CN112522544B - Grain boundary regulation and control method for improving weldability of cast high-temperature alloy and welding process - Google Patents
Grain boundary regulation and control method for improving weldability of cast high-temperature alloy and welding process Download PDFInfo
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Abstract
The invention discloses a grain boundary regulating method and a welding process for improving the weldability of cast high-temperature alloy, belonging to the technical field of high-temperature alloy materials. The grain boundary regulating method comprises the following steps: after carrying out solution heat treatment on the as-cast alloy plate, cooling the as-cast alloy plate by adopting a slow cooling mode in a furnace, wherein the cooling rate is 2-5 ℃/min, cooling to below 300 ℃, and taking out; and welding the base metal after the grain boundary regulation by adopting a TIG welding process, wherein the welding current is 15-35A, the welding voltage is 10-12V, and the welding speed is 3-5 cm/min. In the course of welding, grain boundary precipitation phase M23X6Solid solution occurs at high temperature, and the liquid film grain boundary is continuously and uniformly distributed under the action of B element segregation. In the welding and cooling process, the continuity of the sliding deformation of the grain boundary is ensured by the continuous liquid film, so that the high-temperature alloy has good crack resistance. Meanwhile, the serrated crystal boundary formed by the crystal boundary regulation and control method can effectively hinder the crack damage process in the joint heat affected zone.
Description
Technical Field
The invention relates to the technical field of high-temperature alloy materials, in particular to a grain boundary regulation and control method and a welding process for improving the weldability of cast high-temperature alloy.
Background
Ni-based superalloys are based primarily on coherent L12The-gamma ' phase is strengthened, wherein the gamma ' phase is uniformly distributed in a matrix of an FCC phase in a cubic shape, and dislocation deformation is difficult to cut through the gamma ', so the alloy often has good high-temperature structure stability, creep deformation resistance and excellent fatigue resistance and oxidation resistance. For members such as industrial gas turbines and aircraft engines which are subject to severe service conditions such as high temperature and high stress, the excellent mechanical properties of Ni-based high temperature alloys make them ideal structural materials. The M951 superalloy is a cast superalloy independently developed by the institute of Metal, national academy of sciences, and has excellent high-temperature tensile properties, good creep deformation resistance, excellent high-temperature oxidation resistance, good castability, and low cost. These excellent properties have led to the widespread use of M951 superalloys in the field of industrial gas turbines and aircraft engines.
The fusion welding is an economic, convenient and efficient material connecting and repairing method, the application of the fusion welding method in the high-temperature alloy structural material has obvious economic benefits and important practical significance, and meanwhile, the material with good weldability can also have wider application in practical industrial application. However, in order to maintain excellent high-temperature mechanical properties of the superalloy, the addition of various alloying elements, such as Cr, Mo, Al, Ti, etc., causes a significant deterioration in weldability of the superalloy. During the welding process of fusion welding, due to welding stress and formation of an unbalanced welded structure, welding heat cracks of various kinds such as a liquefied crack, a solidification crack, and the like are very easily formed in a heat affected zone of a joint and a fusion zone. These frequently occurring weld cracking problems are major factors that limit the applicability of the fusion welding process to superalloy materials. In the process of welding temperature rise, the strength of the grain boundary at high temperature is weaker than that in the crystal, so the grain boundary is the main position for forming welding cracks, and the precipitated phase composition, the element segregation state and the morphology of the grain boundary can be effectively improved by adopting a reasonable grain boundary regulation and control method, so that the welding performance of the alloy material is obviously improved, and the welding hot crack resistance of the alloy is greatly improved.
At present, how to improve the welding heat crack resistance of a high-temperature alloy material M951 with high application value and wide potential application range by a grain boundary regulation and control method and an action mechanism of the grain boundary regulation and control on the improvement of the welding crack resistance of the alloy are not clear. In order to expand the application of the M951 high-temperature alloy in certain specific fields and enable the fusion welding method to be successfully applied to the connection and repair of the M951 alloy, a reasonable grain boundary regulation and control method needs to be developed to realize the application of the fusion welding process in the M951 alloy.
