Welding-duplexStainless Steels Successfully.SOLUTIONS with Effective, Practical AdviceWelding-duplex stainless should be performed only after rigorous study of the special requirements imposed by this family of materials, after complete determination of practical welding parameters, and after thorough validation of the properties obtained. A short account of Duplex Stainless Steels, those containing about equal amounts of ferrite and austenite, was given in my page on Stainless Steel Welding. (Sponsored Links) Duplex Stainless Steels are a group of materials offering good corrosion resistance, improved mechanical properties with good ductility and toughness, as compared to austenitic grades, and relative ease of fabrication, including weldability, when due precautions are taking place. Modern duplex stainless steels can be grouped according to similarity of composition to four main Grades identified for ease of reference:
The balanced microstructures must be maintained also when fusion welding. Edges of base metals must be carefully machined to facilitate full penetration without burn through, and filler metal must always be used. Cleaning of base metal and consumables before welding should follow the highest standards. For most of the Welding-duplex stainless, when matching composition of filler metal (similar to base metal composition) is used, a post weld heat treatment (PWHT) at high solutioning temperature (1050 - 1100 0C = 1920 - 2010 0F) should be performed, followed by water quenching. The usual low temperature (600-650 0C = 1100-1200 0F) stress relief treatment as used for carbon steel is harmful and must be avoided. In common practice even the matching filler metals have a slight excess of nickel relative to the wrought base metal products, mainly to avoid high ferrite content of the weld metal. If filler metal with higher (8-10%) Nickel is preferred (called overmatching), weldments can be put into service as welded. The favorable properties of Duplex Stainless Steels, including chloride pitting and crevice corrosion resistance, are obtained by carefully balancing the amounts and the proportions of the different microstructures developed during primary material making but also during fabrication. Proper control required to reach a stable duplex structure, depends on specific chemical composition, due to complex interactions between chromium, molybdenum, nitrogen and nickel. Furthermore the thermal history of the material must avoid the formation at elevated temperatures of detrimental intermetallic structures, called generally sigma and chi phases. The presence of nitrogen in suitable proportions effectively contrasts their appearance. Welding-duplex stainless steel is made easier by the current commercial grades that are low in carbon (less than 0.03%), and therefore free from the risk of sensitization and of intergranular corrosion due to carbide precipitation. Solidification hot cracking risk is reduced by the adequate presence of ferrite and also by the fact that sulfur and phosphorus levels are kept low both in base material and filler metals. Therefore hot cracking is seldom a concern when Welding-duplex stainless steel. Hydrogen cold cracking resistance is satisfactory due to high hydrogen solubility in austenite that is present in the matrix in high percentage. Those responsible for Welding-duplex stainless steel should be aware of the fact that not all the problems likely to occur when processing these materials may be readily apparent in the shop, nor that they are detectable by non destructive inspection. The weld quality, as demonstrated by achieved toughness and corrosion resistance in service, depends on strict observance of proven and qualified welding procedures. Any unauthorized deviation involves unacceptable risks of failure. Preheating is generally not required, except for eliminating moisture in cold weather, and could be harmful. The concerns associated with Welding-duplex stainless steel refer to the Heat Affected Zone, (HAZ) not generally to the weld metal. Primary among these are loss of corrosion resistance and of toughness or post weld cracking. Welding-duplex stainless procedures, in order to avoid these problems, should try to minimize the total time at high temperature in preference to limiting the heat input for any single pass. In fact duplex steels have good tolerance for relatively high heat inputs. Best practice allows for rapid cooling from weld temperature, by keeping the weldment mass relatively cool. This is expressed by the requirement for maximum interpass temperature, usually established at 150 0C (300 0F) to be enforced with suitable instrumentation both for welding procedure qualification and for production. Contrary to common practice for different kinds of steel, limiting the heat input to very low values can have deleterious effects as loss of toughness and corrosion resistance. However very high heat input increases the risk of intermetallic phase forming. For Welding-duplex stainless standard welding processes are used, except that flux cored arc welding (FCAW) is not recommended, especially for the super duplex grades. An Article on Spot Welding Duplex Stainless Steels was published (11) in Issue 81 of Practical Welding Letters for May 2010. To receive at no cost each month your copy of Practical Welding Letter in your e-mail Inbox, please subscribe. Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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