Nickel-based flux-cored wire for 700 DEG C ultra-supercritical unit welding and preparation method

Abstract

The invention discloses a nickel-based flux-cored wire for welding an ultra-supercritical unit at 700 DEG C and a preparation method of the nickel-based flux-cored wire. The nickel-based flux-cored wire comprises the following components in percentage by mass: 50.0 to 52wt% of Ni, 17.5 to 18.5 wt% of Cr, less than 0.06 wt% of C, 1.8 to 2.2 wt% of Co, 2.5 to 3.5 wt% of Mn, 0.5 to 1.2 wt% of Mo, 5.0 to 5.5 wt% of Nb, 0.25 to 0.35 wt% of N, 4.5 to 5.5 wt% of Al, 3.2 to 3.6 wt% of Ti, 0.02 to 0.04 wt% of B, 0.23 to 0.26 wt% of Si, 0.15 to 0.23 wt% of Cu, 1.8 to 2.3 wt% of W and the balance of Fe and inevitable impurities. Compared with the prior art, the nickel-based flux-cored wire has the advantages that components of the nickel-based flux-cored wire are optimized, the nickel-based flux-cored wire has excellent tensile strength within the range of 600-700 DEG C, the high-temperature creep property is remarkably improved, the creep life reaches 1747 hours at 700 DEG C / 200 MPa, the minimum creep life is 3.09 * 10 <-3 >, and the percentage elongation after fracture is 21.85%. The endurance strength of deposited metal of the novel nickel-based flux-cored wire at the working temperature of 700 DEG C for 100 thousand hours is 107.78 MPa.

Description

Technical Field

[0001] The present invention relates to the field of welding technology in materials processing engineering, and particularly to a nickel-based flux-cored wire for welding of 700°C ultra-supercritical units and a preparation method thereof. Background Art

[0002] To improve the efficiency of thermal power generation, the key lies in increasing the steam temperature and pressure of the boiler. Increasing the steam parameters poses more stringent requirements on the steel used in power plants. To ensure the long-term stable operation of the unit under extreme working conditions, the relevant materials need to have excellent corrosion resistance and high-temperature creep resistance. Considering safe operation, there are very strict requirements for the materials of the pressure-bearing components of the boiler, that is, the creep rupture strength at the working temperature for 100,000 hours should be higher than 100 MPa. However, when the steam parameters are increased to above 700°C, the existing heat-resistant steel materials are difficult to meet the usage requirements. Therefore, nickel-based superalloys are generally recommended for these key components abroad, such as GH2107, Nimonic263, IN617, and IN625 alloys. Among them, IN625 has good strength, oxidation resistance, corrosion resistance, and creep resistance under high-temperature conditions of 600°C - 900°C, and has become the preferred material for the superheater and reheater of 700°C ultra (super) critical thermal power units. Therefore, Inconel 625 is also used as its preferred matching welding consumable.

[0003] Based on this background, a nickel-based flux-cored wire for welding of 700°C ultra-supercritical units is developed to solve the problem of the high cost of nickel-based welding wires, and at the same time, it can also meet the requirements of welding and repair welding of 700°C ultra-supercritical units. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a nickel-based flux-cored wire for welding of 700°C ultra-supercritical units and a preparation method thereof, which optimize the components of the nickel-based flux-cored wire and have excellent tensile strength in the range of 600 - 700°C, and significantly improve the high-temperature creep performance.

[0005] To achieve this technical purpose, the present invention adopts the following scheme: In a first aspect, the present invention provides a nickel-based flux cored wire for welding of 700 °C ultra-supercritical units, which comprises the following components by mass percentage: Ni: 50.0 - 52 wt%, Cr: 17.5 - 18.5 wt%, C: below 0.06 wt%, Co: 1.8 - 2.2 wt%, Mn: 2.5 - 3.5 wt%, Mo: 0.5 - 1.2 wt%, Nb: 5.0 - 5.5 wt%, N: 0.25 - 0.35 wt%, Al: 4.5 - 5.5 wt%, Ti: 3.2 - 3.6 wt%, B: 0.02 - 0.04%, Si: 0.23 - 0.26 wt%, Cu: 0.15 - 0.23 wt%, W: 1.8 - 2.3%, and the balance is Fe and inevitable impurities.

