Effects of Micro-Alloying Elements on Microstructure, Element Distribution and Mechanical Properties in Gray Irons
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-019-00402-4
Abstract This paper elucidates the effects of micro-alloying (Cr, Mo, rare earth and Ag) on the microstructure, element distribution and mechanical properties including Brinell hardness and tensile strength in cast gray irons. Experimental results show that the morphology of main graphite transforms from type C to type A and is refined obviously in the modified gray iron after micro-alloying. The pearlite content is also reduced, and the pearlite interlamellar spacing is smaller in the modified gray iron. The distribution and segregation of micro-alloying elements have determined the change of microstructure. More Cr and Mo atoms are tended to dissolve in pearlite and are likely to replace Fe atoms to form M3P in final solidified phosphorus eutectic regions. Meanwhile, segregation of rare earth metals
toward boundaries should be considered into roles in refining graphite and pearlite interlamellar spacing. No obvious enrichment of Ag is found in matrix or at grain boundaries from experimental EPMA results. Multiple micro-alloying in gray irons improves the hardness and tensile strength at room temperature and at 700 °C by modifying the microstructure as well. Finer graphite and smaller pearlite interlamellar spacing results in better hardness, tensile strength and elongation in modified gray irons.
Introduction
can increase the content of pearlites in the matrix and improve the strength and corrosion resistance of casting matrix when added less than 1.0 wt% in gray irons.3,4 Mo is mainly used for refining pearlite and graphite with the amount not more than 0.6 wt%. Furthermore, Mo can also increase the as-cast hardenability of gray iron castings.5–7 The effects of rare earth metals in gray irons have manifested in many aspects such as the roles on refinement and homogenization of microstructure. Furthermore, rare earth sulfide and oxide particles can also be formed to become the base for graphite.8–10 In addition, rare earth metals increase tensile properties and corrosion resistance by optimizing inclusions.11,12 Therefore, multiple alloying elements have different roles in modifying the microstructure and mechanical properties of gray irons.
Cast gray iron, which accounts for significant use in metal castings throughout the world, has been widely used in repetitively loaded conditions due to its excellent castability, vibration damping properties and heat conductivity.1,2 Traditional gray iron contains amounts of alloying elements like C, Si, Mn, S and P. In recent years, researchers have been exploring micro-alloying methods to improve the microstructure and comprehensive properties including mechanical properties and thermal properties. Many studies have shown that the micro-alloying method is an effective means to optimize the microstructure and to improve the mechanical properties of gray irons by adding some specific elements. For example, the addition of Cr
International Journal of Metalcasting
Keywords: gray iron, m
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