Study on the Fatigue Damage Behavior of X80 Pipeline Steel in Hydrogen Environment

Author:An Teng

Supervisor:zheng shu qi


Degree Year:2018





With the development of the domestic economy,the domestic natural gas consumer market will continue to expand.Natural coal gas,which has an increasing significance to alleviate the shortage of domestic natural gas and protect the safety of China’s energy resources,will be used as substitute and supplement for liquefied petroleum gas and natural gas.Natural coal gases(H2,CO,and CO2)must be transported safely and efficiently through gas pipelines.X80 pipeline steels exposed to gaseous hydrogen may exhibit an increased susceptibility to hydrogen-assisted material degradation,going so far as to cause cracks in the pipeline.Therefore,it is highly important to study the fatigue properties of X80 pipeline steels in hydrogen environment.This work takes X80 pipeline steel as the research object to investigates the effects of hydrogen on the fatigue peoperties of X80 pipeline steel which is used to transport coal gas.The tensile tests,fatigue crack growth(FCG),fracture toughness and low-cycle fatigue properties of X80 base metal(BM),heat affected zone(HAZ)and weld metal(WM)in hydrogen atmosphere were determined to investigate the variation of hydrogen pressure,strain rate,stress concentration and their influence on fatigue properties.Finally,hardened layer applied by shot peening as protection against hydrogen embrittlement of X80 pipeline steel were studied.Fatigue properties of X80 pipeline steel may be damaged in hydrogen environments.The low-cycle fatigue life of notched specimens decreased quickly in the low-pressure hydrogen environment and decreased with the increase in hydrogen pressure.Crack initiation occurred at about 3000 cycles in 0.6 MPa hydrogen atmosphere when the fatigue tests were performed at the maximum load of 20 kN.X80steels showed no noticeable deterioration in fatigue properties with and without hydrogen before crack initiated.Fatigue properties were severely damaged in hydrogen atmosphere when cracks propagated slowly,but not in nitrogen.Hydrogen increased the crack growth rate,which was dependent on the hydrogen pressure.Hydrogen accelerated crack growth dominated the reduction of fatigue life below a hydrogen partial pressure of 0.6 MPa.Crack initiation played a more important role in the reduction of fatigue life with increasing hydrogen partial pressure.The purpose is to investigate the synergistic action of hydrogen and stress concentration on the fatigue properties of X80 pipeline steel.The gaseous hydrogen at a pressure of 0.6 MPa did not have a significant effect on the tensile strength,irrespective of the presence of a notch in the specimens.The ductility of the X80 pipeline steel significantly degraded owing to the gaseous hydrogen.The stress concentration accelerated the hydrogen induced-ductility loss.Stress concentrations can result in the severe deterioration of the ductility and fatigue properties of such materials.The degree of HE increased as the Kt value increased,and was at its highest level at a Kt value of2.1.The stress concentration governs the reduction of fatigue life,and hydrogen enhances this reduction.The influence of hydrogen on the HAZ and WM of X80 pipeline steels are investigated.The results show that hydrogen increases fatigue crack growth rate of HAZ and WM.The fracture toughness of WM decreased by 38.55%under the 0.6 MPa hydrogen pressure.The fracture toughness of WM which contain cold cracks decreased by 51.63%due to hydrogen and the fracture mode changing from ductile fracture to brittle fracture when the main crack extended to the cold crack.The slag inclusion(Mn3TiSi)governed the reductions of fracture toughness of WM,and hydrogen enhanced this reduction,the fracture toughness decreased by 76.52%.The HE susceptibility of HAZ is higher than base metal and WM,and increases with increasing current density and decreasing strain rate.Cracks in the surface of HAZ were step-crack,the crack propagation path showed that the five or more cracks may join together to make long cracks.The influence of shot peening treatment on the HE susceptibility of X80 pipeline steel was investigated.The results show that the behaviors of resistance to HE of X80pipeline steels are improved when the specimens are shot-peened for 30 s.However,the resistance to HE of X80 pipeline steels decrease with the increasing shot peening time.The hydrogen diffusion coefficient of specimens increases after shot peening and decreases gradually with increasing shot peening time.Shot peening treatment improves the fatigue life of X80 pipeline steels by inhibiting crack initiation,the fatigue life of notched specimens increased 1.7 times in nitrogen and 1.4 times in hydrogen.Hydrogen accelerated crack growth dominated the reduction of fatigue life of the shot-peened specimens.