Study On Interlaced Multiphase Microstructure And Strengthening And Toughening Mechanism Of Deposited Metals Of Metal Powder-cored Wires For 800 MPa Grade High Strength Steels

The new generation of 690¹200 MPa grade ultra low carbon bainitc?ULCB?steels have good characteristics such as ultra-fine grain,high cleanness,high uniformity,high strength and toughness.As an alternative to high strength low alloy?HSLA?steels,ULCB steels have been gradually developed and applied to cars,ships,petroleum pipelines,offshore platforms,aerospace fields,etc.However,the development and application of ULCB steels are limited because of the lack of corresponding welding consumables with equal high strength and toughness duo to the latter's underdevelopment.The root cause is strengthening and toughening theory for deposited metals?below 690 MPa grade?based on acicular ferrite?AF?,Ti-B and Mn-Si cannot provide guidance or be used for the design of new generation of high strength steel deposited metals any longer.Metal powder-cored wires are widely used in the welding of high strength steels for adjusting alloy composition easily,and having both solid wires' features of less slag and low diffusible hydrogen as well as common flux-cored wires' features of high deposition rate,soft arc,desirable weld bead and low spatter.However,there have been few,if any,reports on metal powder-cored wires with high strength and toughness?above 800 MPa grade?.This paper deals with interlaced multiphase microstructure and strengthening and toughening mechanism of deposited metals of metal powder-cored wires for 800 MPa grade high strength steels.The paper aims to provide a new theoretical basis and design guidance for various welding consumables and strengthening and toughening of their deposited metals,which are urgently needed for the application of 690¹200 MPa high strength steels.Therefore,the study of the paper is of great significance to promote the development of fundamental theory of welding metallurgy and enlarge the application of ULCB steels.In this paper,one metal powder-cored wire for 800 MPa grade high strength steels?AWS A5.28 E120C-K4?,with good mechanical properties?especially with low temperature impact toughness and welding performance?,was developed through optimizing alloy system with Design-Expert software,changing shielding gas compositions and using ultrasonic vibration.On this basis,the interlaced multiphase microstructure and strengthening and toughening mechanism of deposited metals of metal powder-cored wires were studied.The main conclusions are as follows:1.The alloy system of deposited metals of metal powder-cored wires for 800 MPa grade high strength steels was designed with Design-Expert optimization design software.The main alloy system of deposited metals was Mn-Si-Cr-Mo-Ni,and trace element element was Zr-Ti-Ce.The predicted values of Design-Expert agreed well with the experiment actual values.The deposited metal of developed wire no.3-17 was with tensile strength 915 MPa,yield strength 800 MPa,elongation 16.8%,and average value of-40 °C impact absorbed energy 97 J.2.The effect of shielding gas components,that is 95%Ar+5%CO₂,90%Ar+10%CO₂,80%Ar+20%CO₂ and 75%Ar+25%CO₂,on droplet transfer and arc stability was studied.The results showed that as the increase of CO₂ in shielding gas compositions,the droplet transfer form of four wires changed from projected transfer to globular?drop and repelled?or explosive transfer,and the stability of deoplet transfor became poor;the probability density distribution of both arc voltage u?t?and welding current i?t?were single hump curves of normal burning,and the closer the curve toward the center,the smaller the standard deviation and variable coefficient of u?t?and i?t?became,and the more stable the arc became;the arc stability was better with shielding gas compositions of 95%Ar+5%CO₂ and 90%Ar+10%CO₂,and worse with 80%Ar+20%CO₂;when shielding gas composition was 90%Ar+10%CO₂,welding performance of four wires from good to poor was Q1,Q4,Q3,and Q2.The developed wire Q1,that is wire no.3-17,had well welding performance.3.The effect of shielding gas components,that is 95%Ar+5%CO₂,90%Ar+10%CO₂,80%Ar+20%CO₂ and 75%Ar+25%CO₂,on microstructure and mechanical properties of deposited metals was studied.The results showed that as the increase of CO₂ in shielding gas compositions,the oxidability of arc atmosphere became stronger,the O content in deposited metals increased,the content of other alloying elements reduced,tensile strength of deposited metal reduced from 915 MPa to 870 MPa,and average value of-40 °C impact absorbed energy increased from 97 J to 125 J,and its dispersion degree increased;the shape of single-pass weld bead changed from thin and long glass cup,to finger shape,and to basin shape;the river pattern of quasi-cleavage fracture in radical zone of impact fracture became smaller and surrounded by small dimples,and dimples in fibrous zone changed from equiaxed dimples to tear dimples;the microstructure of deposited metal changed from degenerate upper bainite?DUB?and granular bainite?GB?to AF and GB;the shielding gas component of 90%Ar+10%CO₂ was benefit for achieving better matched strength and toughness of deposited metal and lower dispersion degree of impact toughness,and thus the deposited metal got with tensile strength 898 MPa,yield strength 810 MPa,elongation 18.4%,and average value of-40 °C impact absorbed energy 104 J.4.The effect of contact and non-contact ultrasonic vibration on microstructure and impact toughness of deposited metals was studied.The results showed that when using ultrasonic vibration,fish-scale pattern of single-pass weld bead became obvious,the shape of weld bead looked like glass cup and its symmetry became better,and the area of filler metal increased;during single-pass welding,ultrasonic vibration effectively broke coarse microstructure of weld metals and contributed to the formation of fine equiaxed grains;during multi-pass welding,ultrasonic vibration changed the main microstructure of deposited metals from DUB,GB and AF to AF and GB,the interlaced degree of multiphase microstructure increased,grains got refined,and impact toughness of deposited metals was improved and its dispersion degree reduced;the average value of-40 °C impact absorbed energy of deposited metal using contact and non-contact ultrasonic vibration increased by 34 J and 22 J,respectively,and thus contact ultrasonic vibration can better refine grains and improve impact toughness.5.The multiphase segmentation microstructure and strengthening and toughening mechanism of deposited metals of metal powder-cored wires for 800 MPa grade high strength steels were studied.The results showed that excellent strength and toughness match was achieved by a deposited metal having an interlaced multiphase microstructure;AF and GB nucleated prior to DUB and martensite?M?in solid-state phase transformation at higher temperatures,dividing prior austenite into several zones;DUB and M nucleated and grew in the divided austenite zones,forming an interlaced multiphase microstructure of DUB,GB,AF,and M in prior austenite grain;DUB and M acted as main strengthening phases,and GB and AF acted as main toughening phases.At the tensile strength of deposited metals high than 800 MPa,the microstructure consisted of bainite,especially DUB and GB.