U.S. patent application number 11/913596 was filed with the patent office on 2008-11-06 for cast iron with improved high temperature properties.
This patent application is currently assigned to WESCAST INDUSTRIES, INC.. Invention is credited to Gene B. Burger, Delin Li, Gangjun Liao, Robert N. Logan.
Application Number | 20080274005 11/913596 |
Document ID | / |
Family ID | 37397113 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080274005 |
Kind Code |
A1 |
Liao; Gangjun ; et
al. |
November 6, 2008 |
Cast Iron With Improved High Temperature Properties
Abstract
A nodular, compacted graphite or other hybrid or duplex graphite
morphology cast high silicon iron is disclosed which contains up to
1.5% tungsten, up to 0.8% vanadium, and up to 1.2% niobium; and at
least 60.0% iron, all percentages are based on the total weight of
the composition. This cast iron exhibits high strength and good
ductility over a wide temperature range compared to the
conventional SiMo ductile iron. The compositions may further
contain, up to 1.5% molybdenum and up to 1.0% chromium to offer
improvements in material strength. The compositions may include 0.2
to 0.5% by weight aluminum and up to 1.2% chromium for further
oxidation resistance and 0.5 to 5.0% nickel for corrosion
resistance.
Inventors: |
Liao; Gangjun; (Brantford,
CA) ; Li; Delin; (Waterloo, CA) ; Burger; Gene
B.; (Cambridge, CA) ; Logan; Robert N.;
(Brantford, CA) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Assignee: |
WESCAST INDUSTRIES, INC.
Brantford
ON
|
Family ID: |
37397113 |
Appl. No.: |
11/913596 |
Filed: |
May 4, 2006 |
PCT Filed: |
May 4, 2006 |
PCT NO: |
PCT/US06/17341 |
371 Date: |
November 5, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60678950 |
May 5, 2005 |
|
|
|
Current U.S.
Class: |
420/15 ; 420/13;
420/27; 420/9 |
Current CPC
Class: |
C22C 37/04 20130101;
C22C 37/10 20130101; C21D 5/00 20130101; C22C 33/08 20130101 |
Class at
Publication: |
420/15 ; 420/9;
420/27; 420/13 |
International
Class: |
C22C 37/10 20060101
C22C037/10; C22C 37/00 20060101 C22C037/00; C22C 37/06 20060101
C22C037/06 |
Claims
1. A high silicon cast iron composition comprising: greater than
about 60 wt. % iron; between about 2.8 to about 5.0 wt. % silicon;
from about 0.03 to about 1.5 wt. % tungsten; up to about 0.8 wt. %
vanadium; and up to about 1.2% niobium.
2. The cast iron composition of claim 1, further comprising at
least one constituent selected from the group consisting of
molybdenum, chromium, nickel and carbon.
3. The cast iron composition of claim 1, wherein said vanadium is
present in an amount of between about 0.02 to about 0.8 wt. %.
4. The cast iron composition of claim 1 wherein said niobium is
present in an amount of between about 0.02 to about 1.2 wt. %.
5. The cast iron composition of claim 2 wherein said molybdenum is
present in an amount up to about 1.5 wt. %.
6. The cast iron composition of claim 2 wherein said aluminum is
present in an amount of between about 0.2 and 3.0 wt. %.
7. The cast iron composition of claim 2 wherein said nickel is
present in an amount of between 0.5% and about 5.0 wt. %.
8. The cast iron composition of claim 2 wherein carbon is present
in an amount such that the wt. % carbon plus 1/3 the wt. % silicon
is up to about 4.9 wt. %.
9. Products formed from the cast iron composition of claim 1.
10. A molded article comprising a nodular, compacted graphite or
hybrid or duplex graphite having a cast iron composition, wherein
the cast iron composition comprises: greater than about 80 wt. %
iron; between about 2.8 to about 5.0 wt. % silicon; from about 0.03
to about 1.5 wt. % tungsten; from about 0.2 to about 1.2 wt. %
niobium; and up to about 0.8 wt. % vanadium.
11. The molded article of claim 10 wherein said vanadium is present
in an amount of between about 0.02 to about 0.8 wt. %.
12. The molded article of claim 11 further comprising between about
0.2 to about 1.5 wt. % molybdenum.
13. The molded article of claim 11 further comprising between about
0.2 to about 3.0 wt. % aluminum.
14. The molded article of claim 11 further comprising between about
0.5 to about 5.0 wt. % nickel.
15. The molded article of claim 11 wherein carbon is present in an
amount such that the wt. % carbon plus 1/3 the wt. % silicon is up
to about 4.9 wt. %.
16. The molded article of claim 10 wherein the article is an
exhaust manifold.
