The structure of white cast iron - what is the fundamental difference?
The solubility of cementites( Fe3C, where the C content is about 6.5%) in austenites( not a chemical compound of iron and carbon, but an inclusion solution where free carbon is embedded in the iron lattice) on the classical iron-cementite state diagram( according to such diagrams,phase alloy at a particular temperature point) is slightly more than two percent.
In cast iron, carbon can be present as cementite and graphite. Forms of grains of graphites and the ratio of their quantity with cementites( also the number and shape) determine the types of cast iron.
In composition they can have additions of silicon, phosphorus, sulfur, manganese. In addition, in view of brittleness, sometimes alloying with aluminum, nickel, chromium or vanadium is used. Obviously, with changes in characteristics and compositions, in the spectrum of cast iron alloys, carbons can be contained in a larger amount, like lamellar graphites. And the color of the kinks will be gray. Therefore, it is customary to call such varieties gray.
In such alloys of iron, carbons and silicon, sulfur, manganese and phosphorus are also present. Unlike the white alloy, the gray carbon is contained in a free form. While the structure of white cast iron contains carbons in the form of iron carbides.
As already mentioned, white is called cast iron, in which the predominant bound carbon structures are ledeburites( as a kind of state of cementitious melting points), perlites and actual cementites. This combination gives the material a high hardness and brittleness, which makes its direct application difficult. From this application it can be called its use for the production of magnificent balls and cilpels( cylinders) for grinding ore materials, rocks, slags, clinkers, and other materials.
White and Bleached Cast Iron - Application of
The composition of white cast iron described above determines its high hardness, combined with brittleness, which makes it difficult in general to process the workpieces by mechanical methods( cutting, milling, etc.) and requiring special methods of such processinghigh-speed steels).In pure form, the use of white cast iron is rather limited.
The industry produces a special, called pig iron, white cast iron, the application of which is specialized for melting in various steels. The standardization of GOSTs determines the reduced content in its composition of silicon additives and a higher concentration of manganese. Such preforms are produced by injection methods.
Depending on the degree of graphitization in the smelting process, half-step, transition stages of cast irons can be obtained, these alloys are commonly referred to as bleached cast iron. Such an alloy contains along with iron carbides( chemically bonded carbon-Fe3C) graphite carbon( free carbon-C).
And this cast iron is used as friction parts of dry friction mechanisms, such as all known brake pads. Along with that, such alloys are also characterized by high wear resistance, therefore, shafts and rolls are performed on rolling equipment of wide use.
Additives and marking of white cast iron
Additional alloying of white cast iron with chromium and nickel makes it possible to obtain heat-resistant materials that have a fantastically high resistance to wear. They are called: wear-resistant. Such alloyed white chromium cast iron is used for casting parts( for example, blades and blades of various pumping machines and turbines).
After casting, the resulting preforms are normalized by heat treatment. The products obtained are distinguished by a very high surface hardness, surprising resistance to the action of acids( even at temperatures of one thousand degrees Celsius).
In addition to alloying with chromium and nickel, in wear-resistant cast iron special additives( in a smaller amount) can be copper, titanium, molybdenum, vanadium. Such additives give the expected wear resistance. The introduction of such elements into the structure allows several to change these properties depending on the purpose, for use in mechanisms with increased abrasiveness( mills, sandblasts or shot-shot, parts and fittings for conventional and cement kilns).
In this case, the marking of white cast iron can be conducted from the limit( P, PL, PF or PVK), which shows the mass fraction of silicon. For cast iron wear-resistant( HF and IHC) marking will be with the corresponding alphabetic and digital indices.
Apparently, white pre-eutectic cast iron serves as a starting material for many types of metallurgical products. Thus, by annealing, ductile iron is produced from it. Again, the production of numerous doped wear-resistant and heat-resistant materials is also carried out due to its properties and the proximity of the microstructure to the alloying additives.
