Thermal Simulation Research On Microstructure And Properties Of Welding Heat Affect Zone For High Strength Pipeline Steel

In this paper, the microstructure and properties of X90 and X100 pipeline steel welding heated zone under welding thermal simulation test in the laboratory were studied by hardness test, charpy impact test, optical microscopy, scanning electron microscopy. The results showed that:(1) The coarsening temperature of original austenite grain of X90 and X100 pipeline steel were related to effect of the second phase particles pinning the grain boundaries and the second phase particles dissolved in high temperature. When the heating temperature was lower than 1250℃, a large number of second phase particles exited in matrix pinned original austenite grain boundaries and hindered the growth of austenite grain. So the original austenite grain growth slowly, the original austenite grain average size of X90 and X100 pipeline steel were 29.5 μm and 26.9μm. When the temperature reached 1300℃, original austenitic grain began to coarsen as part of the second phase particles dissolved, gathered together and grew up reducing the pinning effect of grain boundaries, the average size were 49.5μm and 48.3μm. When the temperature was 1350℃, the second phase particles in matrix basically dissolved,original austenite grain grew dramatically, the average size were 77.9μm and 69.7μm.(2) In the welding thermal simulation experiment of high strength pipeline steel,with the increase of the cooling rate, the hardness of X90 and X100 pipeline steel shows a gradually increasing trend, which is mainly related to the microstructure of the corresponding samples. For X90 pipeline steel, When the cooling rate is within the range of from 0.5 to 15℃/s, the microstructure by the quasi-polygonal ferrite over to granular bainite and the content of granular bainite increased gradually. lump M-A islands gradually shift to sheet shaped and size gradually decreased. When the cooling rate increases to 30℃/s, the microstructure is mainly for the compact and uniform lath bainite, and fined M-A distributed in the ferrite matrix.When the cooling rate reached 50℃/s, martensite appeared in microstructure.For X100 pipeline steel,When the cooling rate is within the range of from 0.5 to 30℃/s, the microstructures mainly contained granular bainite and lath bainite,and with the increase of coolingrate, lath bainite increases. Lump M-A islands gradually shift to bar or stub shaped.When the cooling rate reached 50℃/s, martensite appeared in microstructure.(3) After welding thermal simulation experiment, X90 and X100 pipeline steel have the similar microstructure and properties change rule under different heat input:under the large welding heat input(35~110KJ/cm), the microstructures mainly contained granular bainite and less acicular ferrite, the impact fracture is brittle fracture, toughness is poor; With the decrease of the heat input(10~25KJ/cm), the microstructures mainly contained granular bainite and less martensite, the impact fracture is ductile fracture, showing good toughness.