International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 18 (2016) pp. 9515-9519
© Research India Publications. http://www.ripublication.com
Production of High Strength Spheroidal Graphite Cast Iron during Cast
Iron Smelting in Gas Cupola Furnaces
Vladimir Alexandrovich Grachev
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS,
31, Bldg 4, Leninsky prospect, 119071, Moscow, Russia.
Abstract
The article compares the quality of cast iron produced in coke and
gas cupola furnaces, including its gas content and mechanical
properties. It has been established that gas cupola furnace was
able to produce high quality cast iron with spheroidal graphite,
which was suitable for the casting of critical parts.
Keywords: high strength cast iron, spheroidal graphite cast iron,
ductile cast iron, nodular cast iron, gas content, mechanical
properties, gas cupola furnace, structural material.
INTRODUCTION
Gas cupola furnaces have been in use for cast iron smelting for
years, including the Penza Compressor Plant, where they have
been introduced in 1963 and are in use to this day. Various
complex critical parts (like cylinders, cylinder liners, pistons,
piston rings, frames and etc.) were cast there from various types
of cast iron. To smelt cast iron at the plant, coke cupola furnaces
with an output of 7 to 10 tons per hour were used. The long
manufacturing experience of casting compressor parts using cast
iron smelted in the coke cupola furnaces has shown that the
castings often did not meet the specifications. While being
smelted in the coke cupola furnace, the cast iron gets saturated
with sulfur, and difficulties in obtaining low-carbon inoculated
cast iron and spheroidal graphite cast iron arise. In casted
cylinders, cylinder liners, pistons and piston rings, defects like
porosity, cavities, chill, discrepancies in mechanical properties
(especially hardness), and discrepancies in the desired structure of
the cast iron were present.
Implementing gas cupola furnaces for cast iron smelting allowed
the plant to increase the quality of the castings. This paper
outlines some features of the production of high strength cast iron
with spheroidal graphite smelted in gas cupola furnaces since
ductile cast iron is one of the most promising construction
materials [1; 2].
METHODS
During production of high strength spheroidal graphite cast iron
(also known as high strength nodular cast iron, high strength
ductile cast iron, or high strength SGCI), the temperature before
the metal is treated with magnesium must be 1420-1450 °C, since
the temperature of the metal decreases by 100-120 °C after
modification. At temperatures below 1420 °C, after the metal is
9515
treated with magnesium, cast iron’s castability decreases,
while the tendency to form slag inclusions and “black spots”
in the castings increases.
The temperature of cast iron in the coke cupola furnace’s
gutter usually does not exceed 1370-1420 °C, which
complicates the production of critical and thin-walled
castings from high strength SGCI.
The temperature of cast iron in the gas cupola furnace
forehearth’s gutter is 1420-1450 °C. Such temperature is
optimal for production of high strength cast iron, because its
temperature remains relatively high after the metal is treated
with magnesium.
The smelting process in the gas cupola furnace is somewhat
different from the smelting process in the coke cupola
furnace. Gas cupola furnace does not have a bed charge,
which eliminates carbonization of cast iron. The oxidizing
environment of the furnace contributes to a carbon loss to the
extent of 8-12% [1]. Cast iron in the coke cupola furnace gets
overheated and gets saturated with sulfur when flowing down
the pieces of hot coke. Cast iron in the gas cupola furnace
gets overheated due to run-off of cast iron on incandescent
lining (100-150 °C more than in the coke cupola furnace), at
the time when drops fall in the countercurrents of gas, and in
the pool, onto the surface of which the torches are pointed at.
The sulfur content of the cast iron decreases. With optimum
gas-to-air ratio, slag has an increased castability and in due
time flows into the forehearth, which clears the metal surface
in the pool. This, in turn, creates favorable conditions for
barbotage of the overheated cast iron.
Besides high temperatures, the chemical composition and the
gas content of the metal affect the quality and properties of
cast iron. Table 1 shows the chemical composition and gas
content of cast irons, smelted in coke and gas cupola
furnaces.
The possibility of utilizing low-carbon cast iron for
producing high strength SGCI has not been sufficiently
studied, because the bulk of the cast iron for further
magnesium processing has been smelted in coke cupola
furnaces, in which it is difficult to obtain low-carbon cast
iron.
