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A Primer on Welding Stainless Steel

07/25/2012

A Primer on Welding Stainless Steel

by Jeff Donaghue

 

I am not a welder, nor am I a metallurgist or a machinist. Unlike 65% of the home brewers I know, I am neither an engineer nor a computer geek. I can change the oil and filter on my truck and the spark plug on my mower, and I am proud of all of these accomplishments. What I am is a home brewer, like you. And we love to tinker, don’t we?

Sooner or later the dedicated (read fanatic) home brewer wants to join stainless to stainless. Pipe fittings on kettles and fermentation tanks, wort chillers, RIMS creations, chopped Cornelius kegs, unitanks. … Amen. Our options are 1) pay a skilled craftsman to do the welding, 2) try to trade brews for welding, 3) beg and give homebrews away to get the job done, or 4) purchase our own equipment and learn how to do it ourselves.

I am a homebrewer — I want to do it myself — so I am currently pursuing number four. The purpose of what follows is to pass along the bits of information I have collected over the past few years on the basic methods and materials for welding stainless steel. This article is not intended to say all there is to say on this very complex subject. It is most of what I have to say. Maybe the information contained here can cut your learning curve. Take a look.

NONWELDED BONDS

First off, welding is not the only way to join stainless to stainless with both strength and safety. Brazing is soldering at high temperatures. Gas is the heat source, and various combinations are used; oxygen-acetylene, oxygen-MAPP (methylacetylene-propadiene, stabilized) gas, and oxygen-propane are all common. Air and MAPP gas can be used, but it’s a stretch to get the heat you need.

In the brazing process, you use a filler material whose melting point is lower than that of the melting point of what you want to join (the parent metal). The parent materials are coated with a flux, which cleans and prevents the formation of oxides, and are heated. When the melting point of the filler metal is reached, the molten filler flows into the joint by capillary action and bonds the parent materials together. The resulting joint is very strong.

For the home brewer, what is important is the choice of filler material and flux. First, either silver solder or a silver brazing rod is used. You must go beyond the name, however, and find out what’s actually in the silver solder. I have seen silver content from 0% to 56%. We want brazing material that has silver (Ag) and copper (Cu) and maybe tin (Sn), but no cadmium (Cd). The flux also should be cadmium-free. Check for NSF (National Science Foundation) or USDA/FDA approval for use in food processing.

The advantages of brazing are that the equipment is usually inexpensive, portable, and easy to operate. You should be able to get everything you need for under $ 200. The primary disadvantage of braizing stainless steel is the difficulty of controlling the heat. Excessive heat can cause annealment leading to brittleness and loss of corrosion resistance.

WELDING 101

We all know deep down inside that real men and real women don’t braze, they weld! Welding differs from brazing in that the parent metal is heated to its melting point, and filler metal, if used, is fed into the molten pool. When the whole mess cools, the parents materials have become fused at the joint.

The heat source for fusion welding can be gas or electric. Forget gas for welding stainless. All electric welding processes are called “arc welding.” The parent material is grounded, and as an electrode is brought close to it the current arcs across the gap creating high heat, between 10,000 and 20,000 °F. Power sources are AC, DC straight polarity and DC reverse polarity, constant current and constant voltage, single and three phase … but forget about all that for now. When you begin, here’s what is important.

SMAW: No, this is not the name of a dragon from Tolkien’s Middle Earth. Shielded metal arc welding (SMAW) is what most of us think of when we think about welding, assuming we think about welding at all. It is also called “stick welding.” The welder itself is a “buzz box,” and the electrode is a “stinger.” The consumable electrode is covered with flux. Heat from the arc burns the flux, and the gases that are released serve to protect the molten pool of metal from oxidizing. At the same time, the electrode itself melts and fills the joint. Lots of sparks, lots of smoke, no good for stainless.