[0006] The nickel-based flux cored wire for welding of 700 °C ultra-supercritical units provided by the present invention improves the creep performance of its deposited metal in a 700 °C environment through specific element ratios. Its high-temperature creep resistance is not achieved by a single element, but through the contributions of alloying elements such as nickel, chromium, nitrogen, tungsten, niobium, etc. Among them, carbon, nitrogen, tungsten, etc. mainly play a role in solid solution strengthening in the alloy. Chromium can combine with supersaturated carbon and nitrogen elements in the alloy to form carbonitrides of chromium; in addition, nickel and chromium can form Laves phases with molybdenum, niobium, iron, etc. in the alloy, and these phases are like the "bones" in the alloy, effectively resisting the slip of grain boundaries and enhancing the creep performance of the deposited metal. Niobium can also combine with supersaturated carbon and nitrogen elements in the alloy to form carbonitrides, which precipitate inside the grains, hinder dislocation slip, and enhance the creep resistance of the deposited metal. The addition of aluminum and titanium can form γ' phases, which can inhibit the movement of lattice dislocations and increase the high-temperature strength and stability of the deposited metal. In addition, it can also enhance the toughness and creep resistance of the deposited metal. These elements cooperate with each other in the alloy to form a complex strengthening mechanism, jointly enhancing the creep resistance of the alloy. At the same time, through reasonable composition design and preparation processes, the distribution and morphology of these alloy elements can be further optimized, thereby improving the creep resistance and service life of the alloy.

[0007] Further, the mass percentage content of C is 0.04 - 0.06 wt%.

[0008] The addition of carbon can improve the high-temperature strength of its deposited metal. By specifically limiting the content of carbon, the weldability of the nickel-based welding wire can be further improved and the tissue stability of its deposited metal during long-term high-temperature service can be enhanced.

[0009] Further, by mass percentage of elements: Al / Ti is 1.35 - 1.65, and the total mass percentage of Al and Ti in the nickel-based flux cored wire is 8.0 - 8.7 wt%.

[0010] By further limiting the mass ratio of Al element and Ti element, the high-temperature creep resistance of the deposited metal can be further improved. When the Al element and Ti element exist in the above-defined ratio, the hot strength of the wire deposited metal can be further improved, and its high-temperature creep resistance can be significantly improved. The inventor analyzed that limiting the mass ratio of Al and Ti is beneficial to the reaction of Al and Ti elements with Ni to form γ'-Ni3(Al, Ti) phase. Among them, about 20% of the Al element added to the superalloy enters the γ solid solution, playing a role of solid solution strengthening, and about 80% of the Al element will form Ni3Al with the Ni element, playing a role of precipitation strengthening. When the Ti element is added to the superalloy, about 10% enters the γ solid solution, playing a role of solid solution strengthening, and about 90% of the Ti element enters the γ' phase. The Ti atom can replace the Al atom in the γ'-Ni3Al phase to form γ'-Ni3(Al, Ti). The γ' phase can significantly improve the tensile strength and creep resistance of the deposited metal. Further regulation of the Al element and Ti element can further improve the high-temperature creep resistance of the alloy.

[0011] Further, by mass percentage of elements: (Nb + W) / N is 20.0 - 25.0.

[0012] The formation of nitrides by N with Nb and W elements has a particularly obvious effect on the creep resistance of the alloy. By further limiting the mass ratio of Nb element, W element and N element, M(C, N) type carbonitrides can be further formed, further strengthening the precipitation strengthening effect on the alloy. On the other hand, the M(C, N) precipitated phase is dispersed in the alloy, hindering the movement of dislocations, thereby improving the creep resistance of the alloy; and limiting the mass ratio of Nb element, W element and N element can further reduce pores and improve the high-temperature performance of the alloy.

[0013] Further, the N source includes MnN and CrN; among them, the molar ratio of MnN and CrN is (0.6 - 0.8):(0.8 - 1.0).

[0014] The source of N needs to take into account the contents of Mn and Cr. According to the element ratio of the nitride, the contents and ratios of Mn and Cr elements are controlled, and the optimal ratio obtained through a large number of experimental tests.

[0015] Among them, the N element is added in the form of MnN and CrN, further dissolving the N element in the deposited metal and adjusting the contents of Mn and Cr elements. Among them, N and Mn elements can expand and stabilize the austenite region, playing a role of dissolving more superalloy elements, and at the same time making up for the influence brought by the reduction of Ni; another purpose of N is to form nitrides with Nb and W, further improving the high-temperature creep resistance of the alloy.

[0016] Further, the creep rupture strength of the nickel-based flux-cored wire deposited metal at a working temperature of 700°C for 100,000 hours is calculated by the L-M parameter model to be 10 7.78 MPa.

[0017] The specific method is as follows: The creep performance of the nickel-based flux-cored wire is tested under the conditions of 600°C (400 MPa, 450 MPa, 470 MPa, 500 MPa) and 700°C (200 MPa, 250 MPa, 350 MPa, 400 MPa). The obtained temperature, stress, and creep life are substituted into the following formula,

[0018] to obtain C lm , C0, C1, C2, C3 to obtain the final L-M parameter model. Then, substituting the actual service temperature and creep life into the formula, the stress value can be obtained.

