Welding terms

I have answered some stainless steel questions in the past and at the time Mr. Wizard was still cloaked by the wispy clouds of anonymity. Now that I have been unveiled I can write with a bit more authority on this topic. I am employed by one of the largest stainless steel equipment fabricators in the United States, Paul Mueller Company.

Mueller chose to build a brewery in 1997 to showcase its products and that’s how I ended up working for a stainless company. Weld quality is a very big deal as stainless steel corrosion and the ability to be cleaned are both affected by weld quality. I want to give a little background on welding because both welded and seamless kettles can be excellent pieces of gear.

As a general rule, welds are only used when required. When big vessels are made, weld seams are required because stainless steel coils come in a standard width of 48 inches (122 cm). To build big tanks, weld seams are required to join the material. The standard method used to build tanks is to form a “tube” and attach bottom and top heads to the tube to complete the tank. Fermenters typically have a cylindroconical bottom and dished top head. The heads and tube all have weld seams and the welds are ground flush on the interior of the vessel. A good weld must have full penetration to provide strength and to eliminate crevices where micro-organisms grow.

Excessive heat applied to a weld leaves a large heat-affected zone around a weld and the alloy composition around this area is different than stainless steel that is not exposed to the heat of welding. Corrosion is more likely to occur in the heat-affected zone, especially when the welds are made using too much heat, and various testing methods, such as radiography, are used to evaluate welds. Tube or pipe welds can be really problematic if the tube or pipe is not properly purged before welding. Improper purging causes “sugaring” inside of the tube and this black, granular oxide is a veritable breeding ground for bacteria.

Most long pipe runs are “fit up” with very small tack welds and the joints taped with a stainless steel tape. The line is then purged with an inert gas, usually argon or a blend of argon and nitrogen, before the welds are made.

A good pipe weld, like that used to attach a valve or ferrule into a line, is fully penetrated, has only one start and stop, is smooth (often having the appearance of a stack of dimes slid off into an even and overlapping row) and has very little discoloration. Pipe and tube welds often cannot be ground on the interior and boroscopes are used to visually inspect the weld for penetration, starts and stops and color.

If welding can be avoided it is because of all the associated concerns. One way to build a “pot” without welding is through metal forming. Stainless steel has very good mechanical properties and can be stretched and formed without tearing the material. Although stainless does not look elastic it is really like a sheet of dough and can be formed into a variety of smooth shapes with the proper forming tools.

Spinning dies can be used to stretch and form the steel into the shape of a bowl or pot, for example, and most stainless steel cookware is made using such methods. “Knuckles” can be added to a dish shaped head using special dyes to add strength to the head. These methods do not require welding and the potential problems associated with welds are avoided.

Here is the take home message: Excellent equipment can be purchased with or without weld seams. The key is to know how to properly inspect equipment before purchasing. All beer contact surfaces should be smooth. Polished surfaces should not be scratched or pitted. Welds should be uniform and free of crevices and discoloration. Once you pick out your gear, you then want to maintain it in this condition. Do not use abrasive scrubbing pads to clean stainless steel and be very careful of the metal tips many scrub brushes have as these things can quickly scar a perfectly smooth stainless surface.

Response by Ashton Lewis.