A: Your question has been asked many times, but I did not have an approach to a solution. Very recently, however, an approach seems to have appeared. To begin, super duplex stainless steels are loosely defined as those containing about 25% Cr, 6% Ni, 4% Mo, and at least 0.20% N. A more quantitative definition comes from calculating the pitting resistance index commonly defined as below. PREN = %Cr + 3.3x (% Mo + % W/2) + 16x%N When the PREN equals at least 40, the steel is generally considered to be a super duplex stainless steel. There are two fairly common super duplex stainless steels, commonly known as 2507 (UNS S32750) and Zeron 100 (UNS S32760). Their compositions (from ASTM A790, Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe), and the composition of the matching filler metal ER2594 (from AWS A5.9/A5.9M:2012, Specification for Bare Stainless Steel Welding Electrodes and Rods), are given in Table 1. The high nitrogen content of these steels is often thought to promote porosity during welding. I note that the more ordinary duplex stainless steel 2205 base metal (UNS S32205) and corresponding ER2209 filler metal are limited to 0.20% N maximum, so nitrogen may have something to do with the porosity problem. But there seems to be something more to the porosity problem than high nitrogen content. At the International Institute of Welding’s Annual Assembly in Melbourne, Australia, last July, a presentation was made of recent research work by Stützer et al. (Ref. 1). The GMA welds of ER2594 filler metal on S32750 and S32760 base metal were produced as overlays that could be radiographed. Six shielding gases were considered. These are listed in Table 2. I note that some addition of a small amount of an oxidizing gas, such as O2, CO2, or NO, to the argon-based shielding gas is normally considered beneficial for wetting and bead shape when welding duplex and super duplex stainless steels. I was a bit surprised to see that oxygen additions (typically 1 or 2%), which are common, were not STAINLESS Q&A 28 WELDING JOURNAL / NOVEMBER 2016 BY DAMIAN J. KOTECKI Q: When gas metal arc welding (GMAW) super duplex stainless steels with ER2594 filler metal, we fairly often encounter unacceptable porosity. We don’t observe this with ordinary duplex stainless steel 2205 base metal and ER2209 filler metal. We have tried a variety of welding parameter changes to reduce the porosity to acceptable levels, but nothing has seemed to work. What is the cause of the porosity, and how can we prevent it? Table 1 — Compositions of Super Duplex Stainless Steel Pipe and GMAW Filler Metal Alloy Chemical Composition, wt-% (single value is a maximum) C Mn P S Si Cr Ni Mo Cu W N S32750 0.030 1.20 0.035 0.020 0.80 24.0 6.0 3.0 0.5 — 0.24 to to to to 26.0 8.0 5.0 0.32 S32760 0.05 1.00 0.030 0.010 1.00 24.0 6.0 3.0 0.50 0.50 0.20 to to to to to to 26.0 8.0 4.0 1.00 1.00 0.30 ER2594 0.03 2.5 0.03 0.02 1.0 24.0 8.0 2.5 1.5 1.0 0.20 to to to to 27.0 10.5 4.5 0.30 Table 2 — Shielding Gases Evaluated (Ref. 1) Gas Code Gas Composition, vol-% Ar He CO2 N2 NO G1 balance 30 — — — G2 balance 30 0.05 — — G3 balance 30 0.5 — — G4 balance 30 2.0 — — G5 balance 30 — 0.007 0.03 G6 balance 30 — 2.0 — — continued on page 30
Welding Journal | November 2016
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