A Study On Welding Process Of 690MPa Ultra High Strength Low Yield Ratio Marine Steel

With the development of ocean engineering towards far-reaching sea and even polar sea area,higher requirements have been put forward for the strength,toughness,fatigue resistance,lamellar tear resistance and weldability of steel plate for platform construction.The research focus has been placed on the low yield ratio ultra-high strength steel with high strength,high toughness,easy welding,high crack resistance,low temperature resistance and large thickness.Anshan Iron and Steel Group has successfully developed the 690MPa class ultra-high strength steel EH690 with low yield ratio,which is expected to realize the high level of domestic replacement of steel used in the manufacture of important bearing parts such as pile legs and rack plates in offshore platforms.Therefore,by studying the welding production process of EH690 steel,the welding production efficiency of Marine engineering platform can be improved to ensure the welding quality and structural stability.In this paper,a new type of ultra-high strength Marine steel EH690 with low yield ratio was studied,aiming at the weldability evaluation and welding process parameter design of this steel,and welding thermal simulation experiment,welding cold crack sensitivity and welding process test were carried out.The microstructure and property evolution of welding heat affected zone were systematically studied by thermal simulation test.By adjusting the peak temperature of the thermal simulation test,the microstructure evolution of different subzones in the welding heat affected zone was studied,and the relationship between microhardness and low temperature impact property and microstructure was discussed.The results show that when t_8/5=15s,the subcritical heat affected zone（SCHAZ）of EH690 steel consists of high temperature tempered lath bainite,the critical heat affected zone（ICHAZ）consists of granular bainite and a few polygonal ferrites,and the fine crystalline heat affected zone（FCHAZ）consists of granular bainite,lath bainite and a few massive ferrites.Coarse crystalline heat affected zone（CGHAZ）consists of coarse lath martensite and a small amount of lath bainite.The microhardness of each subzone in the heat affected zone is CGHAZ>FGHAZ>ICHAZ>SCHAZ.ICHAZ has the highest impact energy at-40℃,while CGHAZ has the lowest impact energy,which is the weakest area of welded joint.The microstructure and properties of CGHAZ under different t_8/5conditions are studied by thermal simulation test.It is found that with the increase of t_8/5,the microstructure of CGHAZ changes from lath martensite and lath bainite to granular bainite,the hardness of CGHAZ decreases continuously,and the low temperature impact energy increases first and then decreases.When t_8/5=60s,the low temperature impact energy of CGHAZ is the highest.It’s 112J.Cold crack sensitivity coefficient calculation and cold crack sensitivity test of inclined Y-groove were carried out on EH690 steel.The results show that EH690 steel has low cold crack sensitivity and is not easy to generate cold crack.FCAW and SAW welding tests were carried out on 38mm thick EH690 steel.The results show that for both FCAW and SAW welding methods,the impact energy in the coarse crystal zone of the heat affected zone is the lowest,indicating that CGHAZ is the weak position of the whole welded joint,but the impact energy of SAW joint is significantly higher than that of FCAW joint.When the heat input is 20.5k J/cm,the impact,tensile and bending properties of FCAW welded joints are the best.Under the condition of low heat input,insufficient slag floating will cause slag inclusion defects and damage the performance of welded joint.Compared with FCAW,SAW welded joints had better performance.There was no significant difference in tensile strength of SAW joints under three kinds of heat input:16.2k J/cm,20.1k J/cm and 25.7k J/cm.Tensile tests were all broken at the base metal position,and no cracks were generated after 180°lateral bending test.