17. The molded article of claim 10 wherein the article is a
turbocharger component.
18. The molded article of claim 10 wherein the article is a
catalytic converter housing.
19. The molded article of claim 10 wherein the article is a fuel
cell component.
20. A high silicon cast iron composition comprising: At least 60
wt. % iron; between about 2.8 to about 5.0 wt. % silicon; from
about 0.2 to about 1.2 wt. % niobium; up to about 1.5 wt. %
tungsten; up to about 0.8% vanadium; and. at least one constituent
selected from the group consisting of molybdenum, aluminum, nickel
and carbon.
21. The cast iron composition of claim 20, wherein said vanadium is
present in an amount of between about 0.02 to about 0.8 wt. %.
22. The cast iron composition of claim 20 wherein said molybdenum
is present in an amount up to about 1.5 wt. %.
23. The cast iron composition of claim 20 wherein said tungsten is
present in an amount of between about 0.003 to about 1.5 wt. %.
24. The cast iron composition of claim 20 wherein said nickel is
present in an amount of between 0.5% and about 5.0 wt. %.
25. The cast iron composition of claim 20 wherein carbon is present
in an amount such that the wt. % carbon plus 1/3 the wt. % silicon
is up to about 4.9 wt. %.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/678,950, filed on May 5, 2006. The disclosure of
the above application is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to cast iron that exhibits
improved strength and high temperature properties. More
specifically, the present invention relates to cast iron alloys
which contain certain amounts of carbide formers selected from the
group including tungsten, vanadium and niobium. Other carbide
formers such as molybdenum, and/or chromium may be employed in
addition to at least one of tungsten, vanadium and niobium. Other
alloy additions of silicon and aluminum for oxidation resistance
are also disclosed. The cast iron may include a graphite morphology
that is primarily nodular, vermicular or a combination referred to
herein as hybrid or duplex.
BACKGROUND OF THE INVENTION
[0003] Cast iron alloys, and parts cast from the alloys are subject
in use to an ever increasing range of challenging environments.
Such parts must operate at high temperatures and withstand
temperature cycling between periods of use. The parts must have
good oxidation resistance and be resistant to mechanical and
thermal fatigue and oxidative cracking at a competitive cost.
[0004] Silicon-molybdenum (SiMo) nodular or compacted graphite
irons such as those presented in the comparative example of Table I
herein are currently employed in the manufacture of exhaust
manifolds of the high volume production engines because they often
have advantages in terms of cost and durability. Such SiMo alloys
exhibit improved high temperature strength and thermal fatigue
resistance over many other known ductile cast irons, as well as
improved high temperature oxidation resistance. However, the high
oxidation rate at high temperatures remains a problem in parts such
as exhaust manifolds and turbocharger turbine housings, where the
in-use temperatures can reach 850.degree. C. and higher. In
addition, cast irons in these applications are also subject to
thermal fatigue cracking. This is due at least in part to the
thermal cycling during heating and cooling. Therefore, in use the
part is cycled up to temperatures associated with engine operation
and then back down to approximately room temperature. The part
undergoes the thermal expansion upon heating and contraction upon
cooling. This continued thermal cycling and associated thermal
expansion/contraction is said to contribute to thermal fatigue in
the part which, in time, leads to cracking.
[0005] In order to improve efficiency and reduce harmful emissions
from internal combustion engines, exhaust gas temperatures have
been gradually increasing. More exhaust manifold applications are
running at close to the practical cracking temperature limit of
SiMo irons. As temperatures increase, the cast iron is more prone
to damage from lower strength at high temperatures, creep and
oxidation resistance. It is desirable to have the sufficient
mechanical strength of the alloy at elevated temperatures, while
maintaining or improving the ductility at both room and elevated
temperatures.
[0006] In an attempt to address the above described concerns, high
strength at elevated temperature, good thermal fatigue resistance,
good oxidation resistance, and adequate ductility is required.
Certain alloying elements have been employed in conventional SiMo
cast iron compositions in an effort to address these concerns.
[0007] For example, according to JP Pat. No. 2002-339033 (Nobuaki),
cast iron alloys containing silicon, molybdenum, manganese and
vanadium have higher heat resistance than the conventional ductile
irons. Nobuaki further indicates that alloys containing vanadium
and manganese (Mn) improve the elevated temperature physical
properties of a nodular graphite iron. Specifically, Nobuaki
defines Mn+V in the range of 0.3-2.0% by weight (preferably
0.4-1.8% by weight) while Mo content is 0.3-1.0% by weight
(preferably 0.3-0.7% by weight).
