Effect Of Silicon On Microstructure And Mechanical Properties Of Low-Temperature High-Ductility Ductile Iron

Spheroidal graphite cast iron is an important metal structural material. It has excellent mechanical performance. It has been used in the field of automobile and wind power widely. Due to wind power equipment works for a long time in cold and serious corrosion of smoke condition, the cryogenic properties and corrosion resistance of materials have been put forward higher requirements. By controlling the silicon content of low temperature high-toughness cast iron, we studied the effects of silicon on microstructure and properties.The cast structure of the smples included globular graphite, ferrite, pearlite and little cementite. The size of graphite ball grade was 6-7, the spheroidization rate was 1-2. The sample with more silicon, had more graphite. The graphite ball size was small and distribution was more uniform. The ferrite in as-cast samples increased in value with the increase of silicon. When the Si content was 1.58%, cementite had appeared in as-cast sample, up to 1%. Cementite had not been found in samples with 1.78%Si,1.96%Si and 2.10%Si.The hardness and tensile of as-cast samples were related to the pearlite content and Si content in the organization. The hardness and tensile of the sample with 1.58%Si were higher, reached 156HBW,425MPa respectively; The hardness and tensile of the samples with 1.78%Si,1.96%Si and 2.10%Si were raised gradually.Selecting the heat treatment process, according to the difference of cast structure. When the Si content was 1.58%, we selected 900℃×4h+750℃×4h high and low temperature annealing. When the Si contents were 1.78%,1.96%and 2.10%, the best process scheme was760℃×6h low temperature annealing. Cementite and pearlite disappeared and becomed into ferrite with annealing.The hardness and tensile strength of heat treatment state samples improved with the increase of silicon content, while the elongation ratio reduced. In the range of silicon concent studied, the impact energy under-40℃condition were not obvious with the increase of silicon. The impact energy of Each sample were more than 14J. Corroded by 3.5%NaCl Solution, ferrite grain boundaries and ferrite graphite interface of low-temperature high-toughness nodular cast iron had been corroded firstly, but graphite had not been corroded. Along with the corrosion time prolonging, corrosion of the ferrite matrix was been aggravated gradually and ferrite graphite interface gradually widened. So that the substrate and graphite appeared more unenen phenomenon.The corrosion rate of material decreased with the increase of silicon, from 0.06122g/m2·hto 0.05562g/m2-h. The corrosion rates of four samples were less than 0.1g/m2h, they belong to 1 grade in five level of corrosion resistance standard.Accoording to the research of heat treatment, microstructure and properties of spheroidal graphite cast iron, we could got the following conclusions:The sample with 1.9%-2.1%Si had excellent comprehensive mechanical properties and corrosion resistance by 760℃×6h low temperature annealing. The final organization was ferrite and small round graphite. The tensile strength achieved 365MPa and the elongation achieved 25%. The impact energy was 14.36J under-40℃condition. The corrosion rate was 0.05842g/m2-·h. The comprehensive mechanical properties reached EN-JGS-350-22U-LT of European standards.