Disclosure of Invention
The invention aims to provide a grain boundary regulating method and a welding process for improving the weldability of cast high-temperature alloy. The invention can obviously improve the weldability of various high-temperature alloys including the casting high-temperature alloy M951, and the welding crack sensitivity is obviously improved. The method has important practical significance in the connection and repair of Ni-based high-temperature alloys such as M951 and the like, and therefore has important industrial application and economic benefit values.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a crystal boundary regulation and control method for improving weldability of cast high-temperature alloy is used for carrying out crystal boundary regulation and control on a cast high-temperature alloy base metal, and comprises the following steps:
(1) preparing M951 master alloy ingots by vacuum induction melting, remelting the master alloy ingots, and casting the master alloy ingots into cast alloy plate samples by a sand casting method;
(2) solution heat treatment: carrying out solution heat treatment on the as-cast M951 alloy plate by using a box type resistance furnace to ensure the homogenization of the components of the alloy plate;
(3) regulating and controlling boundary crystals: cooling the alloy plate sample after the solution heat treatment, wherein the furnace temperature in the cooling process is controlled by adopting a slow cooling mode in the furnace; wherein: the cooling rate is 2-5 ℃ per minute, and the cast high-temperature alloy base metal after crystal boundary regulation is obtained after the temperature is cooled to below 300 ℃.
The M951 alloy comprises the following chemical components (wt.%): c: 0.03-0.15%; cr: 8-10%; co: 4.5-5.5%; al: 5.5-6.2%; w: 2.5-4.5%; mo: 2.5-3.5%; nb: 1.8-2.4%; b: 0.001-0.04%; and the balance of Ni.
In the step (1), in the process of smelting and remelting alloy raw materials by adopting vacuum induction smelting, the smelting temperature is 1550-1500 ℃, the heat preservation time is 10-30 min, the smelting is repeated for 2-4 times, and the casting is carried out at 1460-1430 ℃; smelting the alloy raw materials to obtain an M951 alloy ingot, wherein the diameter phi (80 +/-5) mm and the length are more than or equal to 250 mm; and (3) carrying out 100% surface turning and polishing on the M951 alloy ingot, cutting off a shrinkage cavity part, then smelting again, and pouring an as-cast alloy plate sample with the length of 20-22 mm, the width of 20-22 mm and the thickness of 3-4 mm by adopting a sand casting method.
In the step (2), the solution treatment process comprises the following steps: the furnace temperature of the resistance furnace is kept at 1230-1235 ℃, and the heat preservation time is 2-3 hours.
In the step (3), the microstructure characteristics of the cast superalloy parent metal after grain boundary regulation are as follows: form continuous chain M at the grain boundary23X6A boride precipitate phase and a massive MC phase; the segregation of B element occurs obviously at the grain boundary; the straight grain boundaries are transformed into jagged grain boundaries.
Welding is carried out on the cast high-temperature alloy base metal subjected to grain boundary regulation, welding wires with the same chemical components as the base metal are adopted, a welding process is utilized, in the welding process, the edge of the cast high-temperature alloy base metal (M951 welding plate) is limited and fixed through a clamp, and the welding parameters are as follows: the welding current is 15-35A, the welding voltage is 10-12V, the welding speed is 3-5 cm/min, and the protective gas is argon with the purity of 99.9%.
The preparation process of the welding wire comprises the following steps: preparing raw materials (C, Ni, Al, Nb, Mo, Co, Cr, W pure metal and Ni-B intermediate alloy) of a welding wire according to nominal components of an M951 alloy, putting the raw materials of the welding wire into a water-cooled copper crucible of a vacuum smelting furnace for smelting, keeping the smelting temperature at 1550-1500 ℃ for 10-30 min, and repeatedly smelting for 2-4 times; pouring the alloy melt at 1460-1430 ℃ to obtain an M951 alloy ingot with the diameter phi (80 +/-5) mm and the length more than or equal to 250 mm; and (3) turning and polishing the surface of the alloy ingot by 100%, cutting a shrinkage cavity once, and then cutting the welding wire. Cutting into the standard size of the required welding wire by adopting a linear cutting mode, wherein the diameter of the welding wire is phi 1 mm-phi 3mm, and the length range is 150-400 mm.