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 18 (2016) pp. 9515-9519
© Research India Publications. http://www.ripublication.com
Table 1. Chemical composition and gas content of cast irons smelted in coke and gas cupola furnaces
Chemical composition
Gas content
С, %
Si, %
Mn, %
P, %
S, %
O2,%
O2, cm3/100g
H2, %
H2, cm3/100g
N2, %
N2, cm3/100g
Extracted gas, cm3/100g
The research by Milman (1961), Pisarenko and Philippov (1960)
[2; 3], as well as the practical knowledge of the Penza
Compressor Plant on the production of critical castings, operating
at a pressure exceeding 90 atm have shown that, given similar
conditions during inoculation with magnesium of iron-carbon
alloys with a high carbon content, an identical magnesium uptake
and saturation of the metal is achieved. Therefore, there is no
reason to link the magnesium uptake by cast iron and the
quantitative carbon content of the cast iron. Pisarenko and
Philippov (1960) [3], who investigated the correlation
Coke cupola furnace
3.2-3.6
1.4-2.5
0.5-1.2
0.1-0.2
0.10-0.15
0.002146
1.683
0.000499
5.584
0.016518
13.217
20.584
Gas cupola furnace
2.9-3.3
1.4-2.5
0.5-1.2
0.1-0.2
0.023-0.06
0.0011761
1.231
0.0005999
6.712
0.005362
4.286
12.229
between the carbon content of cast iron and its mechanical
properties, point out that as the carbon content of cast iron
increases, the mechanical properties of the high strength
SGCI decrease.
Research on obtaining high strength cast iron with low
carbon content was conducted at the Penza Compressor Plant
confirms that the obtained castings made from magnesium
cast iron with low carbon content have better mechanical
properties (Table 2).
Table 2. Effect of carbon content on the mechanical properties of cast iron
Tensile strength, kgf/mm2
Component
Carbon content, %
Cylinder 39-5-1-1
2.4
Cylinder 51-5-1-1
2.43
Cylinder 39-5-1-1
2.50
Piston 39-4-1-3
2.6
Piston 39-4-1-3
2.61
Cylinder lid 39-4-1-2
2.8
Cylinder 51-5-1-1
3.0
Cylinder sleeve 39Р-1
3.1
In three samples
57.30
60.00
60.00
53.50
53.50
53.50
56.70
50.90
51.00
51.99
52.20
52.20
51.20
48.20
52.20
52.80
50.00
49.00
52.20
48.40
50.30
49.20
49.00
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Average value
59.10
53.50
52.87
52.10
50.53
50.60
50.30
49.33
Percentage elongation, %
Average
In three samples
value
20.35
13.8
18.43
21.0
17.2
18.00
17.42
17.2
16.2
15.5
14.90
13.0
17.4
15.4
14.60
11.0
11.0
16.3
12.10
10.4
6.0
8.0
9.33
14.0
6.6
4.6
6.80
5.2
6.2
5.80
6.00
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 18 (2016) pp. 9515-9519
© Research India Publications. http://www.ripublication.com
Cylinder sleeve 39Р-1
3.3
Cylinder sleeve 39Р-1
3.4
49.80
48.30
48.50
48.60
54.30
45.00
46.80
The research of castings made from low-carbon high strength
SGCI with the wall thickness of 30 mm and more, which were cut
at various sections (in thermal units and transitions, intersections
of different wall thicknesses), has shown the absence of casting
defects that affect the strength and impermeability of the castings.
Smelting low-carbon cast iron in a gas cupola furnace presents no
difficulties.
Aside from graphitizing elements, spheroidal graphite formation
in cast iron is greatly affected by surfactants, to which sulfur and
oxygen belong.
As it was shown by the researches [2; 4], spheroidal graphite
formation is linked to the desulphuration and deoxidation of the
alloys. As sulfur and oxygen have a high chemical affinity
48.47
45.70
5.2
6.4
7.2
4.4
5.5
2.7
8.2
6.00
5.47
with magnesium, magnesium eliminates the effect of sulfur
and oxygen by decreasing the surface tension in the initial
cast iron. Therefore, the surface stress on the metal-graphite
interface of the melt increases (1.5-2 times as compared with
initial cast iron).
Magnesium chemical affinity with the sulfur and oxygen
contained in the cast iron can be determined by the decrease
of ∆G free energy during the reactions of magnesium sulfide
and oxide formation (Table 3)[2]. Table 3 gives data on
elements that start reacting in the gaseous state in curly
brackets and on dissolved substances at standard
concentration (in %) in square brackets. Pure substances are
given without brackets.
Table 3. Decrease in ∆G free energy in formation reactions of magnesium sulfide and oxide
Reaction
Equation of change in free energy
{Mg}+[O%]=MgOs
{Mg}+[S%]=MgSs
Fel+[O%]=FeOl
Fel+[S%]=FeSl
-153.27-7.37TlgT+76.27T
-135.85-7.13TlgT+72.42T
-27.69+11.4T
-18.24+7.77T
By comparing the value of ∆G calculated for identical conditions,
it is apparent that the reactions will take place in the form of FeO
deoxidation and FeS desulphuration, with formation of MgO and
MgS, as the decrease in free energy is greater during the
formation of magnesium compounds with oxygen or sulfur.
