Abstract: Precipitation behavior of multipass welding metal in X70 pipe-line steel was investigated by Gleeble 1500,SEM and TEM were carried out to analysis the amount,size and morph of various microstructures,especially AF,in welding metal after suffering from secondary heat recycling.The results show that there exit little amount of the grain boundary ferrite and the grain boundary ferrite while the acicular ferrite are predominant in weld metal.The sympathetic AF nucleation nucleated on the primary AF and grown into specific size rapidly,both the primary AF and the sympathetic nucleation AF nucleated and grown in the austenite grain,they play an important role of refining the austenite grain which could improve the toughness of multipass welding metal and HAZ.