Effects Of Welding Thermal Cycle On The Microstructure And Property Of A Fe-Cr-Ni-Mo High Strcength Steel

The marine engineering used in deep ocean area has become the mainstream developing direction of marine engineering.The steels used to build the key parts of the marine offshore structures are required to yield strength higher than 700MPa and high toughness as well as good weldability.A Fe-Cr-Ni-Mo high strength steel is designed and produced by Institute of Metal Research,Chinese Academy of Science.The tensile strength of the high strength steel can reach 1224MPa and the impact toughness keeps high at-50?,which could be an ideal material to build offshore structures.It is very necessary to assess the weldability of the steel as the welding is the main connecting way in the construction of marine engineering.The study is the foundation for application in marine engineering of this steel.The work was carried out around the three main points as following:(1)According to the microhardness distribution and the microstructure characteristics of the welded joint,the HAZ was divided iinto the coarse-grained HAZ(CGHAZ),fine-grained HAZ(FGFHAZ)and intercritical HAZ(ICHAZ).The welding heat source model and the thermal filed of the welding process for the metal-arc welding(MAG)was established through SYSWELD software.The peak temperatures of the four welding thermal cycles are 1320?,1020?,830? and 760 ?respectively.The enlarged simulated HAZs were produced by the thermal-mechanical simulator of Gleeble 3800.(2)The microstructures and mechanical properties of sub-HAZs was studied.After the welding thermal cycles,the tensile strength and hardness of the HAZs are much higher than that of the base metal,while the impact toughness of the HAZ deteriorate seriously.The tensile strength and hardness of FGHAZ are highest among all the sub-HAZs.The average impact energy of ICHAZ(Tp=830?)is highest,while the impact toughness of ICHAZ(Tp=760?)is poorest.The impact energy values of CGHAZ and FGHAZ keep stable while the impact absorbed energy values of ICHAZ fluctuate greatly.The microstructures of CGHAZ and FGHAZ are the quenched martensite.The microstructure of ICHAZ is the mixture of the quenched and tempered martensite.The microstructure interface in the ICHA7Z caused the fluctuation of the impact absorbed energy.In ICHAZ(Tp=760?),the ultra-fine grains and undissolved carbides makes the crack initiation process easy,which takes an adverse effect on impact toughness.(3)The effect of tempering temperatures(550,600 and 650 ?)on the microstructures and mechanical properties of the HAZs was studied.From the as welded state to the tempering condition,the tensile strength decreases due to the microstructural transformation from the tempered martensite to the quenched martensite.With the raise of tempering temperature,the disappearance of low angle boundaries caused the decrease of tensile strength.The size and distribution of the carbides significantly influence the impact toughness.As compared with the as-welded simples,the impact toughness of CGHAZ was not obviously promote after tempering.The impact toughness of FGHAZ was obviously promoted only when tempered at 650 ?.The impact toughness of ICHAZ raised obviously after tempering.When tempered at 550 ?,the large inter-lath M3C M7C3 and M23C6 carbides can be the nucleation sites of cleavage cracks in CGHAZ and FGHAZ,which deteriorated the impact toughness.When temper at 650?,the dispersive distributed M2C and MC carbides promote the impact toughness of CGHAZ and FGHAZ.The fine tempering martensite in ICHAZ influence the shape of the inter-lath carbides,the carbides with smaller length/width ratio in ICHAZ(Tp=760?)failed to trigger the cleavage fracture,the impact toughness of ICHAZ(Tp=760?)is relatively higher as compared with the as-welded state.The fine microstructure in ICHAZ makes the impact toughness relatively higher than that of CGHAZ and FGHAZ at the same tempering condition.