[0008] In addition to the cast iron alloys containing vanadium and
manganese mentioned above, cast iron alloys that contain tungsten
(W) and arsenic are also known for high temperature applications in
JP Pat. No. 10-195587 (Takao). The Takao patent suggests that
exhaust manifolds formed from the subject alloy are excellent in
ductility in the moderate temperature embrittlement region near
400.degree. C., using 0.03-0.2% arsenic to the conventional high
silicon ductile iron containing 3.5-5.2% silicon. The high silicon
nodular or ductile iron has the phenomenon in which ductility falls
to minimum at about 400.degree. C.; this is called the moderate
temperature embrittlement phenomenon (or moderate temperature
brittleness). Arsenic is intentionally added to reduce the moderate
temperature brittleness of all kinds of cast irons including high
silicon nodular irons and alloyed nodular irons.
[0009] While such compositions may address one or more of the
perceived issues with conventional SiMo cast irons, still further
improvements are demonstrated under the present invention.
[0010] Further, because of the high demand of molybdenum in the
steel and iron industries, the price of Mo fluctuates dramatically,
increasing the cost of conventional SiMo cast irons.
[0011] The iron alloys of the present invention including one or
more of tungsten, vanadium and niobium give rise to alloys
exhibiting the combined properties of high mechanical strength and
ductility. Further parts cast from such alloys are readily
machined, abrasively cleaned at room temperature, can withstand
oxidation at high in-use temperatures and can withstand
thermal-mechanical fatigue cracking during cycling.
SUMMARY OF THE INVENTION
[0012] The high silicon iron composition of the present invention
contains up to 1.5 wt. % tungsten, up to 0.8 wt. % vanadium, and up
to 1.2 wt. % niobium, preferably in combination with at least one
of molybdenum and chromium. The cast iron alloys of the present
invention yield high strength and good ductility over a wide
temperature range, compared to conventional SiMo iron having
nodular, compacted graphite iron or other graphite morphologies.
The addition of higher silicon and aluminum offers improved hot
oxidation resistance, compared to conventional SiMo iron having
nodular, compacted graphite iron or other graphite morphologies. In
preferred embodiments, the iron alloy of the present invention
contains, from about 0.02 to 0.8% vanadium, from about 0.03% to
about 1.5% tungsten, from 0.02% to about 1.2% niobium, from about
2.8 to about 5% silicon, from 2.8% to about 3.8% carbon, less than
0.06% magnesium, and less than 0.02% cerium, the balances being at
least 60.0 t % iron and impurities, with all percentages based on
the total weight of the composition. The compositions may further
contain up to 1.5% molybdenum, up to 1.0% chromium, up to 5.0%
nickel and between 0.2 and 3.0% aluminum.
[0013] Articles cast from the compositions of the invention are
ductile and can withstand thermal cycling without failure. Such
articles find use in a variety of automotive transportation and
industrial applications. Such applications include, but are not
limited to, exhaust components such as exhaust manifolds,
turbocharger housings, hot end components such as catalytic
converter housings and fuel cell components. Generally, the cast
iron compositions of the invention may be used in any application
calling for nodular or compacted graphite iron, Ni-Resist ductile
iron, chrome molybdenum steel, or a low grade stainless steel.
[0014] The compositions of the invention provide cast iron articles
having desired combinations of elevated temperature strength,
ductility, high oxidation resistance, and thermal fatigue
resistance. The cast iron compositions of the present compositions
are considered to be a viable alternative to the conventional SiMo
nodular and compacted graphite irons. They are useful generally in
any iron application, particularly high temperature cast iron
applications.
[0015] Further areas of applicability will become apparent from the
description provided herein. It should be understood that the
description and specific examples are intended for purposes of
illustration only and are not intended to limit the scope of the
present disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The drawings described herein are for illustration purposes
only and are not intended to limit the scope of the present
disclosure in any way.
[0017] FIG. 1 is a graph setting forth a comparison of the
influence of tungsten on strength at 800.degree. C. to that of
molybdenum;
[0018] FIG. 2 is a graph setting forth a comparison of the
influence of tungsten on strength at room temperature to that of
molybdenum;
[0019] FIG. 3 is a graph setting forth weight change rate versus
exposure time at the temperature of 820.degree. C. for different
materials measured by daily cyclic oxidation testing; and
[0020] FIG. 4 is a graph setting forth average depth of oxide
scales measured after testing in FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Because the compositions of the present invention are
alternative materials to the conventional SiMo irons used in high
temperature applications, the composition of the invention can be
referred to as "high silicon iron alloys" or "modified high
temperature SiMo" alloys which include more than an impurity level
of molybdenum. Thus, the phrases "modified high temperature SiMo"
and "modified SiMo" will be used interchangeably to refer to the
cast iron compositions and molded articles of the present invention
containing molybdenum.
