STABILIZED FERRITIC STAINLESS STEEL WITH IMPROVED BRAZEABILITY

ACIER INOXYDABLE FERRITIQUE STABILISE SE PRETANT MIEUX AU BRASAGE

English Abstract

Stablilized Ferritic Stainless
Steel With Improved Brazeability
ABSTRACT
A stabilized ferritic stainless steel is wettable
by brazing materials used at temperatures of from 2000°F to
2100°F. The steel consists essentially of, by weight, 10.5% to
13.5% chromium, up to 0.1% carbon, up to 0.05% nitrogen, up to
about 0.12% titanium and at least one other stabilizing
element from the group consisting of niobium and tantalum in
an amount in accordance with the relationship:
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Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A brazeable ferritic stainless steel consisting essent-
ially of, by weight,
10.5% to 13.5% chromium,
0 to 0.03% carbon,
0 to 0.05% nitrogen,
0 to 0.10% aluminum,
0 to about 0.12% titanium,
up to about 0.12% the sum of titanium and aluminum,
0 to 1.25% molybdenum,
0 to 1% manganese,
0 to 1% silicon,
at least one other stabilizing element selected from
the group consisting of niobium and tantalum in an amount in
accordance with the relationship
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with the balance substantially iron,
the steel being wettable by brazing filler materials.
2. The brazeable ferritic stainless steel of claim 1,
comprising substantially no molybdenum, manganese and silicon,
3. The brazeable ferritic stainless steel of claim 1 wherein,
nitrogen is present in amounts up to a, 0.03% and
aluminum is present in amounts up to 0.020%, and
comprising substantially no molybdenum.
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4. The brazeable ferritic stainless steel of claim 1, 2 or
3 wherein the titanium is present in amounts up to about 0.01%.
5. The brazeable ferritic stainless steel of claim 1, 2 or
3 wherein the titanium is present in amounts up to about 0.005%.
6. The ferritic stainless steel of claim 1, 2 or 3 wherein
the titanium is present in amounts of at least 0.001%.
7. The brazeable ferritic steel of claim 1 r 2 or 3 stabil-
ized by niobium.
8. The brazeable ferritic steel of claims 1, 2 or 3 stabil-
ized by tantalum.
9. The ferritic stainless steel of claims 1, 2 or 3 wherein
the steel is wettable by molten copper.
10. The ferritic stainless steel of claims 1, 2 or 3 wherein
the titanium is present in amounts up to about 0.01% and the steel
is wettable by molten copper.
11. The ferritic stainless steel of claims 1, 2 or 3 wherein
the titanium is present in amounts up to about 0.005% and the steel
is wettable by molten copper.
12. A brazed ferritic stainless steel article having the
composition of the steel of claims 1, 2 or 3.
13. A ferritic stainless steel article having the composition
of the steel of claims 1, 2 or 3 wherein the article is brazed
with copper.
-11-

14. The brazeable ferritic steel of claims 1, 2 or 3 stabil-
ized with up to 1.0% niobium.
15. The brazeable ferritic steel of claims 1, 2 or 3 stabil-
ized with up to 1.8% tantalum.
16. A method of brazing the ferritic stainless steel of
claim 1, 2 or 3 comprising brazing the steel with a filler
material.
17. A method of brazing the ferritic stainless steel of
claim 1, 2 or 3 comprising brazing the steel with a copper filler
material.
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Description

6~
1 The invention relates to stabilized ferritic stain-
less steels and is particularly use~ul for ferritic stainless
steel articles which are joined by brazing.
Ferritic stainless steels possess excellent mechanical
properties and oxidation and general corrosion resistance at
elevated temperatures. These steels are ideal for use as the
structural members of heat exchangers, exhaust systems, chemical
process vessels and the like which are exposed to high tempera-
tures ancl stresses and corrosive environments. Fabrication of
these articles frequently requires the joining of the Eerritic
stainless steel with either itself or with another dissimilar
metal at sufficiently high temperatures for the joining method
to be effective. Also, generally speaking, the steel must be
joined in a temperature range exceeding the anticipated service
temperature~ Brazing is a widely practiced me-thod of joining
metals involving temperatures of from ~00 F to the 2000 F -
2100F range which are above the melting point of the brazing
filler material bu-t below the melting point of the base metal
being joined. When the temperature of the brazing filler
material is about the melting point, it becomes molten and wets
the surface of the steel, and then flows by capillary action to
fill a joint. Bonding results from the intimate contact
produced by the dissolu-tion of a small amount of -the base metal
in the molten filler metal.
Ferritic stainless steels to be joined at high
temperatures contain low levels of carbon and small amounts o~
stabilizing elements for combining with carbon and nitrogen to
maintain the ferritic phase and to maintain the oxidation and
corrosion resis-tance of the steel. Stabilizing elements such
as titanium, niobium or tantalum react with the carbon and
-- 1 --
/~t
,. '';

