| High strength low alloy steel?HSLA?is widely used in important steel structure occasion as it has high strength and good toughness,such as pressure vessels,bridges,offshore platforms,and so on.Especailly in recent years,as the demand for HSLA steel become more and more as the bearing load become increasing in pipeline transportation,mining machinery,and construction machinery engineering field.Q960 steel is a kind of high strength low alloy steel which is obtained by tempering on the basis of ordinary low carbon steel by adding appropriate alloying elements.However HSLA has stronger hardening because of adding alloying elements,its welded joints has welding quality problem,such as cold cracks,embrittlement,and softening of heat affect zone.Especially its low-temperature impact toughness can not meet the requirements,which limits its application in a wider field.Metal powder flux cored wire is suitable for welding high strength low alloy steel,because of it has a lot of advantages,such as low slag or no slag,good operation,low diffusive hydrogen content,high deposition efficiency,less spatter,less smoke and dust,good weld shape,and so on.Especially,its powder preparation is simple and easy,and the transition coefficient of alloy elements is higher.In this paper,a kind of metal powder flux cored wire which propitious to welding960MPa high strength steel was developed by adding manganese element,molybdenum element,chromium element,nickel element,titanium element,boron element,aluminium element,and other alloying elements in order to improving low-temperature impact energy of weld metal,which from the weld metal metallurgical treatment aspect.Meanwhile,the mechanism of strengthening and toughening of alloying elements has been studied.Tungsten argon welding was used to weld Q960 steel with this kind of flux cored wire by self-developed in this paper.The preparation process of metal powder flux cored wire and welding process parameter were determined by powder filling rate test,hardness test and impact test,combing with metallographic observation and other means.Metal powder flux cored wire was made by steel strip forming method.Its diameter was 1.6mm,the powder filling rate was 16%,and the protective gas was 99.9%Argon gas.By analyzing the effect of each alloying elements in weld metal,Mn,Mo,Cr were used as the main alloy system of metal powder flux cored wire by self-developed,and Ni,Nb,Ti,B and Al act as micro-alloying elements for adjustment weld metal composition.The transition coefficients of each alloying elements were determined by chemical composition measurement.The results as follows:Mn 53%60%;Mo 25%30%;Cr 39%42%;Ni 40%48%;Nb46%49%;Ti 37%41%;B 96%98%;V 50%64%;Al 25%35%.The effects mechanism and effect law of Mn,Mo,Cr main alloying elements on strength of welded joints and impact energy of weld metal were studied respectively by tensile test,impact test,hardness test,combing with chemical composition analysis,metallographic analysis,scanning electron microscopy and transmission electron microscopy,etc.The optimum addition amount of main alloying elements were 1.0%1.5%Mn,1.0%2.0%Mo and 0.6%1.2%Cr.The interaction relationship of Mn,Mo,Cr three main alloying elements were researched by L9?34?orthogonal experiment design.The contribution effect of three main alloying elements on strength of welded joints and impact energy of weld metal were researched by tensile test and impact test,etc..The optimum composition ratio of main alloying elements was 1.2%Mn,1.5%Mo and 0.9%Cr.Used the optimum composition flux cored wire to weld Q960 steel and tested the mechanical properties of welded joints,the average tensile strength of welded joints was 1020 MPa and the average impact energy of weld metal at-60?was55J.The influence of microalloying elements Ni,Nb,Ti,B and Al in weld metal on mechanical properties of weld metal were studied by hardness test and tensile test and impact test at different test temperature.The effect mechanism of microalloying element on mechanical properties of weld metal were researched by metallographic analysis and scanning electron microscope analysis and transmission electron microscope analysis.The results show that:?1?The impact energy of weld metal increases first and then decreases at each test temperature with the increasing Ni element content in weld metal.The optimum Ni content was 1.22%and the impact energy of weld metal at-60?was 57.9J.This was mainly related to Ni element has effect on decrease transformation temperature of austenite to ferrite,and increase stacking fault energy,and promote dislocation intersecting slip and increase crack propagation energy consumption.?2?With the increase of Nb content in weld metal,the tensile strength of welded joints increases,and the impact energy of weld metal increases first and then decreases at each test temperature.The optimum Nb element content was 0.051%.Microalloying Nb element in weld metal plays a role in grain refinement and precipitation strengthening.Nb element in weld metal can refine weld metal grain by dragging effect of Nb element in the grain and pinning effect of precipitated NbC phase.?3?With the increase of micoralloying B element content in weld metal,the impact energy of weld metal increases first and then decreases.The best addition content of B element was0.005%which was related to the increase of acicular ferrite in weld metal.The free state B element in weld grain boundary can inhibit the precipitation of proeutectoid ferrite,which was favorable to the nucleation of acicular ferrite in the grain.Meanwhile,the addition content of B element in weld metal was related to addition contents of N element and Ti element in weld metal.The best raiton of B element and N element was between 0.050.08.When Ti element content was 0.43%,a small amount of B element can greatly improve the impact energy of weld metal.The Ti element was favorable for acicular ferrite nucleation by forming Mn2TiO4oxide in weld metal grain boundary.?4?The microalloying Al element in weld metal has the function of deoxidizing and denitrification and purifying weld metal.When Al content in weld metal was low,Al2O3 oxide was mainly formed and favorable to acicular ferrite nucleation.When Al content in weld metal was high,AlN was formed mainly in weld metal,which can be used as the crack source.The optimum Al content was about 0.21%.A kind of metal powder flux cored wire with Mn?Mo?Cr as the main alloy elements and Ni,Nb,Ti,B,Al as the microalloying elements was developed for 960MPa grade high strength steel.Q960 high strength steel was welded by gas tungsten arc welding with this kind of flux cored wire.The maximum tensile strength of welded joints was 1016MPa,the maximum impact energy of weld metal at-60?was 120J.The mechanical properties of welded joints which welded by metal powder flux cored wire by self-developed reached and exceeded the same kind of foreign products,which filled the blank of this kind of welding materials in our country. |
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