[0019] Furthermore, through MATLAB software, binary non-linear regression is performed on the test data in the temperature range of 600 - 700°C to obtain the Larson-Miller parameter model. The specific formula is as follows: Logt r =-22.98 + 1000[138.6 - 154.5logσ + 72.59(logσ) 2 -11.64(logσ) 3 / T where T (K) is the temperature, σ (MPa) is the stress, and t r (h) is the time.

[0020] Through the above formula, the creep rupture strength of its deposited metal at a working temperature of 700°C for 100,000 hours is deduced to be 10 7.78 MPa. Therefore, the nickel-based flux-cored wire of the present invention has high creep performance when used at a temperature of 700°C.

[0021] In a second aspect, the present invention provides a method for preparing a nickel-based flux-cored wire for welding a 700°C ultra-supercritical unit, including the following steps: S1. Preparation of powder: The flux raw material powders are mixed evenly according to the aforementioned mass percentages; S2. Powder filling: After cleaning the steel strip outer skin, it is made into a "U"-shaped cross-section through a rolling mill. The powders mixed evenly in step S1 enter the "U"-shaped steel strip groove through a powder feeding mechanism, and then the steel strip is rolled into an "O"-shaped cross-section to obtain a semi-finished wire; S3. Sizing and drawing: The semi-finished wire is lubricated and then drawn to 1.2 mm to finally obtain a nickel-based flux-cored wire for welding a 700°C ultra-supercritical unit.

[0022] Further, in step S2, the outer skin of the steel strip is Inconel 718 (a precipitation-hardening nickel-chromium-iron alloy containing niobium and molybdenum), and the powder filling rate of the flux-cored powder is 35% - 42%. The powder filling rate in this application refers to the ratio of the mass of the flux-cored powder to the mass of the welding wire, and the specific calculation is carried out through the following steps.

[0023] Driven by the conveyor belt, the powder is filled into the U-shaped nickel strip, and the mathematical model of the powder filling rate is calculated through relevant physical quantities. The specific calculation is as follows: Assume that the powder filling rate is F, the volumetric mass of the flux-cored wire is m, the volumetric mass of the steel strip is m1, the loose bulk density of the powder is m2, the cross-sectional area of the flux-cored wire is A, the cross-sectional area of the steel strip is A1, the cross-sectional area of the powder is A2, the diameter of the flux-cored wire after forming is D, the diameter of the steel strip during forming is D1, the diameter of the powder core after forming is d, the width of the steel strip is B, and the thickness of the steel strip is δ.

[0024] Am = A1m1 + A2m2 (1) F = A2m2 / Am = (A2m2) / (A1m1 + A2m2) (2) (1 - F)A2m2 = F·A1m1 (3) A2 = (F·A1m1) / m2(1 - F) (4) Since A = A1 + A2, so there is: A = A1 + (F·A1m1) / m2·(1 - F) (5) A1 = A·m2(1 - F) / [m2(1 - F) + Fm1] (6) A2 = A·Fm1 / [m2(1 - F) + Fm1] (7) Since A = (D / 2) 2 ·π, A2 = (d / 2) 2 ·π, so there is: (d / 2) 2 ·π = (D / 2) 2 ·πFm1 / [(1 - F) m2 + Fm1] (8) Let a = {Fm1 / [(1 - F) m2 + Fm1]} 1 / 2 Then there is: d = a·D (9) Since D = d + 2δ, substituting into formula (9) gives: d = 2aδ / (1 - a) (10) δ = d(1 - a) / 2a (11) From B = D1·π = (d + δ)·π, we get: B = δπ[(1 + a) / (1 - a)] (12) δ = B(1 - a) / π(1 + a) = 0.3183B(1 - a) / (1 + a) (13) As can be seen from the above formulas, under the nominal numerical conditions of each parameter, when the same numerical value changes, the influence of the steel strip thickness δ on the stability of the powder filling rate F is the greatest. That is, when the steel strip thickness decreases, the powder filling rate increases, and vice versa. The width and thickness of the steel strip in this application are fixed values, that is, B = 16 mm and δ = 0.2 mm. After multiple rolling mills, the diameter of the primary welding wire is between 5.0 mm and 5.2 mm, and the diameter after wire drawing is 1.2 mm; the bulk density of the powder is 9.61 t / m 3 ~9.87 t / m 3 Between them, the density of the steel strip is 8.44 t / m 3 . Substituting the above obtained values into the formula, the powder filling rate of the flux-cored wire can be obtained as 41.6%. Considering comprehensively the possible phenomenon that the powder may not be tightly wrapped by the steel strip and powder leakage occurs during the preparation process of the welding wire, as well as the operability during the wire drawing process, after repeated measurements, the wire filling rate is finally determined to be about 38%.

