Effect Of Welding Heat Input On Microstructure And Mechanical Properties Of The Simulated CGHAZ In Nb-V-Ti Microalloyed Steel

With the development of large-sized steel structure as well as the higher efficiency and lower production cost of large heat input welding,structural steel is required to have a good combination of strength and toughness,coupled with superior welding performance characterized by the strengthened and toughened CGHAZ.In this work,effect of heat input and Nb content on microstructural evolution,precipitation behavior and resultant mechanical properties of the simulated CGHAZ in 0.05Nb-0.22V-0.01Ti and 0.10Nb-0.22V-0.01Ti experimental steels was systematically investigated using Gleeble 3800thermomechanical simulator.The thermodynamic calculation result conducting on Thermo-Calc software shown that carbonitrides in the experimental steel at equlibium state were V（C,N）-rich low temperature precipitates（FCC＿A1#2）,Nb C enriched medium temperature particles（FCC＿A1#3）and Ti N-rich high temperature precipitates（FCC＿A1#4）,respectively.Increasing Nb content could enhance the precipitation of Nb C enriched phase and yield the precipitation of Ti N and V（C,N）-rich phases,as well as improving the total quantity of carbonitrides.Moreover,the electron microscope result under non-equilibrium condition was basically consistent with that derived from thermodynamic calculation.The submicron scaled Ti N and Nb C enriched particles could pin grain boundary and induce intragranular ferrite formation,which improved the impact toughness of CGHAZ.The nanoscaled V（C,N）-rich precipitates would distribute in matrix,on dislocations and at grain boundaries,which increased the strength of CGHAZ through precipitation strengthening.From the microstructure analysis of CGHAZ in Nb-1 steel with different heat input,when E was 50 k J/cm,the microstructure mainly consisted of lath bainite and some martensite.The microstructure with E 100 k J/cm was composed of intragranular acicular ferrite coupled with some lath bainite and intragranular polygonal ferrite.As E increased to150 and 200 k J/cm,the microstructure was transformed to grain boundary ferrite,intragranular ferrite,as well as a small amount of ferrite side plate,bainite and pearlite,of which the quantity and size were increased with the increasing heat input.The amount of intragranular acicular ferrite was first increased and then decreased,and the maximum value of 51.5%was obtained with E 100 k J/cm.The prior austenite grain size was coarsened from 39.3μm to 81.2μm.The number fraction of high angle grain boundary was first increased from 0.537 to 0.607（E=100k J/cm）and then decreased to 0.423.The effective grain size was increased from 7.16μm to 19.95μm.The average KAM value was first decreased from 0.763°to 0.548°（E=150k J/cm）and then increased to 0.595°.Based on the mechanical properties variation of CGHAZ in Nb-1 steel with different heat input,as E increased form 50 k J/cm to 200 k J/cm,the microhardness was decreased from 304±22.5 HV0.1 to 269±15.3 HV0.1.The corresponding impact energy at 20°C was first increased from 216±10.3 J to 219±3.7 J and then decreased to 184±7.5 J.The value at 0°C was first increased from 127±4.3 J to 147±8.6 J and then decreased to 111±2.9 J.The value at-20°C was first increased from 58±2.7 J to 64±3.0 J and then decreased to 45±2.1 J.The optimal impact toughness of CGHAZ was obtained with E100 k J/cm,because the highest amount of IAF could remarkably increase the number fraction of high angle grain boundary due to the high misorentation between the interlocked ferrite plates and neibouring microstructure,which effectively restrained the crack propagation and the formation of secondary crack.Under the condition of the same heat input,with Nb content increased from 0.05 wt%to 0.10 wt%,the similar type and different amount of microconstituent was obtained in CGHAZ.With E 50 k J/cm,the content of lath bainite was increased and that of martensite was decreased.When heat input was ranging from 100 k J/cm to 200 k J/cm,the content of intragranular ferrite,especially intragranular acicular ferrite was increased,coupled with the decreased grain boundary ferrite,ferrite side plate and pearlite.Meanwhile,the number fraction of high angle grain boundary was increased,as well as the refined effective grain size and prior austenite grain size and the decreased average KAM value.Consequently,the mircohardness was changed not apparent,but the impact toughness was improved,especially that at lower temperature.The appropriate welding heat input for Nb-V-Ti microallyed steel was 100 k J/cm,of which the optimal combination of microhardness and toughess was obtained in CGHAZ.Increasing Nb content to 0.10 wt%not only enlarged solute drag effect of Nb atoms and pinning effect of carbonitrides,which boosted the grian refinement strengthening,but also promoted the precipitation of submicron scaled particles such as Nb C rich-（Nb,V,Ti）（C,N）inducing the formation of IAF with the orientation relationship[011]_MC//[11 1]_Ferrite.The abovementioned influence tends to enhance the strength and toughness of CGHAZ and be better adapted to large heat input welding.