Knowledge

Trace elements in ductile iron

During the manufacturing process of ductile iron, there are a series of trace elements that interfere with the spheroidization process. Although the mass fraction of these elements in ductile iron is only a few parts per ten thousand or a few parts per hundred thousand, they can significantly affect the spheroidization effect. This interference is closely related to the magnesium content in the melt and the cooling rate, and the effects of various interfering elements are superimposed.

 

Common interfering elements include Sb, Sn, Bi, Te, Pb, Ti, Se and As, etc. These elements can be classified according to their consumption characteristics of magnesium: Te and Se belong to magnesium-consuming type, which are prone to combine with magnesium and promote the formation of vermicular graphite, undercooled graphite and flake graphite; Sb, Sn, As and Ti belong to grain boundary segregation type, which tend to form distorted graphite at the grain boundaries; while Pb and Bi belong to the mixed type, forming distorted graphite at low content and promoting the formation of undercooled graphite and flake graphite at high content.

 

To suppress the anti-spheroidizing effect of these trace interfering elements or to broaden their allowable range, rare earth elements can be added to ductile iron. The addition amount of rare earth elements should be adjusted according to the purity of the molten iron, and it is usually appropriate for the residual cerium mass fraction to reach 0.03%.

 

It is particularly worth mentioning the lead element. When the iron liquid contains lead and the mass fraction is between 0.01% and 0.05%, in order to achieve a spheroidization rate of over 85%, the mass fraction of residual magnesium must exceed 0.6%, and the mass fraction of residual cerium also needs to reach above 0.05%.

 

Next, we will delve into the outstanding one among these trace elements.

 

Sb, a metalloid element, possesses unique physical and chemical properties. Its elemental symbol is Sb, atomic number is 51, and its relative atomic mass ranges from 175 to 176. At room temperature, Sb appears as a silver-white crystalline solid, which is brittle and has a relatively low melting point of 630°C, while its boiling point is as high as 1635°C. Additionally, Sb has relatively stable chemical properties; it does not react with oxygen at room temperature but can combine with oxygen to form Sb trioxide at high temperatures. Moreover, Sb is insoluble in water and common acids and bases, but it can dissolve in aqua regia and hot concentrated sulfuric acid.

 

Physical properties

Sb has distinct physical properties. It is a silver-white crystalline solid that is both brittle and fusible. At room temperature, although its electrical and thermal conductivity are not as good as those of other metals, it still has unique application value. Notably, Sb can react with oxygen at high temperatures to form Sb trioxide. This characteristic makes it particularly crucial in certain applications.

 

Chemical Properties

Although Sb has relatively stable chemical properties, it can still react with various elements and compounds under specific conditions. Sb can form two types of halides, among which the trihalides and pentahalides have different spatial configurations. Additionally, Sb oxides such as Sb trioxide and Sb pentoxide exhibit amphoteric characteristics and can react with acids to form corresponding Sb salts.

 

Sb has four allotropes: gray Sb , black Sb , yellow Sb and explosive Sb . These allotropes exhibit certain differences in physical and chemical properties, which endows Sb with diverse characteristics in applications.

 

Existence and Preparation

In nature, Sb mainly exists in the sulfide mineral stibnite. Industrial methods for preparing Sb include roasting and high-temperature reduction processes. These methods enable the efficient extraction of Sb and its application in various fields.

 

Application

Sb and its compounds are widely used in various fields due to their unique properties. For instance, they possess excellent flame retardancy, corrosion resistance and anti-corrosion characteristics. However, it is worth noting that Sb is not an essential element for life and it poses certain toxicity and carcinogenic risks to living organisms. Therefore, special caution should be exercised when using Sb and its compounds.

 

The role of Sb in ductile iron

During the production of ductile iron, adding an appropriate amount of Sb can significantly improve the roundness of graphite spheres and increase their number. This effect is particularly evident in the production of large-section ductile iron. Through inoculation treatment and adding 0.05% Sb by mass, very round graphite spheres can be obtained, and the number of graphite spheres is nearly doubled compared to the situation without Sb . This discovery provides new ideas and methods for the production of ductile iron.

The addition of Sb can increase the amount of pearlite in the matrix of ductile iron. Generally, the appropriate addition amount of Sb is controlled within the range of 0.06% to 0.1% by mass. To further enhance the performance of large-section ductile iron, in addition to adding an appropriate amount of Sb , a certain amount of rare earth elements should also be added. In conventional ductile iron production, the residual rare earth content is usually set at 0.1% to 0.3% by mass.

 

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