Q: Hello, everyone! Ask a question, about the duplex stainless steel welding, the medium side should be welded first. The opposite of austenitic stainless steel welding. Where is this rule? I've seen it, but I forgot where.

 Answer: The welding order of austenitic steel and dual phase steel is just opposite. The mechanism is different, austenitic steel tends to avoid multiple heating, and double phase steel tends to control the phase ratio.

Abstract: Welding article. Back water battle. 

Text: first of all about " about the duplex stainless steel welding, the medium side should be welded first. The standard terms are the opposite of austenitic stainless steel welding " are as follows:

Screenshot from GB 50236-2011 Construction Code for Site Equipment and Industrial Pipeline Welding Engineering

Screenshot from the "GB 50236-2011 Site Equipment, Industrial Pipeline Welding Engineering Construction Code" article description

In short: in the first step, we set one side near the medium as an A and the other side as A B. Next, the mechanism is described. The welding solidification process of double-phase steel is FA mode, that is, the solidification mode of ferrite-austenite, which in the high temperature state first forms the ferrite, and then partially transforms into austenite. Under the condition of first welding A side "may" A iron body content is higher (please note, is possible, not inevitable, this is related to layer temperature, cooling, and so on), and after welding B side welding thermal process, A side metal in theory can further phase transition, which is part of the solid phase transition A side joint iron body into austenite, which adjust the proportion of A side. Finally, let's take a very, very aside example, of course, this is just to do for good understanding, without any directivity, do not extend, such as, the proportion of high consciousness crowd VS ordinary people,, etc.: 100 high school students in science and science. During the oral declaration period, 30 applied for liberal arts and 70 applied for science, that is, 30% -70% of liberal arts and science classes (proportion). In this way, after the mobilization of the academic affairs office and the mobilization meeting of the female students in the liberal arts class, 20 male students who originally planned to apply for science turned to the liberal arts, that is, the liberal arts class (proportion) 50% -50%. Let me add that the biggest use of austenitic stainless steel is corrosion resistance, which is the most basic characteristics of austenitic stainless steel. However, stainless steel corrosion resistance is conditional, that is, 12.5% of the free Cr exists in the matrix, if the production of Cr compounds, stainless steel is not corrosion resistant. In addition, if the compounds producing Cr are relatively brittle phase, easy to embrittlement, not only corrosion resistant but also very brittle, easy to crack to form micro cracks, which is often said 475℃ embrittlement, Segma phase embrittlement, etc. Now see why austenitic weld why in contact medium after welding, if welding first, subsequent welding high temperature will promote the formation of Cr compound, and because the number of channels and layers after welding more, make the contact medium weld in 400~700 or even higher temperature interval stay time longer, easier to form Cr compounds and brittle phase, so adverse to corrosion resistance. Austenitic stainless steel has good corrosion resistance, but low strength, therefore, with duplex tissue, namely austenite + ferrite (high temperature δ ferrite), not only corrosion resistance, there is also high strength, the general ratio of each phase in 40-60, corrosion resistance and strength can achieve a better combination. At this time, if the relationship of welding makes the proportion imbalance, it is not good. As for welding first or after welding, its basic principle and austenitic stainless steel is similar, the specific practice can refer to the teacher's long text. Further description of the theory: in fact, whether the first generation of double phase stainless steel or the third generation of double phase stainless steel, it is all complete ferrite tissue from the beginning of the liquid phase solidification, the tissue retained to the temperature of the ferrite dissolution curve after the temperature with the further decline in temperature