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Brazing and Welding of 304L Stainless Steel
There are many materials and joining processes currently avalailable that can
be used in constructing both home and micro breweries. Each material or
process has its own limitations and these usually become obvious when the
economics of a situation are examined. One of the best beers in the world,
Pilsener Urquell, is brewed and lagered in pitch-lined oak barrels. While
these materials may be readily available, the care and maintenance of this
brewing system is extensive. Because of its relatively low maintenance,
stainless steel has become widely used in North America and throughout the
world.
The stainless steel of choice in the food services industry is the austenitic
300 series. The stainless used for good pots (like Vollrath) is usually 304.
Less expensive pots are often made of 303 alloy stainless which is less
weldable and is quickly attacked by chlorinated cleaners. Other stainless
kitchen equipment, like utensils, are typically Ferritic stainless, which has
less Chromium and Nickel and is less acid-neutral. The 300 series of
stainless steels was originally developed for use in cryogenics. These steels
also perform well at elevated temperatures and are used extensively for steam
pipes and exhaust systems. It is the resistance to elevated temperature
oxidation and corrosion that makes alloys 304 and 316 the choice for food
preparation equipment, including steam-heated boilers and storage tanks. But
there is always a cloud for every silver lining and when it comes time to join
stainless steel, that cloud is heat. The metallurgy that makes these alloys
corrosion resistant and strong also makes welding more difficult than for
ordinary steel.
What Makes A Steel Stainless? (bold)
The alloying addition of Chromium and Nickel to the iron creates a significant
percentage of those atoms at the surface, which form tenacious oxides that
seal the surface and prevent oxidation of the iron. The process known as
"Passivation" for stainless steel, as discussed in the article "Care and
Feeding of Stainless Steel" by Micah Millspaw in the July/August issue of
Brewing Techniques, is a common means of improving this protective oxide layer
through the use of oxidizing acids. Anodizing aluminum alloys is similar in
that a solid aluminum oxide barrier is created to prevent further corrosion.
Anodized (black) aluminum cookware (Magnalite, Calphalon) is acid neutral /
acidic food resistant like stainless steel because of this heavy oxide layer.
Plain (bright) aluminum cookware does not have the degree of surface oxides
necessary to prevent reaction with corrosive media.
Joining of Steel and Brass/Copper (bold)
Stainless steel is routinely welded, but it must be done under an inert gas
atmosphere. The most reliable method for welding stainless is the Tungsten
Inert Gas (TIG) process, aka. GTAW and Helio-Arc. TIG welding has the
advantage of a small weld head, lower heat input is required and filler metal
is optional. The other common welding methods for stainless steel, Metal
Inert Gas (MIG) and Shielded Metal Arc Welding (SMAW), are not as well suited
for welding thin sections like beer keg walls. MIG is commonly used for all
types of stainless welding but the weld gun must be held close to the work and
this decreases its effectiveness in tight areas. MIG equipment will be more
available to a do-it-yourselfer and should provide a satisfactory joint. SMAW
is commonly used for welding thicker pipe and tanks. It has the disadvantage
of obscuring of the weld joint during the pass and the slag must be removed
between passes. Equipment and electrode filler rods are readily available,
however this welding process is not recommended for this application. The
welder does not have the control necessary to insure a good weld.
Because 300 series stainless steels are prone to High Temperature
Embrittlement and Sensitization, the welder must be careful not to apply too
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much heat for too long during welding. Welding of thin gauge stainless steel
requires a definite skill. Producing defect-free welds without overheating
the steel takes years of practice, no matter which welding process is used.
This is not to say that a serviceable weld cannot be done by a novice. But in
my experience, it is better to take stainless steel weld jobs to an
experienced welder, rather than attempting it yourself. Bad welds are
difficult to correct in stainless steel. It is more economical to get things
done right the first time. The scale of welding that a homebrewer would
require would most likely not exceed a welder's one hour minimum charge. In
fact, to weld 3 pipe nipples into 3 kegs, I was quoted 25 dollars. Not much
to pay for a quality job. See Figure 1. In addition an experienced welder
will know how to produce a good weld without overheating it. Overheating
causes precipitation of the chromium atoms away from the grain boundaries to
form chromium carbides, depleting the steel of its corrosion resistance.