[0025] Furthermore, in the lubrication treatment of step S3, the semi-finished welding wire is lubricated by a dry solid lubricant of sodium stearate or potassium stearate before entering the wire drawing for reducing diameter, and the lubricant may contain a high-pressure lubricant and a softening regulator.

[0026] The main components of the high-pressure lubricant can be: sulfur-containing compounds (such as sulfurized oil), phosphorus-containing compounds (such as phosphate esters), chlorine-containing compounds (such as chlorinated paraffin) or mineral oil, synthetic esters. The main components of the softening regulator can be: hydrochloric acid, sulfuric acid, borax, silicate or phosphate coating, surfactant. Such lubricants can be directly purchased.

[0027] Furthermore, in the wire drawing process of step S3, the drawn flux-cored wire is drawn from a diameter of 5.10 mm to 1.20 mm through seven passes.

[0028] Its design principle is: First, calculate the total logSR (5.10 2 / 1.20 2)(SR) = 1.257, where SR is the area ratio; secondly, divide 1.257 equally into seven passes, that is, 1.257÷7≈0.180, and this average value is the logSR value of the fourth drawing pass; adjust the offset of logSR for each pass appropriately according to the actual drawing conditions of production (this application has been verified many times, and finally it is determined that the offset of logSR between the upper and lower passes increases and decreases by 0.01), so its series is 0.150 + 0.160 + 0.170 + 0.180 + 0.190 + 0.200 + 0.210 = 1.26, which is 0.003 larger than the total logSR, so adjust the first pass to 0.147. Finally, list the calculated drawing route, as shown in Table 1.

[0029] In Table 1, the first column is the logSR of each pass; the second column is the logarithm of the diameter ratio of each pass, that is, logϕR; the third column is the logarithm of the die diameter of each pass, Log die ϕ; the fourth column is the diameter of the flux-cored wire. Among them, the data in the second column is obtained by dividing the data in the first column by 2; obtain the logarithm value as the base of the third column according to the finished product diameter (1.2 mm) in the fourth column, that is, log1.2 to get 0.079 as the base of the third column, and add the corresponding logϕR of each pass in the second column to obtain the logarithm value of the die diameter of each pass in the third column, and then take the antilogarithm of the log die ϕ of each pass in the third column to obtain the diameter of each pass.

[0030] Table 1 Drawing Route

[0031] In view of the physical characteristics of the nickel-based flux-cored wire for welding of 700°C ultra-supercritical units in this application, formulate a dedicated welding process specification: the welding method is GMAW (gas metal arc welding), the shielding gas is 96%Ar + 4%N2, the welding current is (300~320A), the arc voltage is (26~28V), the welding speed is 26 cm / min, and the interpass temperature is controlled at 180°C. Among them, 96%Ar + 4%N2 is the core of the process, which can ensure the nitrogen partial pressure to infiltrate nitrogen into the molten pool, realize the functions of nitrogen addition and nitrogen fixation, and at the same time raise the total heat power of the arc to the optimal value.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention optimizes the composition and preparation method of the nickel-based welding material IN625, and prepares a nickel-based flux-cored wire with low cost and excellent high-temperature creep resistance for welding of 700°C ultra-supercritical units. Compared with the existing nickel-based alloys, it has excellent tensile strength in the temperature range of 600 - 700°C, and the high-temperature creep performance is significantly improved. Among them, the creep life reaches 1747 hours at 700°C / 200MPa, and the minimum creep life is 3.09×10 -3, the elongation after fracture is 21.85%. In addition, the Larson-Miller model parameters are solved by MATLAB software, and the creep rupture strength of its deposited metal at a working temperature of 700 °C for 100,000 hours is extrapolated to be 10 7.78 MPa. Therefore, the new nickel-based flux-cored wire of the present invention has high creep resistance when used in an environment of 700 °C / 35 MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the creep curve of the deposited metal of the nickel-based flux-cored wire in Example 1 of the present invention at 700 °C.

[0034] Figure 2 It is a schematic diagram of the creep curve of the deposited metal of Inconel 625 wire in Comparative Example 6 of the present invention at 700 °C.

[0035] Figure 3 It is a diagram showing the relationship between the creep rate and time of the deposited metal of the nickel-based flux-cored wire in Example 1 of the present invention at 700 °C / 200 MPa.

[0036] Figure 4 It is a diagram showing the relationship between the creep rate and time of the deposited metal of Inconel625 wire in Comparative Example 6 of the present invention at 700 °C / 200 MPa.

[0037] Figure 5 It is a SEM image of the nickel-based flux-cored wire deposited metal in Example 1 of the present invention after creep fracture at 700 °C / 200 MPa; where: point 1 is γ; point 2 is Laves; point 3 is M(C, N); point 4 is γ′; point 5 is M 23 C6.

