Optimum performance from stainless steels, as with other materials, relies on attention to detailed design plus good handling and fabrication practices. Fabrication been recognized internationally as a significant factor in achieving confidence in stainless steel use and much good work has been carried out by stainless steel organizations and others to raise awareness of the importance and to provide support and information.
These aspects are mainly aimed at avoiding crevices which may cause problems during service. The more important include:
- Thorough degreasing is required before welding.
- Marks from oil, crayons, sealant, sticky deposits (including stick-on labels), slag, arc strikes, weld spatter should be removed.
- Tooling, blasting and grinding operations which can leave embedded iron should be avoided, e.g. use stainless steel not carbon steel brushes; or, if unavoidable, a final nitric-hydrofluoric acid pickling treatment should be used.
- Welds should be full penetration, free of cracks, overlaps and cold laps.
- Inert gas back purging is necessary during welding to minimise heat tint. Where formed, surface oxides and heat tint should be removed.
Embedded free iron on stainless steel surfaces causes rust marks to form and in some instances provides the sites for the initiation of corrosion. Pickling with nitric-hydrofluoric acid either by immersion or applied in paste form removes free iron and a thin surface layer of metal that may contain surface defects. The clean metal surface naturally passivates itself when exposed to air or aerated water conditions. This is the most corrosion resistant state. If a nitric-hydrofluoric acid pickle is not practical, the free iron can be removed mechanically. Acceptable methods include the use of medium to fine grit abrasives such as clean flapper wheels, flexible disks, or blasting with clean and iron free abrasives such as glass beads or garnet, or by electropolishing. All these methods are also acceptable for removing heat tint.
Publications & References
These and other fabricating topics including fasteners, cutting and forming, welding, weld specifications and post fabrication cleaning are covered in NI reference Book 11026: Fabricating Stainless Steels for the Water Industry.
Other useful fabrication references in the Nickel Institute Technical library include:
9001 Cleaning and Descaling of Stainless Steel
9003 Stainless Steel Fasteners
10004 Fabrication and Post Fabrication Clean Up of Stainless Steels
10068 Specifying Stainless Steel Surface Treatments
10080 Cleaning Stainless Steel Surfaces Prior to Sanitary Service
10085 MIC of Stainless Steels by Water Used for Cooling and Hydrostatic Testing
11007 Guidelines for Welded Fabrication of Nickel Stainless Steels for Corrosion Resistant Service
11026 Fabricating Stainless Steels for the Water Industry: Guidelines for Achieving Top Performance
14018 Guidelines for Welding Dissimilar Metals
14050 Heat Tint on Stainless Can Cause Corrosion Problems