desired, consider Type 312. As a cautionary note, austenitic stainless steels exhibit a rate of expansion that is about 50% greater than that of carbon steel. When joined, the different rates of expansion can cause cracking due to internal stresses unless the proper electrode and welding procedure are used — Fig. 3. What are proper weld preparation cleaning procedures? As with other metals, first remove oil, grease, markings, and dirt with a nonchlorinated solvent. After that, the primary rule of stainless weld preparation is “avoid contamination from carbon steel to prevent corrosion.” Some companies use separate buildings for their “stainless shop” and “carbon shop” to prevent cross contamination. Designate grinding wheels and stainless brushes as “stainless only” when preparing edges for welding. Some procedures call for cleaning 2 in. back from the joint. Joint preparation is also more critical, as compensating for inconsistencies with electrode manipulation is harder than with carbon steel. What is the proper postweld cleaning procedure, or why does my stainless weld rust? To start, remember what makes a stainless steel stainless: the reaction of chromium with oxygen to form a protective layer of chromium oxide on the surface of the material. Stainless rusts because of carbide precipitation (see next to last question) and because the welding process heats the weld metal to the point where ferritic oxide can form on the surface of the weld. Left in the as-welded condition, a perfectly sound weld might show “wagon tracks” of rust at the boundaries of the heat-affected zone in less than 24 hours. So that a new layer of pure chromium oxide can properly reform, stainless steel requires postweld cleaning by polishing, pickling, grinding, or brushing. Again, use grinders and brushes dedicated to the task. Why is my stainless steel welding wire magnetic? Fully austenitic stainless steel is nonmagnetic. However, welding temperatures create a relatively large grain in the microstructure, which results in the weld being crack sensitive. To mitigate sensitivity to hot cracking, electrode manufacturers add alloying elements, including ferrite — Fig. 4. The ferrite phase causes the austenitic grains to be much finer, so the weld becomes more crack resistant. A magnet will not stick to a spool of austenitic stainless filler metal, but a person holding a magnet might feel a slight pull because of the retained ferrite. Unfortunately, this causes some users to think their product has been mislabeled or they are using the wrong filler metal (especially if they tore the label off the wire basket). The correct amount of ferrite in an electrode depends on the service temperature of the application. For example, too much ferrite causes the weld to lose its toughness at low temperatures. Thus, Type 308 filler for an LNG piping application has a ferrite number between 3 and 6, compared to a ferrite number of 8 for standard Type 308 filler. In short, filler metals may seem similar at first, but small differences in composition are important. Is there an easy way to weld duplex stainless steels? Typically, duplex stainless steels have a microstructure consisting of approximately 50% ferrite and 50% austenite. In simple terms, the ferrite provides high strength and some resistance to stress corrosion cracking while the austenite provides good toughness. The two phases in combination give the duplex steels their attractive properties — Fig. 5. A wide range of duplex stainless steels is available, with the most common being Type 2205; it contains 22% chromium, 5% nickel, 3% molybdenum, and 0.15% nitrogen. When welding duplex stainless steel, problems could arise if the weld metal has too much ferrite (the heat from the arc causes the atoms to arrange themselves in a ferrite matrix). To compensate, filler metals need to promote the austenitic structure with higher alloy content, typically 2 to 4% more nickel than in the base metal. For example, flux-cored wire for welding Type 2205 may have 8.85% nickel. Desired ferrite content can range from 25 to 55% after welding (but can be higher). Note that the cooling rate must be slow enough to allow the JULY 2016 / WELDING JOURNAL 35 Fig. 5 — Duplex steels combine the best properties of austenite and ferrite. This micrograph shows a duplex weld with austenite (light color) in a ferrite matrix. Fig. 6 — Intergranular corrosion takes place in the heat-affected zone on the inside of a tank storing corrosive media. Using low-carbon and specially alloyed electrodes can mitigate the risk of carbide precipitation and the resulting corrosion.
Welding Journal | July 2016
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