[0022] Cast iron articles of the invention are prepared by pouring
a molten composition into a mold. The molten composition is a cast
iron composition containing, in addition to at least about 60% by
weight iron, tungsten at levels up to about 1.5% by weight,
vanadium at levels up to about 0.8% by weight, and niobium at
levels up to about 1.2% by weight. Most often the cast iron
composition includes at least 80 wt. % iron. Vanadium at the
appropriate levels is believed to increase the high temperature
strength of the cast iron articles, but too high vanadium would
result in too much vanadium carbide thus decreasing ductility
significantly. Tungsten at the appropriate levels is believed to
increase the elevated temperature strength of cast irons. More
particularly, tungsten is believed to improve high temperature
creep and fatigue resistance. Tungsten appears to have comparable
strengthening characteristics as molybdenum, and both form very
fine tungsten or molybdenum carbide precipitates. However, higher
tungsten content is generally associated with higher carbide
content. This makes the cast articles tend to be more brittle with
some risk of cracking during thermal cycling, as for example, in
normal automotive engine use, or during simulative or accelerated
engine dynamometer durability tests. Thus, the upper limit of
tungsten should be no more than about 1.5% by weight. The preferred
amount of tungsten is from about 0.03% by weight to 0.8% by weight.
Niobium at the appropriate levels of between about 0.02% and 1.2%
are believed to increase the ductility at room and elevated
temperatures and improve high temperature properties.
[0023] In order to further increase the oxidation resistance and/or
increase the nodular or compacted graphite content of the
compositions of the present invention, up to about 3.0% by weight
aluminum.
[0024] The iron compositions may further comprise silicon and
carbon. Silicon is generally present in an amount of from about
2.8% to about 5.0% by weight. In a preferred embodiment, silicon is
present at a level of from about 3.9% to 4.6% by weight. Carbon is
generally present in an amount such that the weight percent carbon
plus 1/3 the weight percent silicon is numerically equal to a value
up to about 4.9%, preferably up to about 4.7%.
[0025] It is generally preferred that the compositions of the
invention contain less than 0.02% sulfur. Higher sulfur levels tend
to lead to a requirement for additional magnesium additions and
cause more rapid fading of magnesium during the treatment step to
control production of either compacted (vermicular), nodular
graphite structures or other graphite morphologies. For similar
reasons, it is preferred to keep the oxygen content of the
compositions low, typically less than about 0.005% (50 ppm).
Phosphorus should also be kept to minimum, preferably below about
0.04%.
[0026] The desirable properties of ductility and machinability
exhibited by the compositions of the invention are believed to
derive from the microstructure of the modified SiMo alloys. The
graphite present in the molded articles is predominantly present in
either nodular or vermicular form. When greater than 80% of the
carbon is present as graphite nodules, the compositions are
generally referred to as ductile irons. In a preferred embodiment,
the nodularity is greater than about 85% for ductile irons. When
the nodularity is less than about 50% (i.e., when less than about
50% of the carbon is present as graphite nodules), the compositions
are referred to as compacted or vermicular graphite iron. In
compacted graphite irons of the invention, nodularity is generally
about 50% or less, with the remainder of the graphite predominantly
present in vermicular form. High levels of flake graphite are
undesirable.
[0027] If the nodularity is between 50-80% a structure referred to
as hybrid or duplex graphite exists. It is an iron containing
significant fractions of both nodular graphite and compacted or
vermicular graphite. In a preferred embodiment, the hybrid or
duplex graphite iron has a nodularity of from 60% to 75% (i.e.
60-75% of the carbon is present as graphite nodules); the remaining
is in the form of compacted or vermicular graphite.
[0028] Various composition trials as set forth in Table 1 were
conducted to analyze high temperature strength, manifold
durability, ductility and oxidation properties at room temperature
of the formulations of the present invention. The high temperature
performance during simulative durability testing of exhaust
manifolds formed from the alloys of the present invention were also
carried out. It should be understood by those of ordinary skill in
the art that the remainder of the compositions presented in
Examples 1-21 include iron and impurities.
[0029] Keel blocks and/or Y blocks of various compositions were
cast with different alloying elements such as niobium, vanadium,
tungsten and molybdenum as presented in Table 1. The comparative
example is a conventional SiMo iron containing about 4% silicon and
0.8% molybdenum. Examples 1-13 are compositions prepared according
to the processes of the invention.
[0030] The tensile tests of test bars cast from compositions of the
invention give some information and insight into the structure of
the materials of the invention. The ultimate tensile strength,
yield strength and elongation measurements for a comparative
example and Examples 1-13 are given in Tables 2-4 below.