Cutting the welded plate sample along the direction vertical to the welding direction, observing the cross section of a welding joint, and counting the average crack length of each welding surface according to a formula (1); the average crack length of the welded plate sample is reduced to below 46 mu m;
in equation (1):
for the average crack length on each weld face, n is the total number of statistical cracks, LiThe length of the ith crack.
The invention has the following beneficial effects:
at present, aiming at a high-temperature alloy material M951 with high application value and wide potential application range, how to improve the welding heat crack resistance by a grain boundary regulation and control method and design the welding material of the M951 still lack related technical process guidance and specifications. In order to expand the application of the M951 high-temperature alloy in certain specific fields and enable the fusion welding process to be successfully applied to the connection and repair of the M951 alloy, a reasonable grain boundary regulation and control method needs to be developed to realize the application of the fusion welding process in the M951 alloy. The grain boundary regulation and control method has the following beneficial effects that:
(1) after the grain boundary treatment process, the state of the grain boundary microstructure is regulated and controlled. The M23X6 precipitated phase continuously precipitated along the grain boundary and the B element segregated along the grain boundary, and the jagged grain boundary microstructure are the key reasons for the improved welding crack resistance of the alloy. During the heating process of the welding heat cycle, under the action of high temperature in a heat affected zone close to a melting zone, a grain boundary precipitated phase M23X6 and an MC phase are locally dissolved in a solid solution, and because solute elements are difficult to diffuse for a long distance in a short time, local segregation is formed at the grain boundary of the heat affected zone, and a liquefaction process is generated in the heat affected zone during the welding heating process.
(2) Because the B element is distributed in the grain boundary, a skin effect is formed, the solid-liquid interface energy of the liquid film can be obviously reduced, and the ductility of the liquid film is favorably improved, so that the grain boundary movement deformation at high temperature is coordinated, and the crack resistance of the alloy is improved.
(3) The bent crystal boundary effectively inhibits the expansion process of cracks in the liquid film, the resistance of the liquid film after cracking is formed and expanded along the bent crystal boundary is increased, the cracks can not expand or even heal along with the accumulation of residual stress in the solidification process of the welding cooling liquid film, and after the liquid film is solidified, the crystal boundary phase is subjected to the remelting process, so that the total length of the welding hot cracks can be controlled to be an extremely low value. The series of grain boundary microstructures is changed along with the heating process and the cooling process of welding, and a schematic diagram of the process is shown in a figure (3).
The invention discloses a method for improving weldability of M951 high-temperature alloy by regulating and controlling a grain boundary structure and a preparation method of a welding material. At present, in certain specific occasions, the M951 high-temperature alloy structural member needs to be repaired and connected through a fusion welding process urgently, and meanwhile, fusion welding is a flexible, economic and efficient process method, and the application of the process method has important practical significance, so that the grain boundary regulation and control process has important industrial application and economic benefit values.
Drawings
FIG. 1 shows a grain boundary M23X6And (4) characterizing the morphology of the projection microstructure of the precipitated phase.
FIG. 2 is a representation of B element distributed in grain boundary by secondary ion mass spectrometry (TOP-SIMS); wherein: (a) characterizing the SEM microstructure corresponding to the area; (b) distribution intensity of B element.
FIG. 3 shows the chain shape M by regulating grain boundary23X6The formation of precipitated phases controls the principle of welding hot cracks.
FIG. 4 is a M951 base metal obtained by the grain boundary control method in example 1; wherein: (a) a grain boundary microstructure obtained by a grain boundary regulation method; (b) the joint section microscopic structure after TIG welding; (c) grain boundary weld heat checking in the joint heat affected zone.
FIG. 5 is a M951 base metal obtained by the grain boundary control method in example 2; wherein (a) a grain boundary microstructure obtained by the grain boundary regulation method; (b) the joint section microscopic structure after TIG welding; (c) a re-solidified microstructure in the heat affected zone of the joint.
FIG. 6 is an M951 base material obtained by the grain boundary regulation method in comparative example 1; wherein: (a) a grain boundary microstructure obtained by the grain boundary regulation and control method; (b) the joint section microscopic structure after TIG welding; (c) a re-solidified microstructure in the heat affected zone of the joint.
Detailed Description
The grain boundary regulating method for improving the weldability of M951 in the invention is described in detail below with reference to the accompanying drawings and specific examples.
