Note: Descriptions are shown in the official language in which they were submitted.
~0~3~Z~
This lnvention relates to a chromium-aluminum-
sllicon-titanium steel of low alloy content for use as a
substrate for aluminum or aluminum alloy coatings, and
wrought coated products thereof havinÆ good resistance to
oxidation at elevated temperatures up to about 1700F
(925C) and resistance against attack by hydrocarbon combus-
tion products at elevated temperature, together with high
strength.
Increasingly stringent requirements for antipol-
lution controls on motor vehicles has created a need for
relatively low cost alloys which will be oxidation resistant
at elevated temperatures, for use in automotive exhaust sys-
tems, such as catalytic converters, mufflers, and llke
articles.
Aluminum coated carbon steel has proved to be
not completely satisfactory for some hlgh temperature appli-
cations. The automotive industry has substituted stainless
' .
steels such as Armco 409 (containing 0.05% carbon, 11%
chromium,traces of aluminum, residual nickel, 0.5% titanium
and remainder iron) and other stainless steels containing
11% or more chromium. The cost of such steel is high, thus
makin~ it undesirable for proposed use in automotive exhaust
systems, such as catalytlc converters, and the lllce. More-
over, although this stainless steel has fair oxidation re-
25 slstance at elevated temperature, and good formability, it ;~
does not adequately wlthstand attack by molt-en salts and
hydrocarbon combustlon products at elevated temperature.
The provision of an aluminum coating on such a steel has
c/e~?ark
. .
~04~323
been found to result ln a product having the deslred proper-
ties, but this solution obviously adds even ~reater cost.
United States Patent 3,698,964, issued October
17, 1972, to E. .J. Caule et al, discloses an iron base al-
loy with good oxidation resistance at tem~eratures Or
about 700 to 800 C (about 1300 to 1475 F). The alloy
o~ thls patent contains up to 2~ carbon, 1% to 5~ chromium,
1% to 4% aluminum, and/or 1~ to 4% silicon, up to 1.5% man-
ganese, up to 2% copper, up to 0.20% total of nickel, molyb-
denum, vanadium and other alloyin'g elements. Pre~erably,
a combinatlon of 2% chromium and 3~ alumlnum, or 3% chromlum
and 2% silicon, are used, with manganese ~referably up to
0.2%, copper not more than 0.5%, carbon not more than 1% and
most prererably rrom 0.01% to 0.2'5%. ;
The high carbon content of the steel of the Caule
et al patent results in a brittle structure havln~ very li- ;~
mited cold workability, poor welding characteristics, and poor
mechanical properties generally. The optional presence o~ ~
molybdenum and vanadlum adversely afrects oxidatlon scale ~' '
resistance, as well as adding to the cost. The harmful effect
o~ molybdenum on oxidation resistance is reported in "Stain-
less And Heat Resistin~ Steels", by Colombler and Hochmann,
p. 330, St. Martin's Press, Mew York, N.Y. (1968). Moreover, ''
copper, nlckel,and other austenite stabilizers in amounts
~reater than typical re'sidual contents of about 0.2~ each are ! ;' ~ ''
undesirable since it can cause a phase chanGe, with a conse- '
quent change in volume, on heatln~, and cooling. This volume ~'~
change results in s~alling a~ter cyclic heating and coolin~.
-~ 2 8~dr 6~
Unlted States Patent ~ issued March 4,
1958, to E~M. Herzog, discloses a steel, and a heat treatment
3 --
:.. .. .. ~, , , .... ~ . . . ................ . . .... .
, . , , , . . . . ~
:lV413~3
thereror whereln a mlcro-structure ls ~roduced havlng
reslstance to stress-corrosion cracking in wet hvdro~en
sulride atmospheres. The steel of this patent contains
from o.o8% to 0.20~ carbon, from o.60% to 5.0~ chromium,
from 0.15% to 1.20% aluminum, from 0.30% to 1.20% man~a-
nese, rrom 0.10% to 0.50~ sllicon, u~ to 0.50% mol~bdenum,
up to 1.0% vanadium, up to 1.0% titanium and remainder
lron. A heat treatment at 740 to 780 C, a second heat
treatment at about 970 to 1080 C followed by a water-
quench and a tempering treatment at about 625~ to 670 C,
are stated to result in the desired micro-structure and
an ultimate tensile strength Or at least 95 ksi. Oxidation
reslstance at elevated temperature is not contemplated in
this patent, and the composition would not lnherently pro-
duce a steel having thls propert~. Moreover, the ~resence
Or molybdenum and vanadium is deleterious ~or reasons set
forth above~
Other patents disclosing low alloysteels con- -
talning chromium,~luminum, and/or silicon in varying a-
mounts include United States Patents: 3,431,101; 2,835,570;
and 2,770,563.