I should explain some background metallurgy here. All metals are crystalline
materials having specific crystal structures which are dependent on
temperature. These structures are referred to as "Phases" and are given names
such as "Austenite" and "Ferrite". A block of metal is very similar to a block
of salt. A block of salt is really a bunch of grains of salt all fused
together. These grains are oriented every which-way and the interface to the
next grain of salt is called the grain boundary. As you would expect, the
grain boundary is weaker than the grain itself. The crystalline structure of
metals is exactly the same in this respect. (By the way, if you would like to
see a metallic grain structure, go look at an aluminum street-light pole. That
mosaic you see is the grain structure.) Because the grain boundaries within a
metal are the weakest sites, heat and corrosion usually affect these areas
first.
The corrosion resistance of stainless steel depends on the chromium.
Austenitic stainless is a super-saturated solution of chromium and nickel in
iron. It is actually a very high temperature phase that has been quenched to
preserve the distribution of elements. Austenitic stainless does not like
middling-high heat. It performs well up to 600F (315C), but higher
temperatures in the range of 800-1600F (425-870C) cause atom diffusion which
causes the properties to change. Temperatures in this range allow the chromium
to diffuse away from the grain boundaries to form chromium carbides, the
preferred crystalline structure in that temperature range. Exposure to the
temperatures that cause diffusion is referred to as being "Sensitized". The
diffusion of chromium away from the grain boundaries results in un-stainless
grain boundaries surrounded by stainless steel. This situation soon leads to
localized corrosion and rapid cracking of the grain boundaries. To correct
this, the metal must be heated to at least 1900F (1040C) for a period of time
in an inert gas atmosphere and then quenched to retain the austenite crystal
structure. Unfortunately, doing this heat treatment to a welded keg would
result in a lot of warping and distortion. It is better to get another keg
and start over.
Welding is a local melting/freezing process that creates high temperature
gradients in the metal around the weld. This Heat Affected Zone (HAZ) is the
region where unwanted atom diffusion can take place if it is hot enough, long
enough. There are time/temperature curves that describe this, and the curve
for alloy 304 is shown in Figure 2. This chart shows that for type 304
stainless (nominal carbon content of 0.08%), five minutes at 600C (1110F) or
above will cause chromium diffusion that will later cause cracking in service.
Type 304L stainless, "L" denoting less carbon (nominal 0.03%), is more
weldable and can spend about 6 hours at 600C before becoming Sensitized. Most
kegs (in North America) are made from 304L to facilitate the welded
construction. Caution must still be taken in service though. I know of one
case of a homebrewer that experienced cracking on the bottom of a cut-off keg
boiler. The cracks appeared at the flame-line where the flame of the CajunCooker-type propane heater met the keg. This shows that he was running the
flame too hot and that, over time, chromium atom diffusion was taking place.
Diffusion is cumulative. Once this type of cracking occurs, there is no
economical way to correct it. In addition, because modern beer and soda kegs
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are designed thin to save on material, a modified keg should never again be
pressurized. Serious injury could result from a rupture at the weld. Welds
are always weaker than the base metal.
If you wish to do the welding yourself and have access to the necessary
equipment, suggested weld schedules for manual TIG and MIG welding of 304L
steel are shown in Table 1. The MIG weld setup uses a 97.5%/2.5% mix of Argon
and CO2 and 0.030 electrode wire. The TIG welding uses a sharpened (~30¡),
Thoriated (2%), 3/32 inch dia. electrode, and 1/16 inch dia. Filler Rod. The
shielding gas for TIG welding is 100% Argon. Note that the same Filler metal
is used for both processes. As noted in Jeff Donaghue's article, "A Primer on
Welding Stainless Steel" in the September issue of BT, vocational welding
classes are usually available through Adult Education or Community Colleges.
These classes can provide the necessary instruction, equipment and practice
material you will want to have before working on your brewing equipment.
