Be it in the brewing trade or in cheesemaking, extremely strict legal requirements apply in the food and drinks industry. One hundred percent compliance with hygiene regulations is therefore of the utmost importance. In order to prevent germs and bacteria from taking up residence, welded joints must be as smooth as possible and, above all, non-porous. This is why the food and drinks industry relies on stainless steel, which is of course distinguished by its corrosion resistance and the fact that it is very easy to weld. In terms of hygiene, it meets the legal quality requirements for surface structure, hardness, heat resistance, and the cleanability of the material.
TIG welding is also required here, because truly non-porous, smooth weld seams can only be assured in combination with the TIG welding process and its extremely stable arc. The fact that seams produced with TIG are highly resistant to cracking also plays a role. However, there are some things to consider when TIG welding on stainless steel in the food and drinks industry: for example, temper coloration is to be avoided at all costs.
How is temper coloration produced during TIG welding?
Stainless steel forms an oxide layer, also called a passive layer. This is a surface protection film that prevents corrosion. However, this protective film can be damaged during welding where there is no oxidation protection (shielding gas). As a result, burnt, i.e., damaged oxide layers are formed, which are reflected in the form of temper coloration.
In this case, the sensitive passive layer has been damaged, which can lead to corrosion in the affected areas. The corrosion with its roughened surface in turn offers germs and bacteria the chance to take up residence. In order to prevent this, the gas flow rate and the gas post-flow time on the weld seam surface are of crucial importance when TIG welding on stainless steel.
For all TIG-joined stainless steel sheets, it is therefore important – especially in the food and drinks industry – to protect the sensitive oxide layer. However, for root passes in particular, and above all in container and pipeline construction, not only the weld seam surface but also the underside of the root must be suitably protected. Forming gas is used here, similar to shielding gas from the welding torch on the weld seam surface.
Forming in pipeline and container construction
Forming is an indispensable process in the food and drinks industry to prevent corrosion on the underside of the root. The forming gases used , for example Nitrogen (N2), Argon (Ar), or Argon-Hydrogen(H2) mixtures, displace (similar to the shielding gas from the welding torch on the top of the root) the atmospheric oxygen in the welding area. In addition, the shape of the root underside is optimized during welding, as it is formed much more smoothly and is virtually flawless, which saves a lot of rework. This ensures that pipes, containers, and storage containers – for example for breweries – end up with precisely manufactured, ultra-pure surfaces on the inside too.
In pipeline construction, the forming gases are often flushed through the whole workpiece or through the corresponding section of the circumferential weld being welded. For this purpose, special forming gas devices made of silicone are used that virtually seal off the pipe in order to fill the space with gas.
Similarly, in container construction or for pipes with larger diameters, trailing gas nozzles are used, which also flush the gas around the underside of the weld seam, but without having to completely fill the whole workpiece. It should be noted that even after the welding process is over, the forming process continues until the parent material has cooled down to below 200 °C (varies depending on the material).
Pipe forming gas devices:
Notwithstanding this optimization during the welding process, any remaining weld reinforcements on the underside of the root should as a rule be ground flush with the sheet metal. However, such reworking is often impossible with narrow pipes due to accessibility. Here too, the forming process provides ideal support in order to create surfaces that are as clean as possible. Some components that were not formed during welding cannot be reworked at all – and so have to be scrapped.
To summarize, forming is an absolute must in TIG welding in the food and drinks industry. When it comes down to it, the production of ultra-pure seams and stainless steel surfaces would be inconceivable without forming.
Root unformed with temper coloration
Automated TIG welding in the food and drinks industry
The quality criteria for the food and drinks industry clearly also apply to mechanized TIG welding: ultra-pure surfaces and clean, smooth seam structures are of critical importance. The circumferential welds produced must also be flawless and, above all, be 100% reproducible in mechanized applications. Among these automated welding systems, a rough distinction is made between systems in which the welding torch moves around the component and systems in which the torch is fixed but the component is turned.
Welding Automation from Fronius also offers various successful systems for this purpose. For example, the Fronius FCW Compact system is ideal for pipe welding with a fixed welding torch. By contrast, the Fronius orbital welding system enables quick and easy pipe welding where the component is fixed and the welding torch can be moved.
Open welding head
Closed welding head
FCW Compact welding system with HMI system controls and remote control
Interested in knowing more about TIG welding?
More information about mobile TIG welding systems can be found in the article: Five functions that a mobile TIG device should have.
More information about welding in the food and beverage industry can be found here.