Research On The Microstructure And Properties Of The Welding Heat-affected Zone Of X90 Pipeline Steel

The growing energy demand and cost control result in more and more concern of the high grade pipeline steel in the long distance pipeline engineering,since the embrittlement and softening of high grade pipeline steel heat affected zone have a major impact on the pipeline quality,and it is significant for the X90 pipeline steel application to study on microstructure and properties of X90 welding heat affected zone and determine the reasonable welding process.Based on the analysis by microstructure observation and mechanical properties of the tested X90 pipeline steel,through the Gleeble-3500 welding thermal simulation testing machine to carry out the simulation of welding heat cycle process,the X90 pipeline steel microstructure and properties characteristics with different peak temperatures(Tm)has been studied,and the same with the microstructure change and embrittlement phenomenon of X90 pipeline steel welding coarse grained heat affected zone under different heat input(E),by using OM and SEM to observe microstructure,combined with low temperature impact toughness and hardness tests to analyze the mechanical properties,moreover,further to explore the impact toughness and fracture mechanism of CGHAZ by TEM and EBSD.The results show that:(1)The tested X90 pipeline steel is composed of quasi-polygonal ferrite(QPF),acicular ferrite(AF),lath bainite(LB)and M/A,the yield strength is 725MPa and tensile strength is 790MPa,the yield ratio is 0.918,the impact absorbed energy at-20? is 307.00J,the micro-hardness value is 228.OHV,which show the excellent match of microstructure and properties.(2)When the peak temperature is 1350?,X90 pipeline steel microstructure is mainly composed of LB,granular bainite(GB)and large M/A,the grains are coarse and the prior austenite grain boundaries are visible;the QPF shows at 1100`C,the grain size becomes fine,and the prior austenite grain boundaries disappear;at 950?,the microstructure is consist of QPF,PF,a small amount of GB and M/A;the PF disappears at 850?;the microstructure at 650? is similar to the base material,showing obvious characteristics of rolling,and composed of QPF,AF,GB and M/A.With the decrease of Tm,the impact absorbed energy at-20? increases first,then decreases,and increases again,when Tm are 950? and 650?,the impact energy is higher than the base material,wherein the subcritical region(Tm=650?)with optimal impact toughness,however the impact absorbed energy is minimum at 1350?,which represents the obvious coarse-grained brittle phenomenon.The hardness test shows that the coarse grain area has the highest hardness value,and the softening phenomenon occurs at 950? and 850?compared with the base metal.(3)The microstructure of X90 pipeline steel CGHAZ under different heat input are lath bainite,granular bainite and M/A constituents,the grains are obvious coarsening,and there are large differences between grain size with high heat input,while the prior austenite grain size is very close.With the increase of welding heat input,the LB is reduced,the lath boundaries is not clear and the parallel degree is decreased,however the GB increases gradually,as well as the size and aspect ratio of M/A constituents;the impact absorbed energy at-20? increases first and then decreases,the maximum value exists at E=25kJ/cm,all the impact toughness values are lower than that of the base material,characterized by coarse grained embrittlement;the impact fracture represents obvious ductile fracture when the heat inputs are less than 30kJ/cm,but the brittle fracture for E?30kJ/cm;the hardness gradually decreases,and the minimum value of 236.01HV is higher than that of the base material,that is no softening.In order to obtain the good matching of microstructure,impact toughness and hardness,the welding heat input of X90 pipeline steel is recommended to be lower than 30kJ/cm,with the condition of good quality welded joint.(4)At 10kJ/cm,25kJ/cm and 40kJ/cm,with the increasing of heat input,the grain misorientation is anisotropy first and then tends to be uniform;the content of high angle boundaries and low sigma index CSL grain boundaries are 25.4%,25.9%,23.8%,and 10.13%,8.29%,4.13%,individually,the analysis shows the orientation relation of N-W and K-S existed in CGHAZ;the average grain sizes are 3.583?m,3.732?m and 4.271?m;the local misorientation peak are 0.55°,0.65° and 0.45°,the deformation of grain content are 73.51%,78.20%and 43.66%,respectively.The coarse grain embrittlement of X90 pipeline steel CGHAZ is not only related to grain size and M/A constituents,but also depends on the material microstructure type and its crystallographic characteristics.The reasonable welding parameters can achieve better microstructure coordination,smaller grain size and M/A,as well as excellent crystallographic orientation,which improve the impact toughness of the coarse grain zone and reduce the brittleness degree,effectively.