[0038] Figure 6 It is a SEM image of the Inconel 625 wire deposited metal in Comparative Example 6 of the present invention after creep fracture at 700 °C / 200 MPa; where: point 1 is Laves; point 2 is MC; point 3 is γ′.

[0039] Figure 7 It is a TEM image of the nickel-based flux-cored wire deposited metal in Example 1 of the present invention after creep fracture at 700 °C / 200 MPa; where: Dislocations in γ (γ dislocations), Dislocations loop (dislocation loop).

[0040] Figure 8TEM image after creep rupture of the Inconel 625 wire submerged arc welding deposit metal at 700 °C / 200 MPa in Comparative Example 6 of the present invention; where: Dislocations in γ (γ dislocations), Stacking faults (stacking faults). Detailed implementation mode

[0041] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific implementation modes, but the present invention is not limited thereto. Example 1

[0042] This example provides a nickel-based flux-cored wire for welding of 700 °C ultra-supercritical units, which is prepared according to the following steps: S1. Preparation of powder: Mix the flux raw material powders evenly.

[0043] S2. Powder filling: Clean the steel strip outer skin and then roll it into a "U" cross-section through a rolling mill. The powders mixed evenly in step S1 enter the "U" steel strip groove through a powder feeding mechanism, and then the steel strip is rolled into an "O" cross-section to obtain a semi-finished wire.

[0044] In this application, the steel strip is Inconel 718 alloy, and its chemical composition is shown in Table 2. The width of the steel strip is 16 mm, the thickness of the steel strip is 0.2 mm, the filling rate is about 38%, and the diameter of the semi-finished wire after forming is 5.1 mm.

[0045] Table 2 Chemical composition of Inconel 718 steel strip (mass fraction, wt%)

[0046] The nickel-based flux-cored wire includes the following components by mass percentage: Ni: 50.0 wt%, Cr: 18.0 wt%, C: 0.06 wt%, Co: 2.0 wt%, Mn: 3.5 wt%, Mo: 1.0 wt%, Nb: 5.3 wt%, N: 0.35 wt%, Al: 5.0 wt%, Ti: 3.2 wt%, B: 0.03%, Si: 0.25 wt%, Cu: 0.2 wt%, W: 2.3%, and the balance is Fe and unavoidable impurities.

[0047] The N source includes MnN and CrN, and the molar ratio of MnN and CrN is 0.8:0.8.

[0048] S3. Wire drawing with diameter reduction: Lubricate the semi-finished wire, and then draw it from a diameter of 5.10 mm to 1.2 mm through seven passes to finally obtain a nickel-based flux-cored wire for welding of 700 °C ultra-supercritical units.

[0049] During the lubrication process, the semi-finished welding wire is lubricated with a dry solid lubricant of sodium stearate or potassium stearate before entering the wire drawing with diameter reduction. The lubricant contains a high-pressure lubricant and a softening regulator. In this application, the dry drawing powder is used as the solid lubricant. Example 2

[0050] The difference between this example and Example 1 is that the raw material system by mass percentage includes: Ni: 51wt%, Cr: 18.0wt%, C: 0.06wt%, Co: 2.0wt%, Mn: 3.0wt%, Mo: 1.0wt%, Nb: 5.3wt%, N: 0.30wt%, Al: 5.0wt%, Ti: 3.2wt%, B: 0.03%, Si: 0.25wt%, Cu: 0.2wt%, W: 2.3%, and the balance is Fe and unavoidable impurities. Example 3

[0051] The difference between this example and Example 1 is that the raw material system by mass percentage includes: Ni: 52wt%, Cr: 18.0wt%, C: 0.06wt%, Co: 2.0wt%, Mn: 2.5wt%, Mo: 1.0wt%, Nb: 5.3wt%, N: 0.25wt%, Al: 5.0wt%, Ti: 3.2wt%, B: 0.03%, Si: 0.25wt%, Cu: 0.2wt%, W: 2.3%, and the balance is Fe and unavoidable impurities.

[0052] Comparative Example 1 The difference between this example and Example 1 is that the addition amount of C in the raw material system by mass percentage is 0.03wt%. The rest of the raw material system is Ni: 50.0wt%, Cr: 18wt%, Co: 2.0wt%, Mn: 3.5wt%, Mo: 1.0wt%, Nb: 5.3wt%, N: 0.35wt%, Al: 5.0wt%, Ti: 3.2wt%, B: 0.03%, Si: 0.25wt%, Cu: 0.2wt%, W: 2.3%, and the balance is Fe and unavoidable impurities.

