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.

Titanium

Element type: Metal element

Atomic number: Relative atomic mass: # Physical properties

Appearance and luster: Titanium, this silver-white transition metal, shines with the characteristic luster of metals.

Crystal structure: Titanium has two allotropes, α-titanium which belongs to the hexagonal crystal system, and β-titanium which is cubic. The transformation temperature between them is 85℃.

Mechanical properties: Pure titanium has excellent plasticity, although its strength is relatively low. However, by adding alloying elements and strictly controlling impurity content, the mechanical properties of titanium can be significantly improved. ​​​​​​​

 

Chemical Properties

Stability: At room temperature, titanium has relatively stable chemical properties and does not readily react with most substances. However, at high temperatures, it can undergo chemical reactions with various elements and compounds.

Corrosion Resistance: Titanium exhibits extremely strong corrosion resistance, especially in harsh environments such as seawater, aqua regia, and moist chlorine gas. It is important to note, however, that titanium reacts vigorously with dry chlorine gas.

Reactions with Compounds: Titanium can react with fluorides, chlorides, sulfuric acid, and other compounds under specific conditions to form corresponding titanium compounds.

 

Existence and Preparation

 

Existence Forms: Titanium is widely distributed in nature, mainly in the forms of ilmenite (FeTiO3) and rutile (TiO2). Additionally, it is present in living organisms, rocks, water bodies, and soil.

 

Preparation Methods: Extracting titanium from the main ores requires complex smelting methods such as the Kroll process or the Hunter process, involving multiple steps including reduction and refining.

 

Application fields

Aerospace: In the manufacturing of aircraft, engines, and rocket components, titanium and its alloys play a crucial role and are hailed as the "space metal" in the aerospace industry.

Shipbuilding: Due to its excellent resistance to seawater corrosion, titanium is widely used in the shipbuilding industry, such as in the manufacturing of propellers, propeller drives, and submarine antennas.

Chemical and petrochemical: In the manufacturing of chemical equipment such as heat exchangers and wet chlorine gas coolers, titanium is favored for its superior corrosion resistance and thermal stability.

Transportation: In the manufacturing of automobiles and motorcycles, titanium is often used to make exhaust pipes, mufflers, and suspension springs, among other components, to reduce overall weight and enhance performance.

Medical field: Due to its excellent biocompatibility, titanium plays a key role in the manufacturing of medical devices and artificial joints.

 

The application of titanium in ductile iron

In ductile iron, the presence of trace amounts of titanium can lead to the formation of distorted graphite and increase the section sensitivity after magnesium treatment. When the titanium content exceeds 1%, it will cause graphite distortion, thereby reducing the elongation after fracture and impact toughness, and simultaneously causing a significant decrease in tensile strength and yield limit. In addition, titanium has a strong reducing ability and can reduce trace elements such as antimony, bismuth, and lead in the molten iron, thereby interfering with the spheroidization process of graphite.

 

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