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.

 

Tin

Basic Information

Element symbol: Sn

Atomic number: 50

Relative atomic mass: 118.71

Position in the periodic table: Group IV main group element

 

Physical Properties

Appearance and Luster: Tin, a white metal with a slightly bluish luster, exists mainly in the form of white tin at room temperature and has a silvery-white color. The surface luster is closely related to the impurity content and casting temperature; the lower the casting temperature, the darker the surface color of tin.

Density: The density of solid tin at 20°C is approximately 3 g/cm³ (or a similar value, such as 28 g/cm³). It is worth noting that the density of liquid tin decreases gradually with increasing temperature.

Melting and Boiling Points: The melting point of tin is approximately 296°C (or a similar value, such as 288°C to 289°C), while the boiling point is around 2270°C (or a similar value, such as 2260°C to 2507°C).

Hardness and Ductility: Tin has a Mohs hardness of only 75, making it one of the softer metals. Despite this, it has excellent malleability, second only to gold, silver, and copper, and can be easily rolled into extremely thin tin foil. However, it has poor ductility and cannot be drawn into fine wires. When a tin bar is bent, due to the friction and damage between grains, a sound similar to breaking is produced, which is known as the "tin cry" phenomenon.

Allotropes: Tin exists in three allotropes: gray tin, white tin, and brittle tin. Below 2°C, white tin gradually transforms into gray tin, and at -30°C, the transformation rate is the fastest, with tin blocks rapidly turning into powder. This phenomenon is called "tin pest."

Other Properties: Tin is a non-magnetic metal with a relatively large coefficient of thermal expansion. Therefore, its volume changes significantly when the temperature changes.

 

Chemical Properties

Stability: At normal temperatures, tin is relatively stable and does not oxidize in air. It mainly exists in nature in the form of oxides (such as cassiterite) and various sulfides (such as stannite). However, when heated, tin reacts with oxygen to form tin dioxide.

Valence: In compounds, tin can have a valence of either two or four.

Reactivity: Tin has the ability to react with halogens (such as chlorine, bromine, and iodine) under heating conditions to form corresponding halides; it can also react with sulfur to form stannous sulfide. Additionally, tin can dissolve in concentrated acids and hot alkaline solutions, but its dissolution in dilute acids is relatively slow.

 

Application fields

Electronics industry: Tin plays a crucial role in the electronics industry. It is widely used in the production of solder, tinplate, and various alloys. Solder is a key material for electronic soldering, while tinplate is favored by multiple industries for its excellent sealing, preservation, light-blocking, durability, corrosion resistance, and metallic decorative appeal, including food packaging, military, instrumentation, and electrical appliances.

Metallurgy and machinery: Tin has extensive applications in the fields of metallurgy and machinery. It can be used to manufacture important alloy materials such as bearing alloys and bronze, which play critical roles in industries such as shipping, chemical engineering, construction, and currency manufacturing.

 

In the field of chemical engineering, compounds and chemical products of tin are also of great significance. They are widely used in the production processes of industries such as dyes, rubber, plastics, and pesticides.

 

Other fields: The uses of tin extend far beyond this. It can also be used to manufacture electric carbon products, friction materials, oil-impregnated bearings, and powder metallurgy structural materials, etc. These materials play an indispensable role in various industrial fields.

 

Health and Environmental Impact

Although metallic tin itself is non-toxic when used in large quantities, certain tin compounds (such as tin dioxide and tin tetrachloride) may be toxic. Therefore, when using and coming into contact with tin, we must take appropriate protective measures to ensure health and safety. At the same time, the mining and processing of tin may also have certain impacts on the environment, so environmental protection measures need to be taken to reduce environmental pollution.

 

The application of tin in ductile iron

Tin also plays a unique role in the manufacturing of ductile iron. When an appropriate amount of tin is added to ductile iron, it can significantly increase the amount of pearlite in the matrix, thereby improving the material's properties. However, the addition of tin needs to be precisely controlled, as excessive tin may reduce impact toughness, deteriorate the morphology of graphite, and cause an increase in the brittle transition temperature.

 

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