| 5%Ni steel(SA645),as a martensitic cryogenic nickel steel,is mainly applied for manufacturing the storage tanks for liquefied ethane gas(LEG)and liquefied petroleum gas(LPG)due to its excellent strength and low-temperature toughness.However,the material utilized to fabricate other components,such as valves and pipes that directly connect to the tank,is normally 304 stainless steel(AISI304).The dissimilar joints between 5% Ni steel and 304 stainless steel are indispensable in the fabrication of cryogenic storage tanks.When using traditional arc welding to weld martensitic steel and austenitic steel,the problems such as severe magnetic blow,high heat input,low efficiency,and high cost of nickel-based alloy consumables are difficult to solve.Laser welding has great potential and practical significance in achieving the efficient connection of these two dissimilar steels because of its high efficiency and precision,small heat input,small welding distortion,and high welding quality.In this paper,the dissimilar welding of SA645 and AISI304 was realized by autogenous laser welding.Since the full martensite structure is easily formed in the weld metal,the weld metal has higher strength and poor low-temperature toughness.Aiming at the problem of the mismatch of the strength and toughness of the weld,the post-weld heat treatment and the optimization of the elements in the weld are proposed to improve the microstructure of the weld and enhance the cryogenic toughness of the weld.On this basis,the low-temperature toughening mechanism and cryogenic fracture behavior of the weld are studied,which provides experimental support and theoretical basis for autogenous laser welding of SA645 and AISI304.The high-density dislocation entanglement of the martensite structure and the appearance of twinned martensite are the main reasons for the poor low-temperature toughness of full martensite weld metal.Aiming at the deterioration of cryogenic toughness of full martensite weld metal,the effect of intercritical tempering on the microstructure and properties of the welds was studied.It is found that during the intercritical tempering,the content of reversed austenite gradually increased with the increase of the tempering temperature,and reached the maximum when tempering at600℃,and then the content of reversed austenite decreased with the further increase of the tempering temperature.The reason is that as the tempering temperature increases,the enrichment of stable elements in austenite decreases and the austenite grain size increases,resulting in the decrease in the stability of austenite.The EDS analysis results show that the blocky reversed austenite distributed on the high-angle grain boundaries was formed by the diffusion of Cr,Ni,and Mn.The acicular reversed austenite within martensite blocks has a similar chemical composition as martensite matrix.Considering the high driving force of transformation and no obvious segregation of elements,the reversed austenite within martensite blocks may belong to diffusionless shear reverse transformation.The variation of the strength of the weld at different tempering temperatures is mainly determined by the contribution of dislocation strengthening.Tempering at 450℃-600℃ can significantly improve the low-temperature toughness of the weld,which is mainly due to the coordinating effect and TRIP effect of the reverse austenite and the toughening of the martensite matrix.When tempered at 600 ℃,the reverse austenite content is the highest,the coordinating effect and TRIP effect are sufficient,and the martensite matrix recovery is also sufficient,so the low temperature toughness is the best(～40J).When the tempering temperature is higher than 600 ℃,the low temperature toughness of the weld decreases due to the decrease of reversed austenite content and the increase of average dislocation density of martensite.Due to the composition limitation of the weld with nearly equal fusion ratio,the heat treatment regulation is still insufficient to make the low temperature toughness of the joint meet the application requirements.However,the study clarified the low-temperature toughening mechanism of dual phase(a’+g)structure,and provided a theoretical basis and direction for further optimization of weld microstructureIn order to further improve the low-temperature toughness of the weld to meet the application requirements,based on the characteristics of precise and controllable laser welding energy,the laser beam focus is shifted to the AISI304 side to control the melting amount of the two base metals,so as to optimize the alloy composition of the weld and improve the metallurgical properties of the weld.It is found that with the increase of beam offset,the Ni equivalent and Cr equivalent in the weld gradually increase,and the weld transforms from single-phase martensite to a dual-phase structure of martensite/retained austenite,and the content of retained austenite gradually increase.But when the beam offset reaches 0.6mm,due to the higher Cr/Ni equivalent ratio,δ-ferrite begins to appear in the weld metal.The low-temperature impact toughness of the dual-phase weld metal increases significantly.The weld metal with the beam offset of0.3 mm(Fe-12.52Cr-7.07Ni-0.916Mn)exhibits the best low-temperature impact toughness(～72J),which meets the standard requirements.However,when the beam offset is too large,the low-temperature impact energy of weld metal containing δ-ferrite will decrease instead.The reason is that the TRIP effect of retained austenite in the weld reduces the strain or stress concentration at the front of the crack to a certain extent,which makes the crack deflect and hinders the crack propagation.As the content of retained austenite increases,the TRIP effect increases,the crack propagation resistance increases,thus the low temperature toughness of the weld metal increases.However,when δ-ferrite occurs in weld metal,due to its low shear deformation resistance,strain or stress concentration is easy to form at the interface of δ-ferrite during deformation,which weakens the TRIP effect of retained austenite and makes it difficult to effectively prevent crack propagation,therefore the low temperature toughness of weld metal decreases.In addition,when the content of retained austenite is over ～18%,it is difficult to effectively inhibit the transformation of retained austenite due to the great decrease of strength / hardness of surrounding martensite matrix,thus part of retained austenite transforms into martensite during cryogenic treatment.Finally,considering the low-temperature safe service of the joint and the non-uniform structure of the SA645/AISI304 laser weld,it is very necessary to conduct an in-depth study on the low-temperature fracture toughness of the weld metal.Taking the Fe-12.52Cr-7.07Ni-0.916 Mn weld metal(beam offset 0.3mm)with the best impact toughness as the research object,the relationship between fracture toughness,fracture morphology and microstructure of weld metal at low temperature was studied,and the effect of microstructure on fracture behavior of weld metal at low temperature was analyzed from the perspective of crack propagation.It is found that the F-V curve of the weld metal does not fluctuate when the precrack position is far away from the weld center due to due to the feature of columnar dendrites symmetrically distributed along the center of the weld metal,and the CTOD value is about 0.416 mm.The CTOD value is about 0.471 mm when the precrack is along the weld center,but the F-V curve of the weld has local fluctuation in the early stage.The local fluctuation of F-V curve is closely related to the position and propagation direction of precrack.When the pre crack is located at the central of the weld,the crack is easy to propagate rapidly along the central interface of the weld in the crack initiation zone,which leads to the fluctuation of F-V curve.Due to the restraining effect of the unevenly distributed retained austenite and the triple junction of grain boundaries,the crack propagation will quickly be deflected into the stable propagation zone.The high-angle grain boundaries in the stable propagation zone have obvious resistance to crack propagation,and the retained austenite also produces TRIP effect,which makes the resistance to crack propagation significantly enhanced,and thus the F-V curve fluctuation phenomenon disappears.The research on crack propagation behavior of weld metal at low temperature provides theoretical guidance for further optimization of weld microstructure. |
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