Table 1 - Manual Welding Parameters for 304L Stainless Steel
Welding Type Thickness
Current
Voltage
Filler Rod
(inches)
(Amps)
(Volts)
(AWS)
MIG
0.063
85 DCEP
21
ER316L
TIG
0.045, 0.090
37/70 DCEN
12-14
ER316L
Table 1 con't
Weld Type Argon Flow
(ft^3/hr)
MIG
15
TIG
12
Weld Speed
in./min.
19
2-4
Wire Feed
in/min
184
As Req'd
(Shadow Box)
A Few Words About Brass (bold)
Brass is an alloy of Copper and Zinc with some lead thrown in for
machinability. The lead percentage varies, but for the common brass alloys
used in plumbing fittings it is 7% or less. Lead is entirely soluble in
copper, but the presence of zinc changes this. In Brass, the lead exists as
minute globules. These globules act as an intrinsic lubricant during
machining. The result is a micro-thin film of lead being smeared over the
machined surface. It is this lead (a very small amount) that can be dissolved
off by the wort. While this small amount of lead should probably not be a
cause of concern, most people would be happier if if wasn't there at all.
Well, never let it be said that the Space Program never yields technology
applicable to the home. Some chemists working on the International Space
Station Alpha program were consulted for an etchant that could safely remove
the lead from the surface of brass parts. The chemists determined that a 1-to1 volume ratio of Glacial Acetic Acid (98% by vol.) to Hydrogen Peroxide (30%
by vol.) would accomplish this without pitting the brass. This procedure was
performed in the lab using the standard laboratory concentrations of these
chemicals. The process consisted of a 30 second dunk, swirl and rinse at room
temperature, and was successful in removing the lead, as determined by a Lead
Home Test Kit (swabs). In addition, the procedure had the added benefit of
turning the brass into Pure Gold. (Okay, the color of, anyway.)
Because 98% Acetic Acid and 30% Hydrogen Peroxide are not available to the
average brewer, the experiment was repeated using the concentrations available
in the supermarket. These are 5% Acetic Acid (White Distilled Vinegar) and 3%
Hydrogen Peroxide. Due to the difference in concentration, the relative
concentration ratio is changed. For the household variety concentrations, a 2to-1 volume ratio of Acetic Acid to H2O2 is needed.
The process was expected to take longer with the more dilute solution, so the
brass part was immersed for 10 minutes. The results showed the same gold color
and the Lead Test swab indicated the lead had been removed. The buttery yellow
gold color can be used as an indicator that the process has completed. Home
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Lead Test kits should be available at most hardware stores.
This procedure for removing surface lead from brass can easily be conducted at
home. A 10-15 minute dunk, swirl, and rinse in a 2/1 volume ratio of 5% Acetic
Acid and 3% Hydrogen Peroxide has been shown to be effective. By the way, the
solution can be irritating to the skin so either wear gloves or use tongs.
(End Shadow Box)
Soldering (bold)
Soldering and Brazing of stainless steel to itself or to brass or copper works
well also. These processes are good alternatives to the welding of stainless
steel fittings. They allow the attachment of copper tubing and brass fittings
directly onto the stainless steel. There is some potential for galvanic
corrosion of the copper or brass in preference to the silver. (In terms of
electro-chemical activity: stainless steel is more passive than silver solder,
which is more passive than brass/copper.) Available industry service data
indicates that the corrosion rate should be quite small. Many people have
used silver alloys with these metals and have not experienced any galvanic
corrosion problems at all.
The difference between soldering and brazing is temperature. The American
Welding Society defines Soldering as metal coalescence below 800F. (Brazing
being above 800F.) Both processes bond adjoining metal surfaces by completely
wetting the surfaces with molten filler metal and maintaining that bond upon
solidification. The bond is only as strong as the filler metal, but some
braze metals can be very strong indeed.
Stainless steel is difficult for solders and braze filler metals to wet. The
surface oxides that protect it from corrosion also prevent the filler metals
from wetting the surface. Special fluxes are needed to eat through these
stainless oxides. The silver solder commonly sold for home plumbing with
copper pipe will work on stainless but a different flux is needed. Look for a
flux containing hydrochloric acid or one that says it is for fluxing nickel
alloys or stainless. The specifics for two common silver solders are listed
in Table 2.
