technical insight
• Carbon steel should not be welded directly to stainless steel if it is going to be exposed to wet or humid conditions. • When joining carbon steel to stainless steel, an intermediate stainless steel doubler plate shall be used that shall not exceed the parent material’s thickness. • Welds should be located away from highly stressed or cold worked areas such as bends and knuckles. • Welds should not cross, as the residual longitudinal stress in the welds will cause a highly stressed region. • Fillet weld attachments shall not be welded over butt welds. • Lap joints and intermittent fillet welds shall be avoided because of crevice and contamination issues. • Heat tint not only reduces corrosion resistance but also degrades cleanability and may increase microbial activity. It should be removed by either mechanical or chemical means, and the surface roughness in these areas should be restored as closely as possible to the original condition. Consumables: • All welding consumables must be handled by the manufacturer’s instructions and maintained clean and dry. • Protect MIG wire spools - these are easily broken, making the wire unusable. • Maintain packages in unbroken condition as long as possible. • Flux-coated electrodes require careful storage - refer to the instructions from the manufacturer. Finishing welds by restoring the surfaces • Where welds are accessible and are not required to be ground flush, they shall be manually wire brushed, while still warm after welding to remove thick slag and flux residues, followed by pickling. This can be followed up with finer abrasives. • Pickling chemicals shall not be left in contact with the material for a period exceeding that recommended by the manufacturer. All residues shall be completely removed using fresh, clean water. • Alternatively, where required, welds on the product contact side shall be ground flush and polished to the appropriate Ra. The surface is abraded using successively finer grit to the required roughness. The abraded strip along a weld must be wide enough to remove any heat-degraded surface oxides. A final passivation process is required to remove internal and external contaminants. • Note that weld roots and interior heat-affected zones of welds that cannot be ground and polished shall be protected from oxidation by back purging during welding.
This is critical as heavy heat tint caused by poor purging is extremely difficult to remove by chemical pickling alone. • Reduction of parent material thickness during grinding and polishing shall not exceed the greater of 0.25mm or 5% of material thickness. • During grinding and polishing, localized heat build-up or “blueing” of the material shall be avoided. • Iron-free particles shall be used for all shot or abrasive blasting processes. If abrasive blasting is used to remove heat tint, the surface roughness is normally increased, and care is required to ensure the Ra requirements are satisfied. Note that the metal flow caused by the impact of the abrasives may trap blasting debris and form crevices which promote corrosion and limit cleanability. As mentioned, the guidelines and practices stated in this article are just the tip of the proverbial iceberg when starting to look at the full requirement for the fabrication of hygienic process equipment. This article is therefore aimed at raising awareness amongst fabricators on the issues to truly deliver on the expectations of the hygienic industry. Sassda is working on a South African code of fabrication best practices for the hygienic industry, which will assist members to be world- class and globally competitive in the future.
For more information on the development of these codes e-mail michel@sassda.co.za
14
Issue 2 – 2024
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