The nickel-based high-temperature alloy Inconel 617 (Ni-23Cr-13Co-9Mo) is used as the material of choice for supercritical thermal power units and ultrahigh-temperature reactors.
In the manufacturing process of thermal power and nuclear power equipment, welding technology is an essential and important link. Nickel-based high-temperature alloy welding technology is difficult, high quality requirements, is an important factor restricting its application. Laser welding is a high-efficiency and high-quality welding method with concentrated heating, low heat input and large depth-to-width ratio, and has been widely used in aerospace, nuclear power and automotive fields.
In this paper, the nickel-based high temperature alloy Inconel 617 as the object of study, the study of its laser welding process, joint organization and performance, as well as the organization of thermal stability. 5 mm thick Inconel 617 laser self-melting weld, with the laser power to improve, to obtain the full penetration of the weld heat input required to reduce the weld morphology from the Y-shape to the I-shape, the weld dendritic orientation tends to converge, dendritic refinement, carbide content and size. The dendrite orientation of the weld tends to be consistent, the dendrite is refined, the carbide content and size are reduced, and the hardness is increased.
By optimizing the process parameters of Inconel 617 laser narrow gap filler wire welding, we obtained 11 mm thick Inconel 617 welded joints without unfused, porosity and solidification cracks, and its total heat input was reduced by 60-70% compared with that of TIG filler wire welding of 11 mm thick Inconel 617.
The interlayer organization of the weld has a local remelted fine grain area, and the precipitation phase is mainly M_6C, M(C,N) (M=Cr, Ni or Ti) and M_(23)C_6 carbides. There is obvious liquefaction in the heat-affected zone within the grain and at the grain boundaries, and a small amount of liquefaction cracks are generated at the grain boundaries.
There is a small amount of secondary M_(23)C_6 carbide precipitation in the crystal and the dislocation density in the heat-affected zone is significantly higher than that of the base material, resulting in a higher hardness in the heat-affected zone than that of the base material. Through the Inconel 617 laser welding heat-affected zone organization and liquefaction cracking research, the heat-affected zone of the thermal cracking has liquefaction cracking and composite thermal cracking two kinds of liquefaction cracking is dominant. Laser welding, the base material of M_(23)(C,B)_6 carbide composition liquefaction and large angle grain boundary leads to grain boundary continuous Mo-rich and Cr-rich liquid film generation, and in the welding thermal stresses under the action of the continuous grain boundary liquid film is torn, resulting in liquefaction cracking.
Laser welding, improve the heat input, preheating temperature, the amount of focus and the use of appropriate pre-weld heat treatment process can significantly reduce the affected zone grain boundary liquefaction cracks, while improving the heat input can reduce the heat-affected zone composite cracks, or even completely eliminate the composite cracks, and pre-weld preheating can be to a certain extent can be reduced to reduce the composite cracks.
Heat-affected zone grain boundaries have a large number of liquid film migration (LFM), which is mainly due to the M_(23)(C,B)_6 carbide liquefaction process released from the element Mo and the matrix γ mismatch between the degree of the lattice strain caused by the co-lattice. The slow heating and cooling rates and the small size of M_(23)C_6 carbide contribute to the liquid film migration, so that the high heat input and the pre-weld solution treatment (1100°C/1 h) reduce the liquid film thickness at the grain boundaries, and thus reduce the heat-affected zone liquefaction cracks.The large residual stresses and high dislocation densities in the heat-affected zones of the weld states of the 750°C and 850°C holding time of 500 h lead to a large number of fine intra-grain precipitations in the heat-affected zones of the weld states. Zone crystal precipitation of a large number of small precipitation phase (M_(23)C_6 carbide and γ ‘phase), while 950 ℃ insulation small M_(23)C_6 carbide accelerated dissolution of the heat-affected zone of the crystal without a large number of carbide precipitation and the number of significantly smaller than the base material.
With the prolongation of the holding time, the dissolution of M_(23)C_6 carbides in the heat-affected zone crystals, γ’ phase coarsening. 750 ℃ long-term holding time, the weld M_6C-type carbides (η_1-M_6C, η_2-M_(12)C), M_(23)C_6 carbides, nanometer-sized γ’ phase (Ni-3Al) and a small amount of Ti (C, N) and TCP-μ phase. The After a long holding time at 850℃ and 950℃, the weld metal has only irregular lumps of M_(23)C_6, η_1-M_6C and Ti(C,N), and there is no TCP-μ phase and γ’ phase. The direct reaction of solute atoms Mo and Cr with C atoms in the matrix, the transformation of M_6C and M_(23)C_6 carbides with each other, the transformation of (Cr,Ni)(C,N) in the weld and the decomposition of Ti(C,N) in the weld in the weld state lead to the precipitation of M_6C and M_(23)C_6 carbides.
With the increase of holding temperature, the content and quantity of M_6C carbides in the weld decreased significantly, and the M_(23)C_6 carbides were coarsened significantly.After the long-term holding temperature of 750℃ and 850℃, the content of nano-sized M_(23)C_6 carbides in the crystal decreased gradually, and the size and content of M_6C carbides increased, and after the holding temperature of 950℃ the content of M_(23)C_6 carbides increased, and the size and content of M_6C Carbide content did not change significantly, the size increased slightly. 750 ℃ insulation, weld rod M_6C carbide and the matrix to maintain the lattice or semi-lattice to non-lattice transition. The coarsening of M_6C carbides in the weld is mainly based on the Ostwald ripening theory and LSEM theory of M_(23)C_6 and M_6C carbides. The coarsening of M_(23)C_6 carbides is mainly due to the Ostwald ripening of secondary M_(23)C_6 carbides in the crystal, and the coarsening rate of intracrystalline is obviously lower than that at the grain boundary.
During the long-term holding process at 750°C, the μ-phase precipitates from specific crystalline surfaces of the matrix and has specific orientation relationships with the matrix: (111)_γ//(0001)_μ, [110]_γ//[11~-2 0]_μ and (111)_γ//(01~-11)_μ, [011]_γ//[11~-2 0]_μ; laminar growth is the main growth mode of the μ-phase. growth mode. With the increase of holding time, the coarsening of γ’ phase is caused by Ostwald ripening, and the mismatch with matrix γ is very small, and its shape is nearly spherical and has good thermal stability. In the early stage of holding time (0~500 hours), the hard and brittle carbides (M_6C, M_(23)C_6) at the grain boundaries and the dispersed M_(23)C_6 carbides and γ’ phases in the grain can significantly reduce the impact toughness of the weld and improve the room temperature strength and hardness of the joint. When the holding time is more than 500 hours, the coarsening of discontinuous M_6C and M_(23)C_6 carbides at the grain boundary can reduce the room temperature strength; the precipitation of a large number of discontinuous M_(23)C_6 carbides and the increase in the spacing of the M_(23)C_6 carbides at the grain boundary can improve the impact toughness of the weld, while the M_6C carbides will reduce the impact toughness.
The μ-phase of TCP has no significant effect on the mechanical properties of the joints due to its low content.
Post time: Aug-05-2023
