SA213-T91 steel belongs to martensite high-alloy steel. Due to its good thermal strength, good toughness, good weldability, high thermal conductivity, small coefficient of linear expansion and good corrosion resistance, it is currently China's thermal power generation. SA213-T91 steel is widely used in boilers such as boiler medium temperature superheaters, high temperature superheaters and high temperature reheaters for 300MW and above units. Although its welding process has become more mature in the utilization of high-efficiency, high-parameter, large-capacity thermal power units, the root weld is easily oxidized and burned when T91 steel is welded, so it is necessary to use argon-filled back protection.
The welding of the bottom layer of T91 is the key, and the quality of the bottoming depends largely on the quality of the argon filling. Therefore, the difference of the argon filling method seriously affects the welding cost, quality and progress of the construction on site. This article focuses on some different methods of argon filling for the junction box weld of T91 in the field construction. The use of argon-filled argon in the adjacent headers of the header replaces the traditional argon filling of the entire header, which not only simplifies the welding process, improves the welding schedule of the on-site construction, but also saves a lot of cost and improves the economy. benefit. In 2008, all the 1072 T91 header welds of the 2Ã—300MW3# unit of the Pingyi Power Plant Phase II project successfully used this argon-filling method, and the welded joint performance can fully meet the welding procedure qualification requirements. This argon-filling method has also been successfully used in many large-scale units (such as Bocheng Power Plant, Wuxiang Power Plant, and Tashan Power Plant).
2. Argon charging method
For the weld of the header pipe socket, the key to the preparation of the argon-filled zone is the plugging of the pipe socket, because the filling depth can only be 80 mm. In order to prevent the end from burning during preheating, there are the following four methods of filling the argon: Take the junction welds of the first and second sections of the high temperature superheater of the 2Ã—300MW3# unit of the second phase of Pingyi Power Plant as an example. The tube row is shown in Figure 1.
(1) Put all the headers of the header and the outlets of the headers with the caps, and block the other side of the headers with soluble paper (or good water-soluble toilet paper), 250~300mm from the groove. When welding, the entire header can be filled with argon directly from the side of the header. Before welding, the cap at the welded socket is pulled out to start welding.
(2) Cover all non-welded welds (refer to the first-stage welds) and the outlet caps on both sides of the header. When welding a joint weld, the bases are made of soluble paper (or good water solubility). The toilet paper is blocked, and the whole tube is filled with argon at the corresponding short tube (the second stage welding joint).
(3) Filling the venting argon from the groove, and using the soluble paper (or good water-soluble toilet paper) on the side of the welded header and the other side of the short tube to be folded into a conical shape to form a closed air chamber, the bottom surface of the cone The diameter is about 10mm larger than the inner diameter of the steel pipe. The soluble paper is fed into the pipe with a round bar of 250~300mm. The soluble paper should be installed with two layers to prevent the soluble paper from burning during the preheating process, resulting in poor root protection. .
The small tube connected to the argon gas belt can be made by short-circuiting the discarded thermocouple, and the head can be flattened into the weld. The head can be flattened with a thin needle or a thin copper tube, but the discarded thermocouple can also save a certain amount. the cost of. Flat thermocouple bends are customarily used in two bends, as shown in Figure 2 in addition to Figure 1.
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