[0031] Examples 1-3 are irons in which vanadium and/or tungsten is
used instead of molybdenum. The tensile testing for the comparative
example (the conventional SiMo ductile iron) and the example 1
containing 0.3% vanadium and 0.5% tungsten is given in Tables 2-4
from room temperature to 900.degree. C. It can be seen that Example
1 has the mechanical properties which are similar to or even better
than the conventional SiMo iron of the comparative example. This
indicates that 0.8% molybdenum in the conventional SiMo ductile
irons can be completely substituted by vanadium and tungsten while
the tensile properties are maintained or may even be better. The
composition of the invention as demonstrated in Example 1 has up to
18% elongation at room temperature, so the material of the
invention shows ductility indicating the machinability may be
similar to the comparative example. Examples 2 and 3 have 0.1-0.3%
vanadium and 0.4-0.6% tungsten and both have comparable tensile
properties to the comparative example (the conventional SiMo
iron).
[0032] Examples 4-6 are improved SiMo irons and have 0.2-0.3%
vanadium added into SiMo ductile iron containing 0.5-0.6%
molybdenum. It can be seen from Tables 2-4 that the addition of
vanadium and molybdenum may increase the high temperature strength
such as at 800.degree. C., while the ductility at room temperature
is reasonable, i.e. there is about 10% elongation for room
temperature and more than 25% for 800.degree. C.
[0033] Examples 7-10 showed that the compositions of the invention
containing tungsten and molybdenum have mechanical properties
comparable to the conventional SiMo iron after some of molybdenum
in conventional SiMo irons is at least partially replaced by
tungsten.
[0034] Examples 11 and 12 use tungsten, vanadium, niobium, and
molybdenum in the ductile iron containing about 4.2% silicon. It is
shown from the Tables 2-4 that the strength at high temperature is
significantly increased, compared to conventional SiMo cast iron as
set forth in the comparative example. It is also important to note
that the ductility at room temperature is more than 6%.
[0035] Example 13 uses niobium in the ductile iron containing about
4.35% silicon. It is shown from the Table 4 that the ductility at
room temperature and 800.degree. C. is higher than conventional
SiMo cast iron as set forth in the comparative example.
TABLE-US-00001 TABLE 1 Example C, % Si, % Mn, % P, % S, % Mg, % Mo,
% V, % W, % Nb, % Al, % Comparative 3.36 3.97 0.36 0.015 0.007
0.030 0.78 0 Ex 1 3.26 3.99 0.35 0.016 0.010 0.036 0.31 0.51 0 2
3.28 3.90 0.27 0.008 0.012 0.020 0.11 0.6 0 3 3.33 3.92 0.29 0.008
0.011 0.020 0.3 0.4 0 4 3.24 4.12 0.30 0.015 0.013 0.035 0.52 0.21
0 5 3.27 3.98 0.31 0.015 0.011 0.035 0.6 0.21 0 6 3.22 3.98 0.29
0.014 0.011 0.032 0.6 0.3 0 7 3.49 3.92 0.34 0.009 0.010 0.026 0.49
0.59 0 8 3.41 4.04 0.33 0.008 0.008 0.021 0.48 0.78 0 9 3.46 3.97
0.30 0.008 0.008 0.021 0.58 0.21 0 10 3.48 4.25 0.33 0.01 0.0087
0.024 0.58 0.64 0 11 3.28 4.19 0.33 0.011 0.009 0.035 0.86 0.51
0.76 0.88 0 12 3.38 4.18 0.32 0.012 0.012 0.036 1.14 0.75 1.29 0.91
0 13 3.28 4.35 0.29 0.013 0.010 0.030 0 0 0.086 0.58 0 14 3.35 3.99
0.36 0.016 0.011 0.037 0.30 0.51 0 15 3.45 4.15 0.33 0.009 0.011
0.023 0.41 0.43 0 16 3.15 4.45 0.31 0.010 0.011 0.016 0.85 0 0 0 0
17 3.35 3.99 0.36 0.016 0.011 0.037 0.30 0.51 0 18 3.06 4.46 0.26
0.017 0.0082 0.029 0.51 0 0 0 0.4 19 3.41 4.47 0.30 0.016 0.008
0.030 0.59 0 0 0 0.4 20 3.1 4.4 0.30 0.015 0.011 0.028 0 0 0.65 0 0
21 3.16 4.40 0.32 0.012 0.010 0.024 0 0 0 0 0.35 Balance = iron and
impurities.
