Note: Descriptions are shown in the official language in which they were submitted.
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CAST IRON ALLOY WITH GOOD OXIDATION
STABILITY AT HIGH TEMPERATURES
The invention relates to a cast iron alloy for cast
iron products with a high oxidation resistance at high
surface temperatures.
Automobile manufacturers are required to comply with
the new exhaust emission standards. The catalytic
converters operate better when the exhaust gas
temperatures are higher. Palladium can be used instead
of platinum as a catalyst material, and the maximum
exhaust gas temperature will increase from currently
850 C to 950 C. At these temperatures, the hitherto
known cast iron alloys entail problems with scaling
resistance. In the previous ferritic alloys, a phase
transition from a ferritic lattice to an austenitic
lattice takes place at temperatures above about 860 C.
The expansion behavior of a ferritic lattice differs
from the expansion behavior of an austenitic lattice.
Because the thermal expansion coefficient of the
austenitic lattice is greater and changes more strongly
than the thermal expansion coefficient of the ferritic
lattice, a change in volume takes place at the
transition temperature. This volume change leads to a
nonuniform expansion behavior and microcracking of the
cast parts. The cast parts, which are subjected to a
frequent temperature change, are mechanically stressed
by this nonuniform expansion and cracking. As a
consequence of this, thin oxide layers (= scale) become
detached from the surface of the cast part. Ideally a
thin oxide layer, which adheres well in the long-term
and blocks oxygen diffusion, should be formed on the
surfaces of the turbocharger housing and/or exhaust
manifold which are exposed to the exhaust gas.
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EP 076 701 Bl discloses a heat-resistant ferritic cast
iron with spheroidal graphite. The alloy contains up to
3.4 wt% C, from 3.5 to 5.5 wt% Si, up to 0.6 wt% Mn,
from 0.1 to 0.7 wt% Cr, from 0.3 to 0.9 wt% Mo and up
to 0.1 wt% of a component forming spheroidal graphite.
The alloy is used for the production of turbocharger
housings in motor vehicle manufacture.
EP 1 386 976 B1 discloses an alloy for cast iron
products with high thermal stability. The alloy
consists of from 2.5 to 2.8 wt% C, from 4.7 to 5.2 wt%
Si, from 0. 5 to 0. 9 wt% Mo, from 0. 5 to 0. 9 wt% Al, up
to 0.04 wt% Mg, up to 0.02 wt% S, from 0.1 to 1.0 wt%
Ni, from 0.1 to 0.4 wt% Zr, remainder Fe and usual
impurities. The alloy is used for exhaust manifolds and
turbocharger housings in motor vehicle manufacture.
On the basis of this prior art, it is an object of the
invention to provide a cast iron alloy that can be used
at temperatures which are as high as possible, is as
economical as possible to produce and ensures as long.
as possible a service life under frequent temperature
changes.
This object is achieved by a cast iron alloy for cast
iron products with a high oxidation resistance at
surface temperatures of from 800 to 950 C having the
chemical constituents from 2.8 to 3.6 wt% C, from 2.0
to 3.0 wt% Si, from 2.5 to 4.3 wt% Al, up to 1.0 wt%
Ni, up to 0.8 wt% Mo, up to 0.3 wt% Mn, from 0.002 to
0.1 wt% Ce, from 0.023 to 0.06 wt% Mg, up to 0.01 wt%
S, remainder Fe and usual impurities.
Preferred refinements of the invention may be found in
the dependent claims.
It is advantageous for the cast parts to expand
elastically as regularly as possible at the operating
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temperature. This is achieved by the temperature of the
transition from the ferritic phase to the austenitic
phase of the alloy lying above 880 C. It is also
achieved by the thermal expansion of the alloy
specimens as measured by a dilatometer varying
uniformly and constantly up to a temperature of 880 C.
It is also achieved by the alloy having a thermal
expansion coefficient of from 8 to 12 10-6/K at 25 C and
from 13.5 to 15.5 10-6/K at 900 C. These are values
which, plotted against the temperature, are
consistently about 30% lower than the values of so-
called Ni resist alloys with the standard designations
D5S or GJSA XNiSiCr35-5-2.
It is furthermore advantageous for the cast parts not
to be brittle at room temperature. This is achieved by
the alloy having strength values of from 500 to 650 MPa
for the tensile strength Rm, from 470 to 620 MPa for
the yield point Rpp,z and from 2.0 to 4.0 for the
elongation at break A5. These are strengths values
which are about 1.3 to 1.5 times as great as those of
so-called Ni resist alloys. The ductility of the cast
iron alloys proposed here corresponds to the average
value of standard commercial ferritic materials which,
however, cannot be exposed to temperatures of more than
860 C.
It is also advantageous for the cast parts to be
readily processable. This is achieved by the alloy
having a Brinell hardness of from 220 to 250.
It is also advantageous for the alloy to be composed of
elements which are as economical as possible. This is
achieved by the alloy containing less than 0.8 wto Mo,
less than 1 wt% Cr and less than 1 wt% Ni. Ni resist
alloys typically contain about 30 to 35 wt% Ni and
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about 2 to 5 wto Cr. Spherocast alloys alloyed with
molybdenum normally contain about 0.8 wt% molybdenum.
