The microstructure and properties of the heat affected coarse grain zone of steel 07MnCrMoVR with post¬weld heat treated at 460 ~ 660 °C for 2 h are analyzed by means of optical microscope

scanning microscope

transmission electron microscope and impact toughness tester. The results show that with increase of post-weld heat treatment temperature

the - 20 °C impact toughness of post-weld heat affected coarse-grained zone decreases firstly and then increases. The steel with post-weld heat treatment between 580 °C. to 620 ℃ can occur reheat cracks tend

the impact specimen is brittle fracture

cleavage fracture and microcracks along grain boundary

the main reason is that the carbide precipitates along the grain boundary and grows up to weaken binding energy of the grain boundary

which leads to the decrease of the impact toughness at low temperature. With post-weld heat treated ≥620°C

recrystallization occurs in 07MnCrMoVR steel

dislocation disappears

and the low temperature impact toughness of the steel increases again due to the combined growth of ferrite grains. The optimum post-weld heat treatment of steel is 460 ~ 500°C X 2 h.