None of the above patents discloses a low alloy
steel having a completely ferritic structure withln the con-
templated operating temperature ran~e, and exhlbiting in
combination good oxidation resistance at elevated temperature,
good resistance a~ainst attack by hydrocarbon combustlon
products, good weldabllity and rormabilit~ and relativel~
high strength. Hence, there still exists a great need for a
low-cost alloy having the ahove comhination of pro~erties
for fabricatloninto coated welded and ~!rought products such
. ,. ~ ,. . .
.
1(~4~ 3
as space heaters, automotive exhaust systems, e.g., catalytic converters and
mufflers, and the like.
~ ccording to the invention there is provided an alloy steel having
good oxidation resistance at elevated temperature, good weldability and good
formability, consisting essentially of, by weight percent, from 0.01% to
0.13% carbon~ from 0.5% to 3% chromium, from 0.8% to 3% aluminum, from 0.4%
to 1.5% silicon, from 0.1% to 0.6% manganese, a maximum of about 0.05% ;~
molybdenum, a maximum of about 0.05% vanadium, less than about 0.2% copper, ;~
less than about 0.2% nickel, an element chosen from the group consisting of
~ ; :
titanium, columbium, zirconium, and mixtures thereof, titanium when present
being from 0.1% to 1% columbium ~hen present being from 0.1% to 1.5%
zirconium when present being from 0.1% to 1.5%, and mixtures when present
being from 0.1% to 1.5% total, and remainder iron except for incidental
impurities.
The steel of the present invention preferably contains chromium, `
aluminum, silicon and titanium in a total amount of less than about 5% and
provides in sheet form a substrate for aluminum or aluminum alloy coatings,
the coated sheet being readily formable into wrought articles having good ;~ `~
oxidation resistance at elevated temperature, good resistance against attack
by hydrocarbon combustion products at elevated temperature, and relatively
high retained strength at elevated temperature.
The carbon, chromium, aluminum, silicon and titanium percentage
ranges are critical and departure therefrom results in loss of one or more -~
of the above properties. Control of the critically low molybdenum, vanadium,
copper, nickel and other austenite stabilizer contents is also essential.
: . .
Carbon is essential in an amount of at least 0.01% in order to
provide the necessary strength in the steel. More than 0.13% carbon cannot `
be tolerated because of its adverse effect upon the weldability, formability
and general mechanical properties of the steel, and because it is a strong
austenite former. 5
~L04~323
At least 0.5% chromium in combination with at least 0~8% aluminum
and 0.4% silicon is necessary in order to provide good oxidation resistance.
A maximum of 3% chromium should be observed in order to mirimize cost and
avoid processing difficulties.
At least 0.8% aluminum is necessary not onl;y for oxidation resis-
tance at elevated temperature but also to provide adequate tensile strength.
More than 3% aluminum results in a loss of formability and workability.
At leas* 0.4% silicon is essential since it co-operates with the
chromium and aluminum to impart oxidation resistance. However, a maximum
of 1.5% silicon should be observed since amounts in excess thereof also
result in loss of formability and workability.
An amount of titanium,columbium or zirconium of at least 0.1% is
necessary in order to impart good weldability to the steel. Moreover~ excess
titanium over that needed to stabilize carbon has been found to improve the
oxidation resistance at elevated temperature. This excess can be slight in
view of the high cost of titanium and of the relatively low residual sulfur,
nitrogen and oxygen contents of the steel of the invention. Preferably the
titanium content is 8 times the carbon content, and a maximum of about 1%
titanium should thus be observed at the carbon levels contemplated herein.
Since it is known that columbium and/or zirconium generally function in an ;
equivalent manner in stainless steels, it is considered within the scope of ~
the invention to substitute columbium and/or zirconium in whole or in part ~-
for titanium. Such substitution would generally be on a stoichiometric basis,
advantageously with a minimum weight ratio of columbium or zirconium to carbon
of 8:1, preferably at least `
.. ,.,.
~
~ ''`~ .
~04~3~3
about 10:1. Columbi~D and/or zirconium would thlls range from
0.10% ~o 1.5% if substituted for titanium~
Impurities at residual levels normal for ferritic
stainless steels can be tolerated in the steel of the inv~ntion.
