MODICIATORS FOR GREY CAST IRON - Exoterm

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družba za proizvodnjo, inženiring, zastopstvo in trgovino, d.o.o .
EXOTERM-IT d.o.o., Kranj
Struževo 66, 4000 Kranj
Telefon:
++ 386 (0)4 2770-700, -711
Faks:
++ 386 (0)4 2770-716, -777
E-pošta:
[email protected]
Spletna stran: http://www.exoterm.si
MODICIATORS FOR GREY CAST
IRON
Types:
MODIFIKATOR SL 3
MODIFIKATOR SL 6 BA
CAST IRONS
What is a cast iron? In general, cast iron is a family of materials which differ in some
properties. They are alloys of iron, carbon, and silicon. The following types of cast irons
exist:
Grey cast iron contains 2.5 to 4.0 % carbon, 1.0 to 3.0 % silicon, 0.40 to 1.0 %
manganese, 0.05 to 0.25 % sulphur, and 0.05 to 1.0 % phosphorus. In the iron with
such a chemical composition the majority of carbon is in form of free carbon, i.e.
graphite, in a lamellar form. Grey cast iron has always a sooty grey fracture surface.
White cast iron has the following composition: 1.8 to 3.6 % carbon, 0.5 to 1.9 %
silicon, 0.25 to 0.80 % manganese, 0.06 to 0.20 % sulphur, and 0.06 to 0.18 %
phosphorus. In such a cast iron the carbon precipitates as iron carbide - cementite during the solidification. White cast iron has a white crystalline fracture surface.
Malleable cast iron has similar composition as the white cast iron. There is white and
black-malleable cast iron. Carbon is eliminated after ignition as graphite aggregates.
Mottled cast iron is a cast iron which is composed partially of white and partially of
grey microstructure.
Nodular graphite cast iron has the following composition: 3.0 to 4.0 % carbon, 1.8 to
2.8 % silicon, 0.15 to 0.90 % manganese, below 0.03 % sulphur, and below 0.10 %
phosphorus. Graphite is precipitated in spheroidal form.
GRAPHITIZATION OF GREY CAST IRON
Properties and applicability of cast iron mainly depend on two parameters, i.e. carbon and
silicon contents, and their effects on the graphitization process. Both elements cause the
formation of graphite if their percentage in iron is increasing.
Carbon can be expected in cast irons in two forms, as iron carbide (cementite), or as
graphite. In the first case carbon is chemically bound, in the second one it is free. Shape of
free carbon can be lamellar or spherical.
Graphitization is a process in which free carbon is precipitated from iron, or the chemically
bound carbon, in Fe3C, is transformed into free carbon (graphite). Increased percentage of
carbon in iron, especially above 2.00 %, promotes graphitization. Further, presence of
various other elements in iron, e.g. silicon, causes that iron carbide becomes less stable,
thus these elements promote formation of carbide. Next to the chemical composition, the
form of carbon and the graphitization process are also influenced by the process and the
conditions of manufacturing the cast iron and by the conditions of casting - solidification rate.
EXOTERM-IT, d.o.o, družba za proizvodnjo, inženiring, zastopstvo in trgovino, Kranj.
Registrirana pri Okrožnem sodišču v Kranju št. 1/07138/00, SRG 2002/01897, matična št. 1775421. Osnovni kapital 707.584,00 EUR. Davčna št. 91817277.
Telefon:
++ 386 (0)4 2770-700, -711
Faks:
++ 386 (0)4 2770-716, -777
E-pošta:
[email protected]
MODIFIERS FOR GREY CAST
IRON
INFLUENCE OF PRESENT ELEMENTS ON THE GRAPHITIZATION
PROCESS
CARBON
Mass portion of carbon in grey cast iron is between 2,5 and 4,5 %. Chemical analyses give
total amount of carbon, i.e. graphite and carbon in cementite together. It is also possible to
determine separately portion of either carbon form in the iron. White cast iron has total
carbon in form of cementite. If analysis gives that about 0.5 to 0.8 % carbon is present as
cementite, the cast iron has mainly pearlitic microstructure, since in the cast iron with about
2 % silicon pearlite with approximately 0.6 % carbon is formed in eutectoidal decomposition
of austenite.
SILICON
Silicon is alloyed to grey cast iron in amounts of 1.0 to 3.5 %. According to the phase
diagram, the eutectic point is shifted to lower carbon contents with an increased portion of
silicon. Thus, for comparison of cast irons usually a conception of "carbon equivalent" is
applied which is given by the expression
CE = % ogljika +
1
x % silicija
3
Investigations of the variation of mechanical properties of grey cast iron with chemical
composition show that strength of cast iron is increasing with a decreasing carbon
equivalent. Relation is given in Table 1.
