Welding and Brazing Stainless Steel By John Palmer 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 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. To browse our best-selling corny kegs, homebrew kegs, and keg parts, click here! 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. Table I Manual Welding Parameters for 304L Stainless Steel Welding Method Thickness (in). Current (amps) Voltage (volts) Filler Rod (AWS) Argon Flow (ft 3 /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 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. A Few Words About Brass 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 things. In brass, the lead exists as minute globules. These globules act as an intrinsic lubricant during machining. The result is a microthin 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. Although this small amount is probably no cause for concern, most people would be happier if it were not there at all. 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 would safely remove lead from the surface of brass parts. The chemists determined that a 1:1 volume ratio of glacial acetic acid (98% [v/v]) to hydrogen peroxide (30% [v/v]) would accomplish this without pitting the brass. The 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 it successfully removed the lead, as determined by a home lead test kit (swabs). In addition, the procedure had the added benefit of turning the brass into pure gold (the color of gold, 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. Because of the difference in concentration, the relative concentration ratio changes. For household-variety concentrations, a 2:1 volume ratio of acetic acid to hydrogen peroxide was used. The process was expected to take longer with the more dilute solution, so the brass part was immersed for 10 min. The results showed the same gold color and the lead test swab indicated that the lead had been removed. The buttery yellow gold color can be used as an indicator that the process has completed. Home lead test kits should be available at most hardware stores. This procedure for removing surface lead from brass can easily be performed at home. A 10–15-min dunk, swirl, and rinse in a 2:1 volume ratio of 5% acetic acid (white distilled vinegar) and 3% hydrogen peroxide has been shown to be effective. The solution can be irritating to the skin, so it is advisable to wear gloves or use tongs. 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. 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. Table II Common Silver Solders Composition (%) Temperature Silver Tin °F °C 3 97 430 220 2 98 450 230 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. 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 reformation 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. 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 ½-in. NPT with a wall thickness of ¼ 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. Table III Standard AWS Silver-Based Brazing Alloys AWS 508 Spec’s Composition* 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, 3 Ni‡ 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 0.25-0.50 Li ΙΙ 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 10 Sn. 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 <p align="center