Effect of austempering and aluminizing processes on properties of high silicon spheroidal graphite ductile irons

EN 1563 standard covering spheroidal graphite ductile cast irons was revised in March 2012 and the solution strengthened ferritic (SSF) grades of GJS-450-18, GJS-500-14, and GJS-600-10 with higher Si contents (wt% 3.2-4.3) were newly added to the standard. A higher amount of silicon (as solute atoms) makes a solid solution by replacing iron (solvent atoms) in the lattice, contributes to lattice distortion and makes dislocation motion more difficult. Thus strength of the material increases. Solid solution strengthening improves ductility almost two times, and approximately 50-60% more tool life during machining to SSF ductile cast irons compared to conventional ferritic / pearlitic ductile cast irons is obtained. Although ductile cast irons have good features in terms of mechanical and wear properties, further improvement of these properties were needed in line with industry demands. In accordance with these demands, austempering heat treatment is applied to the ductile cast irons. With austempering treatment, the existing matrix transforms into acicular ferrite and high-carbon retained austenite, resulting in enhanced strength and toughness. While the austempering process is a method commonly applied to traditional ductile cast irons, it has also been recently applied to SSF ductile cast irons. The results showed that austempered high silicon ductile cast irons showed higher strength and higher ductility than austempered conventional ductile cast irons. Due to the high amount of silicon in the SSF ductile cast iron, it prevents the formation of cementite (Fe3C), which is likely to occur in the microstructure after the austempering process, and protects the mechanical properties against the detrimental effect of the cementite. Since high silicon in SSF ductile cast irons increases the eutectoid temperature, higher temperatures are required in the austenitizing step of the austempering process than in conventional ductile cast irons. High austenitizing temperature provides more rapid diffusion of carbon to austenite and austenite becomes richer in carbon. Due to the higher enrichment of carbon obtained at the higher austenitization temperature, thermodynamic stabilization austenite are improved. Because the austenitization step of the austempering process occurs at high temperatures (> 850 °C), it is inevitable that the materials are exposed to oxidation and decarburization if the protective atmosphere is not provided at these temperatures. Thanks to hot-dip aluminizing (HDA) coating technique, these materials can be protected from decarburisation during austenitizing as well as providing high-temperature oxidation resistance to these materials. After HDA coating, generally, a top layer of Al (with FeAl3), a middle layer of FeAl3, and an inner layer of Fe2Al5 were formed in the coating. The Al top layer by forming Al2O3 at high temperatures protects the material from oxidation and decarburisation. The mechanical properties of the coating are improved by transforming the brittle intermetallics (FeAl3, Fe2Al5) present in the coating structure into iron-rich and tougher FeAl and Fe3Al structures at high temperatures. Therefore, it is expected that the HDA coating of ductile cast irons before austempering may contribute to the surface properties of the material. In the first phase of the experimental studies carried out within the scope of the thesis, GJS-500-14 and GJS-600-10 grade ductile irons were austenitized at 975 °C for 2 h, and then they were subjected to the austempering process at predetermined temperatures of 270 °C, 330 °C, and 390 °C for 60 min. The effects of different austempering temperatures on the structural, mechanical and magnetic properties of materials were investigated. It was found that volume fraction of retained austenite increases with increasing austempering temperature, the increase in the amount of silicon from 3.7% (GJS-500-14) to 4.3% (GJS-600-10) negatively affect the ductility and toughness, and also the optimum mechanical properties are obtained at austempering temperature of 330 ° C. One of the factors affecting the optimum mechanical properties is the thin-coarse form of the acicular ferrite found in the microstructure and the other one is the amount of retained austenite is in the range of about 20-25%. The magnetic properties of the SSF ductile irons vary depending on the chemical content and microstructural constituents of the materials, namely, saturation magnetization values decrease with increasing silicon content and increasing retained austenite content. The decrease in saturation magnetization with the increase of silicon content can be explained by the effect of alloying elements, which are capable of decreasing the magnetic moment and consequently the saturation magnetization of iron. The decrease in saturation magnetization due to microstructural constituents can be explained by variations in the ferromagnetic (ferrite) and paramagnetic (austenite) phase volume fractions, namely, saturation magnetization reached a maximum value in the ductile irons containing the highest amount of ferrite. In the second phase of the thesis, HDA coating (750 °C, 5 min) was applied to GJS-500-14 grade SSF ductile irons, and then aluminized ductile irons were austenitized at 975 °C for 2 h, followed by austempering at predetermined temperatures of 270 °C, 330 °C, and 390°C for 60 min. The effect of additional HDA coating on the mechanical and corrosion properties of the materials was investigated. Compared to as-cast ductile iron without aluminizing, the hardness and strength values and ductility of HDA coated ductile irons were almost unchanged in the as-cast condition. The strength values decreased slightly but the ductility increased in the aluminized and austempered condition. This can be explained by the brittle phases (Fe2Al5, FeAl2) in the coating structure and the high temperature gradient during cooling from the austenitizing temperature (975 °C) to the austempering temperature (270 °C–390 °C), which was responsible for the crack formation because of the brittle nature of the Al-rich intermetallics in combination with the tensile stress acting on them. After anodic potentiodynamic polarization corrosion tests, the corrosion resistance of the HDA coated and subsequent austempered ductile irons was found to increase by about 3-6 times compared to as-cast ductile iron without aluminizing. This increment in corrosion resistance can be explained by lack of graphite nodules on the coating surface, namely, graphite nodules are not directly exposed to the corrosive environment in the case of coated ductile irons, unlike as-cast ductile irons without aluminizing. Therefore, graphitic corrosion, which is a common corrosion mechanism of as-cast ductile iron, it was not expected to be the driving mechanism of corrosion in the coated ductile irons. The third and final phase of the thesis, another grade of SSF ductile irons (GJS-600-10) were coated with HDA technique (750 °C, 5 min), and then austempering heat treatment has been performed in the austempering condition optimized from previous studies (975 ° C 2 h for austenitization, 330 ° C 1 h for austempering). After the materials were characterized structurally and mechanically, room temperature wear characteristics were predominantly examined. It was found that wear resistance was increased by about 30% when austempering is applied after the HDA coating. The porous Fe2O3 oxide formed by abrasion on the surface of austempered ductile iron without aluminizing, and this led to increment in wear rate with plastically deformed graphite during wear test. In the case of coated and austempered ductile iron, although thin Al2O3 layer produces cracks on the surface during wear, the newly formed Fe-Al intermetallics in the subsurface during austenitizing provides strong support during wear. In conclusion, single austempering without aluminizing significantly improved both strength and toughness of SSF ductile irons (GJS-500-14, GJS-600-10), as expected. With the increasing austempering temperature, the volume fraction of ferrite (ferromagnetic) decreased and accordingly the volume fraction of retained austenite (paramagnetic) increased. Utilizing the change of magnetic properties with increasing austempering temperature, the retained austenite volume fraction of these materials were calculated by a magnetic method and these values were found to be compatible with the measurements made by XRD method. Additional HDA coating before austempering improved the surface properties (corrosion, wear) of the SSF ductile irons. Mechanical properties of the HDA coated and austempered ductile irons satisfied the requirements of grades 1200-3, 900-8, and 800-10. A further achievement of HDA coating is that the surface is prevented against decarburization and scaling without a need for further protective atmosphere during austenitizing.