BOC Purging while Welding
The primary reason for the wide use of stainless steels is their general corrosion resistance.
To ensure the operational safety of weldments of high-alloy steels under corrosive conditions, it is further necessary to maintain the special characteristics of these materials when producing welded joints.
The exposure to heat and oxygen during the welding process results in the formation of temper colours on stainless steels and other materials (e.g. titanium). These are bands of various colours around the penetration bead and adjacent parent material that can severely reduce the corrosion resistance of high-alloy materials, and usually must be avoided or removed.
Methods for removing temper colours are classifed as chemical, electrochemical and mechanical. Pickling, a purely chemical process used to remove temper colours and other impurities from the workpiece surface through treatment with a mixture of various acids can often not be employed for reasons of availability and environmental protection.
Devices for electrochemical removal of discolourations require manual operation and are therefore only practical for use with smaller workpieces.
When blasting or using mechanical techniques (grinding, brushing) accessibility permitting, the required degree of removal or cleaning effect is often questionable. In addition, there is always the risk that stainless and non-alloyed steel materials may be unintentionally mixed up.
Shielding the weld root with an inert gas—via purging—presents an alternative to these methods.
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