Study On Microstructure And Properties Of Low Temperature X80 Pipeline Steel

The China-Russia eastern natural gas pipeline is the first large-diameter natural gas pipeline with an outer diameter of 1422 mm in China and it passes through the cold areas in northe eastern China.To avoid low-temperature brittle fracture of the pipeline,the material needs to have high strength and sufficient anti-brittle cracking ability and crack propagation resistance.In this paper,the two kinds of low-temperature X80 pipeline steels with specifications of ?1422mm × 32.1mm were studied.The Optical microscope and scan electron microscope(SEM)were used for microstructure observation.and the mechanical properties of the test steel were studied by low temperature tensile test,hardness test,Charpy impact test,drop weight tear test(DWTT)and three-point bending test.The results of the study are as follows:(1)The microstructure of low-temperature X80 pipeline steels mainly composed of granular bainite(GB)and granular bainite(GB)+polygonal ferrite(PF).Through the test of the low temperature tensile properties of the test steel,the effect of the structure on the mechanical properties in the thickness direction of the test steel was studied.The study found that from the surface to the center of the wall thickness,the content of GB and M/A islands increased,and the grain size get bigger.As the test temperature decreases,the tensile strength and yield strength at different areas of the wall thickness increase,and the change in elongation after breaking is small;the tensile strength and yield strength of the specimen at the outer surface of the test steel are relatively high.(2)In the Charpy impact test,the fracture shear area of the sample is 100%.The Charpy impact energy decreases with the decrease of the test temperature,and it remains above 380 J at-60 ?;in the three-point bending test,the crack The CTOD value of the crack tip opening displacement showed a trend of decreasing first and then increasing with decreasing temperature,but the fluctuation range was small and the fracture toughness of the material was high.(3)In the drop weight tear test,when the crack propagates to acicular ferrite,GB and small M/A islands,the crack propagation trajectory was more tortuous,which reduces the crack propagation energy and can hinder the crack propagation to a certain extent.With the test temperature decreases,the fracture of the DWTT specimen breaks,and the coordinated deformation and restraint effect of the ferrite on the bainite of the lath is weakened,thereby reducing the toughness of the material.The fracture shear area percentage of the DWTT thinned specimen was greater than the full-wall thickness specimen,and the fracture shear area percentage of the single-sided thinned specimen was greater than the double-sided thinned specimen;as the thickness of the DWTT specimen increases,the fracture toughness was decreases.