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Welding stainless steel to meet hygienic requirements

Inadequate welding can compromise product safety in an otherwise hygienically designed food-processing plant. This paper summarizes guidelines prepared by the Design Principles subgroup of the European Hygienic Equipment Design Group (EHEDG) to increase awareness of the importance of and techniques required to ensure the production of hygienically acceptable welds. This isthetenth in a series of articles featuring the EHEDG to be published in Trends in Food Science & Technology. The EHEDG is an independent consortium formed todevelop guidelines and test methods for the safe and hygienic processing of food. The group includes representatives from research institutes, the food industry, equipment manufacturers and government organizations in Europe.

Incorrect penetration of the weld can be caused by poor welding technique (e.g. poor control of the welding
current) or incorrect parameters. Ideally, the weld metal should exactly fill the joint and remain flush with the surface. Underpenetration leaves a crevice at the joint, which is a hygienic problem both in vessels and pipework; excessive overpenetration can also hold up product in pipework, although the excess can be removed in vessels by grinding. Surface porosity, or excessive inclusions that may become detached thereby creating surface porosity, can trap product and be difficult to clean. Lack of full fusion of the weld metal in the joint to the parent metal results in crevice formation at the interface between weld and plate [associated mainly with MIG (metal inert gas) welding].

Inadequate inert gas shielding (generally nitrogen or argon based) of the reverse surface, when welds are completed from one side only (e.g. pipework welds), results in a roughened weld and heat-affected zone; this promotes the adhesion of soiling and is difficult to clean. 

Many welding processes are in common use, but only a few can deliver welds of hygienic quality free from the types of defects outlined above. The most appropriate welding process is the gas tungsten arc welding (GTAW) process, commonly referred to as TIG (tungsten inert gas) welding. In this process, an arc is struck between a tungsten electrode shrouded with an inert gas and the workpiece. There is often an external feed of filler wire to the joint. although thin sections «3 mm) can be joined without filler wire ('autogenous' welds). The filler wire is usually of the same composition as the parent plate, and special consideration is required if mixed metals are involved. In some cases it may be desirable to use a higher-alloy filler wire.
The TIG process can be used for pipework and for thin sheet up to -4 mm thick; a manual metal arc process, followed by post-weld grinding, would more likely be used for thick sections. For many hygienic applications, thin-walled vessels and pipes are commonly used.

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