TABLE 1:
RELATION BETWEEN TENSILE STRENGHT AND CARBON EQUIVALENT FOR A NOT
INOCULATED CAST IRON
CE- Carbon Equivalent
4,50 %
4,00 %
3,70 %
Tensile Strength
150 ... 230 N/mm2
250 ... 350 N/mm2
350 ... 450 N/mm2
If carbon equivalent is 4,30 %, we say it is eutectic composition. If it is lower, the molten
metal is under-eutectic, on the other hand above- eutectic molten metal has CE higher then
4,3 %.
Silicon moves eutectic point to the lower contents of carbon. That is why perlite in molten
metal contains around 2 % of silicon and only 0,6 % of carbon instead of 0,76 %, as in
eutectic composition in system iron – carbon. Silicon in the iron is below the solidus
temperature in solid solution in ferrite. Ferrite in clean iron has the hardness from 800 to 900
N/mm2, on the other hand ferrite with 2 % of silicon has hardness from 1200 to 1300 N/mm 2
HB. Silicon accelerates graphitization. Low percentage of silicon is not enough for excreting
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IRON
graphite during hardening process, but it causes cementite decomposition, if castings are
annealed, as in tempering process.
SULFUR AND MANGANESE
Molten metal can contain sulfur which impact on structure and mechanical properties of the
molten metal. Molten metal with the content of 0,010 % of sulfur generally graphitize
completely. Higher contents of sulfur cause holding the fully perlite structure of grey cast iron.
Sulfur is an element, which stabilize carbides. Molten metal with around 0,25 % of sulfur are
hard to work with, because of their hardness, which is the result of stabilized carbides,
caused by sulfur.
Sulfur impact depends on manganese in molten metal. Sulfur would bind with iron into iron
sulfide in molten metal without manganese, which is soluble in liquid iron. But during
crystallization process, it is excreted on crystallization borders. If manganese is involved,
manganese sulfides or complex sulfides of manganese and iron are formed. It depends on
content of manganese in molten metal. Those sulfides are insoluble in molten metal and they
are being removed in liquid state and during hardening process as well. They have no order
in hardened metal. Sulfur, bounded on manganese sulfide does not impact on perlitization of
molten metal.
Manganese is strong creator and stabilizer of carbides. Thus it is usually added only in such
amounts, that it binds sulfur and stabilize perlitic structure, where following equations are
valid:
a) Content of manganese needed for binding sulfur
% of manganese = 1,7 x % of sulfur
b) Content of manganese, where we obtain the most of ferrite and the least of perlit
% of manganese = 1,7 x % of sulfur + 0,15
c) Content of manganese, which ensures perlitic structure
% of manganese = 3 x % of sulfur + 0,35
PHOSPHORUS
Phosphorus in grey cast iron is usually seen as in form of steadit – eutectic of iron and iron
phosphide with low melting point. During hardening process it segregate into the parts, which
are hardened last. Usually we find steadit in cell structure. Iron phosphide is really though as
well as cementite. Phosphoric percolates defines up to ten times bigger concentrations of
phosphorus in comparing to molten metal analyze. Because the percolates are eutectic, we
have an impression that phosphorus reacts similar as silicon, although phosphorus reacts
differently. However, we define carbon equivalent, with expressing the impact of phosphorus:
CE = % carbon +
1
x (% silicon + % phosphorus)
3
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IRON
HARDENING PROCESS OF GREY CAST IRON
Majority of perlitic grey cast iron has under-eutectic composition, because during hardening
process primary carbon is removed in form of foam graphite, from above-eutectics. This
causes bad mechanic properties of metal.
During chilling process primary avstenite in shape of spongy dendrite packets is being
removed. When the molten metal reaches the temperature of eutectics, metal becomes solid,
which was caught in dendrites. Graphite and avstenit are removed from eutectic molten
metal. Eutectic hardening process runs shell-endogenously.
Graphite which is being remowed during eutectic hardening process, differs by size and
shape from lamells. We have standards for grading the shape of graphite.
Best mechanical properties are obtained if graphite is shaped as fine lamells without
orientation (type A). Usually orientated lamells of graphite are seen, which are formed from
percolates of graphite from dendrites of primary avstenite (type E). It is harder to distinguish
forming of cell shaped graphite type D. Molten metal of identical composition can contain
either graphite type A, either type D. Most of researchers, who were researching problematic
of grey cast iron graphitization, came to an end that the shape of graphite depends on molten
metal cooling during hardening process. If eutectic crystallization happens at lower
temperatures than usual (from 1046°C to 1100°C), then instead of graphite A or B, graphite
D or E is shaped. Eutectic hardening at temperatures from 1010°C to 1040°C forms white
ledeburitical structure.
