Study On Strengthening And Toughening Mechanicsm Of Alloy Elements On Weld Metal Of Q960 Steel

Q960 high strength steel is widely used in pressure vessels,bridges,offshore platforms and other large structural members because of its high strength and good plasticity and toughness.However,due to the addition of a large number of alloying elements in the high strength steel,it has high hardenability.The weldability problems such as cold cracking,embrittlement and softening of heat-affected zone?HAZ?are easy to appear after welding.Especially,impact ductility of weld metal at low temperature does not meet engineering requirements,so Q960 high strength steel is limited applying in a larger range.Self-shielded flux-cored welding wire is a new type of welding material which can be welded without additional protective measures.And self-shielded flux-cored also has many advantages,such as strong wind resistance,simple welding equipment and good operation in the absence of a power supply.Therefore,high quality welded joints can be obtained.In this paper,self-shielded flux-cored was developed taking Fe,Mn,Mo,Cr,Ni elements as the main alloy system and taking BaF₂,CaF₂,Al-Mg alloy as the basic slag system.The effects regularity and effects mechanism of alloying elements in the self-shielded flux-cored powder on the microstructure and mechanical properties of the weld metal welded by this kind of flux-cored were studied.And the strengthening mechanism and toughening mechanism of weld metal of high strength steel was explored.By means of tensile test,impact test and hardness test,the influence of Mn,Mo,Si,Cr,Ni,Al,Ti,B and other elements in self-protective flux-cored wire on the mechanical properties of Q960 high-strength steel weld metal was studied.By means of metallographic observation,scanning electron microscope?SEM?,transmission electron microscopy?TEM?,chemical composition analysis and so on,the action mechanism of alloy elements was studied.The process properties of welding wire were studied by splashing rate test and slag removal rate test.On this basis,the formation mechanism of acicular ferrite and the strengthening and toughening mechanism of weld metal of high strength steel were studied.The results show that:?1?Mn and Si elements are all metal strengthening elements in weld metal of Q960 steel,which can inhibit the grain boundary ferrite precipitation in a certain range,and facilitate the formation of acicular ferrite.The Mn element in weld metal has the function of solid solution strengthening and fine grain strengthening,and the optimum addition is between 1.17%and1.25%.Si element and Mn element have the same face-centered cubic structure,and the two elements have strong mutual attraction.The best Mn/Si ratio is between 4 and 8.The high content of Si prevents Mn atoms from entering the Ti₂O₃ oxide structure,reduces the complex structure oxides that can be used for acicular ferrite nucleation,and is not conducive to acicular structure.Ferrite nucleus.?2?Within in certain range,Mo,Cr,Ni elements can promote the formation of acicular ferrite in Q960 steel weld metal.Therefore,the problem of low temperature impact toughness of weld metal is improved.Solid solution strengthening and microstructure transformation are the main reasons for the improvement of strength and impact toughness of weld metal.?3?The appropriate amount Ti and B elements can promote the formation of acicular ferrite in weld metal.The addition of Ti can promote the formation of microinclusions and facilitate the nucleation of acicular ferrite.The solid solution B element at grain boundary can inhibit the precipitation of preeutectic ferrite,which is beneficial to the formation of acicular ferrite.However,the formation of BN compound between B element and N element can reduce the content of B element in grain boundary,and the addition of Ti can react with N first,thus ensuring the proper B element in grain boundary,which has an important effect on improving the low temperature impact toughness of weld metal of Q960 steel.?4?Al can form two kinds of inclusions in the weld metal of Q960 steel.When the content of Al in weld metal is low,it is Al₂O₃ inclusion,spherical,small size and dispersed distribution,which is favorable to become the nucleus center of acicular ferrite.When the Al content of weld metal is high,it is AlN,polygonal structure,and the size is larger.The oxide of Al is beneficial to acicular ferrite nucleation because the inclusion as a high energy inert interface can reduce the nucleus barrier of acicular ferrite.Because the expansion coefficient of inclusion and weld metal matrix is different,resulting high stress zone can promote acicular ferrite nucleation.?5?The addition of Zr to the weld metal of Q960 steel can inhibit the precipitation of preeutectic ferrite and bainite and promote the formation of acicular ferrite.At the same time,the microstructure of weld metal can be refined.Due to the high density dislocation and substructure of acicular ferrite,the weld metal has good impact toughness due to its strong anti-crack propagation ability.?6?In order to improve the low temperature impact toughness of the weld metal of Q960high strength steel,acicular ferrite is an ideal microstructure of the weld metal,and its formation is related to the micro inclusions of the weld metal.When there are elements such as Ti and Al in the weld metal,the lattice distortion is caused by the difference of the thermal expansion coefficient between the microinclusions and the austenitic matrix,and the quantity of acicular ferrite is increased by strain-induced mechanism.When there are elements such as Ti and Mn in weld metal,the formation of oxide of Ti and Mn provides cation vacancy for diffusion of metal atoms,and microinclusions act as nucleating particles to promote the nucleation of acicular ferrite.At the same time,the formation of the compound consisting of Ti and Mn element oxygen also causes the phase change equilibrium temperature to rise,forming a poor Mn region and a poor Ti region around the inclusion,so that the ferrite phase change point of the surrounding substrate is increased,the nucleation driving force is increased,and the acicular ferrite is promoted to grow radially in the center of the inclusion.In addition,that structure of the inclusions such as TiN,TiO and MnS in the weld metal is similar to that of the acicular ferrite,so that the shape nuclear energy is reduced,and the nucleation of the acicular ferrite is favorable.