I nitrogen to prevent the formation and precipitation of chromium
carbides and nitrides at grain ~oundaries and the simultaneous
depletion of chromium in the surrounding areas. Stabilizing
elements must be added in amounts e~ceeding the theoretical
requirement to assure complete stabilization o~ carbon and
nitrogen. Titanium is the preferred stabilizing element because
of its very strong affinity for carbon and nitrogen, its low
atomic weight and its availability. Other stabilizing agents
including niobium and tantalum are not favored because they are
1~ more expensive and less effective on a weight basis than titanium
and also because they are accompanied by a tendency toward weld
cracking problems.
Titanium stabilized ferritic steels known in the prior
art (see, e.g., Lula et al. U.S. Patent NoO 3,250,611) cannot
be re~dily brazed with filler materials such as oxygen-Eree
copper and nickel base alloys. These steels form a non-wettable
surface film which prevents proper bonding between the férritic
stainless steel base metal and the brazing filler material even
when furnace brazing under vacuum or in an inert atmosphere. The
oxygen-free copper as a high temperature brazing filler metal
does not pene-tra-te this surface film~ Nickel alloy high tempera-
ture brazing filler me-tals usually contain boron and silicon
additions to penetrate the surface film. ~lthough the steel
wettabilit~ is improved, these nickel base materials will also
penetrate the grain boundaries thereby causing intergranular
attack of the base metal. In addition, brazing operations are
not aided by increased temperatures or by increasecd brazing
times because the high temperature range is beginning to afect
the grain size of the steel and prolonged time tends to increase
3~ film resistance. For these reasons, brazing with copper is
. . ~,

1 impossible and bxazing with nickel base metals is not consistent
enough to be of practical value from a quality assurance view-
point. Thus copper clad ferritic stainless steels are used in
brazing applications when the brazing temperature is to reach
2000F - 2100F. In this process, the copper cladding is brazed
rather than the steel.
The present invention relates to a stabilized ferritic
stainless steel composition which is wettable by conventional
brazing materials used at temperatures of from 2000 F - 2100 F
in furnace brazing practices. In accordance with the invention,
a ferritic stainless steel consists essentially of, by weight,
10.5% to 13.5% chromium, up to 0.1% carbon, up to 0.05% nitrogen,
up to about 0.12% titanium and at least one other stabilizing
element from the group consisting of niobium and -tantalum in
accordance with the relationship:
Wt% Nb + Wt% Ta + Wt% Ti
93 181 48
. ~ , . . . . . -.-- --- -- > 1 .
Wt% C + Wt% N
12 1
The presence of niobium, tantalum and titanium in
accoxdance with this stabilization relationship are sufficient
to effectively stabilize the inters-titial elements in the steel
without forming a non-we-t-table surface film. The niobium and
tantalum are presen-t as additions to the melt. Titanium may be
present in the scrap feed or added to the melt. The titanium is
responsible for the nature of the film which becomes non-wettable
when titanium is present in amounts ~reater than about 0.12%.
Greater amounts of titanium could be tolerated and the effect of
titanium on wettability could he neutralized if titanium compounds
-

1 stable at brazing temperatures such as TiO2, TiS and TiN are
permitted to ~orm. However, oxygen, sulfur and nitrogen have
an undesirable effect on other steel qualities and generally they
will be kept as low as possihle. From a brazing viewpoin-t it is
preferable to have a composition ~ree o~ titanium. For this
reason the titanium is preferably present in an amount up 0.01%
by weight and, most pre~erably, up to 0.005%. The steel may also
contain up to 0.1~ aluminum, up to 1.25% molybdenum, up to 1%
manganese and up to 1% silicon to enhance its mechanical and
corrosion properties. Articles of this composition are wettable
by fillers such as copper, nickel and their alloys and can be
successfully furnace brazed according to conventional practices.
In some cases, however, it may be desirable to both
weld and braze the same article. Therefore, titanium is tole-
rated in controlled amounts up to 0.12% to prevent weld cracking
while maintaining reasonable wettability during brazing operations.
Larger amounts o~ titanium render the steel unbrazeable for
practical purposes.
To illustrate the beneficial results of the invention
specimens from sixteen laboratory heats and two commercial heats
were tested for wettability. The composition oE the laboratory
heats and the commercial heats are identified in Table I as
Nos. 1-16 and Nos. A and B respectively.