Example 1
5Kg of pure Ni, Al, Nb, Mo, Co, Cr, W and master alloy Ni-B metal material in the nominal composition ratio of M951 are placed in a water-cooled copper crucible of an electric arc melting furnace. Smelting the alloy raw materials by vacuum induction smelting, repeatedly smelting for 2-3 times, preserving heat for 20min at 1500 ℃, pouring at 1460 ℃, and pouring the smelted and homogenized alloy melt into a plate-shaped sample.
After the plate-shaped sample vacuum tube sealing completed by smelting and pouring the M951 base metal is carried out, the as-cast M951 alloy plate is subjected to solution heat treatment before welding by adopting a box-type resistance furnace, the temperature of the resistance furnace is kept at 1230 ℃, and the heat preservation time is 2 hours. The sample cooling process is carried out in a resistance furnace, argon is introduced to control the cooling rate, the cooling rate is average 2 ℃ per minute, and after the sample is cooled to below 300 ℃, the sample is taken out of the resistance furnace.
In this example, after the M951 base material was subjected to grain boundary control treatment, grain boundaries formed microstructures having three characteristics: (1) grain boundary formation of continuous M23X6The morphology of the transmission electron microscope bright field phase of the M23X6 phase and the diffraction spots in selected areas are shown in figure 1; (2) b element segregation, which forms a continuous distribution on grain boundaries, is shown in fig. 2; (3) the grain boundary regulating methodCausing a saw-tooth transition at the grain boundaries.
As shown in fig. 4a, the grain boundary microstructure of the master alloy obtained by the grain boundary control method is shown. It can be observed that the grain boundary forms micron-sized M in chain distribution by the grain boundary regulation and control method23X6And separating out a phase, and simultaneously bending the appearance of a grain boundary.
And carrying out single-pass fusion welding on the master alloy plate with the regulated and controlled crystal boundary by adopting a TIG welding method, wherein the components of the filling material are the same as those of the M951 alloy, and the edge of the M951 welding plate is limited and fixed by a clamp. The welding parameters are 35A of welding current, 12V of welding voltage, 5cm of welding speed per minute on average, 99.9% of Ar gas of protective gas and 35mm of welding bead length, after welding is finished, the welding bead is divided by wire cutting, the cross section of the welding bead is observed by SEM, and the length and the number of cracks in a welding melting area and a heat affected area are counted.
FIG. 3 shows the chain shape M by regulating grain boundary23X6The formation of precipitated phases controls the principle of welding hot cracks.
FIG. 4(b) shows the microstructure of the joint cross section after TIG welding, the number of weld cracks generated in the joint is small, the average crack length per surface is 46 μm, and is far lower than the average crack length of the weld cracks generated in the joint after standard heat treatment (2883 +/-365 μm). Fig. 4(c) is a microcrack in the heat-affected zone after TIG welding. After the crystal boundary precipitated phase is subjected to welding heat cycle, the process of dissolution and re-solidification is carried out, the crystal boundary is liquefied, but due to the action of the B element, the wettability of the crystal boundary is improved, and the cracking of the liquefied cracks is inhibited. The weldability of the M951 alloy is significantly improved by the grain boundary regulation method.
Cutting the welded sample along the direction perpendicular to the welding direction, observing the cross section of the welded joint, and counting the average crack length of each surface
Where n is the total number of statistical cracks, LiThe length of the ith crack. TIG welding joint for welding after standard solid solution heat treatment (1230 ℃/2 h/air cooling)The samples were compared and the average crack length of the welded joint of the present example was found
The great reduction is from 6250 μm +/-320 μm (welding after standard solution heat treatment) to 46 +/-5 μm.
Example 2
Composition Ni in M95171.6Cr9Co5Al5.8W3.5Mo3Nb2B0.024C0.025Kg of pure Ni, Al, Nb, Mo, Co, Cr, W and master alloy Ni-B metal materials (the components are in mass ratio) are placed in a water-cooled copper crucible of an electric arc melting furnace. Smelting the alloy raw materials by vacuum induction smelting, repeatedly smelting for 2-3 times, preserving heat for 20min at 1500 ℃, pouring at 1460 ℃, and pouring the smelted and homogenized alloy melt into a plate-shaped sample.
