Correlations Of Microstructural Control And Mechanical Behaviors Of Heavy Thick X80 Pipeline Steels

With the rapid development of the world economy,the demand for energy resources like oil and natural gas has been increasing year by year.The laying of pipeline is towarding to the complicated geological environment and various climate such as ocean and polar.In order to improve the efficiency of oil and natural gas transportation,the pipeline with high steel grade,thick wall and large diameter is growing.In this paper,the plate,pipe and welded joint of API X80 line pipes(OD 1422mm)used in Sino-Russian eastern routed gas pipeline project were studied.Optical microscopy and scan electronic microscopy(SEM)were used for microstructure observation.The Charpy impact test,hardness test,tensile test and drop-weight tear test(DWTT)were performed to study mechanical properties.The results of the study are as follows:In this study,drop-weight tear tests were conducted on acicular ferrite type X80 pipeline steel plates with 27.2mm thickness in order to analysis the relationship between fracture behaviors and microstructures.The result shows that the volume fraction of acicular ferrite reduced gradually in thickness direction and reached the minimum level at the center of thickness,and more volume fraction of granular bainite(GB),polygonal ferrite(PF)and quasi-polygonal ferrite(QPF)appeared.The more fraction of acicular ferrite shows in the plate,the larger share areas appear in DWTT specimens.In addition,the existence of PF,QPF and large size martensite-austenite(MA)constituent could lead to the initiation of cleavage crack,and reduce the fracture toughness of the plates.During the crack propagation process,the stress in the sample would induce the deformation around the main crack,but the hard and brittle MA constituents can hardly deform,it would cause the stress concentration and then lead to the formation of micro voids.The microstructures near the main crack deformed and elongated seriously,resulting in the growth of voids and formation of micro cracks.The secondary crack always deflected or arrested around acicular ferrite boundaries,these results demonstrate that the acicular ferrite has a good toughness,and resists the crack propagation effectively.The microstructure of multi-phase X80 pipeline steel with 25.7mm thickness and 30.8mm thickness were observed and the mechanical properties were tested to study the effect of X80 pipeline steel plate in thickness direction on the mechanical properties.It is found that the microstructure of multi-phase X80 pipeline steel is complex.Microstructures in the steel plate surface is mainly composed of PF and BF,and the microstructure is gradually transformed into multi-phase structures like PF,QPF,GB,BF and MA constituents.In addition,BF can significantly improve the hardness of steel plate.The combination of multi-phase structure makes the excellent strength and toughness of the steel plate.Drop-weight tear test found that the reason for the delamination phenomenon is correlated with the band structures elongated along the rolling direction.The more serious of banded structure of the steel plate,the number of the delaminations were larger in fracture surface.It is the multi-phase structures in the steel plate that made the process of crack propagate never straightforward,and reduced the energy for crack propagation.Several studies have been carried out to understand the relationship between microstructural characteristics and mechanical properties of base metal and welded joint of the multi-phase X80 pipeline steels.The results showed that the multi-phase structure has excellent strength and toughness,and the delamination showed at the DWTT fracture surface has both ductile fracture and brittle fracture.Under the influence of weld thermal cycle,the weld metal(WM)and heat affected zones(HAZ)still have good impact toughness.Under the influence of outside weld thermal cycle,the size and volume fraction of MA constituent at inside welding are smaller than that of outside welding.The presence of necklace-type MA constituent was found along prior austenite grain boundaries at intercritically reheated coarse grain heat affected zone(ICCGHAZ),and the hardness is obviously higher than the regions of reheated coarse grain heat affected zone.However,the width of ICCGHAZ which is too small results in the less risk of embrittlement to the whole weld joint.