What makes a sanitary weld ?
No crack , No voids , full penetration , No sugaring, and a smooth inside.
How do we achieve all of these things? Well…the way to achieve the BEST result possible comes from a few different areas. The little details are what will make a huge difference in the end, very possibly the difference between a good sanitary weld and one that will have to be cut out or re-worked.
Having the correct purge gas and purge pressure is important and can drastically change the way the material welds and also the final weld quality. 100% argon is the most common purge gas. 5-15 CHF (cubic feet per hour) is a good starting point for purge pressure. All inlets and outlets of the piece need to be closed off. Typically with blue painters tape. Works for everything. Make sure to let the oxygen escape by tilting your work piece and having a roughly ⅛ inch hole in the end. Argon flows similarly to water, which makes it fairly easy to understand how it works. We have a much more in depth article about purge welding.
Welding amperage will change depending on the thickness and type of material to be welded. 30-60 amps is a good range to start with. You want to use higher heat because you are able to move faster; however, it is more difficult to control. With being able to move faster, it can actually have less heat input which will result in a less affected area and a better quality weld. There is a small dot in the middle of the weld joint that is a common way to know that the weld is fully penetrated. Post flow of gas over the weld will keep it from oxidizing. So once you are done welding, you will hold the torch over the end of the welded joint which keeps oxygen from entering the weld and creating an oxide layer and making the weld discolor or become weaker.
Filler metal is often not used because the base material is thin enough that it can be fully penetrated without degrading the material. If filler is needed, you would use 308 or 316 (a type of stainless filler), depending on the grade of stainless. The filler thickness will change weld pool characteristics and will cool the weld as it's added, so you might need to increase amperage. The ends of the wire need to be clipped when they become oxidized. If it is oxidized and you do not clip them, then it will cause a much weaker weld.
Material preparation is important. Making sure the weld joint fit up is good and there are no gaps. Both pieces need to be cleaned (typically acetone is used) and no lint or burrs left on material to avoid contamination .
Sugaring comes from a weld not being properly purged. It looks like black or gray sugar on the back of your weld. Sugaring can be removed with a burr bit and in some applications that's acceptable but it's better to get no sugar at all. Sugaring also occurs when oxygen or nitrogen comes into the weld pool.
Let's finish this off by talking about post weld visual inspection and the cleaning the color off of welds. A perfect sanitary weld should have virtually no color, the color spectrum can range from chrome to straw to gold to blue and to purple. The darker the color, the more oxidation that has occurred. For most applications, any color up to blue is acceptable and straw/gold is what we are shooting for most often. We are also looking for any visible cracks and fish eyes at the end of the weld. Fish eyes are little holes or divots. They are able to cause cracking or leaks in the long run.
Now, if the weld has a color, that is acceptable. However, we still want to remove it. The color is caused by an oxidized layer and it compromises the rust resistance of the stainless steel. It's a side benefit that it looks much nicer when it's shining bright like all the other material is. Reference the song Shiny that Tamatoa sings in the movie Moana.
There are 4 main ways to remove the color. A stainless wire brush is a good option, especially when paired with Scotch Brite for any stubborn spots and to get the grain back if that's necessary. Next is a Scotch Brite wheel, also known as a unitized wheel. These can get a near mirror shine very quickly. Next up, a pickling solution which chemically passivates (makes it less active) the weld area. Finally, a tig brush or electrochemical cleaning, which can be made by companies such as Cougartron, Walter Surface Technologies, and Ensitech. These use an electrolyte solution and other wizardry in a box to remove weld color, rust, and other contaminants .
That is a fairly concise, yet basic, breakdown of the way sanitary welding works with manual welding. If you're looking for more good information or need some visual references, Weldmonger and Weld.com are both great places to go to.
Pictures found via google. Sites connected from:
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