After the plate-shaped sample vacuum tube sealing completed by smelting and pouring the M951 base metal is carried out, the as-cast M951 alloy plate is subjected to solution heat treatment before welding by adopting a box-type resistance furnace, the temperature of the resistance furnace is kept at 1230 ℃, and the heat preservation time is 2 hours. The sample cooling process is carried out in a resistance furnace, argon is introduced to control the cooling rate, and the cooling rate is 2 ℃ per minute on average until the sample is cooled to room temperature.
And performing single-pass fusion welding by adopting a TIG welding method, wherein the components of the filling material are the same as those of the M951 alloy, and the edge of the M951 welding plate is limited and fixed by a clamp. The welding parameters are 35A of welding current, 12V of welding voltage, 5cm of welding speed in average minutes, 99.9% of Ar gas of protective gas and 35mm of welding bead length, after welding is finished, the welding bead is divided by wire cutting, the cross section of the welding bead is observed by SEM, and the length and the number of cracks in a welding melting area and a heat affected area are counted.
Fig. 5(a) shows a grain boundary microstructure of the master alloy obtained by the grain boundary control method. It can be observed that the grain boundary forms micron-sized M in chain distribution by the grain boundary regulation and control method23X6And separating out a phase, and simultaneously bending the appearance of a grain boundary. FIG. 5(b) shows a cross-sectional microscopic view of the joint after TIG weldingThe joint was organized and no significant hot weld cracks were observed in the joint. FIG. 5(c) shows the microstructure morphology after remelting and solidifying the grain boundary in the heat affected zone without formation of weld heat cracks, wherein the grain boundary phase M is a chain-like grain boundary phase after the re-solidification process23X6The phase and the MC phase are transformed into a lamellar eutectic structure. After the grain boundary regulation and control method is adopted, the weldability of the high-temperature alloy M951 is remarkably improved.
Comparative example 1
5Kg of pure Ni, Al, Nb, Mo, Co, Cr, W and master alloy Ni-B metal material in the nominal composition ratio of M951 are placed in a water-cooled copper crucible of an electric arc melting furnace. Smelting the alloy raw materials by vacuum induction smelting, repeatedly smelting for 2-3 times, preserving heat for 20min at 1500 ℃, pouring at 1460 ℃, and pouring the smelted and homogenized alloy melt into a plate-shaped sample.
After the plate-shaped sample vacuum tube sealing completed by smelting and pouring the M951 base metal is carried out, the as-cast M951 alloy plate is subjected to solution heat treatment before welding by adopting a box-type resistance furnace, the temperature of the resistance furnace is kept at 1230 ℃, and the heat preservation time is 2 hours. The sample cooling process is carried out in a resistance furnace, and the sample is directly cooled to room temperature without slow cooling in the furnace.
And performing single-pass fusion welding by adopting a TIG welding method, wherein the components of the filling material are the same as those of the M951 alloy, and the edge of the M951 welding plate is limited and fixed by a clamp. The welding parameters are 35A of welding current, 12V of welding voltage, 5cm of welding speed in average minutes, 99.9% of Ar gas of protective gas and 35mm of welding bead length, after welding is finished, the welding bead is divided by wire cutting, the cross section of the welding bead is observed by SEM, and the length and the number of cracks in a welding melting area and a heat affected area are counted.
Fig. 6(a) shows a grain boundary microstructure of the master alloy obtained by the grain boundary control method. It can be observed that the grain boundary forms micron-sized M in chain distribution by the grain boundary regulation and control method23X6And separating out a phase, and simultaneously bending the appearance of a grain boundary. Fig. 6(b) shows the microstructure of the joint cross section after TIG welding, and it can be observed that no significant hot cracks are generated in the joint. FIG. 6(c) shows the heat of fusion not formedRemelting microstructure morphology after solidification of crystal boundary in heat affected zone of crack, and re-solidifying crystal boundary to obtain chain-like crystal boundary phase M23X6The phase and the MC phase are transformed into a lamellar eutectic structure. After the grain boundary regulation and control method is adopted, the weldability of the high-temperature alloy M951 is remarkably improved.
