Super duplex stainless steels have historically been considered difficult to weld. Incorrect welding conditions can lead to a change in the balance of austenite:ferrite, and also lead to the creation of deleterious intermetallic phases. Both phenomena can result in lessened corrosion resistance and impact toughness.
If subjected to temperatures between 450degC and 900degC for enough time, super duplex stainless steels can form several harmful intermetallic phases, including sigma phase. These chromium-rich regions result in areas where the chromium is denuded, which are subsequently prone to pitting corrosion.
To avoid the formation of sigma phase, the power input should be carefully controlled. Also the time between multi-pass welds should be sufficient to prevent the weld region spending too long at too high a temperature. ‘Over-matched’ weld fillers, containing a greater level of nickel, will also help to maintain a desirable balance between austenite and ferrite.
Like every welding procedure, the key to success is in the preparation. Here are a number of key areas to focus on to achieve the best results –
The Area – A specific area should be used when welding duplex and super duplex plus specific brushes and tools.
Joint Preparation – A slightly larger joint angle and root gap is needed than would be used for stainless steel. Please note, a slight loss in austenite will occur during welding.
Pre-Weld Cleaning – Pre-weld cleaning is essential, with the welding groove and joints needing to be frequently cleaned. Acetone must be used to remove all grease, oil and dirt with any rough edges needing to be ground away.
Preheating – Preheating is not required although a hot air blower should be used to remove condensation.
Tack Welding – Oxidation and shrinkage can occur in the weld groove if your tack welding is not precise. Spacers should also be included in the welding joints.
Heat Input – Special attention needs to be paid for both heat input and the inter-pass temperature. If the heat input is too low, rapid cooling could affect the 50:50 composition creating more ferrite that reduces resistance to corrosion. Too much heat can cause embrittlement through σ-phases, χ-phases, and other phases though.
Arc – The technique of creating and extinguishing the arc is essential as part of the conventional arc welding process. Only highly skilled welders should be used to perform duplex and super duplex welds.
If you have any more questions about austenitic or duplex stainless steels, please contact us today. A member of our team will be more than happy to help and can also advise you on the best option for your application.