The welding of the main pipeline of VVER-1000 type nuclear power plant is a key milestone in the installation of the nuclear island. The main pipeline materials are made of composite steels 10ГН2МФА and 15Х2НМФА-А (vessel nozzles) with specifications of

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990 mm×70 mm and

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1000 mm×75 mm. The materials belong to pearlite heat-resistant steel

and the welding and heat treatment processes are complex

making the construction challenging. The original "sequential construction" plan had issues such as a long duration (about 12 months)

harsh working environment

and difficulties in controlling cold cracks and shrinkage deformation. By analyzing the weldability of the base metal

it was determined that cold cracks were the primary prevention target

. Parameters such as preheating at 200 ℃

post-heating at 200-250 ℃ (holding for 12 h)

and stress-relieving heat treatment at 620±20 ℃ (holding for 6.5 h) were established. The welding materials (selecting СВ-08Г2С and other 4 types)

welding methods (argon-electric combined technology)

and groove design (double V-shaped) were optimized. The feasibility was verified through four process evaluations. Innovatively

the welding sequence was changed to "parallel method + lap method"

combined with deformation monitoring and non-destructive testing. Subsequent applications showed that this process resulted in a primary qualification rate of over 96.8% for the main pipeline welding

shortened the construction period by about 2 months

improved the working environment and safety

and it is the world's first for VVER-type reactors (patent application has been filed). It has guiding and promoting value for similar units and other reactor main pipeline weldings.