Study On Microstructures & Properties Of Weld Coarse-grained HAZ Of X100 Pipeline Steels

In this paper, the microstructures and properties of weld heat-affected zone(WI IAZ) of XI00 pipeline steel in the welding process were studied by welding thermal simulation techniques, mechanical properties test and microstructure analysis, and the weakest area in WHAZ on toughness was found. Microstructure and properties of Weld Coarse-Grained HAZ(WCGHAZ) in XI00 Pipeline Steel were discussed under different heat input. In order to make the toughness of WCGHAZ to the best, suitable heat input range was put forward‘ The microstructure and properties of XI00 pipeline steel WCGHAZ heated in diflerent second peak temperature were studied and analysed.To XI00 pipeline steel weld, the toughness and strength are well at peak temperature 650°C,950°C and 1100?C because of the tiny Ferrite(F) inside grain. Lots of Polygonal Ferrite(PF) and massive M-A between grains appeared in the Weld-Intcr-Critical HAZ(WICHAZ) lead to embrittlement of WICHAZ. When peak temperature reach to 1300°C which leading to serious embrittlement of WCGHAZ, the main reason of embrittlement is the large size dendrite and large size M-A between grains ? That is the most weak area in the weld which has a obvious tear edges of the fracture and the worse toughness.The toughness of XI00 WCGHAZ gradually decreased with the increase of the welding heat input. When the welding heat input between 10 KJ/cm and 25 KJ/cm? organization of WCGHAZ mainly is Bainitic Ferrtie(BF) with fine lath structure and AF which precipitates in many positions. So it is a ductile fracture toughness is well. When more than 30 KJ/cm, PF and QF(Quasi-Poiygonal Ferrtie) in the organization of XI00 WCGHAZ increased and grain coarsening seriously. Toughness of WCGHAZ is seriously impacted. Based on this analysis, recommended range of heat input for XI00 pipeline steel changes between 10 and 20KJ/cm.The toughness is best when second peak temperature reach to 1000°C during the both-side welding or multi-pass welding. In two phase region(a+7) to the second process of welding thermal cycle, the partial recrystallization and weld genetic phenomenon are not conducive to the improvement of toughness,the main reason of embrittlement is the large size carbon-rich M-A which is distribued in the boundary of the branch crystal grain unevenly.