Welding technology of casting block valve

The 2 × 330 MW subcritical boiler unit of an aluminum Power Generation Co., Ltd. is sg-1170 / 17.5-m722. It is a subcritical parameter, primary reheat and natural circulation drum boiler produced by Shanghai Boiler Plant of Shanghai Electric Co., Ltd. In 2018, magnetic particle testing was carried out on the valve body of the cold section of Reheater of No.1 boiler. It was found that there were three surface cracks within 90 mm of the shoulder of the right valve body, which were 7 mm, 17 mm and 27 mm respectively, and the maximum depth of excavation was 22 mm (as shown in Figure 1). The plug valve is a casting, the specification is susvsy61h-64-dn600, the maximum thickness of the valve body is 500 mm, and the material is WCB.

WCB steel has certain toughness and plasticity, high strength and hardness, good machinability and good weldability.

The plug valve shell in this repair process belongs to steel casting, and its large thickness, large volume, large rigidity and other characteristics are similar to those of steam turbine steel casting. Therefore, the repair welding process can be formulated with reference to the relevant provisions in the standard DL / t753-2015. Due to the bad construction conditions, it is difficult to ensure the effect of preheating before welding and heat treatment after welding. Some data show that the application of heterogeneous cold welding process can avoid excessive dilution of base metal, ensure the high temperature oxidation resistance and mechanical properties of weld metal, and avoid welding deformation and stress problems; Therefore, combined with the field practice, the heterogeneous cold welding process is adopted for repair welding, that is, no preheating before welding, using nickel based electrode for welding and no heat treatment after welding, which can greatly reduce the difficulty of field construction and improve the success rate of repair welding.

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