If Duplex Stainless Steel does not have a really excellent weld purge, the relatively high chromium content and the relatively low thermal expansion of a Duplex Stainless Steel, make the resulting oxide scale very thin and highly resistant to removal.
It is desirable to remove this heat tint to obtain maximum corrosion resistance and in many tube and pipe welding applications, the methods of removal can be extremely difficult and/or expensive.
Grinding down to bright metal is effective but with small diameters and complex contours, not easily achievable. Blasting can also be effective but because of circumstances, may not be as effective as grinding for removing the oxide layer.
Pickling by solution or by paste is effective, but longer times or more aggressive solutions are required for duplex grades than are typically required for austenitic grades. In addition, with tube and pipework accessibility, volumes of material and disposal difficulties and costs, make this an undesirable method from a cost point of view.
All of these difficulties can be overcome, if the welding and quality engineers specify the weld purging methods and insist on high quality procedures. 
Choosing a high quality inflatable tube and pipe welding system can be easily carried out, ensuring that the product selected has low vapour pressure materials and a means of adjusting the flow of purge gas during welding without the danger of bursting one of the inflatable dams, using an accessory such as PurgeGate™ .
Typical Tube and Pipe Weld Purging Systems >
Including insertion of the inflatable purge system into the welding procedure as well, can ensure that the inflatable dams are not placed too close to the weld root.
< QuickPurge® II system fitted with PurgeGate™
Systems with a centring device such as Rootglo® can be chosen. This is a band of light emitting material that is fixed to the centre of the weld purging system. Rootglo® absorbs enough ambient light outside the pipe, that once it is inside it glows bright in the dark for up to 12 hours. This will allow the weld team to see the purge system coming into place by viewing it through the root gap. Once centred, the root can be taped over*, the purge gas can be turned on and PurgeGate™ will do the rest.
At the other end of the purge system, if practical, the expelled gas tube should be connected to a Weld Purge Monitor® that will read oxygen levels accurately down to 10 parts per million. Such Weld Purge Monitors use the Zirconia cells that are far more accurate than the wet cell instruments that are more suitable for normal stainless steels. In this way it can be seen that the oxygen levels are being maintained below the recommended 500 ppm prior to the start of welding. Low cost Zirconia Cell, Weld Purge Monitor® for reading accurately down to 10 ppm ^
In the event that the exhaust gas cannot be monitored, the Weld Purge Monitor will have a stainless steel probe assembly and hand bulb vacuum pump to take intermittent readings of oxygen level through the taped weld root gap.
Assuming such stipulations can be included in a Welding Procedure, then oxide layers or heat tints will not form.
* When taping a joint gap, avoid placing adhesive onto the pipe exterior close to the heat affected zone of the joint. Contact the writer for a taping method if necessary rasewell@huntingdonfusion.com
