How straightforward is super duplex welding?

Super duplex stainless steels have historically been considered difficult to weld, but they weld reliably when the procedure is controlled. Incorrect welding conditions can shift the balance of austenite to ferrite and create deleterious intermetallic phases, both of which reduce corrosion resistance and impact toughness.

Avoiding intermetallic phases

Held at temperatures between 450°C and 900°C for long enough, super duplex stainless steels can form several harmful intermetallic phases, including sigma phase. These chromium-rich regions leave the surrounding metal denuded of chromium, which then becomes prone to pitting corrosion.

To avoid sigma phase, control the power input carefully and allow enough time between multi-pass welds so the weld region does not spend too long at too high a temperature. Over-matched weld fillers, containing a higher level of nickel, also help maintain a good balance between austenite and ferrite.

Welding procedure for duplex stainless steel

As with any welding procedure, success is in the preparation. The key areas to focus on are:

The area - use a dedicated area for welding duplex and super duplex, with dedicated brushes and tools.
Joint preparation - a slightly larger joint angle and root gap are needed than for stainless steel. A slight loss in austenite occurs during welding.
Pre-weld cleaning - essential. Clean the welding groove and joints frequently. Use acetone to remove all grease, oil and dirt, and grind away any rough edges.
Preheating - not required, though a hot air blower should be used to remove condensation.
Tack welding - oxidation and shrinkage can occur in the weld groove if tack welding is imprecise. Include spacers in the welding joints.
Heat input - pay close attention to both heat input and inter-pass temperature. Too low a heat input causes rapid cooling that can shift the 50:50 balance towards more ferrite and reduce corrosion resistance. Too much heat can cause embrittlement through sigma phases, chi phases and others.
Arc - creating and extinguishing the arc cleanly is essential in conventional arc welding. Only highly skilled welders should perform duplex and super duplex welds.

Frequently asked questions

Why has super duplex stainless steel historically been considered difficult to weld?

Incorrect welding conditions can shift the balance of austenite to ferrite and create deleterious intermetallic phases, both of which reduce corrosion resistance and impact toughness. With a controlled procedure, though, super duplex welds reliably.

How can sigma phase be avoided during welding?

Sigma phase forms when the weld region spends too long between 450degC and 900degC, so power input must be carefully controlled and enough time allowed between multi-pass welds. Over-matched weld fillers with higher nickel also help maintain the austenite-ferrite balance.

Why does joint preparation matter more for duplex welding than for standard stainless steel?

A slightly larger joint angle and root gap are needed than for standard stainless steel, because a slight loss in austenite occurs during welding. Careful pre-weld cleaning with acetone and grinding away rough edges is also essential to a sound weld.

Why should only highly skilled welders work on duplex and super duplex?

Creating and extinguishing the arc cleanly is essential in conventional arc welding, and heat input together with inter-pass temperature both need close attention - too little heat shifts the balance toward ferrite, while too much causes embrittlement through sigma and chi phases.

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