Welding duplex stainless steel: key considerations

Duplex stainless steel, with 1.4462 (2205) as its best known grade, combines high strength with excellent resistance to pitting and stress corrosion cracking. It owes that combination to a microstructure of roughly half ferrite and half austenite. Exactly that balance makes welding critical: every weld disturbs the ratio, and only correct parameters and the right filler metal restore it sufficiently.

This article covers the four pillars of sound duplex welding: maintaining the ferrite austenite balance, the heat input window and associated interpass temperature, the choice of filler metal and shielding gas, and verification through ferrite measurement and corrosion testing.

Why the ferrite austenite balance governs everything

Duplex solidifies fully ferritic. Austenite only forms during cooling, and how much forms depends on chemistry and cooling rate. If the weld cools too fast, excess ferrite remains and chromium nitrides precipitate: toughness and corrosion resistance drop. If it cools too slowly, or dwells too long between roughly 700 and 1000 degrees Celsius, intermetallic phases such as sigma form, embrittling the material and slashing its corrosion resistance.

The target is therefore a weld metal and heat affected zone with broadly 30 to 70 percent ferrite, with many specifications imposing a tighter band such as 30 to 60 percent for the weld metal. Everything in execution, from joint preparation to cooling, serves that balance.

The heat input window and interpass temperature

Unlike ordinary structural steel, duplex has both a lower and an upper heat input limit. For standard duplex 1.4462 a window of 0.5 to 2.5 kJ/mm is common; superduplex is tighter at roughly 0.5 to 1.5 kJ/mm. Too little heat means rapid cooling, excess ferrite and nitrides. Too much means slow cooling, grain growth and the risk of intermetallic phases.

The interpass temperature is equally important: duplex is typically limited to 150 to 250 degrees Celsius, with 150 degrees Celsius the usual limit for superduplex and 250 degrees Celsius often the maximum for standard duplex. Waiting between runs is part of the process. Preheating is unnecessary and undesirable; at most a gentle warm through to drive off condensation. How to calculate heat input correctly is covered in calculating heat input.

Filler metal: over alloyed in nickel

Duplex is almost always welded with a filler over alloyed in nickel relative to the base material; for 1.4462 that is type 22 9 3 N L, commonly called 2209. The extra nickel promotes austenite formation in the weld metal and compensates for the fast cooling of a weld. Autogenous fusion without filler, for example a TIG wash pass, produces a strongly ferritic structure and must be avoided or corrected by solution annealing.

Selection points:

• Superduplex requires its own filler, for example 25 9 4 N L, and is not interchangeable with 2209
• Dissimilar joints to austenitic or carbon steel require a separate filler assessment
• For TIG, adding 1 to 2 percent nitrogen to the shielding gas can offset nitrogen loss from the weld pool

Shielding gas, backing and joint preparation

For TIG, argon with an optional 1 to 2 percent nitrogen is standard; hydrogen bearing gases are prohibited on duplex because the ferrite phase is susceptible to hydrogen embrittlement. For MAG, argon rich mixtures with limited CO2 or O2 are used, and for cored wires the mixtures specified by the manufacturer.

The root deserves particular care: a backing gas with low residual oxygen keeps the root bright and preserves corrosion resistance. Heat tint on the root side means chromium depletion at the surface, inviting local pitting. Acceptance limits for discolouration and post weld pickling and passivation are covered in our article on heat tint on stainless welds. Keep duplex work strictly segregated from carbon steel: dedicated tools, brushes and grinding discs only.

Qualifying the welding procedure

Procedure qualification to EN ISO 15614-1 is the minimum for duplex, but the standard tests say little about phase balance. Serious specifications, such as NORSOK M-601 or ISO 17781 for the petrochemical industry, therefore supplement qualification with:

• Ferrite measurement on weld metal, fusion line and heat affected zone, by point counting on a cross section to ASTM E562 or magnetically to EN ISO 8249 in ferrite number FN
• A corrosion test to ASTM G48 method A, assessing weight loss and pitting at a specified test temperature
• Low temperature impact testing, often at minus 46 degrees Celsius, since excess ferrite reveals itself there immediately
• Microstructural examination for intermetallic phases and nitrides

In the WPS, record the heat input window, the maximum interpass temperature and the backing gas alongside the usual parameters.

Production control and verification

Production is about discipline: keeping heat input per run inside the window, measuring rather than guessing interpass temperature, and not stacking repeated repairs on the same spot, since every additional thermal cycle raises the risk of intermetallic phases. Repairs on duplex warrant their own qualified repair procedure.

For verification, portable ferrite gauges based on magnetic induction are practical: they read the ferrite number directly on the weld. Interpret results knowledgeably, as surface roughness, geometry and calibration all influence the reading. For doubts about the measurement set up, acceptance limits or the inspection and test plan, independent NDT coordination is a sensible step.