Nickel

High-melting, silvery white, polishable heavy metal.

SymbolNi
Atomic number28
Atomic weight58.69
Specific thermal weight at 20°C8.9g/cm3
Melting point1455°C
Boiling point2910°C
Melting heat306 kJ/kg
Specific thermal capacity at 20°C0.445 kJ/(kg · K)
Thermal conductivity90 kJ/(m · K)
Therm. coefficient for linear expansion Coefficient for linear expansion13 10-6/K

Nickel is ferromagnetic and has a high corrosion resistance.. In this regard it is superior to copper. As a main component of numerous nickel alloys, it is also used as an alloy element in cast iron and steel as well as in many copper and light-metal alloys.

Nickel in steel
Nickel is soluble in γ iron in any proportion, in α iron the solubility is 10%. By bonding mixed crystals nickel increases the ferrite strength and toughness and slightly improves the hardenability of the austenite. With higher carbon contents nickel promotes the supercooling capacity of the austenite. Upon addition of nickel, a continuous solid solution series is formed in the steel, i.e. there is no tendency for the occurrence of intermetallic compounds. As nickel significantly decreases the temperature for the transformation of γ into α iron, steels of this type with more than 30% Ni remain purely austenitic even after slow cooling at room temperature. Nickel is not a carbide former and occurs in steel almost exclusively in solid solution.

Nickel does not have a significant influence on the diffusion rate of the carbon in the austenite. However, the diffusion rate of nickel, which itself is much lower, can be influenced by changing the carbon content.

Nickel in cast iron
Nickel is soluble in iron in any proportion. In cast iron, it has a slightly graphitizing effect, but also stabilizes the pearlite and reduces the content of pure ferrite. Consequently, in cast iron, nickel acts towards are more uniform structure and more balanced other characteristics. Cast iron parts containing nickel solidify homogenously and their hardness is only slightly different in thicker and thinner cross sections.

Smaller nickel additions of approximately 0.1 to 1.0% lead to finer pearlite structures; higher nickel contents lead to the formation of martensite and austenite. The fine and resistant pearlitestructure leads to good machining properties and hardness of the cast iron.

Due to its regular structure and the increased densitynickel-alloyed iron is the preferred choice for large castings in the chemical industry, mainly because graphitizing areas and other structural inhomogeneities which reduce corrosion resistance are largely missing.

The graphitization effect of nickel is only one third of the one of silicone. It is therefore advisable to keep the silicone content low in nickel-alloyed castings, if graphitization is to be restricted. However, this is not always possible in foundries, which is why chromium is added in such cases, if possible.

Inocculation of cast iron with a nickel-silicone alloy leads to a fine graphite structure and increases the strength. 

Ni-Resist is a cast iron group with high contents of nickel and copper and a consequently very reistant austenitic structure. A range of special physical and mechanical properties depend on this structure. Ni-Resist has good corrosion resistance, mainly in weakly alkaline solutions, and good thermal resistance if temperatures are not too high. Nicrosilal refers to a nickel-alloyed, austenitic cast iron which has excellent thermal resistance, even at high temperatures. Molybdenum additions repress the transformation of the cast iron in the the pearlite stage and promote the formation of an acicular bainitic structure (referred to as “acicular bainite”). Ni-Hard is a white solidified, weaar-resistant cast iron with nickel. The nickel content is high enough to ensure the formation of a martensitic matrix in the as-cast condition. Excessive nickel contents may lead to residual austenite contents.

 

 

 

 

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