GTAW/TIG: Gas-tungsten arc welding (GTAW) has traditionally been used to join difficult metals like stainless, aluminum, titanium, magnesium, and so forth. Initially, helium was used as the shielding gas, hence the now-obsolete term heli-arc welding. Helium (He), argon (Ar), and various mixes of these are used now, as is the term TIG (tungsten inert gas).

In TIG welding, an arc is struck using a nonconsumable electrode. The electrode is made of tungsten because of its high melting point. What holds the electrode is called a torch. An inert gas flows through the torch and around the arc while the arc is heating and melting the parent metal; the gas shields the arc and prevents oxidation.

As the arc from the tungsten electrode melts the parent metal, a filler metal is fed into the molten pool. The amount of arc, and thus the amount of heat, is controlled with a foot rheostat. The main advantages of TIG welding include the following:

·         You have almost infinite control over the amount of heat applied, thus making precision welding possible.

·         Shielding gases make it possible to weld difficult materials like stainless.

·         You can also do stick welding with TIG machines.

On the negative side, the equipment tends to be big, heavy, expensive, and complicated. In commercial machines, the torch gets so hot it has to be water cooled. You have water controls, gas controls, a foot control for heat, and a high-frequency control for arc starting. All this weighs in at 400-600 lb. Some relatively inexpensive portable units are coming out now in the $ 1200 – $ 1500 range. For the same or less money you can get used single-phase commercial units, but with the above trade-offs. With a good AC/DC TIG unit you can weld anything and know that you are a welder. In a word, niños, muy macho!

GMAW: Gas–metal arc welding, aka MIG (metal inert gas) and sometimes wire feed welding, is what’s hot. With this process the consumable electrode is wire on a big spool. What you hold is called the gun. Gas flows through the gun, shielding the arc which has been struck between the parent metal and the electrode. As the wire electrode melts, a roller mechanism feeds the wire into the molten pool. You adjust the current level (heat), the wire speed, and the flow rate of the gas. Various gases and wires are used for different metals. The advantages are many, including:

·         You can weld almost any metal, including stainless.

·         It is fast and easy to learn. When I told one friend about taking a welding class he described a “metal glue gun with argon gas” that he uses at work. He didn’t even know he was welding!

·         You can get good hardware inexpensively, in the $ 600 – $ 800 range.

·         The hardware can be small and portable.

So what’s the bad news? I’ve been told that for precision work you need the control of a TIG machine, especially for thinner metals. Most home brewers don’t have to worry about complying with the standards set for big breweries and dairies, so just how important all this control is, I don’t know. Although joining materials with strength is not difficult, getting liquid-tight joints requires practice and skill. I’ve seen good and bad welds on stainless with MIG and TIG. You make the call.

Be aware that not all wire feed machines are the same. The cheapest ones are designed for use with flux core wire and don’t use gas. Food-grade stainless welding may well be a problem. Most entry-level machines lack the power to do heavier stainless and can’t be adjusted precisely enough to weld thin metal like that on Cornelius kegs. If the machine is foreign made, you may have to wait months to get parts. Even at half mast I’d rather buy American.

CONCLUSION

Welding is a skill that can only be learned through practice with a good teacher. But almost anyone can learn it, and many machines include video tapes. Lincoln, Miller, and Hobart all make good equipment. Century is more on the low “consumer” end but still reliable. Go to your local library and read up. Go to welding supply stores and ask questions until they throw you out. Talk to welders. Always mention making beer when talking to welders. Finally take a class at your local vo-tech, technical college, or trade school. They may force you to learn gas welding first, but that’s OK because many of the skills are transferable.

The GTAW (TIG) class I’m taking at a local technical college costs a mere $ 120 for 60 hours of instruction using top-quality professional equipment. You can’t get 3 h of professional welding for that. The class fee probably only covers the cost of electrodes, gas, filler metal, electricity, grinders, practice metal, and instructor. At last count I have about 50 stainless projects lined up in my basement which I hope to bring to class, assuming I ever get the hang of it. I may not even need to buy a welder by the time I’m done (not!).

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