TABLE-US-00002 TABLE 2 Ultimate Tensile Strength (ksi) Temperature,
.degree. C. 20 300 400 425 500 600 700 800 850 900 Comparative Ex
88.75 77.85 68.48 68.17 52.35 29.43 14.15 6.8 5.9 7.02 1 91.67 80.5
71.17 67.5 50.6 27.43 14.2 7.38 7.3 2 81 6.51 3 90.3 6.92 4 86.5
7.37 5 83.5 7.36 6 85 7.95 7 83 6.75 8 84 6.85 9 82 6.77 10 83.8
6.89 11 101 79 19.5 7.1 12 104 7.65 13 85.6 5.56
TABLE-US-00003 TABLE 3 Yield Strength (ksi) Temperature, .degree.
C. 20 300 400 425 500 600 700 800 850 900 Comparative Ex 67 59.15
54.81 56.03 42.7 21.74 10.31 4.82 3.85 4.39 1 67.83 59.75 56.5 54.5
40.3 20.9 11.37 5.69 4.42 2 62 4.89 3 69.75 5.39 4 65.5 5.46 5 67.5
5.82 6 67.5 6.01 7 63 4.87 8 63.8 4.87 9 61.3 4.7 10 63.3 4.79 11
75.75 62 15 4.29 12 77.75 5.20 13 66.7 2.75
TABLE-US-00004 TABLE 4 Elongation (%) Temperature, .degree. C. 20
300 400 425 500 600 700 800 850 900 Comparative Ex 16 9.9 5.8 5 25
45.3 49.7 61.4 49 62.6 1 18 11.2 6.8 8.1 29.7 45.1 43.7 45.5 51.8 2
8.5 26.2 3 15.8 42.9 4 11.4 22.4 5 10.5 49 6 10 43.9 7 10.9 61.3 8
10.8 46.7 9 15.3 27.3 10 9.5 59.5 11 9 4.0 24.3 33.4 12 6.1 34.8 13
22.3 82.1
[0036] Comparison of the strength at 800.degree. C. between
molybdenum and tungsten in the high silicon ductile iron containing
4% silicon is now provided. The additions of tungsten range from
0.2% to 0.8% by weight in order to compare the SiMo iron containing
0.2% to 0.8% molybdenum respectively. It has been found that the
1.0% W is equivalent to about 0.8% Mo in the range of 0.77-0.83%
Mo, based on the ultimate tensile strength (UTS) and yield strength
(YS) at 800.degree. C. The Mo equivalent (Mo.E) is found as the
following: Mo.E.=% Mo+0.8.times.% W. The regression curves given in
FIG. 1, with R-squared values (the square of the correlation
coefficient) of 0.988 to 0.997. It was found that a molybdenum
equivalent is Mo %=0.5.times.W %, i.e. the strengthening effect of
1% tungsten in steels is equivalent to about 0.5% molybdenum at
room temperature.
[0037] Comparison of the strength at room temperature is also
provided between molybdenum and tungsten in the high silicon
ductile iron containing 4% silicon. The additions of tungsten range
from 0% to 1.2% by weight in order to compare the SiMo iron
containing 0% to 1.2% molybdenum respectively. It has been found
that the 1.0% W is equivalent to about 1.0% Mo in the range of
0.96-1.05% Mo, based on the ultimate tensile strength (UTS) and
yield strength (YS) at room temperature. The Mo equivalent (Mo.E)
is found as the following: Mo.E.=% Mo+1.times.% W. The regression
curves given in FIG. 2, with R-squared values (the square of the
correlation coefficient) of 0.997 to 0.999, suggest that a
molybdenum equivalent is Mo %=0.5.times.W %. Thus the strengthening
effect of 1% tungsten in steels is approximately equivalent to 0.5%
molybdenum at room temperature.
[0038] The strengthening effect of tungsten (i.e. 1% W=0.8% Mo at
800.degree. C. and 1% W=1% Mo at room temperature) found in the
composition of the present invention is different than in heat
resistant steels in which 1% W=0.5% Mo in terms of strengthening
effect.
[0039] As the atomic weight of tungsten is twice as much as
molybdenum, one would expect the tungsten to have 50% the effect of
molybdenum (i.e. 1% W is equivalent to 0.5% Mo in terms of
strengthening). This is seen in the steels mentioned above, but is
not seen in the alloys of the present invention. In the
compositions of the present invention, the tungsten has 80-100% the
effect of molybdenum (i.e. 1% tungsten is equivalent to 0.8-1%
molybdenum) both at room temperature and 800.degree. C., which was
surprising.
[0040] To test for one or more of elevated temperature strength,
ductility, high oxidation resistance, and thermal fatigue
resistance, manifolds were cast from the formulations set forth as
Examples 14-20. It should be noted that generally thermal cycles of
greater than 1500 without significant distortion of the test
component (exhaust manifold), under the test conditions set forth
are considered to be successful.
