[0001] The present invention relates to a system comprising a substrate and a catalytic
material and to the alloy from which the substrate is formed.
[0002] As the particular material used as a substrate for the catalyst in automotive catalytic
convertors affects the performance of the convertors, there is a need to develop improved
substrates. In particular, there is a need for a metallic substrate to replace the
ceramic substrates presently in use. Ceramic substrates do not have the mechanical
properties; e.g., shock resistance, of metallic substrates. Ceramic substrates are,
in addition, inherently thicker than are metallic substrates.
[0003] A particular metallic substrate is described in United States Patent No. 3,920,583.
It is a ferritic stainless steel which contains up to 15% chromium, 0.5% to 12% aluminium
and 0.1 to 3% yttrium.
[0004] Economic and technical problems have been encountered with the use of yttrium, thereby
detracting from the benefits of the substrate of United States Patent No. 3,920,583.
Yttrium is an expensive material which is not presently available in abundant quantities,
and a material characterized by a low recovery rate during melting.
[0005] Through the present invention there is provided a metallic substrate which minimizes
the difficulties associated with the substrate of Patent No. 3,920,583. The present
invention relates to a substrate wherein the yttrium level of Patent No. 3,920,583
is significantly reduced. It has been determined that high quality substrates can
be produced from iron-chromium-aluminium alloys having less than 0.1% yttrium.
[0006] Other references disclosing ferritic stainless steels with chromium, aluminium, and
yttrium are United States Patent Nos. 3,027,252 and 3,298,826, and reports respectively
entitled, "The Physical Metallurgy And Oxidation Behavior Of Fe-Cr-Al-Y Alloys" and
"Sulfidation - Resistant Alloy For Coal Gasification Service". The first report is
dated June 1, 1966. It was prepared by C. S. Wukusick under United States Atomic Energy
Commission Contract No. AT(40-1)-2847. The second report was published June 14, 1977.
It was prepared by Roger A. Perkins and M. S. Bhat for the U. S. Energy Research And
Development Administration under Contract No. E(49-18)-2299. Neither the reports nor
the patents relate to a substrate for a catalytic material nor to an alloy having
less than 0.1% yttrium.
[0007] Another reference disclosing ferritic stainless steels with chromium, aluminium and
yttrium is United States Patent No, 3,591,365. Although it discloses alloys which
may have less than 0.1% yttrium, its alloys require specific additions of gadolinium
and/or dysprosium. As with the references disclosed in the preceding paragraphs, it
does not pertain to substrates for catalytic materials.
[0008] It is accordingly an object of the subject invention to provide a substrate for a
catalytic material and an alloy from which the substrate is formed.
[0009] The present invention provides a system comprising a ferritic stainless steel substrate
having a tightly adherent oxide coating and a catalytic material thereupon, said ferritic
stainless steel being of a chemistry which forms a tightly adherent non-spalling scale
suitable for application of a catalytic bearing material; characterized in that the
ferritic stainless steel substrate consists essentially of, by weight, up to 26% chromium,
from 1 to 8% aluminium, between 0.01 and 0.1% yttrium, up to 0.1% carbon, up to 2%
silicon, balance essentially iron.
[0010] The present invention further provides a ferritic stainless steel consisting essentially
of, by weight, up to 26% chromium, from 1 to 8% aluminium, between 0.01 to 0.1% yttrium,
up to 0.1% carbon, up to 2% silicon, balance essentially iron.
[0011] The substrate is in most instances of a thickness of from 0.0127 to 0.254mm (0.0005
to 0.01 inch).
[0012] Chromium may be present within the substrate or steel of the present invention as
it is known to improve oxidation resistance. A maximum limit is placed thereupon as
chromium is expensive and renders the alloy more difficult to process. Chromium is
usually present within the range of from 5 to 22%, and preferably within the range
of from 12. to 20%.
[0013] Aluminium is present as it improves the oxidation resistance of the substrate or
steel. A maximum limit is placed thereupon as aluminium, like chromium, is expensive
and renders the alloy more difficult to process. Aluminium is preferably present within
the range of from 3 to 6%.
