Study On The Microstructure And Hydrogen-Induced Cracking Of Welding Joints Of Pipeline Steel

Microstructure and hardness of the domestic X70 pipeline steels and its' weldingjoints as well as the resistance of Hydrogen-Induced Cracking (HIC) were carried out inthis paper. Two different welding processes were used in the girth welding of the domestic X70pipeline steel. One was welded with BOHLER E6010 for root pass and HOBART 81N1self-shielded flux-cored wire for filler passes and map pass. The other was welded withJM-58 wire for root pass and JM-68 wire for filler passes and map pass. The analysis ofmicrostructure and hardness show that the microstructure of the latter is finer than theformer. The hardness values of the entire welding joints meet the requirement except thelatter's map pass. The resistance of HIC of the domestic X70 pipeline steels and its' welding joints werecarried out through HIC standard test method. The results suggest that acicular-ferritehas excellent HIC resistance. Acicular-ferrite is made up of lath ferrite which caninhabit the expansion of crack. In addition, the interaction of precipitated phases anddislocations in acicular-ferrite can supply many sites for H ,which reduce local pressure .Therefore acicular-ferrite steel has excellent HIC resistance. The results of differentposition of X70 strip sheet and different welding joints show that HIC occur in bandedstructure. The influence of banded microstructure and non-metallic inclusion were alsodiscussed in this paper. HIC test for four X60 pipeline steels indicate that if bandedmicrostructure and non-metallic inclusion were not formed pipeline steel would haveexcellent HIC resistance even Mn content is high. Globular non-metallic inclusion canreduce stress concentration and provide nucleation site for acicular-ferrite, consequentlyimproving the HIC resistance.