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The influence of silicon and boron on tensile properties of ductile iron

The influence of silicon and boron content on the tensile strength, 0, 2 % proof strength and elongation of ductile iron in the as-cast condition at room temperature was analyzed in this paper. The base melts for the production of ductile iron were produced in medium frequency coreless induction furnace from a charges consisting of pig iron, steel scrap, ferrosilicon and recarburizer. To obtain desired boron content in the melts, granulated NiB containing 16, 0 wt. % boron was added. The Y- blocks were cast in the vertically-parted molds produced by the CO2 process. The nodularizing treatment of the base melts was performed by In- Mould method via FeSiMg5 alloy containing 0, 50 wt. % lanthanum. Silicon content in ductile irons varied from 2, 65 wt. % to 3, 10 wt. % and boron content varied between 0, 0004 wt. % and 0, 0075 wt. %. Standard tensile test pieces were machined from Y-blocks. Tensile properties of ductile irons alloyed with silicon and boron were compared with the tensile properties of ductile iron grade EN-GJS-400-18-LT, which was also made. The obtained results show that the microstructure of ductile irons alloyed with silicon and boron consists of a ferritic metal matrix and increased nodule count. Apart from promoting graphitization and ferrite formation, the increase of silicon content results in increasing ductile iron tensile strength and 0, 2 % proof strength compared with the grade EN- GJS-400-18-LT, because the ferrite is strengthened by silicon. The high nodule count facilitates the formation of the high content of ferrite in the metal matrix. The results of tensile test show that the ductile iron alloyed with silicon and boron had a high value of the ratio of 0, 2 % proof strength and tensile strength (approx. 0, 75) as compared to the ductile iron grade EN-GJS-400-18-LT (approx. 0, 65). Despite this higher ratio, elongation of ductile irons alloyed with silicon and boron is high (as the elongation of ductile iron grade EN-GJS-400-18-LT). SEM analysis shows that the boron is concentrated mainly in graphite nodules.

ductile iron ; microstructure ; tensile properties

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