[0014] Yttrium is present as it stabilizes the aluminium- bearing scale and makes it both
tight and adherent. It is preferably present in amounts between 0.03 to 0.09%.
[0015] Carbon and silicon are preferably maintained at respective maximum levels of 0.03
and 0.5%. As ferritic stainless steels have inherently high transition temperatures,
which rise with increasing carbon levels, low carbon contents should be specified
in order to obtain a more ductile material.
[0016] The present invention is not dependent upon any particular means for manufacturing
the catalytic system described herein. The system can be produced in accordance with
the teachings of heretofore referred to United States Patent No. 3,920,583, or by
any other process known to those skilled in the art. Platinum, palladium, irridium,
rhodium, and alloys thereof, are typical catalytic materials. The catalyst serves
to provoke oxidation of partially oxidized hydrocarbons; e.g., CO to CO
2.
[0017] The following examples are illustrative of several aspects of the invention.
[0018] Several 0.0508mm (0.002 inch) thick Fe-Cr-Al alloys were subjected to a cyclic oxidation
test in air at 1260°C (2300°F). The alloys were alternately resistance heated and
cooled. Cycles to failure for each were recorded. Some of the samples had yttrium
in excess of 0.1%. some were free of yttrium, and another had a yttrium level between
0.01 and 0.1%. The chemistry of the alloys appears hereinbelow in Table I. The carbon
content for each is less than 0.03%.

[0019] The results of the cyclic oxidation tests appear hereinbelow in Table II.

[0020] From Table II, it is noted that the oxidation resistance of Alloys A, B and C with
yttrium contents in excess of 0.1% was superior to that for Alloys D, E and F which
were devoid of yttrium, and that Alloy G with yttrium between 0.01 and 0.1% fared
well in comparison with Alloys A, B and C. The results clearly show that iron-chromium-aluminium
alloys with yttrium between 0.01 and 0.1% can be used as substrates for catalytic
materials.
1. A system comprising a ferritic stainless steel substrate having a tightly adherent
oxide coating and a catalytic material thereupon, said ferritic stainless steel being
of a chemistry which forms a tightly adherent non-spalling scale suitable for application
of a catalytic bearing material; characterized in that the ferritic stainless steel
substrate consists essentially of, by weight, up to 26% chromium, from 1 to 8% aluminium,
between 0.01 and 0.1% yttrium, up to 0.1% carbon, up to 2% silicon, balance essentially
iron.
2. A system according to claim 1, wherein said ferritic stainless steel substrate
has from 5 to 22% chromium.
3. A system according to claim 2, wherein said ferritic stainless steel substrate
has from 12 to 20% chromium.
4. A system according to claim 1, 2 or 3, wherein said ferritic stainless steel substrate
has from 3 to 6% aluminium.
5. A system according to any one of the preceding claims, wherein said ferritic stainless
steel substrate has from 0.03 to 0.09% yttrium.
6. A system according to any one of the preceding claims, wherein said ferritic stainless,
steel substrate has less than 0.03% carbon.
7. A system according to claim 1, wherein said ferritic stainless steel substrate
has from 12 to 20% chromium, from 3 to 6% aluminium, between 0.03% and 0.09% yttrium
and up to 0.03% carbon.
8. A ferritic stainless steel consisting essentially of, by weight, up to 26% chromium,
from 1 to 8% aluminium, between O.Ol to 0.1% yttrium, up to 0.1% carbon, up to 2%
silicon, balance essentially iron.
9. A ferritic stainless steel according to claim 8, having from 5 to 22% chromium.
10. A ferritic stainless steel according to claim 9, having from 12 to 20% chromium.
11. A ferritic stainless steel according to claim 8, 9 or 10, having from 3 to 6%
aluminium.
12. A ferritic stainless steel according to any one of claims 8 to 11, having between
0.03 and 0.09% yttrium.
13. A ferritic stainless steel according to any one of claims 8 to 12, having up to
0.03% carbon.
14. A ferritic stainless steel according to any one of claims 8 to 13, having from
12 to 20% chromium, from 3 to 6% aluminium, between 0.03 and 0.09% yttrium and up
to 0.03% carbon.