Table 2 - Common Silver Solders
Composition (%)
Silver
Tin
3
97
2
98
Melting Temperature
F
C
430
220
450
230
In my experience, getting the steel hot is the big problem. A propane torch
can be used, but the flame needs to be slightly reducing in nature to prevent
the re-formation of surface oxides. The best method for soldering a copper or
brass fitting onto a stainless steel pipe is to "tin" the fitting first with
the solder. Flux is then applied to the stainless pipe and the two pieces are
fitted together. Then the heat is applied to the joint, and more solder is
fed into the joint once its hot. This way the steel surface is protected from
the air until it is hot enough to be wetted by the solder.
Brazing (bold)
Silver-based brazing alloys have lower melting temperatures than Copper/Zinc
brazing alloys, so the silver-based alloys are the more practical choice for
Do-it-yourselfers. But there are two issues to keep in mind with brazing.
The first is that most brazing temperatures are right in the temperature range
that causes sensitization of the steel. The braze must be conducted
efficiently to ensure that the time limit for the onset of diffusion is not
exceeded. Acetylene and propane are two of the most common gases used for
Torch Brazing. Use a slightly reducing flame and AWS type 3A flux. This flux
has the higher useful temperature range needed for brazing (1050-1600F). Both
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surfaces to be joined must be cleaned and fluxed for best results. As in
soldering, it is a good idea to pre-braze the fitting, since it has the higher
thermal mass in the localized area. Pre-heating the fitting will help
decrease the amount of time that heat is applied to the joint.
A friend of mine recently brazed a stainless steel pipe nipple directly onto
the side of a stainless steel milk can. See Figure 3. The wall of the can
was flattened with a hammer to allow a good flat fit-up with the nipple. The
pipe was 1/2 inch NPT with a wall thickness of 1/4 inch. The pipe was heated
up first since it had a much higher thermal mass than the milk can wall. It
was brazed using flux coated rod and an acetylene torch. The braze was quite
strong, allowing him to torque up a connecting threaded fitting such that he
later had trouble taking it apart!
Silver brazing rod does not contain lead, but some of the alloys
contain Cadmium, which is worse. It will cause severe heavy metal poisoning.
The American Welding Society alloy designations are listed in Table 3; don't
use the alloys containing Cadmium. The best bet is to look for rod that is
made for food industry applications. The AWS BAg-5 is recommended for this
purpose and is readily available from weld supply shops at about $15.00 an
ounce (1/16 inch dia., spooled).
Table 3 - Standard AWS Silver-based Brazing Alloys
AWS
Composition(a), %
spec.
Ag
Cu
BAg-l
44.0-46.0
14.0-16.0
BAg-la
49.0-51.0
14.5-16.5
BAg-2
34.0-36.0
25.0-27.0
BAg-2a
29.0-31.0
26.0-28.0
BAg-3
49.0-51.0
14.5-16.5
BAg-4
39.0-41.0
29.0-31.0
BAg-5
44.0-46.0
29.0-31.0
BAg-6
49.0-51.0
33.0-35.0
BAg-7
55.0-57.0
21.0-23.0
BAg-8
71.0-73.0
Rem
BAg-8a
71.0-73.0
Rem
BAg-13
53.0-55.0
Rem
BAg-13a 55.0-57.0
Rem
BAg-18
59.0-61.0
Rem
BAg-l9
92.0-93.0
Rem
BAg-20
29.0-31.0
37.0-39.0
BAg-21
62.0-64.0
27.5-29.5
Zn
14.0-18.0
14.5-18.5
19.0-23.0
21.0-25.0
13.5-17.5
26.0-30.0
23.0-27.0
14.0-18.0
15.0-19.0
...
...
4.0-6.0
...
...
...
30.0-34.0
...
Others
23.0-25.0 Cd
17.0-19.0 Cd
17.0-19.0 Cd
19.0-21.0 Cd
16 Cd, 3 Ni
1.5-2.5 Ni
...
...
4.5-5.5 Sn
...
0.25-0.50 Li
0.5-1.5 Ni
1.5-2.5 Ni
10 Sn, .025 max P
0.15-0.30 Li
...