~ -- O ~ O ~ L'~ ~-- C ~
~ ~ V . .
Cl~
O 0~ ~ O O
~1 ~ ~ '~:c~ zz
Z Z Z 2 Z Z Z Z Z Z 0 ~1 ~I O 0
r
o o o f~ ~ O C O ~ ,~
o o ~ 1` o o o o o o o o
....
Z ~ ~ ~ O O O C O C O O
~ V
u~ N N C0 O O O
O O O o 0 ~1 ~ ~ ~ O o o O~ o
_~ o o o o o ~I r~
E~ .. . . .. . . O G O -
0000 000 000 000 00
O ~ ~ N O ~ O ~ ) 0~ ~ r ~ ~ ~ ~
~`J t`l N N tN ~1 N ,~
N O O O O O O O O O O O O O O O O O O
OOOO ooO OOO OoO OoO Oo
~ r.~l r,~l ~ r~ r~) ~ r~ r~ r~ ~ r,~
3 oooo OOO O OO O O O OOO OO
~) ,o............ ... ... ... ... ..
0000 ooo 000 000 000 00
o o o o ~ 1--r- ~ ~ ~ r~) r~ r o
~ ~ ~ ~r r~ ~ N
O 0 000 000 0 0 0 0 00 0 C O O O
~ . , . . . . , , . . . . ~ , . . . .
0000 000 000 000 000 00
~1 o 0 o 1-- ,t r~ O N
OO_O OOO Oo~1 ~,~--( ~1~o r,~lr,~
OOOO OOO OOO OOO OCO OO
1~:;
0000 000 000 000 000 00
r,~ 9 ~ ~ ~ r~7 ~ ~ O a~
Ll') ~ r~) ~ r~) r~) ~ ~1 ~ ~ ~ a~ r~ 10 ~ 11`~ r.~ o
_ O O O O O O O O O O r,~ r~ l O O O~ r,~
Z .... ... ... ... ... ..
0000 000 000 000 000 00
O ~ ~ ~ ~ r~ C 1~ ~ C~ o r; ~ ~ ~ ;
r,~ ro r- ~ r~ ~ r~ ~ ~ ~ ~D O ~ Cr~ o ~
L~ .... ... ... ... ... ..
O O O O O O ~ ~ ~ ~ Ltl U~ O O O_~ ~
N o ) C~ CD ~ O o~ a:) ~ N C ~ C~ OO ~9
.~ ~ c r~l r~ r ~ ~ r~) r~
U~ .... ... ... ... ... ..
0000 0 00 00 0 000 0 0 0 QO
~D ~ ~D ~D CO ~ ~ D ~ ~ ~ ~ C r~ r~~1 r,~l
, 0000 000 00 0 0 00 000 00
~n oooo ooo ooo ooo ooo oo
o oo o o o o oo o o oo o o o oo
O o o o N ~ N ~ N ~ r~ r~) ~ c ~ r~
: o o o o o o o
o o o o o o o o o o o o o o o o o o
c~ ~I c ~ ~ ~ u) ~ c ~ r~ to L'~
C ~:r L~ Lr~ r c r~- r~ r~- ~ r~
. ~ c
o o o o o o o o o o o o o o o o o o o
o a~ ~ ~ Lr~ Ln ~ r~
O O O Cl ~ ~ ~ ~ '' O ~ C` ~ O O O ~ O ~
o o o o o o o o o o o o o o c o o G C)
C~
¦ ~ N 1~1 C Lt"l ~) r-- C~ C. o _ ~ ~ Lr ~ C~
Z ~ ~ ~ ~ ~ ~ ~ Z
-- 5 --

1 Samples from the laboratory heats were hot rolled to
about 0.100 inch and cold rolled to 0.020 inch. The commercial
samples were also cold rolled to 0 020 inch. ~he cold rolled
samples were then annealed and pickled in accordance with stan-
dard practices. Circular specimens of 1 l/2 inch diameter were
stamped from the cold rolled strips and tested for brazing
wettabilit~ in a resistance heated cold wall vacuum furnace.
The test generally consisted of placing a brazing
filler material on each specimen and heating the specimens and
filler materials to the melting point of the filler mat0rial.
The wettability o~ the specimens were evaluated according to the
parameter "d2/h", where "d" is the average diameter of the drop
in inches which formed on the surface of the specimen and "h" is
the height of the drop in inches, wettability being proportional
to the area covered by the drop and inversely proportional to the
height of the drop.
Specimens of the heats were tested at 2050F in
conventional furnace atmospheres with oxygen-free copper as a
brazing filler matexial. No flux was applied because this would
be an uncommon practice in furnace brazing operations. Short
l/8 inch lengths of 0.010 inch diameter wire with square ends
were placedon end at the center of each specimen heated. In
vacuum -testsr the furnace was evacuated cold, heated to 1050 F,
held at a vacuum of one micron of mercury or less while heating
to the brazing temperature. In inert gas tests, the furnace was
evacuated cold, heated to 1050F, held at a vacuum of one micxon
ox less while heating to 1200 F, pressurized with nitrogen to
1500 microns and heated to the brazing temperature.
In the reducing atmosphere tests r the furnace was
evacuated cold, heated to 1050F, held at a vacuum of one micron
~. .