Speed of cooling and number of crystallization nuclei impact on shape and distribution of
carbon. If we want to obtain graphite A and B, large number of nuclei and relatively slow
cooling must happen, so cementite can decompose and carbon can difunde to the point of
excretion.
Temperature of molten metal impact on shape of graphite as well. Overheated molten metal
(temperature close to 1500°C) is strongly inclined to white and marl hardening in thinner
layers. At eutectic crystallization graphite D and E are produced.
Eutectic crystallization is placed in shape of so called eutectic cells. Eutectic cell is defined
as graphite rosette, which has perlit removed from center-based lammels. Shape of graphite
is in close relation with number and size of eutectic cells. Generally, more eutectic cells we
have per volume unit of molten metal, better mechanical properties of molten metal we have.
Oligo elements have a strong impact on number of eutectic cells in molten metal.
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IRON
MODIFICATION OR INOCULATION OF GREY CAST IRON
By modification of grey cast iron we define the procedure, where we add inoculation to the
molten metal, which cause crystallization changes. These changes are bigger than changes
of chemical composition without additive. Most effective inoculations include the elements,
which are strong phosphorus in oxide removers (aluminium, calcium, barium, magnesium,
strontium or zircon). Using those elements we get new crystallization nuclei for graphite.
Main ingredient of inoculations is silicon, which as researches showed, does not effect on the
number eutectic cells and it does not effect on graphite shaping as well. Some researchers
determine that silicon impacts on increase of mechanical properties because of inoculation.
Silicon has an impact on shaping the primary dendrite avstenite, which provides armature in
molten metal structure.
By modification we reach the following effects:
- we lower the wall thickness, at which molten metal is hardened white
- we increase of eutectic cells
- we lower the differences between mechanical properties of different wall thicknesses
- we increase ductility strength at practically unchanged composition
- we assure crystallization of graphite A and B
- we lower the impacts of oligo elements on properties and crystallization structure of molten
metal
IMPLEMENTATION AND SUCCESS OF THE MODIFICATION
For successful inoculation we have to add the inoculation on the channel or in a stream of
molten metal during the streaming into the mold pot. We use 0,2 to 0,3 % of inoculation,
depending on quantity of molten metal. If we add inoculation into the pot and we re-pour it,
the efficiency of modifier is bad.
Since the effect of inoculation decreases with time, we have to cast the molten metal in
shortest time. For the same reasons, inoculation of molten metal with thick walls is
problematic as well.
We can find the less seen results at castings, which have eutectic or near eutectic
composition.
Telefon:
++ 386 (0)4 2770-700, -711
Faks:
++ 386 (0)4 2770-716, -777
E-pošta:
[email protected]
IRON
TYPES OF INOCULATIONS
In EXOTERM-it d.o.o. we produce two types of inoculations:
MODIFIKATOR SL-3, which has following composition:
Ca content
8–9%
Si content
50 – 55 %
C content
20 – 22 %
Granulation
0,4 – 6,0 mm
MODIFIKATOR SL Ba, which has following composition and granulation:
modifier
SL 6 Ba 1-7 mm
SL 6 Ba 1-3 mm
SL 25 Ba 0,2–0,7 mm
SL 12 Ba 0,2-0,7 mm
SL 50 Ba 1-3 mm
SL 100 Ba 1-7 mm
Ba content
2,2% min
2 – 2,5%
2,2% min
1,2% min
4,5 – 5,5%
9 – 11%
Si content
67 – 72%
67 – 72%
67 – 72%
67 – 72%
67 – 72%
65 – 70%
Ca content
1,5% min
2 – 2,5%
1,4% min
1,4% min
1,4% min
1,4% min
Al content
1,9% max
1,9% max
1,9% max
1,9% max
1,9% max
1,9% max
granulation
0,63 – 7 mm
0,63 – 3 mm
0,2-0,7 mm
0,2-0,7 mm
0,63 – 3 mm
0,63 – 7 mm
Both types of inoculations have their excellences at lowering the hardness at same strengths
in comparison with other inoculations, which contain zircon. MODIFIKATOR SL Ba has an
extended time of effecting.
PACKING
Inoculations are packed in natron bags with polyethylene bags inside at net 40kg each. 25
bags are packed on the pallet, wrapped with PVC foil.
STORING AND TRANSPORT
MODIFIKATORs SL must be stored in covered, dry stores, protected against atmospheric
influences. No ignition sources, open fires and smoking are allowed in its surrounding.
According to the European regulation for international road transport, MODIFIKATORs SL is
a dangerous substance. It is classified in ADR Class 4.3, packing group III. Amount allowed
to be carried by vehicles without ADR equipment is 1000 kg max.

MODICIATORS FOR GREY CAST IRON - Exoterm

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