Methods for post fabrication and welding clean-up of stainless steels are well documented. One issue is whether the heat tint discolouration in the heat-affected zone of stainless steel welds should be removed.
What is heat tint
Heat tinting is a thickening of the naturally occurring oxide layer on the surface of the metal. The colours formed are similar to ‘temper colours’ and are the result of ‘light interference’ effects. This is due to ‘optical path length’ differences between light reflected from the surface of the oxide film and that reflected from the oxide / metal ‘interface’, which results in a range of colours, depending on the oxide layer thickness.
When should heat tint be removed
If the intended application relies on the heat or oxidation resistance of the stainless steel, then removal of heat tint is not important, as, depending on the service temperature, oxidation heat tinting will occur and blend into any localised weld tinting.
As heat tint colours are formed on stainless steel chromium is drawn from below the surface of the metal to form a chromium rich oxide surface layer. This leaves the metal just below the surface with a lower chromium level. In cases where the application involves ‘aqueous’ ie a low temperature corrosion hazard environment, the local reduction in sub-surface chromium can affect the corrosion resistance of the steel.
Although some sources suggest that the removal of heat tint is not always essential, it is vitally important that weld heat tint is removed so that the full corrosion resistance of the finished product is restored after welding. This will help avoid unnecessary service corrosion problems in fabrication weld areas.
The recently published Drinking Water Inspectorate Code of Practice (Operational Guidelines and Code of Practice for Stainless Steel Products in Drinking Water Supply) states:
In order to achieve the optimum corrosion performance of stainless steel welded joints, crevice features, contamination and at least all weld heat tints deeper in colour than a pale yellow shall be removed by mechanical dressing followed by acid pickling of the joint.
So as a general rule, if you can see a disclouration on the surface due heat tint, this must be removed for drinking water applications and should be adopted as good post weld cleaning practice for any stainless steel welded joints.
Removing heat tint
The removal of heat tint from stainless steel fabrications using brush-on pastes or gels, spray or immersion acid pickling or electrolytic methods will normally be satisfactory.
The nitric acid used in these treatments will also leave the steel surface in the ‘passive’ condition.
A combination of finishing techniques may be needed, especially as nitric acid treatments alone cannot be relied on to remove sufficient metal from the surface. This may include mechanical treatments (grinding or abrading) followed by nitric acid cleaning (passivation). passivation of stainless steel
It is important to follow the preparation supplier’s instructions, as excessive contact times with these hydrofluoric acid containing products can result in pitting damage to the stainless steel.
Where better surface finishes are required, either for aesthetic appearance or to further optimise corrosion resistance, then final electropolishing can be used.
These finishing techniques will also improve the overall appearance and presentation of the finished product.
It is equally or perhaps more important to consider the inside weld faces of fabrications. Although these area may be out of sight, they are intended to be in direct contact with the service environment for which the stainless steel was selected.
Even though weld backing gas systems may have been used, post weld heat tint removal in these areas is perhaps more important than the outside faces.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)