[0053] Comparative Example 2 The difference between this example and Example 1 is that the addition amount of W in the raw material system by mass percentage is 3wt% and the addition amount of Nb is 6wt%. The rest of the raw material system is Ni: 50.0wt%, Cr: 18.0wt%, C: 0.06wt%, Co: 2.0wt%, Mn: 3.5wt%, Mo: 1.0wt%, N: 0.35wt%, Al: 5.0wt%, Ti: 3.2wt%, B: 0.03%, Si: 0.25wt%, Cu: 0.2wt%, and the balance is Fe and unavoidable impurities.

[0054] Comparative Example 3 The difference between this example and Example 1 is that in the raw material system by mass percentage, the addition amount of Al is 6 wt%, and the addition amount of Ti is 3 wt%. Ni: 50.0 wt%, Cr: 18.0 wt%, C: 0.06 wt%, Co: 2.0 wt%, Mn: 3.5 wt%, Mo: 1.0 wt%, Nb: 5.3 wt%, N: 0.35 wt%, B: 0.03%, Si: 0.25 wt%, Cu: 0.2 wt%, W: 2.3%, and the balance is Fe and unavoidable impurities.

[0055] Comparative Example 4 The difference between this example and Example 1 is that in the raw material system by mass percentage, the addition amount of N is 0.4 wt%. The remaining raw material system is Ni: 50.0 wt%, Cr: 18.0 wt%, C: 0.06 wt%, Co: 2.0 wt%, Mn: 3.5 wt%, Mo: 1.0 wt%, Nb: 5.3 wt%, Al: 5.0 wt%, Ti: 3.2 wt%, B: 0.03%, Si: 0.25 wt%, Cu: 0.2 wt%, W: 2.3%, and the balance is Fe and unavoidable impurities.

[0056] Comparative Example 5 The difference between this example and Example 1 is that the molar ratio of MnN to CrN is 3:2.

[0057] Comparative Example 6 The difference between this comparative example and Example 1 is that the composition of each element of the Inconel 625 welding wire includes the following components in parts by weight: C: 0.01 wt%, Ni: 63.0 wt%, Si: 0.55 wt%, Mo: 8.7 wt%, Nb: 3.7 wt%, Cr: 22 wt%, Ti: 0.4 wt%, Al: 0.1 wt%, Mn: 1.4 wt%, and the balance is Fe.

[0058] Comparative Example 7 The difference between this comparative example and Example 1 is that the composition of each element of the Inconel 617 welding wire includes the following components in parts by weight: C: 0.05 wt%, Co: 13 wt%, Ni: 44.5 wt%, Si: 0.65 wt%, Mo: 9.0 wt%, Cr: 23 wt%, Ti: 0.4 wt%, Al: 1.3 wt%, Mn: 0.6 wt%, and the balance is Fe.

[0059] Test Example 1 Measure the tensile strength, yield strength, and elongation after fracture of Examples 1-3 and Comparative Examples 1-7 at a temperature of 700°C. The test temperature is set at 700°C, and the specimen is heated to the set temperature at a heating rate of 10°C / s and then held for 10 min. The specimen is stretched at a tensile speed of 0.1 mm / min until all specimens are fractured and the test ends. During the test, the radial temperature difference of the specimen should not be too large, preferably not exceeding ±3°C. At least 2 parallel specimens are used for the tensile test under each set of tensile test parameters, and the final tensile property results are the average values of the measured results under the same test parameters. The test results are shown in Table 3.

[0060] Table 3 Tensile properties at different temperatures

[0061] Test Example 2 Measure the creep life, elongation after fracture, minimum creep rate, etc. of Example 1 and Comparative Example 6 at a temperature of 700°C. The creep test temperature range is 700°C, and the stress range is 200 MPa - 400 MPa. Before the test starts, a stress (about 10 MPa) that can fasten the specimen to be tested is applied to the test bar. The test furnace is heated to the test temperature (such as 700°C) at a rate of 3°C / min and held for 12 min. During the creep test, a fixed load (such as 200 MPa) is applied to the test bar until the test ends. The fractured specimens are taken out for standby after being cooled to room temperature with the furnace. Ensure that the radial temperature difference of the specimen is within ±3°C during the creep process. At least 2 parallel specimens are used for the creep test under each set of test parameters, and the final creep properties are the average values of the measured results under the same test parameters. The test results are shown in Table 4, Figures 1 - 4 as shown.

[0062] Table 4 Creep life and minimum creep rate under different creep conditions

[0063] Figure 1 and Figure 2 are the creep rupture curves of the deposited metals of Example 1 and Comparative Example 6 at 700°C. It can be seen from the figure that the deposited metals of the two nickel-based welding wires have typical creep curve characteristics under different creep conditions, namely the initial creep stage, the steady-state creep stage, and the accelerated creep stage. At the same temperature, the creep life of the material is greatly affected by the applied stress. As the stress increases, the minimum creep rate increases while the creep life decreases sharply. In addition, with the increase of the creep temperature, the plasticity improves.

