Study On Tempering Stability And Corrosion Resistance Of Acicular Ferrite In Weld Metals

A large number of acicular ferrites will be formed during welding using “oxide metallurgy technology” and the heat affected zone can keep good strength and toughness so as to meet the technical requirement of the welded joint.The welding of steel containment as an assembly of the third generation nuclear power station also use“oxide metallurgy technology” to produce a large number of acicular ferrites to ensure the strength and toughness of the welded joint.Considering the use environment of the steel containment,the technical standards require the welded joint to maintain good stability of mechanical property at a certain high temperature when hot strengthening and stress release are conducted after welding.Because of the lack of intensive study of the stability of acicular ferrite structure,it is therefore the present work will focus on it.The main research results are as follows:In consideration of the practical technical standard,acicular ferrite in weld metals was obtained by using homemade flux cored wires.The tempering experiments were respectively carried out at 580℃,610℃ and 640℃ for various tempering times.Microstructure characterization and mechanical property tests were conducted on weld metals.The results show that the tempered microstructure in weld metals is almost the same as that of as-welded.It is mainly composed of acicular ferrites.The grain size is basically unchanged,being approximately 3.5μm.The microstructure remains good stability when the tempering temperature is lower than 610℃.When the tempering temperature rises up to 640℃,the strength would increases obviously and the toughness drops at some extent.The results of transmission electron microscope observations show that many nanoscale precipitates is precipitated at the dislocations of acicular ferrite grains.This is main reason for the increase of tensile strength.The thermodynamics and kinetics analysis of tempering stability of the acicular ferrite structure in weld metals was carried out.The results show that when the tempering temperature reaches 640℃,the nucleation rate of precipitates which is mainly composed of titanium and carbon increases greatly and the precipitation start time is shortened obviously.Many nanoscale carbides are precipitated and thus the strength is increased remarkably.In addition,the electrochemestry test was conducted to study the corrosion resistance of the acicular ferrite in weld metals.The results of potentiodynamic polarization curves and the fitting of impedance show that the charge-transfer resistance increases along with the rise of the tempering temperature or the prolonging of the tempering time,which means that the corrosion resistance of acicular ferrite in weld metals is improved after tempering.The above results indicate that acicular ferrite has excellent tempering stability and the corrosion resistance also be improved after tempering.It can be seen that reasonable control of chemical composition and tempering condition,mechanical properties and corrosion resistance can be optimized,which will provide technical support for material design and engineering implementation.