Study On Embrittlement Mechanism Of Heat Affected Zone In Laser-arc Hybrid Welding Of EQ70 Steel

Laser-arc hybrid welding（LAHW）technology is a high-quality and efficient welding process,which has been used in the welding of low strength offshore engineering steels,such as E36 and S355 steels.And LAHW has remarkable effect on improving welding efficiency,guaranteeing joint quality,controlling structural deformation and so on.However,there is a lack of systematic research on the LAHW of ultra-high strength offshore engineering steels,and the toughness fluctuation and local embrittlement exist in the heat affected zone（HAZ）.The influencing factors and embrittlement mechanism are not clear yet.EQ70 ultra-high strength steels were studied in this paper,the local brittle zone（LBZ）of the LAHW joint of EQ70 steels were determined through the microstructure and property investigation.The welding thermal cycle characteristics of the LAHW and its effect on the HAZ microstructure transformation were studied by welding thermal cycle measurement and finite element numerical simulation.The LAHW HAZ homogeneous specimens of EQ70 steels were obtained by the Gleeble thermal simulation technology.With the help of microstructure and property characterization,the microstructure unit which caused the toughness reduction of the LBZ was determined,and the relationship between the LBZ microstructure and toughness was analysed.The formation mechanism of embrittlement microstructure and its influence on the initiation and propagation of cracks was analysed,the embrittlement mechanism of the LBZ was clarified through the multi-scale substructure and crystallographic orientation investigation.Firstly,the LAHW joints of EQ70 steels without obvious defects were obtained.The impact test,micro-shear test,fracture surface analysis and in-situ microstructure observation of the LAHW joint showed that the coarse grained HAZ（CGHAZ）and the inter-critically reheated CGHAZ（ICCGHAZ）were the LBZs.The secondary crack analysis results showed that the crack propagation deflected or terminated at the grain boundaries above 15°,the high angle grain boundaries（HABs）above 15°can effectively hinder the crack propagation,and the block effect of the HABs above 45°was more obvious.The multi-scale substructure analysis showed that the block was the microstructure unit controlling the fracture process of the lath martensite（LM）.The large grain size together with the necklace blocky Martensite-Austenite（M-A）constituents on the grain boundaries were the main reasons for the HAZ toughness reduction.Finite element numerical simulation technology was used to simulate the temperature field of the LAHW joint of EQ70 steels.The double ellipsoid and heat flux peak increasing cone combined heat source can accurately simulate the LAHW temperature field,and the welding thermal cycle parameters obtained by the numerical simulation were basically the same as those obtained by the thermal cycle measurement.The average heating rate of the LAHW thermal cycle was above 400℃/s,the high temperature residence time t₁₁₀₀ was only 0.8 s～1.3 s,and t_8/5 was only 4 s～6 s.The same micro-zones of the arc-zone,overlapping,laser-zone of the EQ70 LAHW joints had the similar welding thermal and microstructures.The different HAZ micro-zones homogeneous specimens of the single-pass LAHW joint of EQ70 steels were obtained by the Gleeble thermal simulation technology.The instrumented impact test and fracture surface analysis showed that the total impact energy cannot truly reflect the toughness difference between the specimens.The crack initiation energies of the inter-critically HAZ（ICHAZ）,fine grained HAZ（FGHAZ）and CGHAZ were similar,both lower than that of the base metal and the sub-critically HAZ（SCHAZ）.The CGHAZ specimen had the lowest crack propagation energy and worst resistance to crack propagation,with a quasi-cleavage brittle fracture surface.The crack initiation energy and crack unstable propagation energy of the complete quenching zones remained unchanged as the increase of the peak temperature,while the total impact energy and the crack stable propagation energy reduced.The multi-scale substructure analysis showed that the prior austenite grain size（PAGS）,packet width,block width increased as the increase of the peak temperature,but the lath width barely changed.The quasi-cleavage facet size of the CGHAZ was similar to the block width,which proved that the block was the microstructural unit controlling the LM crack propagation process.The HAZ microstructure characterization of the double-pass LAHW joint simulated HAZ of EQ70 steels showed that the un-altered CGHAZ（UACGHAZ）composed of coarse LM,and the ICCGHAZ composed of LM with blocky M-A constituents distributed at grain boundaries and sub-grain boundaries.Necklace blocky M-A constituents distributed at prior austenite grain boundaries when the second peak temperature was 760℃.As the second peak temperature reached to 800℃,the necklace M-A constituents began to unlink and some distributed at the sub-grain boundaries.When the second peak temperature reached to 840℃,the prior austenite grain boundaries disappeared,the microstructure transformed into fine LM,and the grain boundary M-A constituents further dispersed.The impact test and fracture surface analysis showed that the ICCGHAZ and UACGHAZ owned relatively low impact energies,with cleavage and quasi-cleavage brittle fracture surfaces.The influence of the morphology,distribution,size and volume fraction of the M-A constituents on the ICCGHAZ impact toughness were studied by changing the welding thermal parameters.The results showed that the volume fraction of M-A constituents had little effects on the crack initiation energy.The large size of M-A constituents led to a low crack propagation energy.The necklace blocky M-A constituents distributed along grain boundaries significantly reduced the crack initiation energy.The toughness was improved when the M-A constituents were dispersed at grain boundaries or within grains.The CGHAZ martensite variants and the prior austenite were approximately K-S oriented.Variant combination occurred during the formation of LM in CGHAZ,forming sub-block substructure.The sub-block boundaries were low angle boundaries（LABs）.The extremely fast cooling rate of the LAHW process weakened the tempering effect,which resulted in the retention of twinned structure or twinned lath formed at the initial stage of the martensite transformation.The embrittlement mechanism of CGHAZ can be described as below.The fracture was crack propagation controlled,the large grain size,the twinned structure,the twinned lath and the sub-block both reduced the proportion of HABs and weakened the ability to resist the crack propagation,the micro-cracks propagated rapidly and finally led to the occurrence of brittle fracture.The microstructure characterization and crystallographic analysis showed that the ICCGHAZ inherited the large grain size,twinned structure,twinned lath and sub-block of the CGHAZ.The crystallographic relationship between martensite and prior austenite in the grain boundary M-A constituents was similar to K-S orientation.The M-A constituent was composed of the first sub-region with large grain size and the second sub-region with small grain size and twinned structure.The embrittlement mechanism of the ICCGHAZ can be described as follows.Under the combined action of stress superposition effect and non-cooperative deformation,the grain boundary blocky M-A constituents were separated from the matrix,the micro-cracks initiated at the boundaries between the M-A constituents and the matrix.The grain boundary M-A constituents together with the large grain size,twinned structure,twinned lath and sub-block,both reduced the HABs proportion and weakened the ability to resist crack propagation,leading to the rapid propagation through the grain,and finally brittle fracture occurred.