[0064] Figure 3 and Figure 4 are the minimum creep rates of the deposited metals of Example 1 and Comparative Example 6 at 700°C. Figure 3 andFigure 4 is a further explanation of Figure 1 and Figure 2 , and the specific minimum creep rate obtained can be found in Table 3. It further verifies that the creep performance of Example 1 under this creep condition is better than that of Comparative Example 6.

[0065] Test Example 3 Determine the microstructure after creep fracture of Example 1 and Comparative Example 6 at 700°C. After the long-term creep fracture, a 5-mm metal block was cut along the longitudinal section direction of the specimen fracture surface using wire cutting, embedded, ground, polished, and then etched with aqua regia for 40 - 50 s, and then observed using an S-3400N type scanning electron microscope. The results are shown in Figure 5 and Figure 6 .

[0066] Figure 5 is the SEM morphology of Example 1 under the creep condition of 700°C / 200 MPa. It can be observed from the figure that the size of the Laves phase precipitated at the "point 2" position on the grain boundary becomes significantly smaller and its distribution on the grain boundary becomes more uniform. At the same time, the size of the M 23 C6 type carbide at the "point 5" position becomes fine and its distribution on the grain boundary is uniform. The number of M(C, N) type carbonitrides at the "point 3" position in the grains increases significantly, the size slightly increases, and its distribution in the grains is more uniform in the shape of ellipses and blocks. In addition, more g' phases with a size less than 1 μm and diffusely distributed in the grains can be seen, that is, at the "point 4" position. Among these precipitated phases, the Laves phase and M23C6 are mostly distributed on the grain boundary, and the M(C, N) and γ′ phases are mainly precipitated in the grains. In the actual service of materials, the microstructure corresponding to the best performance is always unstable. With the extension of service time, the unstable structure will gradually transform into a thermodynamically stable structure, but the performance of the material deteriorates after the transformation, which is caused by the weakening of the dislocation structure and the matrix strengthening effect during the transformation process. The precipitated phases can pin the dislocations and grain boundaries, thereby delaying this transformation.

[0067] Figure 6 is the SEM morphology of Comparative Example 6 under the creep condition of 700°C / 200 MPa. The precipitated Laves phase at the "point 1" becomes fine and forms a line on the grain boundary. The precipitated MC type carbide at the "point 2" is more uniformly distributed in the grains. Finer γ′ phases at the "point 3" are found in the figure.

[0068] Test Example 4 Measure the dislocation morphology after creep fracture at 700 °C for Example 1 and Comparative Example 6. Use wire cutting to cut a 0.5 mm thick metal sheet from the specimen. Manually grind the sample with 600# sandpaper to 100 μm, then use 1000# sandpaper to grind it to 70 μm, and finally use 2000# sandpaper to grind it to 50 μm. Punch the ground sample into a disc with a diameter of ϕ3 mm. Use a TJ100-SE type electrolytic twin-jet thinning instrument to prepare a transmission sample. The solution is a mixture of alcohol and perchloric acid with a volume ratio of 9:1, the temperature is about -25 °C, the light transmittance is 80, the current is 120 mA, and the voltage is about 39 V. In this article, all transmission samples are characterized by a JEM-2100 type transmission electron microscope. The results are shown in Figure 7 and Figure 8 。

[0069] Figure 7 is the dislocation morphology of Example 1 under the creep condition of 700 °C / 200 MPa. It can be seen that there are a large number of dislocations in the matrix in the microstructure, and the distribution is relatively uniform. In addition, dislocation loops are also observed. This indicates that the dislocation bypass mechanism has been activated, and the deformation mechanism has changed from dislocation cutting of the precipitate phase to dislocation bypassing of the precipitate phase. The reasons for this phenomenon are as follows: First, as the temperature increases, some of the precipitates in the matrix begin to dissolve back; second, as the temperature of the alloy increases, the deformation mechanism has undergone a major change.

[0070] Figure 8 is the dislocation morphology of Comparative Example 6 under the creep condition of 700 °C / 200 MPa. Different from Example 1, at this temperature, stacking faults can be seen in the microstructure, but dislocation loops are not observed. For face-centered cubic crystals, the full dislocation with the lowest energy is the unit dislocation with a Burgers vector of a / 2<110> in the {111} plane. When the unit dislocation slips along the {111}<110> direction under the action of shear stress, it will slip along the direction of a / 6<112> with lower energy. This slip will form a stacking fault, and two partial dislocations will inevitably form between the stacking fault and the normal region. This dislocation reaction can be decomposed as follows:

