Research On Microstructure And Mechanical Properties Of Double-sided Double Arc Welding Joint Of 10CrNi3MoV Steel

For high strength low alloy(HSLA)steel structure,the conventional welding process is filling one side firstly,then backgouging with carbon arc,polishing.The other side will then be filled when flaw inspection is passed.To avoid cold cracks,preheating is usually needed.The conventional welding process is of low productivity,unstable welding quality and labour-intensive.Double-sided double arc welding(DSAW)has been used for the welding structure of HSLA thick plates years ago.Backgouging with carbon arc has been cancelled in this method and the two sides can be filled at the same time,thus the productivity is improved significantly.10CrNi3MoV steel plates which were 35mm thick were welded by DSAW in this thesis.The welding parameters were obtained by experiments,the effect of thermal process of DSAW on joint microstructure and mechanical properties were investigated.The premises and mechanism that DSAW improve the mechanical properties and fracture resistance of welding joint were revealed according to the relationship between microstructure and toughness as well as fracture resistance.The DSAW welding specification of 10CrNi3MoV steel were determined by tests based on the welding process stability and weld appearance.Good joint formation could be obtained within the determined specification.With the increase of heat input,the microstructure of coarse grained HAZ(CGHAZ)of TIG welding were transformed from lath martensite(LM)to the mixture of LM and lath bainite(LB).With the post heating of subsequent weld passes,the microstructure of TIG pass were transformed from acicular ferrite to polygonal ferrite.The CGHAZ of MAG weld pass contained both lath martensite and bainite,the composition of HAZ of DSAW behaved the same as that of conventional multi-layer and multi-passes welding joint.The microstructure of MAG passes were mainly acicular ferrite.The thermal process of HAZ and its effect on microstructure and mechanical properties were investigated by finite-element simulation and thermal simulation.With the pre-heating of the fore arc,the cooling rate of HAZ of the rear side in double-sided double arc backing welding was decreased,lath bainite was formed in CGHAZ and the toughness was improved.With the decrease of arc distance in backing welding,the second peak temperature Tp2 of weld toe in the fore side rose from temperature below Ac1 to that above Ac3.When Tp2 was below Aci,tempering would occur in weld toe.When Tp2 was between Ac1 and Ac3,the weld toe would transform to intercritically reheated coarse grained HAZ(IRCGHAZ).The existence of M-A constituents in IRCGHAZ reduced to just 46.6J at-50?,which was lower than that of CGHAZ in single arc welding joint(109.9J at-50?).When Tp2 was higher than Ac3,the weld toe would transform to supercritically reheated coarse grained HAZ(SCRCGHAZ).The microstructure of SCRCGHAZ was fine and contained more lath bainite,which made the impact toughness increase to 143.8J at-50? and higher that that of CGHAZ in single arc welding joint.When 21.7kJ/cm heat input was used for each pass,the microstructure of unaltered coarse grained HAZs(UACGHAZs)of double-sided double arc backing welding joint was the mixture of LM and LB and the toughness was higher than that of CGHAZ of single arc welding.When 33.2kJ/cm heat input was used for each pass and the plates were preheated to 150?,the toughness of UACGHAZs were at the same level as that of CGHAZ.The formation of brittle GB in UACGHAZs negated the improvement of LB to impact toughness.The thermal processes of double-sided double arc filling welding were analyzed based on the measured thermal cycles.The microstructure and mechanical properties of CGHAZs in filling welding were investigated by welding thermal simulation.For 10CrNi3MoV steel plates in 35mm-thick,the arc in one side had effect on thermal cycles of the other side in the whole welding process.The heating of the rear arc would reduce the cooling rate of CGHAZ in the fore side obviously,if the cooling rate reduces too much,granular bainite would form in CGHAZ.The existence of hard and brittle M-A constituents and the reduction of high angle grain boundaries made the impact toughness of granular at-50?decrease to just 19.1J.To avoid the formation of brittle GB,the rear should heat the CGHAZ of the fore side should be cooled down at suitable rate.It is suggested that the fore side be reheated when it is cooled down to temperature below the martensite end transformation temperature Mf or the bainite end transformation temperature Bf and above 200?,and the rear arc just has temperi ng effect.When the welding specification determined in this thesis were adopted,all the 10CrNi3MoV steel DSAW joint fractured at base metal.The maximum hardness was 357HV and in the CGHAZ of the last pass.The minimum and maximum impact toughness of different zones of DSAW joint at-50? were 81.8J and 210.9J,respectively,which indicates the good joint toughness.There were no cracks when samples were bend to 120° and no fracture occurred when samples were bend to 180°,the joint showed well plastic deformation capacity.The results of cracking test of large-scale rigid sample demonstrates that DSAW can achieve non-preheating welding for 10CrNi3MoV steel.No cracks generated during and after welding.