What should be paid attention to when producing nodular cast iron?

The alloying treatment of cast iron can be traced back to 1930s and 1940s. Alloying treatment has made a qualitative leap in the properties of cast iron, and at the same time, some special cast irons with wear resistance, corrosion resistance and heat resistance have been born. It was also during this period that cast iron was produced by inoculation. At the end of 1940s, the inoculated spherical graphite cast iron replaced the common flake graphite cast iron, which we called nodular cast iron. Classification of spheroidizing elements and anti-spheroidizing elements. Spheroidizing elements are generally divided into three groups according to their spheroidizing effect. The first group: Mg, y, Ce, La, Pr, Sm, Dy, Ho, Er. The second group: barium, lithium, cesium, rubidium, strontium, thorium, potassium and sodium. The third group: aluminum, zinc, cadmium and tin. The first group has the strongest spheroidizing ability, followed by the second group and the third group is the weakest. When magnesium is used as spheroidizing element, the third group of elements often produce anti-spheroidization. Anti-spheroidizing elements: Sulfur and oxygen are common anti-spheroidizing elements in cast iron, while titanium, aluminum, boron, arsenic, lead, tin, antimony, bismuth, tellurium and selenium are common anti-spheroidizing elements in hot metal.

Problems needing attention in production of nodular cast iron. The sulfur content and other trace elements in molten iron should not be too high. If the content of trace elements such as sulfur in molten iron is too high, more nodulizer or nodulizer with higher rare earth content is needed, so the cost of nodulizer will increase, and too much nodulizer will cause more slag inclusion, which is not conducive to the stability of casting quality. Excessive rare earth content is easy to produce crushed stone ink on large cross-section castings. Stability of spheroidizing treatment. Spheroidizing treatment process is the key process in the production of ductile iron. Only when the spheroidizing treatment process is stable can the casting quality be stable. According to different products and different sulfur content of hot metal, the addition amount of nodulizer and inoculant should be written into the work instruction and strictly implemented. Avoid long waits. After spheroidizing inoculation, it should be poured immediately. Because with the extension of time, the residual magnesium will burn out and the inoculation effect will decrease. Avoid excessive residual magnesium content. Higher residual magnesium content will increase the shrinkage tendency of castings. For general ductile iron, the residual magnesium content (mass fraction) should be controlled between 0.035% and 0.045%, and for high nickel ductile iron, the residual magnesium content should be controlled between 0.06% and 0.07%. Use better inoculants for castings with higher requirements. For wind power parts and high-speed rail parts with high requirements, stream inoculation agent with strong inoculation effect (such as patented Ultraseed/Ce) should be selected. Its characteristic is that the number of graphite balls can be significantly increased, and the graphite balls are round.