Structure and properties of cast iron
The microstructure of cast iron consists of a metal basis (matrix) and graphite
inclusions. Properties of cast iron are defined by properties of a metal basis and
nature of inclusions of graphite.
Cast iron contain the following structural components:
- graphite (G).,
- pearlite (П).,
- ferrite (Ф).,
- ledeburite (L).,
- phosphide eutectic.
Formation of a microstructure of cast iron depends on its chemical
composition and speed of cooling of casting. The structure of a metal basis
defines hardness of cast iron.
Carbon in structure can be present at a type of chemical connection – cement
carbide, graphite or their mix. In comparison with a metal basis graphite has low
durability. It is possible to consider places of its position as violations of a
continuity of metal. Cast iron is as though penetrated by the inclusions of
graphite weakening its metal basis. In process of a rounding off of graphitic
inclusions (at the expense of modifying by additives) their negative role as cuts
of a metal basis decreases and mechanical properties of cast iron grow.
For example, gray cast iron (a lamellar form of graphite) has low characteristics
of mechanical properties as plates of inclusions of graphite play a role of
concentrates of tension in casting. However gray cast iron has a number of
advantages: possesses high fluid and a small casting shrinkage., inclusions of
graphite do shaving fragile, allowing it easy to process cast iron cutting., thanks
to greasing effect of graphite cast iron possesses good antifriction properties.,
well extinguishes vibrations and resonant fluctuations. From high-strength cast
iron (a spherical form of graphite) make responsible details: cogwheels, cranked
shaft.
Silicon promotes a cast iron graphitization. Changing its contents and speed of
cooling of casting, it is possible to receive cast iron of various structures.
Manganese interferes with a graphitization and will neutralize an adverse
effect of sulfur, forming with it refractory connections of MNS.
Phosphorus doesn't render essential influence on graphitization process. At
the raised content of phosphorus in structure of cast iron firm inclusions
phosphide eutectic which increases its foundry properties are formed.
Sulfur is harmful impurity. It causes deterioration of foundry properties of
cast iron, increase of shrinkage , increase of tendency to a crack formation,
decrease in temperature of the red-shortness of cast iron.
Gray cast iron and high-strength cast iron.
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Gray cast iron is an alloy of Fе system - S-Si, containing in quality of impurity
manganese, phosphorus, sulfur. Carbon in gray cast iron mainly is in a type of
graphite of a lamellar form.
According to SS gray cast iron mark the letters "G" - gray and "CI" - cast iron.
The number after an alphabetic reference shows average value of strength of
cast iron at stretching. For example, GCI 20-cast iron gray, strength at stretching
of 200 MPas.
On properties gray cast iron is possible to distribute conditionally on the
following groups:
- ferrite and ferrite-pearlite cast iron (brands GCI10, GCI15), apply to
manufacturing of the low-responsible not loaded details of cars.,
- pearlite cast iron (brands GCI20, GCI25, GCI30), use for manufacturing of the
wear-resistant details maintained at big loadings: pistons, cylinders, blocks of
engines.,
- modified cast iron (brands SCh35, СЧ40, СЧ45), receive addition before a
casting in liquid gray cast iron of additives, such cast iron have a pearlite metal
matrix with a small amount of the isolated plates of graphite.
Distinctive feature of high-strength cast iron are its high mechanical properties
caused by existence in structure of spherical graphite. Cast iron with spherical
graphite possesses not only high durability, but also plasticity.
Receiving spherical graphite in cast iron is reached by modifying melt the
additives containing Мg, Sa, Xie and other rare-earth metals.
Chemical composition and properties high-strength cast iron are regulated by
SS and marked by the letters "HS" - high-strength, ―CI‖ - cast iron and the
number designating average value of strength of cast iron at stretching. For
example, HSCI 100-high-strength cast iron, strength at stretching of 1000 MPas.
High-strength cast iron with spherical graphite is the most perspective foundry
alloy by means of which it is possible to solve successfully a problem of
decrease in weight of designs at preservation of their high reliability and
durability.
High-strength cast iron use for manufacturing of responsible details in
mechanical engineering (cranked shaft, cogwheels, cylinders, etc.).
Malleable cast iron and alloyed cast iron.
Malleable cast iron receive by annealing of white cast iron of the certain
chemical composition, differing the lowered maintenance of graphitizes.
Mechanical properties and recommended chemical composition of malleable
cast iron are regulated by SS. Malleable cast iron mark the letters "M" -
malleable, "CI" - cast iron and figures. The first group of figures shows strength
of cast iron at the stretching, the second - its relative lengthening at a gap. For
example, MCI 33-8 means: malleable cast iron with strength at stretching of 330
MP as and relative lengthening at a rupture of 8 %.
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For acceleration of process of annealing of MCI use various receptions:
increase endurance temperature, modify and micro-alloyed cast iron additives of
aluminum, pine forest, the titan and bismuth.
Malleable cast iron use for manufacturing of small and average thin-walled
casting of the responsible appointment working in the conditions of dynamic
sign-variable loadings (a detail of driving mechanisms, transmissions, brake
shoes, gear wheels, naves etc.). However malleable cast iron is a low-
perspective material because of difficult technology of receiving and duration of
a production cycle of manufacturing of details from it.
Depending on appointment distinguish wear-resistant, bearing, heat resisting
and corrosion-resistant cast-iron.
Wear resistant cast iron, alloyed by nickel and chrome apply to manufacturing
of the details working in abrasive environments. Cast iron with addition of the
titan, copper, vanadium posses the increased wear resistance in the conditions of
a friction without lubricant. Them use for manufacturing of brake drums of cars,
disks of coupling, sleeves of cylinders.
Heat resisting cast iron apply to manufacturing of details of contact devices of
the chemical equipment, turbo-compressor, maintained at temperature 600?С
and 700?С.
Corrosion-resistant cast-iron posses the increased corrosion firmness in gas, air
and alkaline environments. They are applied to manufacturing of details of knots
of the friction working at increased temperatures (piston rings, blocks and heads
of cylinders of internal combustion engines.).
Bearing cast iron are used as bearing alloys as represent group of the special
alloys which structure satisfies to a rule of Sharpi, capable to work in the
conditions of a friction as sliding bearings.
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