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
2004 SAE World CongressDetroit, MichiganMarch 8-11, 2004
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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
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.
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  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  can be used up to the material temperatures of 840 to 850oC  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 101030 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
the CRS facility and testing procedures have been given elsewhere . 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 , 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 . 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 m in diameter were precipitated around the grain boundaries, as observed by Black et al. . 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 sh