In a Technical Paper Written by Dr Barbara K Henon of AMI Arc Machines Inc , entitled “Orbital Welding of Corrosion Resistant Materials” Section 3.5 Discolouration/Heat Tint, she laid out details of standards relating to weld purging in the Bio/Pharm/semi-conductor Industries.
She states that “Proper inert gas purging to prevent the formation of heat tint during welding is critical to maintaining corrosion resistance of 316L”. This of course, applies to all of the stainless and duplex steels as well as with titanium, zircalloy and nickel alloys.
The BPE standard specifies colour free welds referencing the colour chart of AWS D18
.
Readers wishing to have a copy of that colour chart, along with those of other standards e.g. Den Norsk Veritas, can contact rasewell@huntingdonfusion.com.
It is also noted that welds in the pharmaceutical industry are put into service in the ‘as welded condition’, not permitting any cleaning.
This stresses the need for an excellent weld purging technique that can only be achieved buy using a “Weld Purge Monitor®” that will indicate accurately down to 50 parts per million of oxygen.
The PurgEye® 300 Nano is a typical instrument to carry out these measurements and is in fact the lowest priced Weld Purge Monitor® able to read accurately down to 10 ppm
.
Passivation with nitric or citric acid-based solutions may be carried out, however that does not remove heat tint that can extend below the surface where it is not affected by passivation.
Dr Henon goes on to state that “achieving an adequate i.d. Purge with minimal heat tint discolouration helps to retain the corrosion resistance of the material during welding”.
A really adequate oxygen level for this is below 50 ppm
