Microstructure And Mechanical Properties Of Flux-cored Wire Deposited Metal Processed By High Heat Input

In last decades,shipbuilding industry,large oil storage tanks,high rise building,marine engineering,and other equipment manufacturing filed were booming enough,and the welding technology had drawn a great attention from us as key process with the development of steel construction with large scale and large span To improve efficiency and cut down cost during welding process,some advanced welding processes characterized by high heat input,i.e.gas welding,submerged arc welding,electroslag welding,was widely used.While,the temperature was increased in the welded joints in the process of high heat input welding,and weld metal stayed a long time in high temperature environment.The cooling rate was small,which resulted in the microstructural coarsen of heat affected zone and weld metal.The mechanical properties of weld metal were also deteriorated.The security of utility of construction was affected seriously.Hence,research and development of the supported steel and welding material for high heat input welding based on the oxide metallurgy technology had drawn a great attention from us.In present study,the selection of alloy elements,optimization of physical metallurgy process,transformation behavior of weld metal,precipitation of inclusion,and nucleation mechanism of intra-granular acicular ferrite associated with flux-cored wire deposited metal under high heat input welding were investigated,systematically.The predominant research works and results were listed as below:(1)The effects of different chemical elements on the mechanical properties of weld metal processed by high heat input welding were investigated.The chemical composition of flux-cored wire used for high heat input welding was optimized in the basis of basic theory of oxide metallurgy technology,and the chemical composition of flux-cored wire used for high heat input welding was obtained.The manufacture process of flux-cored wire was also obtained.(2)High heat input welding with heat input energy 85 kJ/cm was carried out utilizing the developed flux-cored wire for high heat input welding,and the effects of Mn content on the microstructure and mechanical properties were studied.The results indicated that large amount of small-sized inclusions were distributed in the deposited weld metal with 1.85%Mn,and certain volume fraction of AF were obtained by initiating at the inclusions,which contributed to excellent low temperature toughness.With the decreased of Mn content,the volume fraction of AF in the microstructure was decreased,and volume fraction of bainite was increased,which finally resulted in high strength and low toughness for the deposited weld metal processed by high heat input welding.(3)The effects of oxygen content on microstructure and toughness of deposited weld metal processed by heat input value 341 kJ/cm were investigated.The results indicated that when the oxygen was lower than 610 ppm,the micro-inclusions distri-buted in the weld deposited metal was increased with the increased oxygen amount.The probability between the collapsed inclusions was increased,and the nucleation site of AF in the austenite grain was increased,which resulted in small sized grains due to sufficient transformation of AF.Eventually,the toughness of deposited weld metal was inproved.Besides,when the oxygen content in the weld metal was higher than 610 ppm,the amount of small-sized inclusions was sharply increased with increased content of oxygen The austenite grain size was significantly decreased,and the transformation tenperature was increased.Large amount of grain boundary ferrite(GBF)was formed,which deteriorated the low temperature toughness of deposited weld metal.(4)The transformation behaviors of AF including the nucleation process and growing process were investigated under different high heat input value by utilizing electron back-scattered diffraction technique(EBSD).The results indicated that when the high heat input value was higher than the critical value,and the cooling rate was lower than critical cooling rate,the AF was nucleated in the form of polygonal ferrite.The AF grain size was decreased with decreased high heat input value,and there was a deviation for the KS crystallographic orientation relationship(KS OR)between parent austenite and AF.When the temperature of weld metal was decreased to a value below the transformation start temperature,the AF grows in the form of needle shape and acicular shape,and the parent austenite kept KS OR with AF.When the heat input value was lower than critical value,it is too late to nucleate for AF,and the AF rapidly grows.The AF kept KS OR with the parent austenite.(5)The microstructure and mechanical property variation discipline of weld metal during reheating process were investigated by MMS-300 simulation equipment.The results indicated that when the heat input value was 200 kJ/cm,the GBF in the weld metal was disappeared,and the volume fraction of AF was decreased.There were large amount of polygonal ferrite in the micro structure,and the toughness of the weld metal processed by reheating process was lower compared to the weld metal.With the increased heat input value,the form of ferrite was always transfer to equiaxed grains.The volume fraction of polygonal ferrite was increased,and AF could not be observed absolutely.The toughness of weld metal processed by reheating process was slightly improved.(6)The flux-cored wire developed in the laboratory was used in the heat input value range of 205?425 kJ/cm,and the relationship between toughness of deposited weld metal and welding heat input value was investigated systematically.The results showed that the mechanical properties of deposited weld metal processed in the heat input value range of 205?425 kJ/cm were satisfied with the requirements of EH 36 to EH460 ship building steel in accordance with the GB 712-2011 standard.When the heat input value was lower 276 kJ/cm,the mechanical properties of deposited weld metal were satisfied with the requirements of 12 MnNiVR oil tank steel in accordance with the GB19189-2003 standard.The developed chemical composition,manufacturing process,and corresponding mechanism of flux-cored wire in present study was beneficial to the in-depth research and industrial development of welding material processed by high heat input welding process in domestic.