[0071] It can be found from the reaction formula that there is a stacking fault, that is, an extended dislocation, between the two dislocations after decomposition. This dislocation reaction is feasible in terms of both geometric and energy conditions. The distance of the stacking fault region in the extended dislocation is defined as the width d of the extended dislocation, which can be obtained by balancing the repulsive force between two Shockley partial dislocations and the stacking fault energy of the dislocation:

[0072] Wherein K is a constant; φ is the angle between the total dislocation line and the Burgers vector; γ is the stacking fault energy. It can be seen from Equation II that the width of the extended dislocation is inversely proportional to the stacking fault energy per unit area. That is, the larger the stacking fault energy, the smaller the width of the extended dislocation. As the temperature increases, the stacking fault energy continuously increases, and the width of the extended dislocation becomes narrower, even only 1 to 2 atomic spacings, and actually it can be considered that no extended dislocation will form. It can be speculated that when the tensile temperature exceeds 700 °C, the stacking fault energy of the deposited metal of the nitrogen-containing nickel-based flux-cored wire will increase significantly.

[0073] Finally, it should be noted that: the above-listed are only the preferred embodiments of the present invention. Of course, those skilled in the art can make changes and modifications to the present invention. If these modifications and variations fall within the scope of the claims of the present invention and its equivalent technologies, they should all be considered as the protection scope of the present invention.

Claims

1. A nickel-based flux-cored wire for welding of 700°C ultra-supercritical units, characterized in that, It comprises the following components by mass percentage: Ni: 50.0 - 52 wt%, Cr: 17.5 - 18.5 wt%, C: below 0.06 wt%, Co: 1.8 - 2.2 wt%, Mn: 2.5 - 3.5 wt%, Mo: 0.5 - 1.2 wt%, Nb: 5.0 - 5.5 wt%, N: 0.25 - 0.35 wt%, Al: 4.5 - 5.5 wt%, Ti: 3.2 - 3.6 wt%, B: 0.02 - 0.04%, Si: 0.23 - 0.26 wt%, Cu: 0.15 - 0.23 wt%, W: 1.8 - 2.3%, and the balance is Fe and unavoidable impurities.

2. The nickel-based flux-cored wire for welding of 700 °C ultra-supercritical units according to claim 1, wherein The mass percentage of C is 0.04 - 0.06 wt%.

3. The nickel-based flux-cored wire for welding of 700 °C ultra-supercritical units according to claim 1, characterized in that, By elemental mass percentage: Al / Ti is 1.35 - 1.65, and the total mass percentage of Al and Ti in the nickel-based flux-cored wire is 8.0 - 8.7 wt%.

4. The nickel-based flux cored wire for welding of 700 °C ultra-supercritical units according to claim 1, characterized in that By elemental mass percentage: (Nb + W) / N is 20.0 - 25.

0.

5. The nickel-based flux-cored wire for welding of 700 °C ultra-supercritical units according to claim 1, characterized in that The N source includes MnN and CrN; among them, the molar ratio of MnN to CrN is (0.6 - 0.8):(0.8 - 1.0).

6. The nickel-based flux-cored wire for welding of 700 °C ultra-supercritical units according to claim 1, characterized in that, The creep rupture strength of the deposited metal of nickel-based flux-cored wire calculated by the L-M parameter model at a working temperature of 700°C for 100,000 hours is 10 7.78 MPa.

7. A preparation method of a nickel-based flux-cored wire for welding of a 700 °C ultra-supercritical unit according to any one of claims 1-6, characterized in that, It includes the following steps: S1. Preparation of powder: Mix the flux raw material powders evenly according to the required mass percentages. S2. Powder filling: Clean the steel strip outer skin and roll it into a "U"-shaped cross-section through a rolling mill. The powders mixed evenly in step S1 enter the "U"-shaped steel strip groove through a powder feeding mechanism, and then roll the steel strip into an "O"-shaped cross-section to obtain a semi-finished wire. S3. Sizing and drawing: Lubricate the semi-finished wire, and then draw it to 1.2 mm to finally obtain a nickel-based flux-cored wire for welding of 700°C ultra-supercritical units.

8. The preparation method of the nickel-based flux-cored wire for welding of a 700 °C ultra-supercritical unit according to claim 7, characterized in that, The steel strip outer skin in step S2 is Inconel 718, and the powder filling rate of the flux is 35% - 42%.

9. The preparation method of the nickel-based flux-cored wire for welding of a 700 °C ultra-supercritical unit according to claim 7, characterized in that, In the lubrication treatment of step S3, the semi-finished wire is lubricated by a dry solid lubricant of sodium stearate or potassium stearate before entering the sizing and drawing.

10. The preparation method of the nickel-based flux-cored wire for welding of a 700 °C ultra-supercritical unit according to claim 7, characterized in that, In step S3, the drawn flux-cored wire is drawn from a diameter of 5.10 mm to 1.20 mm through seven passes.

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