Duplex stainless steels in the pipe production - a historical review and new findings

    From exotic material to commodity

    ...continuation of our previous newsletter

    Duplex stainless steels


    Weldability of Duplex Stainless Steels during Pipe Production
    All common arc welding processes are applicable during the production of welded pipes made of duplex stainless steels. If the process is applied without filler metal, as in mechanized TIG and Plasma welding, electron beam or laser beam welding, special precautions need to be taken. The high cooling rates after electron beam and laser welding affect the ferrite/austenite transformation and are beneficial for an equilibrium ratio of ferrite and austenite. However, should the cooling rates be too high, a surplus of ferrite in the weld can be developed resulting in a reduction of toughness, ductility or corrosion resistance.
    Welding without filler metal is generally followed by a solution annealing in the pipe production.

    During inert gas welding processes, the use of nitrogen containing inert gases helps to balance the austenite/ferrite ratio in the weld. The austenite/ferrite ratio in the heat affected zone (HAZ) can only be influenced by means of a precise control of the heat input during welding. High temperatures during welding lead to ferrite formation in the weld area. During heating above 1,100 °C, the zone of stable ferrite is reached, which is transformed to austenite with
    an amount of 50 % during cooling. The degree of austenite formation is a kinetically influenced process, thus depending on the cooling rate. In order to achieve a balance of 30 to 65 % ferrite in the weld filler, metals with higher nickel contents - of up to about 2 or 5 % higher than the base metal - are commonly used.

    Chemical composition of duplex filler metals

    Chemical composition of duplex filler metals

    Welding Parameters are crucial
    The following parameters influence the ferrite/austenite ratio during welding:

    • the energy input per unit length
    • the interpass temperature
    • the chemical composition of the filler metal
    • the composition of the inert gas
    • the final heating after welding

    The arc energy during welding is directly influenced by the welding parameters (amperage, voltage and travel speed). During the mandatory welding procedure qualification, the threshold values are determined. The energy input per unit length has less influence on the ferrite/austenite ratio in the weld than the chemical analysis of the filler metal. However it significantly influences the grain size in the weld and heat affected zone (HAZ) as well as the austenite/ferrite ratio at the fusion line and the heat affected zone.
    The maximum allowable interpass temperatures, as published in the literature, are:

    • 150 °C for lean duplex stainless steels,
    • between 175 and 250 °C for standard duplex stainless steels and
    • maximum 150 °C for superduplex stainless steels.

    From that, it can be concluded that lean and superduplex stainless steels with a chemical composition near the thresholds of austenite/ferrite duplex stability show less tolerance towards the allowable interpass temperatures. Today, the chemical composition of filler metals is designed in order to suppress ferrite formation in the weld. Due to an increased nickel content of 9 up to 11 % Ni, the austenite ratio in the weld lies above the base material.

    Using Shielding Gases
    Shielding gases with 1 up to 3 % nitrogen in argon or helium support the austenite formation and increase the pitting resistance in the weld. Unfortunately, during TIG welding with shielding gases containing nitrogen between 2 and 3 %, the Tungsten electrode shows a reduced life time. Additionally, the risk for pores in the weld increases with the nitrogen content in the shielding gas. 1 up to 2 % nitrogen in the shielding gas has been proven to be an excellent compromise for a trained welder. The welding of higher wall thicknesses with several layers may transform the austenite/ferrite ratio to austenite. In this case the pipe should be protected with a nitrogen containing gas from the inside.
    Solution annealing after welding is always beneficial to adjust the optimum structural properties of duplex stainless steel. In cases where mechanized TIG or Plasma welding processes are applied, it is recommended to use argon/helium gas mixtures. Argon/helium gas mixtures containing amounts of nitrogen combine the beneficial effects of balancing ferrite and austenite: increasing the weld velocity, increasing the pitting resistance in the weld as well as reducing weld defects such as pores and increasing the service time of the Tungsten electrode.

    During GMAW, gas mixtures containing up to 2.5 % CO2 and 20 up to 50 % helium show good results in practice. Principally, the quality of the welding wire is of high importance but during mechanized Gas Metal Arc (GMA) welding, really no compromise regarding the quality of the welding wire should be accepted.

    Submerged Arc Welding is the process of choice for heavy wall pipes made from heavy plates. In this case, the heat input needs special attention. In principle, there is no need for preheating, but if the welding is interrupted, it does make sense to preheat the weld area up to interpass temperature, in order to reduce remaining stresses. Both powder types, highly and moderately alkaline, can be used. Highly alkaline powders reduce the risk of cracks.

    Quality of the Welding Process
    An adequate root protection against oxidation during pipe welding is mandatory in order to avoid repairs. During tack welding, in the root pass and also from the top pass, a perfect shielding is always a must. For root pass protection, all inert gases - nitrogen containing gases and with some limitations reducing gases - can be used. The welder may not begin to weld before the remaining oxygen content is low enough to ensure that there is no visible oxidizing during welding. As an average, the remaining oxygen should be below 50 ppm. If the specification requires the precise calibration of the shielding chamber, the measurement devices need to be able to measure down to 1 ppm oxygen in argon as well as in nitrogen/hydrogen. A lack in shielding leads to heavy oxidation of the root area, which is normally the starting point of corrosion attacks in later service.

    Read about the "Heat Treatment of Duplex Pipes" in our next newsletter...

    Authors:
    Dr I. Rommerskirchen
    T. Schüller, Managing Director
    R. Soelch, Senior Welding Manager
    R. Hoffmann, Head of QA
    E. Flohr, Deputy Head of QA

    References

    1. DIN EN 10217-7 Welded steel tubes for pressure purposes-Technical delivery conditions-Part 7: Stainless Steel Tubes, Beuth Verlag GmbH, Berlin, May 2005
    2. VdTÜV Material Data Sheet 418, 03/2006 edition, Ferritic-austenitic rolled and forged steel X2CrNiMoN22-5-3 Material NO. 1.4462, TÜV Media GmbH Cologne 2006
    3. ASTM A 928/ A 928M-09a Standard Specification for Ferritic/Austenitic (duplex) Stainless Steel Pipe, Electric Fusion Welded with Addition of Filler Metal, ASTM International, West Conshohocken, 2008