Furthermore, it is also advantageous for the cast parts
to be as insensitive as possible to heat. This is
achieved by the alloy specimens having a thermal
conductivity of 25 W/mK at 25 C and a thermal
conductivity of 26 W/mK at 900 C. Ni resist alloys have
a thermal conductivity which is 20 to 50% lower at
400 C.
The key concept of the invention is to provide a cast
iron alloy which allows as high as possible a working
temperature with a high scaling resistance in
turbocharger housings and exhaust manifolds, and which
can be produced as economically as possible and as
simply as possible in a casting process. Previous
standard solutions for higher working temperatures
reside in the use of expensive cast steel and
austenitic cast iron or in the use of elaborately
produced sheet metal designs.
Example
An exhaust manifold made of spherocast for a combustion
engine of an automobile with the following chemical
composition in percentages by weight: 3.02 C, 2.96 Si,
2.53 Al, 0.79 Ni, 0.65 Mo, 0.23 Mn, 0.04 Cu, 0.031 P,
0.026 Cr, 0.023 Mg, 0.017 Ti, less than 0.01 S and
0.002 Ce, has a ferritic lattice. The exhaust manifold
is cast directly into the molds from a melt, which was
pretreated with magnesium in a GF converter. Subsequent
time-consuming heat treatment, such as solution
annealing or austempering, is not necessary.
The treatment with magnesium has a favorable effect on
the sulfur content of the alloy and ensures the
formation of graphite in the spheroidal or vermicular
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form. Magnesium has a desulphurizing effect, although
sufficient Mg must remain in solution in order to
promote growth of the spheroliths (= spheroidal
graphite particles). An Mg content of about 0.025 wt%
is ideal for the present Al content of about 2.5 wt%.
The alloy specimens have a density which is at least 5%
less than the density of comparable conventional cast
iron alloys.
The carbon content of from 2.8 to 3.6 wt% ensures a
composition which lies close to the eutectic. Less than
2.8% C is unfavorable for the feedstock of the cast
parts. More than 3.6% C is unfavorable for the high-
temperature properties of the alloy.
Cerium is added in amounts of from 0.002 to 0.1 wt% as
a nucleation promoter. More than 0.1% Ce is unfavorable
and leads to the formation of so-called chunky
graphite.
The silicon content of from 2 to 3 wt% in the present
alloy has a positive effect on formation of the
ferritic phase, improves the fluidity of the melt,
raises the yield point and improves the heat resistance
of the cast parts. Less than 2% Si is unfavorable for
the chill depth. More than 3% Si increases the
brittleness of the cast parts.
The aluminum content of from 2.5 to 4.3 wt% likewise
has a positive effect on formation of the ferritic
phase and neutralizes the nitrogen. Less than 2.5o Al
is unfavorable for the graphite stabilization. More
than 4.3% Al is unfavorable for the formation of
spheroidal graphite.
The nickel content of from 0.1 to 1 wt% raises the
yield point without substantially increasing the
brittleness and improves the corrosion resistance. Less
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than 0.1% Ni is unfavorable for the graphite
stabilization. More than 1% Ni is unfavorable for the
formation of bainite and martensite in thinner regions
of the cast parts. Nickel is a comparatively expensive
alloy element.
The molybdenum content of from 0.4 to 0.8 wt% has a
positive effect on increasing the yield point, the
thermal stability, the creep strength and therefore the
thermal cycling stability. Less than 0.4% Mo is
unfavorable for the graphite stabilization. More than
0.8% Mo is unfavorable for the formation of carbides
and gas bubbles. Molybdenum is a very expensive alloy
element.
The manganese content of up to 0.3 wt% has a positive
effect on the binding of sulfur. More than 0.3% Mn is
unfavorable for the formation of grain boundary
carbides and impairs of the nucleation state. Too much
Mn promotes the formation of perlite in the crystal
lattice. The. bainitic lattice becomes increasingly
brittle.
The chromium content of up to 1 wt% has a positive
effect on the creep strength and the thermal stability
of the castings.
In general, lower contents of the alloy additives are
favorable for reducing the formation of grain boundary
carbides and the brittleness at room temperature. This
is the case for example with the copper and titanium
contents.
Compared with cast steel, the melting temperatures for
spherocast are about 100 to 200 C lower. This means
that less energy is consumed and less alloy elements
are released to the environment by evaporation.
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Figure 1 represents the transition of the present alloy
from the ferritic phase to the austenitic phase as a
function of temperature. It may be seen here that an
equilibrium phase transition takes place at about
900 C. The way in which the alloy changes aggregate
state at a melting temperature of from 1240 to 1280 C
may also be seen here.
Figure 2 represents the thermal expansion coefficient
of the new alloy with the designation SiMol000plus,
measured as a function of temperature, compared with
other cast iron alloys.
Figure 3 represents the thermal conductivity of the
alloy SiMol000plus compared with other cast iron alloys
as a function of temperature. Here, D5S stands for the
so-called Ni resist alloys, and GJV SiMo and SiMoNi
stand for the previously known spherocast alloys
alloyed with about 1% Mo.
