Abstract：The morphology of bainite and toughness was investigated in CGHAZ of the high Nb X80 pipeline steel after simulated weld thermal cycle. The result show that the microstructure formed after thermal cycle was mainly lath bainite and granular bainite in CGHAZ .When the cooling time is 10 seconds, impact toughness is highest for appropriate granular bainite which make the lath bainite finer for the function of partition，and fine martensite-austenite constituent (MA ) .The toughness will decrease whether the value of cooling time is lower or higher .The reason is that the percent of coarse lath bainite increase and MA constituent is strip for cooling time of less than 10s and the percent of granular bainite increase sharply for cooling time more than 10s.
2011, Vol. 46 