EXAMPLE 14
[0041] A manifold for a 6.0 liter engine was cast from an iron
composition containing 3.35% carbon, 3.99% silicon, 0.3% vanadium,
0.51% tungsten, with additions of Mg, Ce, rare earths and the
remainder being iron plus impurities, all percentages being
presented as percentages by weight. The microstructure displayed
good nodularity (about 95%), nodule count of about 400
nodules/mm.sup.2, no pearlite and about 3% carbide. The carbide is
blocky vanadium carbide and some tungsten-rich precipitate which is
similar to the molybdenum-rich precipitate in the SiMo irons.
[0042] The manifold was evaluated in an engine exhaust simulation
test. The test consisted of 1810 thermal cycles before failure. The
test included heat shields applied with an exhaust gas temperature
of 1616.degree. F. (880.degree. C). A thermal cycle consisted of a
6 minute heating portion with burners on followed by a 6 minute
cooling period with burners off. During heating, the exhaust gas
had a temperature of about 860-900.degree. C. and portions of the
surface of the manifold reached temperatures varying from
760.degree. C. to around 780.degree. C. After the burners are
turned off, the exhaust gas and manifold cool down within a period
of 4 or 5 minutes to a uniform temperature of about 70.degree. C.
The manifold showed good stability and heat resistance in the
engine exhaust simulation test. These results were comparable to
tests run with SiMo chemistry of the comparative example.
EXAMPLE 15
[0043] A manifold for a 6.0 liter engine was cast from an iron
composition containing 3.45% carbon, 4.15% silicon, 0.43% tungsten,
0.41% molybdenum with additions of Mg, Ce, rare earths and the
remainder being iron plus impurities, all percentages being
presented as percentages by weight. The microstructure displayed
good nodularity (approximately 94%), nodule count (approximately
350 nodules/mm.sup.2), 6 to 10% molybdenum-rich and tungsten-rich
precipitates, very low pearlite levels (below 5%) and carbide
(approximately 1%) levels.
[0044] The manifold was evaluated in an engine exhaust simulation
test. The test consisted of 1790 thermal cycles prior to failure.
This test included heat shields applied with an exhaust gas
temperature of 1616.degree. F. (880.degree. C.). A thermal cycle
consisted of a 6 minute heating portion with burners on followed by
a 6 minute cooling period with burners off. During heating, the
exhaust gas had a temperature of about 860-900.degree. C. and
portions of the surface of the manifold reached temperatures
varying from 760.degree. C. to around 780.degree. C. After the
burners are turned off, the exhaust gas and manifold cool down
within a period of 4 or 5 minutes to a uniform temperature of about
70.degree. C. The manifold showed good stability and heat
resistance in the engine exhaust simulation test. These results
were comparable to tests run with SiMo chemistry of the comparative
example.
EXAMPLE 16
[0045] A manifold for a 6.0 liter engine was cast from an iron
composition exhibiting a hybrid/duplex graphite microstructure
containing 3.15% C, 4.45% Si, and 0.85% Mo with additions of Mg,
Ce, rare earths, and the remainder being iron plus impurities. This
test included heat shields applied with an exhaust gas temperature
of 1616.degree. F. (880.degree. C.). A thermal cycle consisted of a
6 minute heating portion with burners on followed by a 6 minute
cooling period with burners off. During heating, the exhaust gas
had a temperature of about 860-00.degree. C. and portions of the
surface of the manifold reached temperatures varying from
760.degree. C. to around 780.degree. C. After the burners are
turned off, the exhaust gas and manifold cool down within a period
of 4 or 5 minutes to a uniform temperature of about 70.degree. C.
The test consisted of 2012 thermal cycles prior to failure.
EXAMPLE 17
[0046] A manifold for a 6.0 liter engine was cast from an iron
composition exhibiting a nodular graphite microstructure containing
3.35% carbon, 4% silicon, 0.3% vanadium and 0.51% tungsten with
additions of Mg, Ce, rare earths and the remainder being iron plus
impurities, all percentages being presented as percentages by
weight. This test included heat shields applied with an exhaust gas
temperature of 1616.degree. F. (880.degree. C.). A thermal cycle
consisted of a 6 minute heating portion with burners on followed by
a 6 minute cooling period with burners off. During heating, the
exhaust gas had a temperature of about 860-900.degree. C. and
portions of the surface of the manifold reached temperatures
varying from 760.degree. C. to around 780.degree. C. After the
burners are turned off, the exhaust gas and manifold cool down
within a period of 4 or 5 minutes to a uniform temperature of about
70.degree. C. The test consisted of 1977 thermal cycles prior to
failure.
