BrewingTechniques
Brazing and Welding 304L Stainless Steel
By John Palmer
Republished from BrewingTechniques' November/December 1994.

Last issue, one brewer presented a brief overview of options for welding stainless steels. This issue follows up with some nitty-gritty details about welding and brazing options for breweries large and small.

Many materials and joining processes are currently avalailable for use in constructing both home and microbreweries. 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. Although wood and pitch are readily available, the care and maintenance of such brewing systems can be extensive. Because of its relatively low maintenance requirements, stainless steel has become widely used in North America and throughout the world.

STAINLESS STEELS FOR FOOD-GRADE APPLICATIONS

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 their 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 every silver lining has its cloud, and when it comes to joining stainless steel, that cloud is heat. The metallurgy that makes these alloys corrosion resistant and strong also makes welding more difficult than is the case with ordinary steel.

METALLURGY BASICS

What makes a steel stainless? The addition of chromium and nickel to the iron creates a significant percentage of chromium and nickel atoms at the surface. These atoms form tenacious oxides that seal the surface and prevent oxidation of the iron. The process known as passivation for stainless steel (see the article by Micah Millspaw in a previous issue of BrewingTechniques [1]), is a common means of improving this protective oxide layer through the use of oxidizing acids. Anodizing aluminum alloys is a similar method in that a solid aluminum oxide barrier is created to prevent further corrosion. Like stainless steel, anodized (black) aluminum cookware (Magnalite, Calphalon) is acid neutral and resistant to acidic food because of this heavy oxide layer. Plain (bright) aluminum cookware lacks the degree of surface oxides necessary to prevent reaction with corrosive media.

Stainless steel's Achilles heel: All metals are crystalline materials that have specific crystal structures that 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 supersaturated solution of chromium and nickel in iron. It is actually a very high temperature phase that has been quenched (quick-cooled) to preserve the distribution of elements.

Austenitic stainless does not like middling-high heat. It performs well up to 600 °F (315 °C), but higher temperatures in the range of 800-1600 °F (425-870 °C) cause atom diffusion and change the metal's properties. Such high temperatures allow the chromium to diffuse away from the grain boundaries to form chromium carbides, its preferred crystalline structure at that temperature range. If exposed to these high temperatures and chromium diffusion occurs, the metal becomes sensitized and prone to cracking. The diffusion of chromium away from the grain boundaries results in non-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 1900 °F (1040 °C) 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 considerable 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. Time/temperature curves describe this phenomenon, and the curve for alloy 304 is shown in Figure 1. The figure shows that for type 304 stainless (nominal carbon content of 0.08%), 5 min at 600 °C (1110 °F) or higher 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 600 °C before becoming sensitized. Most kegs (in North America) are made from 304L to facilitate welded construction.

Caution must be taken when heating stainless steel equipment. I know of one home brewer whose cut-off keg boiler began cracking at the bottom. The cracks appeared at the flame line where the flame of the cajun cooker-style 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.

Table I   Manual Welding Parameters for 304L Stainless Steel
Welding
Method
Thickness
(in.)
Current
(amps)
Voltage
(volts)
Filler Rod
(AWS)
Argon Flow
(ft3/h)
Weld Speed
(in./min)
Wire Feed
(in./min)
MIG 0.063 85 DCEP 21 ER316L 15 19 184
TIG 0.045, 0.090 30/70 DCEN 12-14 ER316L 12 2-4 As Required

JOINING OF STEEL AND BRASS OR COPPER

Stainless steel is routinely welded, but it must be welded under an inert gas atmosphere. The most reliable method for welding stainless is the tungsten inert gas (TIG) process, also known as GTAW or helio-arc. TIG welding has the advantage of a small weld head, it requires lower heat input, 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. (Note: Never weld on vessels that you intend to use under pressure [i.e., kegs that will be used as kegs]. Because modern beer and soda kegs are designed thin to save on material, a modified keg should never again be pressurized. Welds are always weaker than the base metal, and at least one death has been attributed to a keg exploding after modification.) MIG is commonly used for all types of stainless welding, but the weld gun must be held close to the work, which 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 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 brewing equipment. The welder lacks the control necessary to ensure 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 much heat for too long during welding. An experienced welder will know how to produce a good weld without overheating it.

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 critical 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 home brewer would require would most likely not exceed a welder's 1-h minimum charge. In fact, I was quoted $25 to weld three pipe nipples into three kegs - not much to pay for a quality job.

If you wish to do the welding yourself and have access to the necessary equipment, refer to the suggested weld schedules for manual TIG and MIG welding of 304L steel shown in Table I. The MIG weld setup uses a 97.5%/2.5% mix of argon and carbon dioxide and 0.030 electrode wire. The TIG welding uses a sharpened (~30#161#), thoriated (2%), 3/32-in. diameter electrode, and 1/16-in. diameter filler rod. The shielding gas for TIG welding is 100% argon. Note that the same filler metal is used for both processes. As Jeff Donaghue noted in his article, "A Primer on Welding Stainless Steel" in the September issue of BrewingTechniques (2), vocational welding classes are usually available through adult education programs and community colleges. These classes can provide the necessary instruction, equipment, and practice material needed to get you working on your brewing equipment.