The given data shows that chemical interaction of magnesium
with sulfur and oxygen, which are present in molten cast iron and
have high surfactant properties, is the reason for a substantial
increase in surface tension of cast iron. High surface tension is
apparently one of the main conditions for formation of spheroidal
graphite in cast iron.
The oxygen and sulfur content of cast iron smelted in a gas cupola
furnace is 1.2-1.3 and 3-5 times lower, respectively, as compared
with cast iron smelted in coke cupola furnace.
A low sulfur and oxygen content of cast iron smelted in a gas
cupola furnace (Table 1) allows reducing magnesium
consumption. With a decrease of sulfur and oxygen content of
cast iron the stability of the process of obtaining spheroidal
graphite increases, the amount of nonmetallic inclusions’
formation in the metal decreases, the thermal loss during
inoculation is reduced.
This statement can be confirmed by calculating the decrease in
magnesium requirement for binding sulfur and oxygen, as well as
by calculating the decrease in thermal loss during magnesium
inoculation of cast iron smelted in a gas cupola furnace.
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∆G, kcal/g mole of the forming
compound at 1450 ºС
-63.40
-28.33
-8
-4.84
The magnesium content for binding sulfur and oxygen is
determined by the formula:
(1)
where MgS and MgO – magnesium consumption for binding
sulfur and oxygen, respectively, in %;
0.76 and 1.52 – magnesium consumption for binding 1% of
sulfur and 1% of oxygen, respectively, in %;
S1 and S2 – amount of sulfur in cast iron before and after
magnesium treatment, in %;
O1 and O2 – amount of oxygen in cast iron before and after
magnesium treatment, in %;
The magnesium amount (in %) for binding sulfur and oxygen
during treating cast iron smelted in a coke cupola furnace is
determined by the formula:
(2)
where MgcS and MgcO – magnesium consumption for binding
sulfur and oxygen, respectively, contained in cast iron
smelted in a coke cupola furnace, in %;
S1 = 0.125% – average value according to the Table 1;
S2 = 0.007% – average value according to the plant’s data;
O1 = 0.002146% – according to the Table 1;
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 18 (2016) pp. 9515-9519
© Research India Publications. http://www.ripublication.com
O2 = 0 – to simplify the calculations, as the oxygen content of cast
iron after magnesium treatment decreases by a factor of 10 and
more and is equal to a ten-thousandth of a percent.
The magnesium amount (in %) for binding sulfur and oxygen
during treating cast iron smelted in a gas cupola furnace is
determined by the formula:
(3)
where MggS and MggO – magnesium consumption for binding
sulfur and oxygen, respectively, contained in cast iron smelted in
a gas cupola furnace, in %;
S1 = 0.041% – average value according to the Table 1;
S2 = 0.07% – average value according to the plant’s data;
O1= 0.007761% – according to the Table 1;
О2 = 0.
Therefore, to bind sulfur and oxygen contained in cast iron
smelted in a gas cupola furnace, approximately 3.28 times less
magnesium is needed, to bind sulfur – 3.5 times less, to bind
oxygen – 1.22 times less. Magnesium savings amount to 0.065%.
Magnesium consumption during inoculation of coke cupola
furnace’s cast iron in the sealed ladle amounts to 0.25-0.35%.
During processing cast iron in a gas cupola furnace, magnesium
consumption decreases by a factor of 1.25 or more.
1% magnesium melting and evaporation theoretically
decreases the temperature of the metal by 100 °C. The
decrease in thermal loss (in °C) due to a decrease in
magnesium expenditure will be:
(4)
RESULTS
Practically, when using cast iron smelted in a gas cupola
furnace for production of high strength SGCI, the
temperature of this cast
iron after magnesium treatment is 50-70 °C higher than the
temperature of cast iron smelted in a coke cupola furnace
after magnesium treatment.
Other gases also affect magnesium consumption [4]. Gases
have a significant effect on mechanical and casting
properties, as well as on the quality of the magnesium cast
iron castings. The gas content of cast iron smelted in a gas
cupola furnace is 1.5-1.7 times lower as compared to the gas
content of cast iron smelted in a coke cupola furnace (Table
1). The decrease of total gas content of cast iron smelted in a
gas cupola furnace leads to an increase in mechanical
properties of both gray and high strength SGCI (Table 4).
