Effect Of Nb And Heat Input On Microstructure And Property In Coarse-grained Heat-affected Zone Of X70Pipeline Steel

Steels are widely used in different applications because of their good mechanicalproperties. However, the applications of steels unavoidably involve welding and the propertiesof weld joint, especially for the toughness, may be deteriorated seriously because the coarsemicrostructures and some other factors. The toughness of the heat affected-zone is stronglyinfluenced by factors such as the microstructure of the matrix, the volume fraction, size anddistribution of martensite/austenite constituents etc.In this thesis, the influence of Nb and heat input on the microstructure,martensite/austenite constituents and grain size were investigated by means of thermalsimulation experiment on a Gleeble3800. Meanwhile, the colour metallography, scanningelectron microscopy, transmission electron microscopy and electron backscatter diffractiontechnique were used in the present work.For the specimen with low-level Nb (0.039%), when it was simulated with the heat inputof20kJ/cm, the microstructures in the CGHAZ consisted of mainly plate-like bainitic ferrite;and the M/A constituents were small and evenly distributed in the matrix. This is the mainreason for good toughness of the CGHAZ. When the specimen was simulated with the heatinput of100kJ/cm, the formation of acicular ferrite laths or plates and small and film-like M/Aconstituents were also responsible for the good toughness. In addition, the high angle grainboundaries (>10°) kept a higher level in the specimens simulated with the heat input of20and100kJ/cm, which could inhibit the propagation of cracking and thus improve the toughness.When the specimen was simulated at the heat input of200kJ/cm, the microstructure in theCGHAZ consisted of predominantly granular bainite. The poor toughness of the CGHAZ wasattributed to large austenite grains size and massive M/A constituents. Similar results wereobtained for the specimens containing0.073%and0.11%, respectively.For the specimen with high-level Nb (0.14%), the mixed microstructure of acicular ferriteand granular bainite, fine M/A constituents and high angle grain boundaries were responsiblefor the fact that the toughness of the CGHAZ simulated with20kJ/cm was better than that ofthe CGHAZ simulated with100and200kJ/cm.Under the low heat input welding, the toughness of the CGHAZ increased with theincreasing Nb content, it reached an optimized one when the Nb content was0.11%. This isbecause the formation of the acicular ferrite and evenly distributed fine M/A constituents. Meanwhile the acicular ferrite can refine austenite grain and form many high angle grainboundaries, which could inhibit the propagation of cracking and thus improve the toughness.The formation of upper bainite was promoted when the Nb content was0.14%, which was notgood to the toughness. At the same time, high Nb content limited the heterogenous nucleationof ferrite on TiN by forming unstable (Ti,Nb)N or (Ti,Nb)(C,N) precipitates and also made thecarbonitride particles coarsened, which weakened the pinning effect and made the austenitegrain coarsened and impacted the toughness.When the large heat input welding was applied, big austenite grains and coarsemicrostructures were formed in the CGHAZ, and thus the toughness of the CGHAZ wasseriously reduced. With the increase of Nb content, the toughness of the CGHAZ did notobviously change under large heat input welding.