Currently, conventional ductile iron, which is based on ferrite and pearlite, still accounts for the vast majority of ductile iron production.
1.1.1 Strengthen the control of factors affecting the quality of ductile iron. The microstructure and properties of ductile iron depend on the composition and crystallization conditions of cast iron, as well as the quality of the spheroidizing agent used. Research suggests that in order to ensure the mechanical properties of ductile iron, strict control must be carried out on the specific wall thickness of the casting, pouring temperature, spheroidizing agent used, spheroidizing treatment process, cooling parameters, and effective slag discharge measures. Proper reduction of carbon equivalent, alloying, and heat treatment are effective measures to improve ductile iron.
1.1.2 Effective control of production of ferritic ductile iron and ductile iron. The main factors controlling the ductile iron matrix include the composition of cast iron, the type of spheroidizing agent and inoculant used, the method of addition, and cooling conditions. The composition control of cast ferrite nodular iron is slightly hypereutectic, with slightly higher carbon content but no graphite floating, slightly lower silicon content, and less than 3% silicon content in the inoculant. The lower the manganese content, the better. Mn should be less than 0.04%, and sulfur and phosphorus should be low, with S ≤ 0.02% and P ≤ 0.02%. This is because silicon can improve the microstructure and corresponding plasticity of nodular iron, and Si=3.0-3.5% can obtain the entire ferrite structure.
Some studies have shown that when Si=2.6-2.8%, cast iron has the highest elongation and impact toughness. However, the microsegregation of silicon in iron becomes more severe with the increase of phosphorus content, and has adverse effects on mechanical properties, especially when the temperature is below zero degrees. For low sulfur content, low magnesium and low rare earth spheroidizing agents can be used to spheroidize and reduce the occurrence of “black spot” defects, which are mainly aggregates of magnesium, cerium sulfides, and oxides. In addition, low silicon spheroidizing agents should be used to ensure multiple pregnancies.
For pearlite nodular iron, the manganese content in cast iron can be increased to 0.8-1.0% during production. For some castings used as wear-resistant crankshafts, the manganese content can be increased to 1.2-1.35% to produce as cast pearlite element copper. When the addition amount is greater than 1.8%, it hinders graphite spheroidization but promotes complete pearlite transformation of the matrix. Generally, the copper content in ductile iron should be less than 1.5%. Tin is a strong pearlite element, and its influence on hardness is greater than that of copper and manganese. However, when Sn ≥ 1.0%, it causes graphite distortion, so its content should be limited to below 0.08%.
1.1.3 The role of rare earths in ductile iron Rare earths can promote the spheroidization effect of magnesium alloys (spheroidization rate and roundness of spheres), and their effect on preventing spherical graphite distortion in thick walled ductile iron parts has been emphasized. This is also one of the main reasons why rare earths are included in spheroidizing agents both domestically and internationally.
Some elements in castings can damage and hinder graphite spheroidization, known as spheroidization interference elements. Interference elements are divided into two categories: one is the consumption of spheroidization element type interference elements, which react with magnesium and rare earths to form MgS, MgO, MgSe, RE2O3, RE2S3, RE2Te3, etc., reducing spheroidization elements and thus damaging the formation of spherical graphite; Another type is intergranular segregation type interference elements, including tin, antimony, arsenic, copper, titanium, aluminum, etc. During eutectic crystallization, these elements are enriched at grain boundaries, promoting the formation of deformed dendritic graphite in the later stage of eutectic carbon. The larger the atomic weight of spheroidization interference elements, the stronger their interference effect. Many studies have now found the critical content of interference elements in cast iron. When the content of these elements is less than the critical content, distorted graphite cannot be formed.
In cast iron with interfering elements, adding rare earth elements can eliminate their interference effect. Research reports indicate that the sum of interfering elements in cast iron should be less than 0.10%, that is, z=Ti+Cr+Sb+V+As+Pb+Zn+…<0.10%. Studies have shown that neutralizing Al, Sb, TI, Pb, Bi, etc. in molten iron only requires adding 0.005-0.04% Ce, for example, neutralizing Ti, Pb, Sb, Al, etc. only requires adding 0.005-0.007%, 0.014%, 0.15%, and 0.008% Ce, respectively. Interference elements have a greater destructive effect when the casting wall thickness and cooling rate are slow. Interference elements also have an impact on the ductile iron matrix, with Te and B strongly promoting the formation of white spots, Cr, As, Sn, Sb, Pb, and Bi stabilizing pearlite, and Al and Zr promoting ferrite.
At present, some composite spheroidizing agents of spheroidizing elements and interfering elements are being developed to improve the processing effect of large section ductile iron and the roundness of graphite balls.
1.1. Strengthening ductile iron testing is an important measure to ensure its quality. Currently, research is being conducted on the development of line analysis, which involves analyzing products during the production process to determine their quality. Many units have already used ultrasonic waves to analyze the quality of castings under large-scale production conditions.
When using ultrasound to measure the microstructure of cast iron, the sound velocity of flake graphite is 4500m/s, that of vermicular graphite cast iron is 5400m/s, and that of ductile iron is 5600m/s. In addition, the change in high-frequency attenuation rate in cast iron can also determine the type of cast iron. The center frequency of ductile iron is 5MHz, while that of flake cast iron is only 1.5MHz.
At present, some units are using ultrasound to determine the level of spheroidization, which can determine qualified spheroidization levels and unqualified products (between levels 3 and 4), but cannot conduct more detailed grading determination yet. This method is being improved.
1.2 Austenitic ductile iron In the 1970s, the Netherlands, China, and the United States independently announced their successful research on bainitic ductile iron almost simultaneously. China’s successful research was on lower bainite, the United States on lower bainite+martensite, and the Netherlands on upper bainite+austenite. The most representative achievement of the Netherlands is now known as Austenitic ductile iron. In 1977, M. Jokason announced that Karkkila Foundry, a subsidiary of Kgmi Kgmmene in the Netherlands, had developed a new type of cast iron with excellent properties, known as austenite bainite ductile iron. He presented a paper on this invention at the 45th International Annual Conference held in 1978. This invention was patented in 13 countries including the United States, the United Kingdom, France, and Canada (US Patent No. 3860457, Dutch Patent 1996/72, and former West German Patent 2852870), which attracted attention from various countries and was hailed as one of the major achievements in cast iron metallurgy in recent decades. Austenitic ductile iron combines high strength, high toughness, and high wear resistance. The standards in the UK include NE-GJS-800-8, EN-GJS-1000-5, and EN-GJS-1400-1. The composition of austenitic ductile iron is the same as that of conventional ductile iron, and the spheroidizing agent and treatment process are also the same. The difference is that isothermal quenching treatment must be carried out. Different isothermal quenching temperatures can obtain different matrices such as upper bainite+austenite, lower bainite+austenite, and lower bainite+martensite. This type of cast iron is expensive and difficult to produce. Although its application is constantly expanding, its total quantity is not large, and it is known as the material of the 21st century.