Table 4. Mechanical properties of gray and high strength SGCI smelted in coke and gas cupola furnaces
σf, kgf/mm2
Coke cupola furnace
43.8
40.7
45.1
41.7
43.8
40.7
44.27
41.30
2.9
2.6
2.9
2.6
3.0
2.6
2.93
2.6
51.3
45.1
56.4
43.8
54.5
43.0
57.07
43.97
3.0
3.0
3.0
3.0
3.1
3.0
3.03
3.00
52.5
53.5
52.2
58.6
52.2
53.5
53.30
55.2
14.0
15.0
14.0
14.6
8.0
12.9
12.00
10.83
In three samples
Average
GCI 18-36
f, mm
In three samples
Average
σf, kgf/mm2
In three samples
Average
GCI 21-40
f, mm
In three samples
Average
σt, kgf/mm2
In three samples
Average
SGCI 45-5
δ, %
In three samples
Average
An increased hydrogen content of the initial metal does not
substantially affect the quality and properties of magnesium cast
iron (hydrogen content of cast iron after magnesium treatment
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Gas cupola furnace
48.2
47.6
48.2
51.3
44.6
57.7
47.00
50.20
3.1
2.8
3.2
3.2
3.0
3.2
3.1
3.07
57.6
52.6
57.6
56.6
57.6
57.6
57.6
55.67
3.3
3.2
3.4
3.3
3.3
3.3
3.33
3.27
57.6
58.7
52.9
57.3
56.7
60.2
55.73
58.73
13.8
16.0
16.6
8.3
12.3
18.6
14.23
14.30
decreases to 3.0-3.5 cm3/100 g). It should be noted that
according to Kashirsky (1964) [5], hydrogen up to 3-4%
increases the mechanical properties of cast iron.
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 18 (2016) pp. 9515-9519
© Research India Publications. http://www.ripublication.com
With a decrease in sulfur and gas content of the initial metal, and
consequently in magnesium consumption for inoculation, the
concentration of nonmetallic inclusions in high strength cast iron
decreases. The calculations show that the concentration of
nonmetallic inclusions (MgS, MgO, Mg3N2) in magnesium cast
iron, which was obtained from cast iron smelted in a gas cupola
furnace, is lower than in high strength cast iron, which was
obtained by smelting cast iron in a coke cupola furnace, by a
factor of 3.55, 1.2 and 3.1 times, respectively.
Gas cupola furnaces allow smelting the initial metal for a
subsequent inoculation with magnesium and obtain high strength
SCGI of high quality with low magnesium consumption.
The mechanical properties of high strength SGCI can be changed
by means heat treatment. At the plant, large castings made from
high strength cast iron (cylinders, cylinder lids, pistons) together
with control samples are subjected to high-temperature annealing
in the following mode: slow heating up to 940-950 °C in the
course of 9 hours; thermal retardation at this temperature for 5
hours; furnace-cooling to 700-720 °C and thermal retardation at
this temperature for 7 hours; furnace-cooling to 500 °C and
subsequent air-cooling. Cast iron of ferrite class SGCI 45-5.
For castings like sleeves and sealing rings, in order to increase
wear resistance and to obtain a perlite structure, normalizing
followed by tempering is used in the following mode: heating up
to 920-930 °C; thermal retardation at this temperature for 3-5
hours; air-cooling. Tempering: heating up to 550 °C; thermal
retardation at this temperature for 2-3 hours; air-cooling.
For components made from high strength cast iron that is utilized
in frictional conditions, a method of isothermal hardening was
tested in the following mode: heating up to 880 °C; thermal
retardation at this temperature for 3-5 hours; cooling in an alkali
tank at a temperature of 350 °C; air-cooling.
The wear resistance testing of samples was performed on a set
cast iron sealing ring / thermally treated steel rod. The testing has
shown that the wear of the sealing ring made of high strength
SGCI, which was isothermally hardened, was 2-3 times lower
than the wear of a sealing ring made of gray cast iron, which was
subjected to similar heat treatment. The wear of the rod has
decreased approximately by a factor of 2 as compared to the set
with gray cast iron sealing ring.
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REFERENCES
[1]
[2]
[3]
[4]
[5]
I. A. Dibrov. “Development of the foundry
engineering in Russia”. Liteishchik Rossii, vol. 8,
pp. 1-48, 2015
B. S. Milman. “Theory and practice of producing ascast ductile cast iron with nodular graphite”.
Moscow: TsNIITMASh, 1961.
G. A. Pisarenko & A. S. Philippov, “Metallurgical
equipment castings from nodular cast iron”.
Sverdlovsk: Metallurgizdat, 1960.
K. I. Vashchenko & L. Sofroni, “Magnesium cast
iron”. Moscow: Mashgiz, 1960.
A. V. Kashirsky. “Effect of hydrogen on physical
and chemical properties of cast iron”. In All-Union
Conference on the use of natural gas in iron
resmelting.
Kharkov:
Kharkov
University
Publishing, 1964.