6 Sn, 2.5 Ni
Notes: (a) Total maximum allowable impurities in each alloy is
0.15%.
Ag is Silver, Cu is Copper, Zn is Zinc, Cd is Cadmium, Ni is
Nickel, Sn is Tin, Li is Lithium, and P is Phosphorus.
Table 4 - Standard AWS Brazing Alloy Usage Temperatures
AWS
Brazing
spec.
temp.
F
C
BAg-l
1145-1400
618-760
BAg-la
1175-1400
635-760
BAg-2
1295-1550
702-843
BAg-2a
1310-1550
710-843
BAg-3
1270-1500
688-816
BAg-4
1435-1650
779-899
BAg-5
1370-1550
743-843
BAg-6
1425-1600
774-871
BAg-7
1205-1400
652-760
BAg-8
1435-1650
779-899
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BAg-8a
BAg-13
BAg-13a
BAg-18
BAg-l9
BAg-20
BAg-21
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1410-1600
1575-1775
1600-1800
1325-1550
1610-1800
1410-1600
1475-1650
766-871
857-968
871-982
718-843
877-982
766-871
802-899
Soldering, brazing and welding are all useful methods for joining stainless
steel. Consider the joint design when choosing the process. Each process has
its own limitations, but the most important is the nature of alloy 304L
itself. Soldering is most useful for joining pipes and small fittings.
Brazing is useful for making high strength bonds between dissimilar metals.
These two methods may be accomplished fairly easily at home. But if the goal
is to seal a stainless pipe through the wall of a keg, then welding is the
best way to go. If you want to do the welding yourself, ask yourself the
following questions:
1. Can I produce a quality weld (no defects or overheating)?
2. What resources do I have available?
Weigh the economics of your decision. Time, effort, and equipment investment
are some factors to be considered. If you have the interest and knack for
learning new skills, then give it a try. Check out vocational welding classes
in your area. With some instruction and practice, a serviceable weld is not
difficult. If you want to get right to the brewing, then you may want to hit
the Yellow Pages.
References:
M. Jackson, The New World Guide to Beer (Running Press, Philadelphia,
Pennsylvania, 1987).
ASM Metals Handbook, 9th Ed., Vol. 3, Properties and Selection: Stainless
Steels, Tool Materials and Special-Purpose Metals; Fabrication of Wrought
Stainless Steels, (American Society for Metals, Metals Park, Ohio, 1980).
ASM Metals Handbook, 9th Ed., Vol. 6, Welding, Brazing, and Soldering; Arc
Welding of Stainless Steels, (American Society for Metals, Metals Park, Ohio,
1983).
ASM Metals Handbook, 9th Ed., Vol. 6, Welding, Brazing, and Soldering; Torch
Brazing of Steels, (American Society for Metals, Metals Park, Ohio, 1983).
ASM Metals Handbook, 9th Ed., Vol. 6, Welding, Brazing, and Soldering; Brazing
of Stainless Steels, (American Society for Metals, Metals Park, Ohio, 1983).
ASM Metals Handbook, 9th Ed., Vol. 6, Welding, Brazing, and Soldering;
Soldering, (American Society for Metals, Metals Park, Ohio, 1983).
ASM Metals Handbook, 9th Ed., Vol. 13, Corrosion; Environmentally Induced
Cracking, (American Society for Metals, Metals Park, Ohio, 1987).
ASM Metals Handbook, 9th Ed., Vol. 13, Corrosion; Corrosion of Stainless
Steel, (American Society for Metals, Metals Park, Ohio, 1987).
ASM Metals Handbook, 9th Ed., Vol. 13, Corrosion; Corrosion in the Brewery
Industry, (American Society for Metals, Metals Park, Ohio, 1987).
John Palmer is a metallurgical and welding engineer for McDonnell Douglas
Aerospace in Huntington Beach, California. He is a frequent contributor to
the Home Brew Digest on the Internet and the author of "How to Brew Your First
Beer", available at several BBSs and FTP computer sites around the world. He
is an enthusiastic member of the Crown of the Valley Brewing Club in Pasadena,
CA.
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