Cracking Reasons of X70 Steel Submerged Arc Weld Pipe during Guide-Bend Test

By the methods of chemical composition analysis,mechanical properties test,microstructure and crack morphology observation,the cracking reasons of X70 steel submerged arc weld pipe during guide-bend test were analyzed.The results show that the heat affected zone(HAZ) of cracked steel pipe erxhibited obvious softening,and the plastic deformation concentrated on softened HAZ during bend test.Superfluous plastic deformation caused the stress concentration on the interface between coarse M/A island in coarse grain zone and lathy ferrite,when the stress exceeded the tensile strength of the material,bending cracking happened.It was suggested that the steel pipe plant should establish corresponding welding process according to the chemical composition,microstructure and mechanical properties of the steel sheets.