More specifically, a maximum of about 0.03% sulEur and a maximum -
of about 0.04% phosphorus do not adversely affect the properties
of the steel. Molybdenum and vanadium are undesirable in the
steel of the invention as explained above, and are maintained at
the minimum practicable levels. Copper and nickel are maintained
10at a maximum of less than 0.2% each for reasons set forth above. ;
While the desirable novel combination of properties `~
is achieved in a steel having the broad composition ranges
hereinabove set forth, optimum properties are obtained in a
steel having the following preferred analysis by weight percent:
Carbon 0.04% to 0.06% ~;
Chromium ~ 1.7% to 2.1%
Aluminum 1.7% to 2.0%
Silicon 0.6% to 0.9%
Manganese 0.2% to 0.4%
Titanium 0.1% to 0.6%
and remainder iron except or incidental impurities.
In order to investigate the effect of the relative
proportions of chromium, aluminum, silicon and titanium on the
properties of the steel, a series of experimental heats was
prepared and tested. For purposes of comparison, tests were
also conducted on plain carbon steel coated with aluminum and
aluminum alloys containing up to 10% silicon, and on Armco
Type 409 stainless steel. The compositions of the experimental
heats are set forth in Table I below.
,:
7 ~
: : ~: :. - :,. . :: :. . : : .. : ,
::: ........ : : : ~ : :
.. . - . ~- ..
1C~4~3~3
TABLE I
Sample
Code C Cr Al Si Mn Tl
0.048 1.0 <0.1 0.2 ~.~ o.4
41* o.o33 '5 1.7 0.8 0.4 0.4
42~ O.Q33 l.o 1.7 o.7 o.4 o.4
43* 0.034 1.5 1.7 0.7 0.4 0.4
61* 0.037 l.o l.o o.7 o,4 0.3
62* o.o38 1.7 .9 o.8 0.4 o.3
0-054 2.5 1.9 0.47 0.21 0
86~ 0.073 2.0 1.92 0.81 0.32 0.48
Steels of the invention - also containin~ 0.1% Cu, 0. 04
Mo, and 0.03% V.
The above materials were hot rolled rrom 21nO ~
(1149~ C) from 1 lnch by 3 inch ingots to 0.1 inch thlckness.
Samples were annealed at 1700 F (927 C) for 10 minutes,
descaled and cold rolled to 0.05 lnoh thickness. It should ~ -
be recognized that annealin~ the hot rolled material is option- ''
al. Tensile stren~ths were determined on the cold rolled sam-
.
ples at this stage while the remainder of the cold rolled stri~
was annealed at 1600 F (871 C) for 6 minutes and pickled.
This material was tested for the remalnin~ mechanical properties
reported below in Table II.
A consideration of the mechanical properties
..
reported in Table II lndicates that the steels of the inven- ~-
tion have ultimate tensile strengths equivalent to those o~
comparable prior art allo~s but have imDroved elon~ation.
Thls is believed to result from the relatively low carbon
contents (rangin~ from about 0.03% to about 0.07~). The yield
stren~th and tensile stren~ths o~ ~he alloys containin~ about
;;~
8 ~
, . ~ ' ~ . , , :. . ' !
~4~Z3 ; ~
l~ aluminum were about 5 ksi and 4 ksi, respectively, less
than those containln~ 2~ alu~inum. Of greater si~nificance
is the comparison with the 1% chromium alloy (Sample Code 15)
also havlng low aluminumand silicon contents. It will be
noted that the yield ~trength of the 1% ch:romium alloy was
:. :
about 12 ksi lower and the tensile strength about 8 ksi lower ~ -
than the steels of the invention containing from about l.7%
to about 2% aluminum with chromium ranging from 0.5% to 2%.
.
At the same time the elongation values of these steels of
the invention were about equivalent to that of Sample Code
15. For an optimum comblnation of mechanical propertles,
it is thus apparent that the carbon content should not ex~
ceed about 0.06%, that aluminum should ran~e from about
l.7 to about ?% in combination with at least about l.7%
chromium and at least about 0.6% silicon.
,
TABLE II
Olsen
Sample 0. 2% YS UTS ~ Elon~. Hardness Cup Test
Code (ksi) (ksi) in 2~ (Rockwell B) Hei~ht-Inches
:,
23.65 53. g5 35.5 57.0 .390, .4~0
41* 36.40 61.85 36. o 73.0 400, .410
42* 35.65 62.15 34.0 73.0 . 410, .405
43* 35.80 62.55 35.5 73.0 .400, ~4~0
61* 29.95 58.25 37.0 68.o .380, .400
62* 30.45 58.60 34. 5 68.5 .3g5, .44
39.70 61. 25 20.5 73.0 .390
86~ 37.15 61.10 26.0 7~.0 .408 - -
* Steels of the present invention
.
Samples of the steels Or Table I ln the cold rolled,
30 annealed and Dickled condltion were surface ~,round, and a full
: ~ . . -
104i323
Denetration auto~enous ~TA weld was run down the lon~ltu-
dlnal axis of the strlp of each samole. 180 bend and
Olsen cu~ test specimens were cut from the sam~le~s and
tested wlth hoth root and face side ln tension. These
tests are reported in Table III.
