Research On Fracture Toughness Of Welded Joints Of DH36 Steel Thick Plate In Offshore Structures

With the development and application of marine resources,the strength and thickness of the steel used in marine structure increases. Fracture toughness of high-strength steel welded joints is becoming more and more important. Study on fracture toughness of thick plate welded joints, not only can improve safety and reliability of the structures, but also can shorten fabrication time and reduce build costs.The Paper is based on BS7448, ISO12135 and ISO15653 standards about crack tip opening displacement CTOD test method, fracture toughness of welded joints DH36 steel thick plate used in marine structures were studied. Results show that: The CTOD values of weld zone and HAZ of double-wire submerged arc are lower than the single-wire submerged arc weld, and it did not meet the minimum value requirement of exempt post weld heat treatment; if the superior toughness of wire used in double-wire SAW, CTOD values of weld metal increased, but CTOD values of HAZ did not increase. After analysis the reasons of poor toughness in double-wire submerged arc welding, design process of double-wire submerged arc welding. The welding heat input is 2.9kJ/mm, and with the reasonable process parameter, the CTOD value of weld metal and heat affected zone increased substantially.Yield strength and hardness of heat affected zone in wire submerged arc welding joints decreased slightly, after the stress relieved annealed; Fracture toughness of the weld zone increased significantly, CTOD values of conventional welding processes increased from 0.02~0.2mm to 0.4~1.0mm, the CTOD value of optimization process increased from 0.2~0.4mm to 0.9~1.5mm; However, after post-weld heat treatment, the CTOD values of HAZ are quite dispersed, and CTOD value of some sample increased, while others are reduced.Assessed the Pop-in phenomenon and analyzed its causes, research found that slag defects, coarse ferrite side plate, network carbide appeared in crack tip caused local brittle zone, then Pop-in phenomenon appeared; with the maximum fatigue pre-cracking and temperature increased, Pop-in phenomenon disappeared in CTOD test of FCAW weld metal.