STAINLESS Q&A BY DAMIAN J. KOTECKI Q: I have been told that nitrogen is an essential ingredient in duplex stainless steel weld metal. But I understand that nitrogen is only an accidental impurity in other stainless steel weld metal. Is this correct, and if so, why is it essential in duplex stainless steel? A: You have been told correctly. There are two factors involved. The first has to do with pitting corrosion resistance. There is a well-accepted formula for a Pitting Resistance Index (PREN). PREN = %Cr + 3.3×(%Mo + %W/2) + 16×%N As the PREN increases, the pitting resistance increases. Duplex stainless steels are often used in chloride-containing environments, including the higher alloy grades in seawater. The higher alloy grades of duplex stainless steel, often referred to as “superduplex,” have a PREN greater than 40. As you can see, the coefficient for nitrogen in the above formula is much larger than for any other element. The superduplex stainless steels typically contain about 0.25%N, or more. But pitting corrosion resistance is only part of the story concerning nitrogen in duplex stainless steels and their weld metals. In weld metal in the as-welded condition, nitrogen is the critical element for obtaining a proper phase balance between ferrite and austenite. There remains a lot of discussion about what amount of ferrite (remainder austenite) is appropriate for best properties, but most engineers will agree that the range of 30 to 70 Ferrite Number or 22 to 50% ferrite (higher ferrite content is allowed with inert gasshielded processes) provides the best combination of properties, particularly corrosion resistance, toughness, and ductility. Nitrogen is critical because it is the only useful alloying element that is an interstitial atom rather than a substitutional atom. Interstitial atoms are much smaller than the matrix alloy element atoms iron, chromium, nickel, molybdenum, and possibly tungsten. As a result, nitrogen can diffuse more than 100 times faster than the other atoms. Nitrogen promotes austenite formation by diffusing out of the ferrite as the virtually 100% ferrite weld metal cools at temperatures above 1040°C (1900°F). Under ordinary arc welding cooling conditions, only nitrogen diffuses fast enough to partition appreciably between ferrite and austenite. This was very well demonstrated in the work of Ogawa and Koseki (Ref. 1). B C D E Fig. 1 — 2205 base metal as hot-rolled. A — Microstructure; B — 22% Cr; C — 6% Ni; D — 3% Mo; E — 0.12% N. Fig. 2 — Autogenous 2205 GTA weld metal, as-welded. A — Microstructure; B — 22% Cr; C — 6% Ni; D — 3% Mo; E — 0.12% N. Unfortunately, when the work of Ogawa and Koseki was published in the Welding Journal, color printing in the Research Supplement was not in use, and the element partitioning was illustrated by color-coded maps, so it was difficult to appreciate exactly what was going on in the black and white reproductions, and I think very few people did appreciate it. However, Ogawa and Koseki also presented their work a year later in Commission IX of the International Institute of Welding, as IIW Document IX-1600-90, and I was fortunate enough to obtain a copy with color. Three figures extracted from that work serve to illustrate the importance of nitrogen, and these are reproduced herein. 14 JANUARY 2013 A A B C D E
Welding Journal | January 2013
To see the actual publication please follow the link above