The Study On Toughness Of X100Pipeline Steels And Heat-affected Zone

In this paper, the designed toughness of XI00pipeline steels was calculated and predictedcombining fracture theory with pipeline project examples, the toughness requirements of theXI00pipeline steels which were intended to be used in the third West-east gas pipeline wereproposed, and the factors influencing the prediction model of fracture arrest toughness wereanalyzed. At the same time, the energy characteirstics and fracture characteirstics wererevealed atfer DWTT and CVN impact test, by which the relationship between CVN （CharpyV-notch） and DWTT （Drop weight tear test） energy of XI00pipeline was established,evaluating the superiority of the CVN and DWTT energy to characteirze material toughness.Finally, the microstructures and properties of the heat-affected zone （HAZ） of high-gradeXI00pipeline steel were studied by welding thermal simulation technology, mechanicalproperties test and material microscopic analysis method, and investigated the clfect rules ofthe ifrst welding thermal cycle and the second one on the microstructurc and toughness in thecoarse-grain HAZ （CGHAZ）.The research results indicated that the brittle fracture toughness of the XI00test sectionintended to be used in the third West-east gas pipeline were respectively213J and85%sheararea （S.A） calculated by energy criteiron and velocity criteiron, the initiation toughness andarrest toughness of ductile fracture were112J and390J predicted by the CVN-ac relationcurves and Battelle Two Curve （BTC） method combined with the corresponding correction.Based on the BTC fracture arrest prediction model, it was found that the grand, pipediameter, wall thickness, pipeline pressure and gaseous media were important factors to affectthe arrest toughness. The arrest toughness of pipeline steels would increase with the increasesof the grand, pipe diameter, wall thickness and pipeline pressure, while the heavy hydrocardoncomponents in the gaseous medium would also lead to the increase of arrest toughness.The CVN and DWTT test results showed that the chevron notch （CN） DWTT sampleused to characteirze high strength and high toughness pipeline steels was more accurate thanCharpy V-notch sample. Compared to PN DWTT, CN DWTT was easyer to crack initially andthe ductile fracture was higher at room temperature, which could effectively evaluate thedynamics of high toughness pipeline crack. The Batlcllc relationship between DWTT and CVN energy was （E/A）_DlV7r-3（E/A）_CVN+0.63veriifed by the analysis of DWTT and CVNenergy.According to the research results of thermal simulation test, the toughness of XI00pipeline steel CGHAZ decreased with the increase of welding heat input. The mainmicrostructure obtained in the CGHAZ were benitic ferrite （BF） with ifne lath structure andacicular ferirte （AF） with multidirectional distirbution when the welding heat input was10-25kJ/cm, which procured CGHAZ with good toughness. While the welding heat input washigher than30kJ/cm, the grains grew up and the polygonal ferrite （PF） or quasi-polygonalferirte （QF） were main microstructurc of CGHAZ, which resulted in the decrease of thetoughness in the XI00CGHAZ,In the sccond welding thermal cycle，the toughness of XI00pipeline steel decreaseddrastically when the second peak temperature was in the critical range of Ac]^Ac3, exhibitinglocal embirttlement of ICCGTIAZ. The large size grain and large martensite-austeniteconstituent （M-A） with rich carbon are the main reasons leading to local embirttlement ofICCGHAZ.