| Low-carbon bainitic high-strength steel has been widely utilized in many critical facilities and major projects,e.g.petrochemical pressure vessels,oil and gas pipelines,magnificent bridges,bathyscaphes etc.,because such steel possesses sound mechanical properties and weldability.The strength-toughness mismatch problem between weld metal(WM)and base metal is always caused by too many uncertainties during welding,which impairs the macroscopic properties of weldments.It is urgently significant to solve the mismatch problem by enhancing the mechanical properties of WMs,which can be realized by improving microstructures in WMs by adjusting the chemical components of welding consumables.Hence,it is of hugely-scientific value to investigate the relationship between microstructure and mechanical property,and then to provide solid theoretical background for researchers to optimize the mechanical properties of weldments and to develop a new generation of low-carbon bainitic high-strength steel.Based on previous introduction,mechanical properties of various WMs with four contents(i.e.0,2,4,6 wt.%)of Nickle(Ni)were analyzed.Strengthening and toughening mechanisms were revealed as a result of microstructure characterization by means of OM,SEM,EBSD,TEM and XRD techniques.In addition,bainite transformation kinetics were investigated via the dilatometric curves obtained by Gleeble thermal simulator.Laser scanning confocal microscope(LSCM)was used to in-situ observe bainite transformation behavior while EBSD was also introduced to analyze the growth orientation and the growth rate difference among various bainite laths.The main results are listed as follows:The effects of Ni addition on microstructural evolution in WMs are:(1)Both columnar grains and prior austenite grains are refined,however,both of them are coarsened with higher content of Ni(>4%).(2)The primary microstructures of WMs transfer from proeutectoid ferrite(PF),granular bainite(GB),acicular ferrite(AF),lath bainite(LB),and lath martensite(LM).(3)Retained austenite(RA)formation was enhanced.It is indicative that a larger number of martensite-austenite(M-A)constituents can be found in Ni0 and Ni2 specimens,because the content of carbon in Ni0 and Ni2 specimens are higher than those in other specimens.The effects of Ni addition on mechanical property development in WMs are:(1)The microhardness values of WMs are increased in all specimens.(2)Both yield strenghth(YS:?y)and ultra-tensile strength(UTS:?b)are gradually enhanced in Ni0,Ni2 and Ni4 specimens.Grain refinement acts as the most significant role among all strengthening mechanisms.(c)The ductile-brittle transition temperature(DBTT)is decreased and low-temperature toughness is improved.(4)The strength and toughness are deteriorated due to the resultant hot cracks during welding in the specimen with 6%Ni.(5)The critical fracture stress(?f)is increased,the critical value of stress triaxiality for cleavage cracking(Tc)is increased and then dropped,the critical plastic strain for cleavage fracture(?pc)remains close to 0.DBTTs in different specimens are determined by?f,in comparison with the surface energy of unit crack area(?).In other words,the stronger fracture resistance corresponds to the higher value of?f,which indicates that more external work is required to cause fracture.Both the nucleation and the growth of bainite were in-situ observed.From morphological perspective,there are some potential nucleation sites:twin boundary,prior austenite grain boundary,the broad side of previously-formed laths,the particles in the grain and some flat zones.Besides,Six kinds of bainite lath growth modes were in-situ observed:(i)successive single laths growing straight into a grain,(ii)a single lath growing along a grain boundary,(iii)a number of nuclei simultaneously growing in parallel at a high rate in a grain,(iv)a number of nuclei successively growing at a very low rate in a grain and growing parallel with each other,(v)laths simultaneously growing in different directions and forming a latticed frame and(vi)simultaneous nucleation and growth of a triangle latticed frame.Bainite start transformation temperature is reduced due to Ni addition.In addition,Ni addition enhances the possibility of nucleation in the flat zone in one grain,promotes the formation of effective inclusions and refines bainite laths.The laths that belong to one crystallographic packet can be nucleated on various habit planes and grow at various orientations,which causes resultant variants.Although different bainite laths can be identified in different crystallographic packets and in the same Bain zone,the growth rates of these laths vary hugely(i.e.2?m/s-2000?m/s).Such a wide range of speeds can be attributed to the following causes:(1)the density of the high-angle misorientation in it,(2)the included angle between habit planes of different variants,and(3)the direction of the displacement growing vector respective to the free(polished)surface.In terms of bainite transformation,the activation energy at higher cooling rates(30~oC/s-10~oC/s)is lower than that at lower cooling rates(3~oC/s-0.3~oC/s),which can be attributed to the sympathetic nucleation on the broad side of bainite laths obtained at higher cooling rates.The growth of bainite interface may not be controlled by carbon diffusion and the carbon concentration gradient at the interface does not follow local equilibrium conditions.Hence,bainite transformation occurs without diffusion characteristics.With regard to bainite transformation during isothermal holding,the growth of lath bainite is rather faster than the movement of?/?interface caused by the diffusion of solute atoms,which indicates that bainite transformation is diffusionless transformation. |
PDF Full Text Request