Acicular Ferrite Formation In HAZ Of Pipeline Steel

Microstructure in welding Heat Affected Zone (HAZ) of pipeline steel, when applied to large heat input welding process, will become much coarse. This leads to greatly decrease in toughness of pipeline steel, which severely restricts its applications. This thesis focused on the transformation of microstructure in HAZ of X70/80 pipeline steel. The behavior that beneficial inclusions inducing acicular ferrite nucleation thus refining microstructure in Coarse Grain Heat Affected Zone (CGHAZ) of X70/80 pipeline steel under large heat input welding conditions was investigated detailedly.Due to the fact that welding of pipeline steel is always adopted by multipilewire large heat input welding methods, transformation behavior of microstructure in HAZ of X80 pipeline steel was investigated with the selected heat input parameters 29.8 KJ/cm, 40.0 KJ/cm, and 50.3KJ/cm. It was found that when large heat input automatic submerged-arc welding was applied to X80 pipeline steel, grains in CGHAZ of the steel would grown up which lead to decrease in toughness. If beneficial inclusions in the steel can be utilized to induce acicular ferrite nucleation, microstructure in CGHAZ will be refined and toughness be improved correspondingly.Using physical simulation technology, microstructure in oxides metallurgy steel and its CGHAZ as well as the character of beneficial inclusions in the process of large heat input welding process was investigated. Results indicated that the role of beneficial inclusions with a diameter of 0.2~0.6μm that inducing acicular ferrite nucleation and sympathetic nucleation woud not lose. They have the ability of restraining growth of grains in CGHAZ.Self refining grain behavior in HAZ of acicular ferrite microstructure pipeline steel was studied systematically by welding themal simulation method. Changes on size and distribution of inclusions which induced acicular ferrite nucleation during the welding thermal circle were analyzed completely. Relations between character of beneficial inclusions and microstructure in HAZ were disclosed. Those results provided a basis for optimizing welding parameters of acicular ferrite microstructure pipeline steel.