NICKEL ALLOYING OF FOIL IN BRAZED METAL CATALYTIC CONVERTER SUPPORT

ALLIAGE AVEC DU NICKEL DE FEUILLE DANS UN SUPPORT DE CONVERTISSEUR CATALYTIQUE A BASE DE METAL BRASE

English Abstract

A method is provided for enhancing the crack resistance of an iron chromium
aluminum foil by
localized alloying of the structure. According to the method a continuous
strip of amorphous
nickel foil is placed on a surface of a sheet of iron chromium aluminum foil.
The combined foil
layers so formed are heated to a temperature sufficient to cause the amorphous
nickel foil to
diffuse into the iron chromium aluminum foil to an extent sufficient to yield
a microstructure and
composition having enhanced crack resistance as compared to unalloyed iron
chromium
aluminum foil.

Claims

3
CLAIMS
1. A method for enhancing the crack resistance of an iron chromium aluminum
foil by
localized alloying of the structure. The method comprises the steps of:
(i) obtaining a sheet of iron chromium aluminum foil;
(ii) placing a continuous strip of amorphous nickel foil on a surface of the
iron
chromium aluminum foil;
(iii) heating the combined foil layers of step (ii) to a temperature
sufficient to
cause the amorphous nickel foil to diffuse into the iron chromium
aluminum foil to an extent sufficient to yield a microstructure and
composition having enhanced crack resistance as compared to unalloyed
iron chromium aluminum foil.

Description

CA 02490481 2004-12-17
1
TITLE: NICKEL ALLOYING OF FOIL IN BRAZED METAL CATALYTIC
CONVERTER SUPPORT
Description of the Invention
[0001 ] Honeycomb matrices made from high temperature steel foil are used as
support
structures for catalytic coatings for catalytic converters used in automotive
and industrial
applications. The matrix is formed by winding previously corrugated stainless
steel foil into a
spiral shape to form a multitude of channels and passages. The corrugated foil
is usually
interspersed with a flat foil and in fact the two are generally wound
simultaneously so that the
spiral of flat foil acts as a partition between adjacent corrugated layers.
[0002] The resulting assembly may be brazed together wherein the peaks and
valleys of
the corrugations are brazed to the respectively adj acent flat foil.
[0003] A common problem with substrates which are assembled from brazed layers
of
flat and corrugated stainless steel is that cracks appear in the flat foils,
which propagate until
failure of the entire system. The cracks are believed to be the result of
thermal stresses which
arise due to the different unit lengths of the flat and corrugated foil and
the thermal cycling of the
substrate. The cracks most commonly appear in the flat foil midway between two
brazed joints.
This believed to be the result of the highly restrained geometry and cyclical
buckling.
[0004] The foil generally used for catalytic converter substrates is an
aluminum
chromium stainless steel comprising approximately 20% chromium, 5% aluminum
and the
balance being iron. While such a material is generally not regarded as being
as tough as a nickel
containing stainless alloy, it does have the advantage of bonding well to a
catalyst bearing
material, which is the underlying purpose of the substrate.
[0005] It is an object of the present invention to modify the metallurgy of at
least the flat
foil in a brazed metal catalytic converter support in order to enhance the
service life of the unit
by mitigating cracking of the flat foil between the brazed joints.

CA 02490481 2004-12-17
2
Descriution of Preferred Embodiments
[0006] Amorphous nickel foil is alloyed by a diffusion into a base metal of
iron
chromium aluminum. The base metal may be a foil such as used for catalytic
converter catalyst
support structures.
[0007] The nickel foil may be laid as a strip onto the base metal and heated
to a
temperature of and for a duration sufficient to cause its diffusion into the
base metal foil. The
amorphous nickel foil may be a brazing foil such as sold under the trademark
METGLAS. A
METGLAS specification sheet is attached as Figure 3.
[0008] The reaction may be carried out under vacuum conditions and at
temperatures of
the type typically used for brazing iron chromium aluminum flat foil and
corrugated foil
together.
[0009] The modified metallurgy when used for catalytic converter applications
need not
extend the full breadth of the foil. For example, on a 90 millimetre wide
foil, a continuous strip
approximately 5 millimetres of amorphous nickel foil may be used.
[0010] The resulting alloy has a desirable microstructure as illustrated in
the attached
Figure 1.
[0011 ] Figure 2 illustrates cracking problems associated with the prior art
structure.
[0012] Figure 4 is illustrative of the method of the present invention.