Research On Structure Control And Performance Of Ductile Iron For Efficient Wind-power

As wind turbines continue to grow in size,wall thicknesses of ductile iron parts such as bases and hubs for wind turbines continue to increase,and the thickness of certain parts of high-efficiency wind power ductile iron parts reaches 300 to 500 mm;Due to thick sections,the cooling rate is slow and the solidification time is long.The gestational recession in the thick parts is more serious,which affects the organization and performance of these parts.At the same time,the use of ductile iron castings for offshore wind turbines tends to have a large pitting corrosion rate in the salt spray environment,which will also seriously affect the service life of components.In this paper,aiming at highly efficient ductile iron castings for wind power,the influence of casting temperature,cooling conditions and heat treatment on the microstructure and properties of large-section ductile iron was studied to improve the strength and toughness of ductile iron.The effects of ductile iron in different environments on the chloride ion environment were studied.Corrosion resistance,revealed the corrosion mechanism of ductile iron parts for offshore wind power.Properly lowering the casting temperature can improve the structure and performance of ductile iron for efficient wind power.At 1380°C,the spheroidization grade is 3,the graphite sphere size is 5,and the pearlite content in the weak zone is 15% or more and contains a large number of inclusions caused by the spheroidizing element segregation;the "weak zone" has a tensile strength of 333.3MPa,the elongation after fracture was 15.9%,and the impact toughness at-20°C was 7.8J.At 1320°C,the spheroidization grade is Grade 1 and the graphite sphere size is Grade 7,which is improved by 2 grades.The pearlite content is reduced to less than 10%,the inclusions are obviously eliminated,and only the weak region contains a very small amount of inclusions;The tensile strength was 356.4MPa,an increase of 7.0%,the elongation was 24%,an increase of 51%,and the low temperature impact toughness at-20°C was 9.1J,an increase of 16.0%.The thickness of the inner and outer surfaces of the casting mold was increased by about 1/3 of the thickness of the cast iron,which further optimized the microstructure and properties of ductile iron.The level of spheroidization near the surface of the casting reached the highest level,the size of graphite spheres reached the finest level 8,and the ferrite content was higher than 98%,greatly improving the ductile iron structure.After adding cold iron,the tensile strength of the surface area of the ductile iron reaches 407.0 MPa,and only 373.0 MPa when the cold iron is not added,which is 9.1% higher.The elongation rate of each part is increased,and the elongation is increased near the casting surface.The more significant,the surface elongation of the test piece was 26.0% when cold iron was added,only 11.4% when cold iron was not added,which was increased by 128.1%;the impact toughness of each part at-20°C was greatly improved.The average cold impact toughness improvement effect of the Ministry was the most significant.The average impact toughness of the core without cold iron cooling scheme was 6.3J,and the average impact toughness of the core with cold iron cooling scheme was 15 J,an increase of 137.0%.The effects of low temperature and high temperature graphitization annealing processes on the structure and properties of highly efficient ductile iron for wind power are different.After low temperature graphitization annealing,pearlite is reduced,and the spheroidization rate is significantly increased.After high temperature annealing,ductile iron contains a large amount of pearlite,and the spheroidization rate is low,but after two-stage graphitization annealing,Ferrite is mainly composed of ferrite,with less pearlite and a higher degree of spheroidization rate.Comprehensive evaluation of the main mechanical parameters,the best process of low temperature graphitization annealing is 740 °C × 6h,the tensile strength of the nodular cast iron obtained is 364.9MPa,which is 7.1% higher than the as-cast 340.6MPa,and the highest impact toughness at-20°C is 13.3J.8.3J increased by 59.7% when compared with the as-cast condition,and 920°C×2h+740°C×5h achieved the optimum process for the hightemperature graphitization.The obtained ductile iron has a tensile strength of 379.0MPa,which is 11.3% higher than the as-cast condition.The highest toughness is 12.0J,which is 44.1% higher than the as-cast state.The corrosion resistance of ductile iron castings in the Cl ion environment is influenced by the graphite balls and the matrix.When the matrix structure is the same,the larger the graphite sphere diameter is,the better the corrosion resistance is;when the graphite sphere size grade and the spheroidization rate are similar,the ductile iron of the pure ferrite matrix structure has good corrosion resistance,with the pearlite content increasing.Its seawater corrosion resistance is weakened;and with the increase of retained austenite content,its corrosion resistance is enhanced.The corrosion of the matrix in the vicinity of the graphite ball is the most serious and gradually spreads to the surroundings.When the graphite spheres are unevenly distributed,the graphite ball is the most prone to corrosion.