Study On Low Temperature Toughness And Fracture Separation Of Heavy Thick Multi-phase Microstructure X80 Pipeline Steels

The demand of natural gas in the domestic has been increasing year by year,natural gas pipelines are beginning to develop towards ultra-large volumes Increasing pipe diameter and thick wall is an effective way to improve conveying capacity.As the pipe diameter and wall thickness increase,the design coefficient and the conveying pressure become higher and higher,which puts higher requirements on the low-temperature fracture toughness of pipeline steel.The Drop Weight Tear Test(DWTT)is used to study the crack arrest performance of thick steel plate.Fracture separation usually occurs on the fracture surface,which reduces the low temperature toughness of steel plate.In this paper,heavy thick multi-phase microstructure X80 pipeline steel with 25.7 mm thickness and 32.1 mm thickness were studied.Microstructures of X80 pipeline steel and separation cracks of DWTT specimens were studied by optical microscopy(OM)and scan electron microscopy(SEM)in order to establish the relationship between the fracture behavior and microstructures of X80 pipeline steel.The research results are as follows:Microstructures and fracture morphology of heavy thick multi-phase microstructure X80 pipeline steel with 25.7 mm thickness were observed in order to analysis the relationship between fracture behaviors and microstructures.The results showed that microstructures of X80 pipeline steel were composed of granular bainite(GB),polygonal-like ferrite(PF),quasi-polygonal ferrite(QPF),bainite ferrite(BF),and acicular ferrite(AF).The inclusions could cause the generation of voids and the secondary cracks.The secondary cracks nucleated inside microstructure,and arrested on the small martensite-austenite(MA)constituent and AF boundaries,thereby improves the fracture toughness effectively.The separation cracks are easily generated in the band structure.The separation cracks of DWTT specimens with 19.0 mm reduced thickness were observed to analyze the relationship between microstructures and cracks.The results showed the band structure of GB worked as the initiation sites of separationcracks.Separation cracks passed through GB in transgranular fracture,the crack path were close to straight,which indicates that GB has less effect on hindering the crack propagation.While separation cracks propagated along the grain boundaries of PF in intergranular fracture,the directions of separation cracks propagation were constantly deflected,thereby increaing the crack propagation energy and the fracture toughness of the plate.The small MA constituent can passivate the crack tip,and retard the crack propagation.A small amount of GB and an appropriate amount of PF are beneficial to reduce the generation of separation cracks in multi-phase microstructure steel.The separation cracks of DWTT specimens with 32.1 mm full thickness were observed and the mechanical properties were tested to study.The results showed the inverse cleavage fracture area of the DWTT specimens of 32.1 mm multi-phase structure X80 pipeline steel is larger,and the number of separation cracks is smaller.The tissue near the separation crack on the side of the large shear lip was greatly deformed,while the other side was hardly deformed.For 32.1 mm thickness heavy thick multi-phase microstructure,separation cracks extend along small-sized PF grain boundaries,but straight through large-size PF interiors.The separation cracks expend straight through GB,and are arrested on the small MA constituent.