SOLDERING

Stainless steel can also be soldered or brazed to itself or to brass or copper, with good results. These processes provide good alternatives to the welding of stainless steel fittings. They allow the copper tubing and brass fittings to be attached 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 electrochemical activity, stainless steel is more passive than silver solder, which is more passive than brass or copper.) Available industry service data indicate that the corrosion rate should be quite small. Many people have used silver alloys with these metals and have experienced no galvanic corrosion problems.

Table II
Common Silver Solders
Composition (%)         Temperature
Silver Tin °F °C
3 97 430 220
2 98 450 230

The difference between soldering and brazing is temperature. The American Welding Society defines soldering as metal coalescence below 800 °F and brazing as metal coalescence above 800 °F. Both processes bond adjoining metal surfaces by completely wetting the surfaces with molten filler metal and maintaining that bond until solidified. 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 II.

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 solder. Next, apply flux to the stainless pipe, and fit the two pieces together. Then heat the joint, and feed more solder into the joint once it is hot. By using this method, the steel surface is protected from the air until it is hot enough to be wetted by the solder.

Table III
Standard AWS Silver-Based Brazing Alloys
Composition
AWS 5.8
Spec's
 
Silver
 
Copper
 
Zinc
 
Others
BAg-1 44.0-46.0 14.0-16.0 14.0-18.0 23.0-25.0 Cd  **
BAg-1a 49.0-51.0 14.5-16.5 14.5-18.5 17.0-19.0 Cd
BAg-2 34.0-36.0 25.0-27.0 19.0-23.0 17.0-19.0 Cd
BAg-2a 29.0-31.0 26.0-28.0 21.0-25.0 19.0-21.0 Cd
BAg-3 49.0-51.0 14.5-16.5 13.5-17.5 16 Cd, 3N*
BAg-4 39.0-41.0 29.0-31.0 26.0-30.0 1.5-2.5 Ni
BAg-5 44.0-46.0 29.0-31.0 23.0-27.0   
BAg-6 49.0-51.0 33.0-35.0 14.0-18.0   
BAg-7 55.0-57.0 21.0-23.0 15.0-19.0 4.5-5.5 Sn
BAg-8 71.0-73.0 Remainder    
BAg-8a 71.0-73.0 Remainder    
BAg-13 53.0-55.0 Remainder 4.0-6.0 0.5-1.5 Ni
BAg-13a 55.0-57.0 Remainder   1.5-2.5 Ni
BAg-18 59.0-61.0 Remainder   10Sn. 0.125 max. P#
BAg-19 92.0-93.0 Remainder   0.15-0.30 Li
BAg-20 29.0-31.0 37.0-39.0 30.0-34.0  
BAg-21 62.0-64.0 27.5-29.5   6 Sn, 2.5 Ni
*Total maximum allowable impurities in each allow is 0.15%.
Cd=cadmium   Sn=tin   P#=phosphorus   Ni=nickel   Li=lithium

BRAZING

Silver-based brazing alloys have lower melting temperatures than copper or zinc brazing alloys, so the silver-based alloys are the more practical choice for do-it-yourselfers. Two issues must be kept in mind when brazing. First, most brazing temperatures are right in the temperature range that causes sensitization of the steel. The braze must be done 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 reduced flame and AWS-type 3A flux, which has the higher useful temperature range needed for brazing (1050-1600 °F). Both surfaces to be joined must be cleaned and fluxed for best results. As in soldering, it is a good idea to prebraze the fitting, because it has the higher thermal mass in the localized area. Preheating 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 (Figure 2). He flattened the wall of the can with a hammer to allow a good flat fit-up with the nipple. The pipe was 1/2-in. NPT with a wall thickness of 1/4 in. The pipe was heated first because it had a much higher thermal mass than the milk can wall. It was brazed using a 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 contains no lead, but some of the alloys contain cadmium, which is worse. Cadmium will cause severe heavy metal poisoning. The American Welding Society alloy designations are listed in Table III. Do not use the alloys containing cadmium. Look for rods that are 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/oz (1/16-in. diameter, spooled). Table IV shows the usage temperatures for the alloys listed in Table III.

USE THE RIGHT TOOL FOR THE JOB

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 used 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.

  
Table IV   Standard AWS Brazing Alloy Usage Temperatures
Brazing Temperature    
AWS 5.8
Spec's
 
°F
 
°C
BAg-1 1145-1400 618-760
BAg-1a 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
BAg-8a 1435-1650 779-899
BAg-13 1575-1775 857-968
BAg-13a 1600-1800 871-982
BAg-18 1325-1550 718-843
BAg-19 1610-1800 877-982
BAg-20 1410-1600 766-871
BAg-21 1475-1650 802-899

If you want to do the welding yourself, ask yourself the following questions: Can I produce a quality weld (no defects or overheating), and 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

(1) Micah Millspaw, "The Care and Feeding of Stainless Steel," BrewingTechniques 2 (4), 44-47 (1994).

(2) Jeff Donaghue, "A Primer on Welding Stainless Steel," BrewingTechniques 2 (5), 52-54 (1994).

FURTHER READING

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).

Issue 2.6 Table Of Contents
[Home]  [BrewingTechniques Library]  [Contact Us]  [Order]