1 or less while heating to 1200 F, pressurized with dry hydrogen
(having a dew poin-t of less than -80 F) to a pressure of 3no,000
microns and hea-ted to the brazing temperature. The wettability
ratings (d /h) of the specimens are shown in Table II. The
letter "C" indicates that the specimen was completely wetted.
TABLE II
Dry N2 Dry H2
No. ATMOSPHERE ATMOSPHERE Vacuum
1 C ~.292 C
2 C 4.836 C
3 C 5.55g C
lO 4 C ~.930 C
2.0~0 0.721 22.003
6 1.836 0.182 9.840
7 0.286 0.174 0.296
8 0.304 0.199 0.325
9 0.253 0.187 0.256
0.228 0.171 0.178
~ 0.573
12 - - 0.579
13 - - 0.568
14 - - 30.502
- - 24.807
16 - - C
A 0.896 0.207 0.586
B 0.361 0.219 0.,25~
The wettability o~ the laboratory melted compositions can be
compared with each other and with the prior art compositions of
He~ts ~ and B to determine the adverse e~fects of titanium.
The prior art compositions are clearly non-wettable. The
stabili~ed compositions of Heats 1-4 and 14-16 contain up to
0.005 of titanium ancl exhibit superior wettability under all
atmospheres. The efEec-t of increasiny amounts of titanium i5
most clearly shown by the compositions of Heats 5-7. The
composition of Heat 5 contains 0.008 wt% titanium and has
superior wettability characteristics under all atmospheres.
The composition o~ Heat 6 contains 0.11 w-t~ titanium and has
improved wettability characteristics under inert gas and vacuum

67
atmospheres, however the adverse effect of titanium is evident
in a reducing atmosphere. Heats 7-13 contain lar~e amounts of
titanium and have no better wettability characteristics than do
the prior art compositions.
The difference in wettability between the specimens
brazed with oxyyen~free copper in a dry nitrogen atmosphere is
seen from Figures 1 and 2. Figures 1 and 2 are the perspective
and top views, respectively, of a brazing table supporting the
specimens identified in Tables I and II. Specimens A and B are
the cornmercial steels and illustrate the problem where the filler
material does not wet the surface beyond the periphery of the
molten drop. Similarly, specimens 7, 8, 9 an~ 10 are also not
wetted by the filler material~ Specimens 1, 2, 3 and 4 are
completely wetted by the oxygen-free copper. Specimens 5 and 6,
although containing increasing titanium concentrations of 0.008%
and 0.11% respectively, are clearly wetted by the copper beyond
the periphery of the molten drop.
Specimens of the heats were tested at ~000F under
vacuum conditions with a nickel alloy as a brazing filler
material. In these tests nickel alloy powder (A~S BNi-2) was
mixed with a plastic cement which vaporized completely be~ore
reaching 1000F. The mixture was formed into pellets of
approximately 3/16 inch diameter by 3/16 inch height and the
pellets were placed on the specimens. The furnace was evacuated
cold and heated to the brazing ternperature~ No flux was applied
because this is uncommon practice in furnace brazing at high
temperatures. The wettability ratings of the laboratory melted
specimens are shown in Table III~ The letter "C" indicates that
the specimen was completely wetted.
-- 8 --

i7
1 Table III
Heat d2/h
2 C
C
6 35.917
7 5.329
9 2.~88
0 2.188
12 ~.929
13
1~ C
C
16 C
A
The prior art compositions were not tested but.they would have
a rating approximating those of Heats 7 and 9 respectively in
view of their titanium con-tents. The compositions o~ Heats 3
5 and 14-16 all contain less than .01 wt% titanium and have
superior wet-tability characteristics. The composition of Heat
6 contains 0.11 w-t~ titanium and has superior wettability
characteristics in comparison to the other compositions contain-
ing 0.18 wt~ (Heat 12) or more tikanium (Heats 7 and 9).
It will be apparent to those skilled in the art that
the novel principles o~ the invention disclosed herein in
connection with specific examples thereof will suggest various
other modifications and applications of the sc~me. It is
accordingly desired that in construing the breadth of the
appended claims they shall not be limited to the speci.fic
examples of the invention described herein.
_ g ~
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