Effect of relaxation processing parameter on microstructure and properties of ultra-heavy gauge EH47 crack-arrest steel
The effects of relaxation times of 60-360 s after rolling on the microstructure and properties of ultra-thick EH47 steel were studied using tensile testing,Charpy impact testing,nil-ducility transition(NDT)temperature drop-weight testing,combined with optical microscopy,scanning electron microscopy,and electron back scattering diffrac-tion.The results showed that there was no significant difference in yield and tensile strength at 1/4 thickness(t/4)of the experimental steel with a relaxation time of 60 s and 180 s,yield strength and tensile strength were 518,538 MPa and 641,655 MPa,respectively.However,at a relaxation time of 360 s,the yield and tensile strength of t/4 of the experimental steel decreased significantly to 474 MPa and 553 MPa,respectively.The low-temperature tough-ness of t/4 position and surface P-2 type NDT drop-weight performance of experimental steel first decreased and then increased with the relaxation time increasing from 60 s to 360 s.The impact energies at-40℃for t/4 of the experimental steel relaxed for 60 s,180 s,and 360 s were 248 J,78 J,and 340 J,respectively.The P-2 type NDT drop-weight for surface of the steel plate at-50℃were no cracking,cracking,and no cracking,respectively.The microstructure characterization results indicated that the relaxation time has a significant impact on the microstructure of the experimental steel from surface to t/4 position.When the relaxation time is 60 s,the surface of the experimen-tal steel is lath bainite,and the t/4 position is acicular ferrite plus bainite;at a relaxation time of 180 s,the surface had bainite and a network distribution of ferrite,and t/4 position consisted of a small amount of quasi-polygonal ferrite and bainite;at a relaxation time of 360 s,the surface was polygonal ferrite with a small amount of bainite,and t/4 was quasi-polygonal ferrite with acicular ferrite.Further investigation revealed that the strain during low-temperature drop-weight was mainly concentrated in the grain boundary distributed ferrite for the experimental steel with a relax-ation time of 180 s,resulting in the formation of micro-voids within the network of ferrite.These network micro-voids were easily connected to form cracks,leading to poor drop-weight property at low temperature.
crack arrest steelEH47 heavy plate steelrelaxationNDTmulti-phase microstructure
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