Microstructure And Properties Of Coarse Grained Haz Of High Grade Pileline Steels

With the increase of demand for oil and gas, the modern pipeline steel withhigher performance. Pipeline steel welding heat affected coarse grain zone is the worstperformance in the process of welding parts. Improve the performance of CGHAZ isparticularly important. Therefore, in this paper, the mechanical properties of CGHAZof several high grade pipeline steel were studied.In this thesis, experiment was carried out by Gleeble-3500thermal simulationmachine. Through expansion method, the three kinds of welded steel CGHAZ CCTcurve were constructed. We were discussed the transformation law of microstructureand M-A constituent in the CGHAZ of tested steel on different cooling rates. Theeffect of microstructure and M-A constituent on the mechanical properties of materialswere analyzed. The results show that the cooling rate of0.25℃/s~60℃/s range, CCTcurve mainly include polygonal ferrite and quasi-polygonal ferrite, acicular ferrite andbainite ferrite. With the cooling rate increases, the M-A constituent content decreased,but MA constituent content as low as10%, influence to toughness of the material isnot very obvious. When the cooling rate is0.25℃/s， The material have more MAconstituent content，the mean length longer than2μm，which is irregular large blockwith sharp corners， which is usually as the origin of crack occur, leading to thedecrease of material toughness. With cooling rate increases, the M-A constituentaverage size decreased, when the cooling rate is20℃/s, the microstructure is mainlyacicular ferrite, the average size of M-A constituent is1μm, which can prevent thecrack growing, that increase the toughness of material significantly. With high coolingrate, the lath bainitic ferrite formed, the M-A constituent distributed in the originalaustrian crystal boundaries and internal ferrite M-A constituent becomes long-thin, inother words, the average size increased, which lead the toughness of materialdecreased.Select the three kinds of high Nb but different Mo content pipeline, take twodifferent welding heat input, simulate its coarse grain heat affected zone. The effects of different heat inputs and different Mo content on transformation law about test steelwere studied. And the effect of different Mo content on the phase transitiontemperature and mechanical properties of materials were analyzed. The results showthat increase the Mo element content of the pipeline, the phase transformation starttemperature can be reduced. With Mo element content of material increase, theacicular ferrite content increasing, and effective grain size of acicular ferrite and thegrain size of original austenite are reduced. Meanwhile, M-A constituent distributionis more diffuse, but the content is increased. Increasing the content of Mo, canstrongly increase strength and reduce the yield ratio test steel. However, with thecontent of the M-A constituent increased, the toughness of materials in lowtemperature had decreased. Since Mo can reduce the toughness of pipeline steel, Mocontent of pipeline steel is usually limited to0.3%.