The simulation on circulation process of heat of low temperature steel 9Ni (/% : 0.049C

0.22Si

0.58Mn

0.0006S

0.0029P

9.16Ni

0.020Al

0.003Ti

0.0030N

0.0006O) with different cooling time (6-100 s) from 800 °C cooling to 500 ℃(t

8/5

) has been carried out by using Gleeble-3500 simulator

and combined with metallography the simulated heat affected zone continuous cooling transformation (SHCCT) curves of tested steel are established. The structure in tested specimen with different cooling time of t

8/5

is observed by optical and scanning electron microscope to study the effect of cooling time on original austenite grain size

M-A group area

size

number and shape factor (rati

o of massive M-A). Results show that the structure of simulated heat affected zone of continuous cooling specimen consists of bainite (B) and martensite (M)

the relative content of M depends on cooling time of t

8/5

with increasing cooling time from 800 °C to 500℃ (t

8/5

) the original austenite grain size

particle bainite content and M-A group size increase while the M-A group content and ratio of massive M-A decrease

it is available to improve the low temperature toughness of coarse grain heat affected zone of steel 9Ni.