.5 liters/min was introduced through the furnace. Same scale was used for the five pictures. For each micrograph, the far left and right edges represent the mounting material and the iron matrix respectively, and the middle regions represent the oxide layers.

-20

-10

0

10

20

800 840 880 920 960 1000

Temperature (C)

Alp

ha, 1

0^(-6

)

200 µm

(b) (c)

(d)

(e)

(a)

Fe3O4 FeO+Fe2SiO4

Figure 11: Coefficient of thermalexpansion alpha versus temperaturemeasured from dilatometer testing at aheating rate of 5oC/min: (a) SiMo of4% Si and (b) high-Si SiMo of 4.9%Si. The critical temperature A1 can bedetermined from the curves.

(a)

(b)