Inoculation of cast iron generally involves two steps: the first is an inoculation before the iron is poured into the furnace and the second is an inoculation after the cast iron has been cast. The first stage is also known as early or ladle inoculation. The second stage is also called late or instantaneous inoculation. The first step is important, as a long gap between the addition of inoculant and the pouring of the metal can reduce the inoculation. In these cases, the second process is used to overcome the decline. A secondary inoculation is called flow inoculation.
Calcium is a component of cast iron that can enhance the inoculant effect of Ba and Al. The amount and method of calcium addition in a cast iron inoculant depend on the desired result. Calcium content in a cast iron inoculant is a proportion of the inoculant weight in the final solid product.
The most common casting inoculant is ferrosilicon, which contains calcium and aluminum. Without calcium, ferrosilicon has very low effect on gray cast iron. However, when combined with calcium, it reacts with oxygen and nitrogen to form a high melting point compound.
Carbides in white cast iron are unstable and tend to decompose into graphite, ferrite, and austenite. Rapid cooling or inoculation with the carbide stabilizing alloying element Cr can suppress this tendency. Other methods include inoculation with Te or Bi. In addition, white cast iron undergoes several changes when cooled to ambient temperature. Among these changes is the rejection of hypereutectoid C. This material can appear in the form of spines or particles within the austenite grain.
Inoculation is a critical part of cast iron manufacturing. It suppresses carbide formation and enhances ductility and strength. It is an ideal method for improving structural properties of cost-effective alloys.
Microporosity in ductile iron can be controlled by inoculation of the iron with a nutrient or other substance. It is an efficient way to reduce the tendency of the iron to develop cracks and chills in thin sections. This process also promotes fast cooling.
Inoculation of cast iron is an important practice for production of gray and ductile cast iron worldwide. The process is ideal for improving the structure of these cast iron alloys and augmenting their mechanical properties.
Cast iron is one of the most common materials, and it is widely used in many applications. Its high sulphur content is due to a variety of factors, including the presence of coke in the iron-making process. Sulphur content below 0.15 percent tends to promote Type A graphite, while a higher content can cause blowholes in green sand castings. For these reasons, most foundries keep sulphur content below 0.15 percent. The typical range for cupola-melted irons is 0.09 to 0.12 percent.
The composition of cast iron largely depends on the type of pig iron used, the conditions of melting in the cupola, and any special alloying additions that may be used. Grey cast irons have low sulphur content, while black cast irons are largely free of these impurities.
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