Figure 1 shows the microstructure of the 2205 duplex stainless steel base metal as hot-rolled. The lighter etching phase is the austenite and the darker etching phase is the ferrite. Then it includes Cr, Ni, Mo, and N distributions obtained by a scanning electron microscope (SEM) of exactly the same area as the microstructure, color coded so that red indicates high concentration, yellow to green indicates nominal composition concentration of each element, and blue indicates low concentration. The nominal composition is also given in the figure. It is easily seen that Cr and Mo are concentrated in the ferrite, while Ni and N are concentrated in the austenite. It is noteworthy that, in the case of nitrogen, the blue color indicates virtually zero percent nitrogen in the ferrite. That is, the nitrogen has almost entirely left the ferrite in favor of the austenite during hot-rolling. Figure 2 shows the microstructure and alloy element distribution of an autogenous GTA weld made in the same base metal, containing 0.12% nitrogen, using the same color coding. Austenite appears only as particles along the grain boundaries of the very large ferrite grains that formed during solidification, and as a few scattered particles within the ferrite grains. Then the color maps of Cr, Ni, and Mo indicate virtually no partitioning of those alloy elements — they are distributed at virtually the nominal composition level throughout the microstructure — but the nitrogen map clearly shows nitrogen concentrated in the grain boundary austenite. In the ferrite immediately beside the grain boundary austenite, the blue color indicates virtually zero nitrogen as the nitrogen there had enough time to diffuse to the austenite. Farther from the grain boundary austenite, the yellow to green color indicates near-nominal nitrogen concentration on average. In fact there are scattered chromium nitride particles that appear as dark specks in the microstructure (Fig. 2A) that formed within the ferrite when the nitrogen could not escape from the ferrite to the austenite during cooling. Even though there is 0.12% N present in this composition, the phase distribution is not acceptable; the weld is brittle. Figure 3 shows the microstructure and alloy element distribution of an autogenous GTA weld made in an identical base metal composition except that the nitrogen is increased to 0.18%. As in the case of the 0.12% N weld metal, austenite appears as particles outlining the original ferrite grain boundaries, but there are also numerous austenite plates throughout the interior of the ferrite grains. Then the color-coded maps of Cr, Ni, and Mo indicate only very slight partitioning of these elements (most clearly seen in the Mo distribution where the grain boundary austenite regions are more clearly blue). But the nitrogen partitioning is complete — the ferrite regions are all blue while the austenite regions are all red. The higher nitrogen of the Fig. 3 weld metal as compared to the Fig. 2 weld metal made the aswelded weld metal ductile by producing much higher austenite content. In practice, filler metal manufacturers tend to include extra nickel to improve toughness and assist in the development of austenite, but the nitrogen is the essential alloy element for providing weldability. ◆ Reference 1. Ogawa, T., and Koseki, T. 1989. Effect of composition profiles on metallurgy and corrosion behavior of duplex stainless steel weld metals. Welding Journal 68(5): 181-s to 191-s. DAMIAN J. KOTECKI is president, Damian Kotecki Welding Consultants, Inc. He is treasurer of the IIW and a member of the A5D Subcommittee on Stainless Steel Filler Metals, D1K Subcommittee on Stainless Steel Structural Welding; and WRC Subcommittee on Welding Stainless Steels and Nickel-Base Alloys. He is a past chair of the A5 Committee on Filler Metals and Allied Materials, and served as AWS president (2005–2006). Send questions to damian@ damiankotecki.com, or Damian Kotecki, c/o Welding Journal Dept., 8669 Doral Blvd., Ste. 130, Doral, FL 33166. WELDING JOURNAL 15 Fig. 3 — Autogenous 2205 GTA weld metal, as-welded. A — Microstructure; B — 22% Cr; C — 6% Ni; D — 3% Mo; E — 0.18% N. For info go to www.aws.org/ad-index A B C D E
Welding Journal | January 2013
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