Research On Microstructure And Properties Of Welding Heat Affect Zone For High Strength Pipeline Steels

In this thesis, the microstructure and mechanical properties of welding heat affect zone (HAZ) of X80 pipeline steels with different compositions made in homeland and outland and a kind of X120 pipeline steel made in homeland, were studied. The purpose is to manufacture pipeline steels and tube of high Nb X80 and X120 steels.Toughness and embrittlement behavior of HAZ after different welding thermal cycle processes for four kinds of X80 grade pipeline steels with different chemical composition and a kind of X120 grade pipeline steel were studied by means of the welding thermal simulation technique in Gleeble-3500 thermal simulator. The influences of welding thermal cycle on the microstructure of HAZ were studied by optical microscope and electron microscope. Furthermore, the effects of welding process parameters and chemical compositions on the microstructure and properties were discussed.The results show that the coarse-grained heat-affected zone (CGHAZ) of X80 pipeline steels has higher toughness in the condition of once welding process with lower welding heat input energy, because the mixed low-carbon martensite and bainite microstructure is obtained. With the welding heat input energy increases (E≥40 kJ/cm), the toughness of CGHAZ is worsen, because the size of prior austenite grain grows up, and the larger size of bainite ferrite and M-A constituent is obtained at lower cooling rate after welding. Comparing four kinds of X80 pipeline steels, the toughness of CGHAZ high Nb steels is generally higher than the toughness of Mn-Mo-Nb system pipeline steel with lower Nb. The toughness of CGHAZ of the X120 pipeline steel is little change with the increase of the welding heat input energy, and it keeps in higher level.In double side or the multi-pass welding processes, as the peak temperature is in two phase area (α+γ) 800°C, it is found the local brittle zone of inter-critically reheated coarse-grained HAZ (IRCGHAZ) for the five kinds of pipeline steels. The reason is that some necklace type M-A constituents distribute along the coarse austenite grain boundary formed after once welding heat cycle process. When the peak temperature increases to 1000°C and 1200°C, the toughness of the reheated coarse-grained of the testing steels has small discrepancy. The higher value of impact toughness is acquired at 1000°C and 1200°C for the X80 pipeline steels and X120 pipeline steel, respectively.