From the data ln Table III it is evldent that
optimum as-welded ductillt,y i3 exhlbited with about 1%
aluminum and chromium in excess Or 1%. At the 2% alumlnum
level better results apParently are obtained with chromium
at about 2%.
The necessity for the presence of titanium for
~ood weld properties is demonstrated b,y Samole Code 85,
wherein a poor Olsen cup value was obtained. In addition,
Sample Codes 85 and 86 were sub,~ected to a further bend
test across the weld (not reported in Table III), and it
was found that Sample Code 85 cracked at 90 across the
weld, whereas Sample Code 86 passed 180 flat across the
weld.
. ~.
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23
Oxidation resistance tests were conducted on
the samples of Ta~ble I in the cold rolled, annealed and
pickled condition, both on coated and uncoated specimens.
For the coated specimens, a pure aluminum coating was ap-
5 plied by hot-dipping using a flux of ~luorides of zirconium ~-
and/or titanium, details of whlch are disclose~ in United
States of America Patents 2,6~6,354 and 2,686,355, issued
August 17, 1954 to H. Lundin. Coating weight was about 1~2
ounce per square foot of sheet or 0.064 gram per square
centimeter (total coatlng weight on both surfaces). For
purposes of comparison, oxidation tests were also run on
plain carbon steel coatèd with pure aluminum and with alu-
minum alloy containing up to 10% sllicon, uncoated Armco
Type 409 stainless steel, and an uncoated commercial alloy
contalning 5% chromium, 0.5% molybdenum, 0.06% carbon, 0.35% --
silicon, 0.4% manganese, residual aluminum and nickel, and
balance substantially iron. The initial tests comprised
100 hours in still air at 1600F, and 1700F, respectively.
These tests are reported in Table IV below.
Since still air tests are not necessarily defi-
nitive, further specimens of coated and uncoated materials
were sub~ected to cyclic testing, using a cycle Or 25 minutes
in and 5 minutes out o~ the rurnace for a total of 130-135
cycles. These results are reported in Table V below.
,
12
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3~3
TABL~ V
Oxl~ation Tests
"
Cycllc 25/5 ;
Sample Wei~ht Increase
Code Condltions m~/ln2.
T,ype 409 stainless 135 c,ycles - 1500F 118
15 uncoated 135 cycles - 1500F 666
41 uncoa~edx 135 cycles - 1500F 315
42 uncoated]* 135 cycles - 1$00F 270
43 uncoated9 135 c,vcles - 150nF 250
61 uncoated~ 135 cycles - 1500F 382
62 uncoated* 135 cycles - 1500F 376
85 uncoated 13Z c.ycles - 1500F 198
86 uncoated* 132 cycles - 1500F 117 ;~ :~
15 Al coated 135 cycles - 1500F 37.2
41 Al coated* 135 c~cles - 1500P 17.4
42 Al coated* 135 c,ycles - 1500F 15.0
43 Al coated* 135 c~cles - 1500F 12.3
61 Al coated* 135 cycles - 1500F 14.3
62 Al coated* 135 c~cles - 1500F 18.2
86 Al coa~ed* 130 cycles - 1600F 10.2
' ~ .
* - Steels Or the invention
- '` ' ' '
:,
,. . . ~ . .
:. . , . . ~ . . ~ ~
13~3
The oxldatlon tests lndicate that all the steels
o~ the ~resent inventlon exhibited good scalin~ reslstance
both in still air and cycllc tests without coatin~s. With
aluminum coatlngs, the steels of the lnvention are superior
to Armco Type 409 ln uncoated condltion. It should further
be noted that the stlll air tests on aluminum and aluminum
alloy coated plain carbon steel base metal or substrate
showed this material to be completely unacce~table for oxi-
datlon reslstance at elevated temperatures of the order of
1500 - 1700 F, because of blistering and warpa~e.
Optimum oxidation resistance ls achieved in a
steel of the inventlon containing about 2~ chromium, about
2% aluminum, about 1% silicon and about 0.5% titanium (with
titanium about 8 tlmes the carbon content). Sa~le Code 86,
having thls approximate analysis, was superior to other sam-
ples in the coated condltion des~lte the fact that lt was
sub~ected to cyclic test temperatures 100 F degrees hi~her
than any of the other materials tested. In the uncoated
condition it was at least equivalent to uncoated Type 409
stainless steel and superior to the other uncoated samples.
:; .
16
...... .. . .. .. . . ..
. . .
... .... .. . .. . .. . : . . . .
