1 Overview
The function of the steam generator in the primary circuit of the nuclear power plant is to transfer the heat of the heat carrier in the reactor to the secondary circuit, so that the steam of the second loop side medium generates a certain pressure, and the steam is driven by the first and second steam separators. The primary side of the steam generator is connected to the reactor pressure vessel. The heat carrier is radioactive, and the dry steam generated on the secondary circuit side directly affects the power and efficiency of the power station. Therefore, the equipment is the most critical in the nuclear power plant. One of the devices. The utility model mainly comprises a lower head, a tube plate, a high middle and lower cylinder assembly, a transition cone body, a sleeve assembly, a tube bundle assembly, an upper head, an upper barrel tube assembly and a steam separator. Its main material is low alloy steel 20MND5.
2. Introduction to 20MND5 forgings
20MND5 is a special material for nuclear island main pressure equipment, and belongs to Mn-Ni-Mo low alloy high strength steel. Due to the action of elements such as Mn, Ni, Mo in steel, such steels have good hardenability, high temperature performance, low temperature temper brittleness and low ductile transition temperature. Through the control and heat treatment of the manufacturing process, it is possible to obtain better low temperature and high temperature mechanical properties required for engineering, and to manufacture ultra-large and ultra-thick components. The forging is used for both the casing and the head of the steam generator in the nuclear island of the nuclear power plant. Due to the special nature of nuclear power plants, safety requirements are of the utmost importance, so the requirements for welding these materials are extremely strict. The chemical composition and mechanical properties of the forgings required by RCCM standards and technical conditions are shown in Tables 1 and 2, respectively.
Table 1 Chemical composition (mass fraction) of 20MND5 forgings (%)
Table 2 Mechanical properties of 20MND5 forgings
3. Welding of 20MND5 forgings
20MND5 forgings are low-alloy high-strength steels with bainite structure and high alloying elements. According to the formula recommended by the International Welding Society, Ceq=0.60%, indicating that the steel has a higher tendency to harden and cold crack. The addition of Mn, Ni, and Mo alloy elements increases the hardenability, but also increases the difficulty in soldering. For example, the cooling rate after soldering is too fast, and cold cracks are likely to occur. In particular, the steam generator belongs to a thick-walled container. When the thick plate is welded, the total amount of heat input is large, and after welding, a large welding stress is formed, which increases the possibility of forming cold cracks. Therefore, strict welding measures are taken during welding. Ensure the quality of the weld, such as cleaning the groove, controlling the heat input of the weld, preheating before welding, and heat treatment after welding.
(1) Welding method The steam generator cylinder and the head are both forged, and the maximum thickness is 131mm. For the main annular seam, due to the large wall thickness and large welding workload, in order to reduce the filling amount of the welding material, a narrow gap submerged arc welding method is proposed. Narrow-gap submerged arc welding is an advanced technology in the field of thick plate welding. Compared with submerged arc welding of ordinary groove, the narrow gap welding groove is narrow and the weld metal filling amount is small, which can save a lot of welding consumables and welding man-hours. At the same time, due to the relatively low heat input during the narrow gap welding, the microstructure of the weld metal and the heat affected zone is remarkably refined, thereby improving its mechanical properties, especially plasticity and toughness. However, it is very difficult to achieve high quality welding in the narrow bevel of the base metal. In addition to the precise preparation of the welding groove, a series of problems must be solved from welding methods, welding tracking, and process measures. Due to the narrow gap between the grooves, it is difficult to repair after the welding, so it is necessary to strengthen the control of the welding process to ensure that the welding pass rate is 100%.
(2) The welding groove is selected from the usual narrow gap groove, and the outer U-shaped inner V-shaped groove is generally selected. The main ring slit of the steam generator is selected to be single-sided U-shaped. Considering that the groove width is too narrow, it is inconvenient to operate and easy to produce defects; if it is too wide, the welding filling amount is greatly increased, so that the advantages of the narrow gap welding cannot be fully exerted, and the width of the bottom of the groove is set to 16~17mm, both sides There is a 3~4mm arc transition; the groove angle is 0°~1°, which is smaller than the conventional groove angle, as shown in Figure 1.
(a) welding indication
(b) clearing the root
For more information, please refer to the fourth issue of Metalworking (Hot Processing).
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