In order to study the microstructure properties of different regions of welding heat affected zone(HAZ)of 1 000 MPa grade ultra-high strength steel

the samples of test steel at different peak temperatures of thermal cycle were prepared by welding thermal simulation technique

and the impact toughness of different regions of HAZ was studied through Charpy impact tests. The results showed that

in the subcritical region of HAZ(SCHAZ)

the intercritical region of HAZ(ICHAZ) and the fine-grained region of HAZ(FGHAZ)

the samples had relatively high impact absorption energy

crack propagation energy and dynamic impact toughness

and a large area of fiber region and shear lip formed on the fracture surface. Toughness dimples of different sizes can be seen at the microscopic level. The samples had good impact toughness. In the coarse-grained region of HAZ(CGHAZ)

all impact data of the samples sharply decreased

and the fracture showed a macroscopic brittle fracture

almost all of which are radiological regions. At the microscopic level

it showed quasi cleavage fracture characteristics

indicating that the resistance to crack propagation decreased

and the time for stable propagation decreased after crack initiation

and the instability propagation was fast. The impact toughness of the samples deteriorated

and the CGHAZ region was a ductile valley region in HAZ. The results showed that the coarse grains and the coarse M-A island were the main causes of embrittlement in the CGHAZ region. The conclusion lays a theoretical foundation for the selection

development and engineering application of 1 000 MPa grade ultra-high strength steel in hydropower projects.