Study Of Weldable Ultra-high Strength Steel By Copper Age Hardening

Quenched and tempered NiCrMoV steel has been used for naval ship hull structure because of ultra-high strength with lath martensite.However,the weldability of high alloying martensite steel is poor and the requirement for an increasingly large ship structure can’t be fulfilled.Copper precipitation hardening improves weldability by lowering carbon equivalent,while keeps high strength and toughness.The welding condition of copper hardening steel for large hull structure can greatly be improved by cancling the pre-welding and post-welding heat treatment due to low carbon equivalent.The copper precipitation strengthening effect depends on density and size of precipitates.Single copper phase particles coarsen easily during aging and thus the strengthening effect would fade.To increase strength further,combined precipitation hardening steels with nanoscale Cu and M2C or NiAl have been developed.The major object of this paper is therefore to design new prototype weldable steels with yield strength above 785 MPa based on conventional copper precipitation strengthening steel.Experimental studies were carried out to optimize the combination of chemical composition and aging process.The desired strength,toughness and weldability are archieved.This provides possible technology guide for future industrial practice.Four steels noted as CuMn,CuMnTi,CuAl,and CuMoAl are investigated in this paper.They can be divided into either CuMn or CuAl categories.The major results include:(1)The property diagram and precipitates of CuMn and CuAl steels are remarkable different:only a partial δ-iron zone is shown for both CuMn and CuMnTi steels,while a full δiron zone exists in both CuAl and CuMoAl steels.Multiple type phases including Cu,NbC,M2C,M23C,and M6C could precipitate in both CuMn steels.In contrast,Only Cu,NbC and cementite can be found in CuAl steel,and varied carbides such as M2C,M23C,and M6C are introduced instead of cementite.(2)Varied transformation behavor during continuous cooling in four steels:lath and granular bainite are formed in both CuMn and CuMnTi steels at less than 2℃/s and lath martensite can be observed at more than 5℃/s;for CuAl steel,ferrite and bainite are the major microstructures at less and more than 10℃/s,respectively;addition of Mo alters the microstructure to granular bainite at less than 2℃/s and to lath bainite at more than 5℃/s.(3)The variation of mechanical properties vesus aging temperature is similar for four steels.The strength increases with aging temperature from 300℃ to 500℃ and peak strength as well as tempering embrittle occurs at around 500℃;while aged at the temperature above 500℃,the strength drops with increasing toughness;the optimized combination of strength and toughness is attained by aging at 550℃ for 1h,with the yield strength of 994 MPa for CuMn steel,1055 MPa for CuMnTi steel,1000 MPa for CuAl steel and 944 MPa for CuMoAl steel,respectively.(4)Heavily dislocated lath structure and high density nano-scale granular precipitates are observed in as-aged CuMn steel.The average diameter of the precipitates is around 8.3 nm.A refinement in microstructure is observed in CuMnTi steel with greater fraction of large misorentation grain boundaries than that of CuMn steel.This therefore improves the strength and toughness.CuAl steels are much less hardenable than CuMn steels so that lath bainite is formed instead of lath martensite.Massive precipitates with average diameter of 10.3 nm are observed within bainite laths.While in CuMoAl steel,granular bainite is the major mircrostructure with lower fraction of large misorientation grain boundaries.Consequently,the strength and toughness is deteriorated although the strength stability with aging temperature is improved by adding Mo.(5)Welding simulation experiments were conducted with 17.5 kJ/cm heat input.The impact energies of HAZ are all higher than 40 J at-40℃ using 5 mm-thick specimens,which meets the designed requirement.