Nickel in cast iron is mainly manifested in the following areas: nickel is graphite elements, in eutectic transformation promote liquid iron turning according the stable lines; nickel can expand the austenite region, increase the stability of the austenite; improve the cast iron hardenability, for alloy cast iron, it helps to get the martensite-based matrix organization; nickel refinement of crystal organization.
In the Fe-C-Ni ternary alloy, the main role of nickel is to reduce the carbon content of the co-crystal nickel, increasing the temperature range of liquid iron by transformation mediated transformation and stable lines of stable lines, thereby suppressing the eutectic transformation carbonization precipitation, effectively reduces the chilling tendency of the cast iron.
The cast iron content 13.5 to 36% of nickel called nickel austenitic cast iron. Nickel austenitic cast iro’s graphite structure can be a sheet-like, also can be spherical. Its base tissue for a single austenite plus a small amount of carbides. Nickel austenitic cast iron’s matrix organization is stable austenite phase, so it is a heat-resistant, corrosion resistant materials, has also been used as a material of low thermal expansion coefficient, the non-magnetic material and the low temperature and high toughness material. According to the information provided by metal powder supplier, in order to give full play to the potential of nickel austenitic cast iron, nickel austenitic cast iron usually contains small amounts of other elements. For instance, they are often added to the chromium, molybdenum, copper, etc. to improve its corrosion resistance and abrasion resistance, by adding niobium to improve the weldability, and so on.
Nickel austenitic cast iron, after graphite spheroidization, thought it has no effect on its corrosion resistance, but can significantly improve its abrasion resistance. When the corrosive medium for suspended solids, choose the D-2 and D-3-type nickel austenitic cast iron is more appropriate; when casting has thermal shock resistance, should select the D-2, D-3, D-4 type nickel austenitic cast iron; When high strength is required and requires a low coefficient of thermal expansion at the same time, it preferably choose D-5 type.
Because of nickel powder with the ability of improving cast iron’s hardenability, so can add nickel into cast iron which containing chromium to improve its hardenability to obtain pearlite, martensite as white cast iron matrix organization. This cast iron both containing nickel and chromium called hard cast iron. Ni-hard cast iron mainly have two kinds: one is containing 4% nickel, 2% chromium. This Ni-hard cast iron microstructure by M3C carbides and martensite and austenite mixed matrix. Another is containing 6% nickel, and containing about 9% of chromium. This Ni-hard cast iron carbides of M7C3.
In ni-hard cast iron, due to the role of nickel, castings in the cooling conditions of the type within martensite-based matrix organization can be obtained. But nickel reduce MS points, the high amount of nickel matrix organization will contain more austenite, so that the wear resistance decreased. Under normal circumstances, the nickel content must be determined according to the wall thickness of the casting as well as the heat treatment process. In addition, the content of the other metal element in the Ni-hard cast iron should be determined according to specific circumstances, for example, chromium has the role of graphitization, and affect the type and morphology of the carbides. The silicon help graphitization in the subsequent cooling process and affect the hardenability, therefore the silicon content should be as low as possible. Manganese is a element to steady austenite, so its content must be limited too.
Source:http://www.mhcmp.com
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