Welding duplex and super duplex: ferrite content, filler metal, gas and heat control

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

Duplex steels consist of roughly half ferrite and half austenite. The austenite provides toughness and general corrosion resistance, the ferrite strength and resistance to stress corrosion cracking. In welding, the key is to preserve this balance.

In the delivery condition, the microstructure is set by solution annealing at 1020 to 1100 °C and rapid cooling: a ferritic matrix with about 45 to 60 % austenite. Compared with austenitic steels, duplex steels offer:

• considerably higher strength, which allows thinner walls,
• very good resistance to pitting,
• higher resistance to stress corrosion cracking, especially in chloride-containing media,
• lower thermal expansion.

In the welded condition, duplex steels are generally used between −50 and +250 °C, super duplex steels up to +220 °C.

They are distinguished by the pitting resistance equivalent PRE, which describes the resistance to pitting: duplex below 40, super duplex from 40. Lean duplex grades contain little or no molybdenum.

PRE = % Cr + 3.3 · % Mo + 16 · % N

Grades according to EN 10088 with guide values of the pitting resistance equivalent according to DVS 0946. For tungsten-containing grades, the equivalent is sometimes calculated including tungsten. On the pitting resistance equivalent, see also What makes stainless steel stainless.

Duplex steel first solidifies fully ferritic. Only on cooling does part of the ferrite transform into austenite. In welding there is little time for this, so the weld contains more ferrite than the base material.

• Cooling time t12/8: the decisive factor is the time between 1200 and 800 °C. It must be long enough for sufficient austenite to form.
• Over-alloyed filler metals: filler metals contain more nickel and nitrogen than the base material. Both promote austenite and compensate for the short cooling time.
• Staying hot too long is also bad: with prolonged dwell between about 300 and 1000 °C, sigma phase, chi phase, chromium nitrides and carbides form; around 475 °C there is a risk of embrittlement. This costs toughness and corrosion resistance.

The basics of the microstructure are covered in Microstructure, delta ferrite and hot cracking.

DVS 0946 recommends 30 to at most 70 FN in the weld metal. Below that, resistance to stress corrosion cracking falls; above it, toughness, ductility and corrosion resistance suffer.

• Weld metal diluted with the base material generally has more ferrite. In TIG weld metal, up to 85 FN is generally still considered acceptable.
• For the austenite content, DVS 0946 likewise gives 30 to 70 % if no solution annealing is carried out.
• Measuring: magnetically according to ISO 8249. Evaluation on a metallographic section has proved unreliable in round-robin tests because the ferrite is very fine and unevenly distributed.
• Estimating: with the WRC-1992 diagram, which extends to 100 FN and takes nitrogen and copper into account. Schaeffler and DeLong do not reliably cover the duplex range: Schaeffler, DeLong and WRC calculator.

For duplex, type 22 9 3 N L (AWS 2209) is the most widely used filler metal, for super duplex 25 9 4 N L (2594). Both are over-alloyed with nickel compared with the base material.

According to DVS 0946, ISO 14343 and ISER information sheet 823. Basic-coated stick electrodes give higher toughness than rutile ones and are preferred for roots before submerged arc welding. The instructions of the steel and filler metal manufacturers take precedence.

Filler metal selection for all stainless steels: Choosing the filler metal.

Nitrogen in the shielding gas promotes austenite and compensates for nitrogen losses. Hydrogen has no place in the welding gas, because it can embrittle duplex steel.

• Not too much nitrogen: excessive contents in the weld metal lead to pores and chromium nitrides. In TIG welding the weld picks up less nitrogen than in MIG/MAG welding, and more with many passes.
• Oxygen in the gas can slightly reduce the toughness of the weld metal.
• Super duplex with MIG/MAG is prone to pores. Investigations showed the best results with argon, about 30 % helium and very little CO₂, around 0.5 %. Even less CO₂ makes the arc unstable. The wire batch also plays a role.
• Root protection with argon, nitrogen or argon-nitrogen. Hydrogen-containing gases only with at most 10 % H₂, preferably 5 %, and only without a gap, so that nothing gets into the arc.

More in Shielding gases and Root protection and purging.

Duplex needs a heat window: enough heat for the austenite, but not so much that sigma phase forms.

• Arc energy according to DVS 0946: duplex 5 to 25 kJ/cm, super duplex 2 to 15 kJ/cm.
• Interpass temperature: duplex at most 250 °C according to DVS 0946 (DVS 0931 gives 150 °C), super duplex at most 150 °C.
• Super duplex with high requirements: for TIG welding, the technical literature also recommends limiting the interpass temperature to 100 °C and the arc energy to 10 kJ/cm. If the root is welded over with high energy, its pitting resistance can decrease.
• Preheating is usually not necessary, because modern duplex steels contain more than 0.15 % nitrogen. From a wall thickness of about 10 mm, 100 °C can be useful to reduce residual stresses, as can roots and different wall thicknesses under high restraint.
• Do not cool artificially: compressed air or water between passes is not permitted.

Calculating the arc energy: Heat input and interpass temperature.

Little dilution, always with filler metal, larger groove angles.

• Groove angle 70 to 80° instead of 60°: the weld pool is more viscous, and a narrow angle promotes lack of fusion.
• Add enough filler metal in TIG welding. Avoid welding without filler and cosmetic remelting of the weld, because the weld metal otherwise becomes too ferritic. Without filler it only works on thin, freely shrinking parts, with subsequent solution annealing, crack testing where possible and nitrogen-containing shielding gas.
• Tack welds with filler metal: they cool quickly, become ferrite-rich and can crack. Preheat if necessary.

More on this: Joint preparation and tack welding.

As a rule, no heat treatment is needed. If it is, only solution annealing comes into question.

It dissolves intermetallic phases and restores the ferrite-austenite ratio: heat up quickly, 1020 to 1050 °C, hold for 5 minutes after complete through-heating, then quench in water. If solution annealing is carried out, a lower austenite content in the weld metal beforehand is also acceptable, but cracks in almost fully ferritic weld metal must be avoided.

Can I weld duplex with 316L?

Not duplex to duplex: the weld would have lower strength and corrosion resistance than the base material. For joining 1.4462 to an austenitic steel, 22 9 3 N L and 19 12 3 L are both suitable.

Why does my weld have more ferrite than the plate?

Because the transformation into austenite does not run to completion during the rapid cooling after welding. Over-alloyed filler metals, enough heat and nitrogen in the gas counteract this.

Can I purge duplex with forming gas?

To a limited extent: at most 10 % hydrogen, preferably 5 %, and without a gap. Argon, nitrogen or argon-nitrogen are simpler.

What is lean duplex?

Duplex grades with little molybdenum and nickel, such as 1.4362. They are less expensive, and their pitting resistance equivalent is lower. They are welded with filler metals of type 23 7 N L or 22 9 3 N L.

The series from the material via process, gas and root protection to the tested weld. For an overview, see Welding of stainless steel.

1. What makes stainless steel stainless
2. Microstructure, delta ferrite and hot cracking
3. Choosing the filler metal
4. Processes at a glance
5. Shielding gases
6. Heat input and interpass temperature
7. Joint preparation and tack welding
8. Root protection and purging
9. Temper colours, pickling, passivation
10. Joining stainless to carbon steel
11. Weld defects and corrosion damage
12. Testing, health and safety