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Microstructure and properties of weld heat-affected zone in pipeline steel
- Feb 02, 2018 -

The microstructure of weld heat affected zone can be divided into two types: hard quenching steel structure and quenching steel structure. The melting zone, coarse grain region, recrystallization zone, incomplete punching crystallization zone and aging embrittlement zone are formed after the change of the microstructure of the hard quenching steel.The quench steel easily formed to Cui acquisition, incomplete quenching zone and tempering zone.

Pipeline steel belongs to hard quenching steel, the fusion zone and coarse grain area of weld heat affected zone have great damage to the base material property, easy to form embrittlement, the damage degree depends on the alloy system of the base material, the original microstructure state of the base material before welding and the welding specification parameters. For pipeline steels below X65 Steel, when online energy is low, in addition to producing ferrite and pearlite, it is also easy to produce martensite (M), Upper Bainite (Bu) and granular bainite (BG), when the online energy is high, the coarse grain region is easy to produce ferrite and pearlite, and it is also easy to produce the common analysis ferrite and the organization. It is generally believed that the upper Bainite, the first common analysis of Ferrite and the organization of Wei is a brittle phenomenon of harmful tissue. For the steel-grade acicular Ferrite pipeline steels above X70, the microstructure of the coarse grain region is mainly bainite (lath bainite and granular bainite), bulk ferrite and first ferrite. Ma Island is deposited on the substrate of lath or mass ferrite or massive ferrite. The main factors contributing to the reduction of the toughness of the coarse grained steel zone are:

(1) The relative amount, size and shape of the MA composition. The more MA, the size is too thick or too long, as well as uneven distribution, so that the brittleness of the phenomenon is serious.

(2) The effective grain size or the grain growth tendency of the base metal. With the increase of line energy, not only the grain size of the original austenite increases, but also the two crystalline microstructure becomes thicker and larger. The reduction of lath ferrite and the increase of bulk ferrite become one of the main causes of embrittlement in coarse grain region.