This is especially important for the successful restoration of the passive layer. Cougartron weld cleaning machines are designed to initiate and induce the formation of chromium oxide film on stainless steel surfaces and welds. Thus, you can be sure that rust and other contaminating factors will not reappear after they are successfully eliminated. The heat-affected zone HAZ is an area on the metal surface whose microstructure has been altered under the influence of heat during welding.
The change in the microstructure makes the steel weaker and less resistant to corrosion and stress cracking. Proper post-weld treatment cleaning and passivation is the only way to recover the structural stability of steel and prevent further decay. But to what extent? How much of the surface do you actually need to treat? The entire metal assembly or just the heat-affected zone and the weld? The best approach is to actively clean and passivate only the part of the surface that was actually affected by the welding process.
This in all cases includes the actual weld and the heat-affected zone areas where discoloration and other post-weld imperfections usually appear. The application of weld cleaning fluids on unaffected areas will not damage them. In any case, you will always reach parts of the metal surface outside the heat-affected zone.
However, the point is that these areas do not require focused and active treatment. Although mild, acids often leave a white residue on the surface of the steel after cleaning and passivation. Removal of these white acid marks is done for two main reasons — lowering the pH value and preserving the attractive appearance of the surface.
Common passivation treatments include nitric acid HNO 3 solutions or pastes which will clean the steel surface of free iron contaminants. Care must be taken in selecting and using passivation treatments to ensure the selected treatment will target the contaminant.
Passivation will also aid in the rapid development of the passive oxide film on the steel's surface. Passivation does not usually result in a marked change in appearance of the steel surface. Both pickling and passivation solutions can employ dangerous acids that can damage both the operator and the environment if not handled correctly. Stainless pickling acids are highly corrosive to carbon steel. It is essential that all acids are thoroughly removed by rinsing the component after completing the process.
Residual hydrofluoric acid will initiate pitting corrosion. The corrosion resistance of the stainless steel is affected by the roughness of the surface after polishing, with a marked decrease of the corrosion resistance as the surface roughness increases above a R a value of about 0. Liquid petroleum gas particularly creates a large amount of water when burnt.
Each welding run must be thoroughly cleaned to remove slag and spatter before proceeding with the next run. The cleaning method used chipping, brushing, grinding will depend on the welding process, bead shape, etc. Any cleaning equipment should be suitable for stainless steel and kept for that purpose. During welding, a gas purge on the reverse side may be advantageous.
After welding, weld spatter, flux, scale, arc strikes and the overall heat discolouration should be removed. This can involve grinding and polishing, blasting and brushing with a stainless steel wire brush, or use of a descaling solution or paste. The preferred procedure is usually dictated by end use.
Grinding and dressing is to be carried out with iron-free brushes,abrasives, etc. Rubber and resin bonded wheels are satisfactory. Wheels should be dressed regularly to prevent them becoming loaded thereby producing objectionable scratches. In any blasting process steel shot shall not be used. Efficient arc striking is essential in welding all types of stainless steels as indiscriminate arc striking tends to scar or burn the surface of the steel thus providing areas for premature chemical attack.
Care and skill are required to obtain satisfactory weld starts and restarts on continuous seams. As both burn-through on light section material and cold lack of penetration starts on heavier material can easily result from imperfect technique, the suggested procedure is to strike the electrode in the joint approximately mm forward of the actual start point. Efficient tack welding on austenitic steels is essential in controlling distortion.
The length of tack welds varies from mm depending on sheet or plate thickness. Generally tack welds are more closely spaced than when welding mild steel of similar dimensions. Knowing how to reduce spatter when MIG welding is crucial when you want neat and professional-looking welds.
Accreditations Product Literature Technical Reports. Skip to content. What is Passivation for Stainless Steel Welds? November 8, pm. What is passivation, and why is it so essential for your stainless steel welds?
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