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The
Acute Technological Services’ Approach To The Fabrication of Duplex Stainless Steel Issue: 15 September, 2008 During the last twenty years there has been a large increase in the use of duplex and super duplex stainless steels. These are the "second generation" of duplex stainless steels. These grades are distinguished from the first generation by their ability to retain a good balance of austenite and ferrite, and thereby toughness and corrosion resistance, in the welded condition. This improvement results from the use of nitrogen as an intentional and precisely controlled alloying addition. Second generation duplex stainless steels have enjoyed wide acceptance because they offer excellent combinations of strength, pitting and crevice corrosion resistance as well as chloride stress corrosion cracking resistance, for a very economical cost. As fabrication experience with these alloys increases, it has become recognized that these technically complex grades require increased qualification and care in production and fabrication to assure safe and economical results.
Acute Technological Services, Inc., (ATS) has spent over fifteen years in the research, development and implementation of high tech welding procedures that facilitate the successful fabrication of these alloys. The Acute Systems approach to orbital piping fabrication has been refined and focused in order to dominate Duplex Stainless Steel Welding Technology, starting with a thorough understanding of the physical metallurgy of these alloys. In the annealed condition duplex stainless steels typically have a balanced 40-60 percent austenite and ferrite structure. As they are heated to higher temperatures, this structure transforms to a progressively higher volume fraction of ferrite. When a duplex stainless steel experiences a rapid thermal cycle such as that which occurs in the heat affected zone (HAZ) adjacent to a weld, the equilibrium phase balance with its excellent toughness, strength, and corrosion resistance may not be restored to equal those of the original alloy. Improper welding techniques, which produce thermal gradients that are either too rapid or too slow, will generate undesirable secondary and intermetallic phases, as well as chromium nitrite precipitation. Acute Technological Services has developed highly specialized manual and automatic orbital welding procedures that consistently maintain all relevant welding parameters within a range that yields optimum weldment properties and minimizes the deleterious effects previously mentioned.
In order to achieve the best combination of quality and productivity in the fabrication of duplex stainless steel piping the following important tenets, inherent to the Acute approach should be followed: 1. Careful selection of base material from the standpoint of chemistry, heat treatment, and manufacturer. 2. Detailed pre-production planning for optimization of welding materials and equipment. 3. Automatic welding with preprogrammed weld schedules to achieve maximum parametric control and reproducibility is preferred over manual welding of Duplex Stainless Steels. However, where automatic welding is not practical, manual welding must be performed by highly skilled, trained technicians with special consideration for the difficulty of parametric control inherent to manual welding. Technician performance must be evaluated to the same extent that welding procedures are qualified in order to verify that individual techniques are consistent with the required parametric control. 4. Utilize mechanized beveling systems to ensure reproducibility of welding procedures. 5. Selection of welding consumables to achieve optimum weld metal properties. 6. Electronic monitoring and recording of welding parameters, thermal profiles, and ferrite contents as an in-process quality control tool. |
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