Study On Microstructure And Mechanical Properties Of Heavy Section Ferrite Ductile Iron Applied For Wind Power Casting

The microstructure defects in the thick position of heavy section ductile iron(HSDI), such as rare graphite, abnormal nodularizing and high percent of pearlite, are responsile for the negative effect on the tensile properties and low temperature impact properties due to the slow cooling rate, thus, the practical application of HSDI in wind power field is limited.This paper focuses on the improvement of toughness of HDSI at low temperature. Through the investigation on the test castings with different modulus, the effect of post inoculation process, spheroidizing process, pretreatment process and the content of Sb on the microstructure and mechanical properties of HSDI are explored. Based on the optimized casting process, the elongation and low-temperature impact toughness are significantly improved on the premise of ensuring sufficient tensile strength.The thick position of HSDI exhibits heterogeneity on both microstructure and mechanical properties. Interestingly, the center of HSDI, which is usually considered to exhibit the worst properties due to the longest solidification time, shows relatively good microstructure and tensile properties except poor low-temperature impact toughness. Actually, the "weak area" of HSDI locates near the edge of the center section which forms a circle of annular region. The microstructure of weak area can be characterized as the minimum number and coarsest graphite nodules and the largest amount of pearlite, leading to the worst tensile properties and low-temperature impact toughness. Therefore, the criterion to optimize the casting process must combinatively consider the the microstructure and mechanical properties of both weak area and the center of HSDI.The inoculation effect and anti-fading ability of inoculants which significantly affect the microstructure and mechanical properties of HDSI depend on the trace element added in the inoculants. Bi can substantially improve the inoculating effect, but the anti-fading ability of the inoculants which contains Bi are relatively poor. In addition, adding 0.002wt% Bi could obviously leading to the formation of chunky graphite at weak area of casting with 5cm modulus. Inoculants containing trace S/O have excellent anti-fading ability. The graphite nodules at center of the HSDI adopting 0.2wt% S/O inoculants are dense and highly rounded, and the graphite nodules at weak area also have high roundness except some coarse primary graphite.The RE content and Ce/La ratio in light RE spheroidizer have a great effect on the microstructure and mechanical properties of HSDI. The spheroidizing effect of 2# spheroidizer with RE%=0.5%, Ce=0 is relatively poor. The graphite nodules of the casting spheroidized with this nodulizing agent are less and poorly rounded, leading to the poor impact toughness at low temperature. Compared to the 2# spheroidizer, the spheroidizing effect of 3# spheroidizer with RE% =0.5%, Ce/La=2 is improved, however, it can cause the lowest low temperature impact resistance of casting due to the formation of a large amount of pearlite in the microstructure. By maintaining the Ce/La ratio, the spheroidizing effect is enhanced significantly when RE content increased from 0.5% to 1%, and the most nodules, the least pearlite and the best comprehensive mechanical properties could be obtained when adopting this agent as spheroidizer.Pretreatment process can significantly improve the microstructure and mechanical properties of HSDI. For the weak area of 360mmx360mmx360mm casting, the number of graphite nodules is increased by 14%, the quantities of pearlite are decreased from 5.8% to 1.6% and the elongation is enhanced from 9.4% to 12% compared to the casting process without pretreatment. At the same time, the impact toughness value at-20℃ is improved from 7.4J to 10.2J, and the-40℃ impact toughness value is increased by 26compared to original process.Trace elements Sb can simultaneously improve the roundess of graphite and inhibit the formation of chunky graphite, resulting in the improvement of the mechanical properties of castings, but excessive content of Sb will lead to the formation of large amount of pearlite at the weak area of casting, leading to the reduction of the elongation and toughness. The optimized addition content of Sb is related to the modulus of the casting, i.e., when the modulus of casting is less than or equal to 4cm,50ppm Sb would obtain the best comprehensive mechanical properties, and when casting module is above 4cm, if 100ppm Sb were added into the molten, the tensile properties and low-temperature impact toughness of casting would be more outstanding.After improving the technology, for 240mmx240mmx240mm casting, the tensile strength and elongation of casting center is of 378.6MPa and 21.7%, and the achieved as-cast Charpy impact strengths are as follows:13.07J(-30℃),9.4J(-40℃); the tensile strength and elongation of the weak area is of 369.8MPa and 16.3%, and the achieved as-cast Charpy impact strengths are as follows:11.1J(-30℃),8.0J(-40 ℃). For 360mmx360mmx360mm casting, the tensile strength and elongation of casting center is of 376.1MPa and 20.2%, and the achieved as-cast Charpy impact strengths are as follows:12.89J(-30℃),9.09J(-40 ℃); The tensile strength and elongation of the weak area is of 364.2MPa and 14.3%, and the achieved as-cast Charpy impact strengths are as follows:9.09J(-30℃),7.23J(-40℃).