Impact toughness of the joint is an important factor to the comprehensive mechanical property evaluation for the main piping. During welding procedure qualification(WPQ) for VVER-1000 main piping in the supervision process

it found that some partial impact toughness value(-10 ℃) of the filling and cosmetic butt-welding filler metal samples which located at horizontal fixed round welding vertically up area could not meet the standard requirement. Combining with the influence factors such as welding position

welding procedure parameter and electrode chemical composition

and according to the results of the impact toughness test

microscopic examination

hardness test and fracture appearance observation

the reasons for the low impact toughness of the welding filler metal samples could be attributed to the coarse microstructure caused by lower welding speed and higher welding heat input at the vertically up welding position. The higher Si content in the welding rod and the lower impact value of welding rod were also the major reasons. After small welding heat input was adopted during the PQR process

the impact toughness of the filler metal of the obtained joints all met the standard. On the basis of the above analysis

some preventive measures were put forward

such as strictly controlling the Si content in the welding rod during purchase

strengthening the skill training for welding personnels

strictly controlling the energy input of the welding line

and strictly controlling the interlayer temperature of vertical welding area near the lower limit value to ensure the cooling rate of weld metal after welding is accelerated.