1. Overview
SA213-T91 steel is a type of martensitic high-alloy steel known for its excellent thermal strength, toughness, weldability, high thermal conductivity, low linear expansion coefficient, and good corrosion resistance. It is widely used in China’s thermal power generation systems, particularly in boilers for medium-temperature superheaters, high-temperature superheaters, and high-temperature reheaters in units with a capacity of 300MW or more. Despite the maturity of its welding process in large-scale, high-parameter thermal power plants, T91 steel is prone to oxidation and burning at the root weld during welding. Therefore, argon back-purging is essential to ensure quality and prevent defects.
The root pass is critical in T91 welding, and the success of the entire weld largely depends on the effectiveness of the argon filling. Different argon-filling methods can significantly impact welding costs, quality, and project progress. This article discusses various argon-filling techniques used in field construction for T91 header joint welding. One innovative approach involves using argon-filled gas in adjacent headers instead of purging the entire header, which simplifies the welding process, improves efficiency, and reduces costs. In 2008, this method was successfully applied to all 1,072 T91 header welds in the second phase of the Pingyi Power Plant's 2×300MW unit. The welded joints met all required welding procedure qualification standards. This technique has since been adopted in several large-scale power plants, including Bocheng, Wuxiang, and Tashan Power Plants.
2. Argon Filling Methods
For the socket weld of header pipes, the key to preparing the argon-filled area lies in sealing the pipe socket, as the filling depth is limited to 80mm. To prevent burning during preheating, four common argon-filling methods are used. Let’s take the junction welds of the first and second sections of the high-temperature superheater from the 2×300MW unit in the second phase of Pingyi Power Plant as an example. The layout of the tubes is shown in Figure 1.
(1) All headers and their outlet caps are sealed, and the other side of the headers is blocked with soluble paper (or water-soluble toilet paper) placed 250–300mm away from the groove. During welding, the entire header is directly filled with argon from the header side. Before welding, the cap at the socket is removed to start the process.
(2) All non-welded joints (such as the first-stage welds) and the outlet caps on both sides of the header are covered. When welding the joint, the base is sealed with soluble paper (or water-soluble toilet paper), and the entire tube is filled with argon through the corresponding short tube (the second-stage weld).
(3) Argon is introduced directly into the groove, and the soluble paper (or water-soluble toilet paper) on both sides of the welded header and the short tube is folded into a conical shape to create a closed air chamber. The base of the cone should be about 10mm larger than the inner diameter of the steel pipe. The paper is inserted into the pipe using a round bar of 250–300mm. Two layers of paper are used to prevent burning during preheating, which could compromise the root protection.
The small tube connected to the argon gas line can be made by repurposing discarded thermocouples. The tip can be flattened and inserted into the weld. A thin needle or copper tube can be used to flatten the thermocouple bend. Using discarded thermocouples not only saves cost but also promotes resource recycling. These bends are typically custom-made with two bends, as shown in Figure 2 along with Figure 1.
For more detailed information, please refer to the attached documents.
Fixed Ball
Fixed Ball,Hard Sealed Ball,Hard Sealed Sphere,Hard Sealed Fixed Ball
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