Research On Welding Technological Parameter On Microstructure And Toughness Of Welding Heat Affected Zone For Three Pipeline Steels

With the increase of demand for oil and gas, the requirement of pipeline steel isincreasing, such as strength, toughness and weldability. Generally, welding process is anecessary in the laying of pipelines. Welding heat affected zone (HAZ) of pipeline steelexperienced different welding thermal cycle, leading to different microstructure andperformance. Pipeline steel welding heat affected coarse grain zone and the two-phaseregion are considered as the weak parts in the process of welding, especially the loss oftoughness. Therefore, it is important to improve the performance of CGHAZ andtwo-phase region effectively. In this paper, three kinds of test steels were used to study theeffects of welding processes on microstrures and properties, which would be useful inpipeline welding field.The experiments were carried out on a Gleeble-3500thermal simulator. The CCTcurves of three kinds of steels were constructed via expansion method. By usingmicroscope, transmission electron microscope and electron backscattered diffraction, themicrostructure and related properties were analyzed. In addition, the effects of peaktemperature on microstructures and properties were investigated. The results showed thatin steel A, austenitic coarsening was obvious and the M/A constituents with a large sizedistributed between the lath beam and the original austenite grain boundary in the coarsegrain region. However, in the two phase region, the composition of austenite wasinhomogeneous and M/A constituents with block shape distributes along the rolling region,which easily became the initiations of cracks. Compared with steel A, the M/Aconstituents were relatively smaller and dispersedly distributed, which hardly affectedtoughness.The results show that the CCT diagrams of steels were composed of PF、GB and BF.Under the little cooling rate, microstructure is most of PF, a small amount of GB. With thelarge cooling rate, the microstructure is consist of BG and BF. Under large cooling rate,the steel C has a single orientation and thick lath beam, which leads to lower toughness.High angle boundary can effectively prevent crack propogation, so that can improves the material toughness. The toughness in CGHAZ of three pipeline steels was investigatedby means of electron backscattering diffraction (EBSD).The results showed that thetoughness of CGHAZ is poor. The reason is that the low desitity of high angle boundaryand large effective grain size.