Research On Heat Affected Zone Of High Grade Pipeline Steels

Welding as the best way of pipeline-steel molding and connection, the welding heataffected zone microstructure and properties of pipeline steel has been an importantresearch issue. Different grade pipeline steel because of the different chemicalcomposition and the original microstructure, the microstructure and mechanical propertiesof welded heat affected zone are quite different.By using Gleeble-3500thermal simulation testing machine measured three differentgrade pipeline steel welding CCT curves, and analysis from the alloy elements and aftercooling microstructure affects of its properties. The results show that the CCT diagrams oftest steels were composed of quasi-polygonal ferrite region、granular ferrite and bainiticferrite region in the range of test cooling rate. C, Mn, Cr, Mo and other alloying elementscan restrain the polygonal ferrite transition, and promote the granular ferrite and bainiteferrite transformation. The critical cooling rate to obtain all acicular ferrite is1oC/s forX80and X100grade pipeline-steel, and the critical cooling rate is0.5oC/s for steel B.With the increase cooling rate increases, M/A component transform from massive to tinyfilm or sheet. In the coarse grain zone peak temperature, cooling rate was0.25~5oC/srange, with the cooling rate increases, the tensile strength, hardness and low temperatureimpact toughness change a little of three test steel. As the cooling rate is higher than5oC/s,strength, hardness and toughness of steel A and steel B increased significantly, while thesteel B due to the nonuniform microstructure and coarse M/A component, the propertieschange insignificantly.The microstructures and properties of sub-zone in HAZ of four different gradepipeline steel were investigated by means of weld simulation with different peaktemperature. The results show that with the increase of peak temperature, themicrostructure transform from the tiny quasi polygonal ferrite to the coarse bainite ferrite,the form of M/A component form the coarse massive to sheet which formed along theoriginal austenite grain boundaries. The significant embrittlement zone of test steel occursat coarse grain HAZ (CGHAZ) of1350oC and inter-critical HAZ (ICHAZ) of750oC, and the peak temperature between800oC of900oC soften obviousously. Refining themicrostructure and optimizing content of base metal can improve the weldability.