Research On Microstructure And Mechanical Properties Of Heat Affected Zone Of X80Pipeline Steels

The evolution of the microstructure and mechanical properties of weld heat affectedzone (HAZ) is always a major subject under investigation in pipeline steels. X80gradpipeline has been already widely used in second-stage construction of West-East naturalgas transmission. But further research of the microstructure and mechanical properties ofHAZ is needed.In this thesis, the CCT diagrams of three X80grade pipeline steels were measured bythermal simulation performed on a Gleeble-3500thermal simulator, and the effects ofalloyed compositions on microstructure and properties after cooling to room temperaturewere analyzed. The results show that the CCT diagrams of test steels were composed ofpolygonal ferrite region and acicular ferrite region in the range of test cooling rate.Elements of C, Mn, Cr and Mo can restrain the polygonal ferrite transition, and promotethe acicular ferrite and bainite transformation. Alloying elements have comprehensiveeffects on the microstructure and mechanical properties. The critical cooling rate to obtainall acicular ferrite is1oC/s for steel A and C which have the same Ceq0.42, and thecritical cooling rate is0.5oC/s for steel B with Ceq0.47. Increase of cooling rate, theamount of M/A decreases, and the style of M/A transited from bulk to granular or thin film.As the cooling rate is in the range of0.25~5oC/s, the strength, hardness and toughness ofHAZ phase increase with the increase of the cooling rate, slightly. As the cooling rate ishigher than5oC/s, strength, hardness and toughness increase with the increase of thecooling rate. The toughness reaches maximum when the cooling rate is20oC/s. Thetoughness decreases with the increase of the cooling rate, while the strength and thehardness have a little change when the cooling rate is higher than20oC/s. At lowercooling rate, coarse ferrite grain and M-A structure cause the decrease of toughness. Athigher cooling rate, the structure based in lath bainite led to the decrease of toughness.The microstructures and properties of sub-zone in HAZ were investigated by meansof weld simulation with different peak temperature. The results show that theembitterment field in HAZ occurs coarse grain HAZ (CGHAZ) and inter-critical HAZ (ICHAZ), while the toughness of refined grain HAZ and aged HAZ is improved. Thecoarse bainite caused by the growth of austenite grain is reason of embitterment inCGHAZ. However, the embitterment in ICHAZ is due to a few brittle M/A islands formedin transition process. Softening region appears at the region that the peak temperature isabout800~900oC. It is due to the alloy elements in the austenite solution insufficient,resulting in a higher transition temperature and non-uniform microstructure in that zone.The chemical compositions and microstructures of base metal have an effect on theembitterment and softening of the HAZ. Refining the microstructure and optimizingcontent of base metal can improve the weldability.