Claims (6)
1. A grain boundary regulation and control method for improving the weldability of cast high-temperature alloy is characterized in that: the method for regulating and controlling the grain boundary of the cast high-temperature alloy parent metal comprises the following steps:
(1) preparing M951 master alloy ingots by vacuum induction melting, remelting the master alloy ingots, and casting the master alloy ingots into cast alloy plate samples by a sand casting method; the M951 alloy comprises the following chemical components in percentage by weight: c: 0.03-0.15%; cr: 8-9%; co: 4.5-5.5%; al: 5.5-6.2%; w: 2.5-4.5%; mo: 2.5-3.5%; nb: 1.8-2.4%; b: 0.001-0.04%; residual Ni;
(2) solution heat treatment: carrying out solution heat treatment on the as-cast M951 alloy plate by using a box type resistance furnace to ensure the homogenization of the components of the alloy plate; the solution treatment process comprises the following steps: keeping the furnace temperature of the resistance furnace at 1230-1235 ℃ and keeping the temperature for 2-3 hours;
(3) regulating and controlling boundary crystals: cooling the alloy plate sample after the solution heat treatment, wherein the furnace temperature in the cooling process is controlled by adopting a slow cooling mode in the furnace; wherein: the cooling rate is 2-5 ℃ per minute, and the cast high-temperature alloy base metal after crystal boundary regulation is obtained after the cooling is carried out to the temperature below 300 ℃; the microstructure characteristics of the cast high-temperature alloy parent metal after grain boundary regulation are as follows: form continuous chain M at the grain boundary23X6A boride precipitate phase and a massive MC phase; the segregation of B element occurs obviously at the grain boundary; the straight grain boundaries are transformed into jagged grain boundaries.
2. The method for regulating and controlling the grain boundary of the parent metal for improving the weldability of the cast superalloy as recited in claim 1, wherein: in the step (1), in the process of smelting and remelting alloy raw materials by adopting vacuum induction smelting, the smelting temperature is 1550-1500 ℃, the heat preservation time is 10-30 min, the alloy raw materials are repeatedly smelted for 2-4 times, and casting is carried out at 1460-1430 ℃; smelting the alloy raw materials to obtain an M951 alloy ingot, wherein the diameter phi (80 +/-5) mm and the length are more than or equal to 250 mm; and (3) carrying out 100% surface turning and polishing on the M951 alloy ingot, cutting off a shrinkage cavity part, then smelting again, and pouring an as-cast alloy plate sample with the length of 20-22 mm, the width of 20-22 mm and the thickness of 3-4 mm by adopting a sand casting method.
3. A welding process for the cast superalloy base material after grain boundary conditioning according to any of claims 1-2, comprising: the welding process is carried out by adopting a welding wire with the same chemical composition as the base metal and utilizing a chemical welding process, wherein in the welding process, the edge of a cast high-temperature alloy base metal M951 welding plate is limited and fixed through a clamp, and the welding parameters are as follows: the welding current is 15-35A, the welding voltage is 10-12V, the welding speed is 3-5 cm/min, and the protective gas is argon with the purity of 99.9%.
4. The welding process of claim 3, wherein: the preparation process of the welding wire comprises the following steps: preparing a welding wire raw material according to the nominal composition of the M951 alloy, putting the welding wire raw material into a water-cooled copper crucible of a vacuum smelting furnace for smelting, keeping the smelting temperature at 1550-1500 ℃ for 10-30 min, and repeatedly smelting for 2-4 times; pouring the alloy melt at 1460-1430 ℃ to obtain an M951 alloy ingot with the diameter phi (80 +/-5) mm and the length more than or equal to 250 mm; and (3) turning and polishing the surface of the alloy ingot by 100%, cutting a shrinkage cavity once, and then cutting the welding wire.
5. The welding process of claim 4, wherein: cutting into the standard size of the required welding wire by adopting a linear cutting mode, wherein the diameter of the welding wire is phi 1 mm-phi 3mm, and the length range is 150-400 mm.
6. The welding process of claim 3, wherein: cutting the welded plate sample along the direction vertical to the welding direction, observing the cross section of a welding joint, and counting the average crack length of each welding surface according to a formula (1); the average crack length of the welded plate sample is reduced to below 46 mu m;
in equation (1):
the average crack length on each weld face, n is the total number of statistical cracks,L i is as followsiLength of the bar crack.
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