EXAMPLE 18
[0047] A manifold for a 6.0 liter engine was cast from an iron
composition exhibiting a nodular graphite microstructure containing
3.15% carbon, 4.46% silicon, 0.4% aluminum and 0.51% molybdenum
with additions of Mg, Ce, rare earths and the remainder being iron
plus impurities, all percentages being presented as percentages by
weight. This test included heat shields applied with an exhaust gas
temperature of 1616.degree. F. (880.degree. C.). A thermal cycle
consisted of a 6 minute heating portion with burners on followed by
a 6 minute cooling period with burners off. During heating, the
exhaust gas had a temperature of about 860-900.degree. C. and
portions of the surface of the manifold reached temperatures
varying from 760.degree. C. to around 780.degree. C. After the
burners are turned off, the exhaust gas and manifold cool down
within a period of 4 or 5 minutes to a uniform temperature of about
70.degree. C. The test consisted of 1515 thermal cycles prior to
failure.
EXAMPLE 19
[0048] A manifold for a 6.0 liter engine was cast from an iron
composition exhibiting a nodular graphite microstructure containing
3.41% carbon, 4.47% silicon, 0.4% aluminum and 0.59% molybdenum
with additions of Mg, Ce, rare earths and the remainder being iron
plus impurities, all percentages being presented as percentages by
weight. This test included heat shields applied with an exhaust gas
temperature of 1616.degree. F. (880.degree. C.). A thermal cycle
consisted of a 6 minute heating portion with burners on followed by
a 6 minute cooling period with burners off. During heating, the
exhaust gas had a temperature of about 860-900.degree. C. and
portions of the surface of the manifold reached temperatures
varying from 760.degree. C. to around 780.degree. C. After the
burners are turned off, the exhaust gas and manifold cool down
within a period of 4 or 5 minutes to a uniform temperature of about
70.degree. C. The test was stopped at 1565 thermal cycles because a
fastener was sheared off during testing and the engine head
actually failed in tensile due to distortion of the manifold. So
the test was incomplete.
EXAMPLE 20
[0049] A manifold for a 6.0 liter engine was cast from an iron
composition exhibiting a nodular graphite microstructure containing
3.1% carbon, 4.4% silicon and 0.65% tungsten with additions of Mg,
Ce, rare earths and the remainder being iron plus impurities, all
percentages being presented as percentages by weight. This test
included heat shields applied with an exhaust gas temperature of
1616.degree. F. (880.degree. C.). A thermal cycle consisted of a 6
minute heating portion with burners on followed by a 6 minute
cooling period with burners off. During heating, the exhaust gas
had a temperature of about 860-900.degree. C. and portions of the
surface of the manifold reached temperatures varying from
760.degree. C. to around 780.degree. C. After the burners are
turned off, the exhaust gas and manifold cool down within a period
of 4 or 5 minutes to a uniform temperature of about 70.degree. C.
The test was stopped at 1321 thermal cycles because a fastener
failed during testing and the engine head actually failed in
tensile due to distortion of the manifold. Thus, the test was
incomplete.
[0050] Oxidation resistance was also tested in accordance with the
below described evaluation.
EXAMPLE 21
[0051] High temperature oxidation resistance was measured. 16 mm
thick Y-blocks were cast and cut into rectangular-shaped specimens
with three as-cast surfaces and three machined surfaces. The coupon
dimension for oxidation testing is approximately
22.times.20.times.16 mm. FIG. 3 shows the weight gain rate as a
function of exposure hours at 820 C for four materials whereas
small squares stand for 4.0% Si-0.6% Mo, solid circles for 4.4%
Si-0.6% Mo, empty circles for 4.4% Si-0% Mo, and triangles for
0.35% Al-4.4% Si-0% Mo. After the total exposure time of 512 hours,
the depth of oxide scales was measured, as shown in FIG. 4. From
FIGS. 3 and 4, the following findings can be listed.
[0052] Oxidation resistance is improved when the Si content is
increased from 4.0% to 4.4%. The resistance consists of weight
gain, depth of oxide scales, and oxide adhesion. There is little
change in oxidation resistance when molybdenum is increased from 0
to 0.6%. For the non-Al containing samples, the difference is
evident between the as-cast and machined surfaces in the oxidation
behavior. With the addition of 0.35% Al alloyed specimens
significantly improved oxidation resistance (weight change, depth,
and especially oxide adhesion). In contrast to non-Al specimens,
there is much less difference between as-cast and machined surfaces
for the 0.35% Al alloyed materials.
[0053] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *