LARGE-SIZED HIGH-NB SUPERALLOY INGOT AND SMELTING PROCESS THEREOF

Abstract

Disclosed in this application are a large-sized high-Nb superalloy ingot and a smelting process thereof. The smelting process includes: vacuum induction melting to prepare a plurality of vacuum induction melting ingots with the same composition which are used for preparing electroslag electrodes with the same number as the vacuum induction melting ingots for use in electroslag remelting, preparing a consumable electrode from the prepared consumable electroslag electrodes, and performing vacuum consumable arc remelting for a plurality of times by using the consumable electroslag electrodes as raw material. A large-sized high-Nb superalloy ingot having a weight of 15 tons or above and a diameter of 800 mm or above can be prepared from such process.

Claims

1. A smelting process of a large-sized high-Nb superalloy ingot, comprising the following steps of: conducting vacuum induction melting (VIM) on pure metal raw material and/or recycled material as raw material, and pouring to form consumable VIM ingots with the same composition; and preparing the same number of electroslag electrodes as that of the prepared VIM ingots; electroslag remelting (ESR) all the prepared electroslag electrodes under protection of argon, followed by cooling down and demoulding to obtain an ESR ingot; and conducting consumable vacuum arc remelting (VAR) for a plurality of times: performing a first annealing and a second annealing on the obtained ESR ingot and forging by stretching to a predetermined size to obtain a primary consumable electrode, in which the second annealing is performed at a temperature higher than that for performing the first annealing temperature; and performing VAR for at least twice by using the primary consumable electrode as starting material, in which a resulting VAR ingot obtained after each VAR is used as a consumable electrode to be used in a next VAR, with a diameter of the resulting VAR ingot being increased after each VAR until fluctuation of a melting rate during a last VAR fall within 10% of a steady-state melting rate, and a VAR ingot obtained in the last VAR is used to prepare an ingot with a target diameter.

2. The smelting process according to claim 1, wherein in the step of VIM, the raw material comprises 2.8-5.5 wt % Nb, 0.2-1.0 wt % Al and 0.5-2.0 wt % Ti.

3. The smelting process according to claim 1, wherein in the step of VIM, the raw material is melt at a temperature of 1300-1550 C. until melting down, refined under electromagnetic stirring at 1350-1550 C. for 15-120 min, cooled for 1-10 h, and demoulded to obtain the VIM ingots; and the VIM process is performed for a plurality of times to obtain the VIM ingots with the same composition.

4. The smelting process according to claim 1, wherein preparing the electroslag electrodes comprises directly stress-relieve annealing each VIM ingot, in which the temperature is initially raised to 600-800 C., then raised to 800-1000 C. at a rate of 5-45 C./h, kept constant for 4-32 h, lowered to 600-800 C. at a rate of 1-35 C./h and kept constant for 4-32 h; air cooling; polishing; and flattening at the head and the tail to obtain the electroslag electrodes.

5. The smelting process according to claim 1, wherein a quaternary slag system (CaF.sub.2CaOAl.sub.2O.sub.3TiO.sub.2) comprising 60-75 wt % CaF.sub.2, 10-25 wt % CaO, 10-25 wt % Al.sub.2O.sub.3, and 1-10 wt % TiO.sub.2 is adopted; a steady-state melting rate of ESR is controlled to be 5-15 kg/min, before exchanging each electrode, when the remaining weight of one electrode is 500 kg-1000 kg, the melting rate is increased by a slope of 0.5-2 kg/min to 12-25 kg/min on the basis of the steady-state melting rate and kept stable until the electrode exchanging begins, during which the smelting parameters remain the same as those before the exchanging and the exchanging time does not exceed 2 min; after exchanging each electrode, the melting rate is reduced by a slope of 0.5-2 kg/min to a steady-state melting rate of 5-15 kg/min for continuing the remelting when 100 kg-500 kg of a next electrode is melted, and hot topping is carried out when the remaining weight of a last electrode is 200-600 kg; and after ESR, cooling down is carried out for 2-10 h and demoulding is carried out to obtain the ESR ingot.

6. The smelting process according to claim 1, wherein in the step of VAR for a plurality of times, performing the first annealing and the second annealing on the obtained ESR ingot and forging by stretching to a predetermined size to obtain the primary consumable electrode comprises the following steps of: initiating the first annealing to the ESR ingot within 0.5-2 h after demoulding, in which the temperature is initially raised to 300-550 C., kept constant for 12-32 h for achieving homogeneous temperature distribution, raised to 600-750 C. at a rate of 1-25 C./h, kept constant for 4-32 h, raised to 800-1000 C. at a rate of 5-35 C./h, kept constant for 4-32 h, lowered to 550-750 C. at a rate of 1-35 C./h, and kept constant for 4-32 h, followed by air cooling; performing the second annealing on the ESR ingot after the first annealing, in which the temperature is raised to 800-1000 C. at a rate of 5-35 C./h, raised to 1050-1150 C. at a rate of 1-25 C./h, kept constant for 4-32 h, raised to 1150-1250 C. at a rate of 1-25 C./h, kept constant for 24-72 h, lowered to 800-950 C. at a rate of 1-35 C./h, and kept constant for 4-32 h, followed by air cooling; forging the ESR ingot after the second annealing, in which the ESR ingot is heated to 1100-1180 C. for 4-12 h before forging, and subjected to a free forging on a high-speed forging press of 3000 tons or above by stretching in one direction, in which a reduction per pass in one direction is 5-30 mm and a final forging temperature is in a range of 850-1000 C.; and polishing the ESR ingot stretched during free forging and flattening at the head and the tail to obtain the primary consumable electrode.

7. The smelting process according to claim 1, wherein in the step of VAR for a plurality of times, a first VAR and a second VAR comprises the following steps of: performing the first VAR at a steady-state melting rate of 3.5-7.5 kg/min; initiating helium cooling when 800-2000 kg of the primary consumable electrode is melted; decreasing a current and adjusting the melting rate to 3.0-7.0 kg/min when the weight of the remained primary consumable electrode is 1500-5000 kg; and starting hot topping when the weight of the remained primary consumable electrode is 200-1000 kg, thereby obtaining a first VAR ingot; for the second VAR, first polishing the first VAR ingot, and flattening the first VAR ingot at the head and the tail to obtain a secondary consumable electrode; performing the second VAR at a steady-state melting rate of 4.0-8.5 kg/min; initiating helium cooling when 1000-3000 kg of the secondary consumable electrode is melted; decreasing the current and adjusting the melting rate to 3.0-7.5 kg/min when the weight of the remained secondary consumable electrode is 2000-5500 kg; and starting hot topping when the weight of the remained secondary consumable electrode is 250-1500 kg; and after the second VAR, vacuum cooling for 1-8 h and then starting stress-relief annealing within 2 h, wherein the temperature is raised to 300-750 C., kept constant for 4-32 h for achieving homogeneous temperature distribution, raised to 800-1000 C. at a rate of 5-50 C./h, kept constant for 4-32 h, lowered to 550-750 C. at a rate of 1-35 C./h, and kept constant for 4-32 h, followed by air cooling.

8. The smelting process according to claim 7, wherein, if fluctuation of the melting rate during the second VAR is beyond 10% of the steady-state melting rate, an obtained second VAR ingot is used to prepare a consumable electrode to be used for a next VAR by the same way as for preparing the primary consumable electrode, including the same first annealing, second annealing, and forging by stretching, and then the second VAR is repeated.

9. A large-sized high-Nb superalloy ingot prepared by the smelting process according to claim 1, wherein the large-sized high-Nb superalloy ingot is an Alloy 706 with a diameter of 800 mm or above, by mass percentage, having a chemical composition of: C0.02%, Cr 15.5-16.5%, Ni 40.0-43.0%, Nb 2.8-3.2%, Ti 1.5-1.8%, Al 0.1-0.3%, Si0.10%, Mn0.20%, P0.015%, S0.0013%, Co0.30%, Mo0.20%, B0.006%, Cu0.30%, Ca0.005%, N0.006%, O0.005%, and Fe the balance.

10. A large-sized high-Nb superalloy ingot prepared by the smelting process according to claim 1, wherein the large-sized high-Nb superalloy ingot is an Alloy 718 with a diameter of 800 mm or above, by mass percentage, having a chemical composition of: C 0.005-0.04%, Cr 17.0-19.0%, Ni 52.0-55.0%, Nb 4.9-5.5%, Ti 0.75-1.15%, Al 0. 35-0.65%, Si0.10%, Mn0.15%, P0.008%, S0.002%, Co0.50%, Mo 2.8-3.3%, B0.006%, Cu0.10%, Ca0.005%, N0.01%, O0.003%, and Fe the balance.

Description

DESCRIPTION OF THE EMBODIMENTS

[0034] This application will be described in detail below in combination with examples.

EXAMPLE 1

[0035] This example is used to illustrate a method for preparing Alloy 706 (a VAR ingot with a diameter of 1050 mm).

[0036] The target Alloy 706 had a composition of (by mass percentage):

C 0.018%, Cr 15.8%, Ni 41.5%, Nb 3.01%, Ti 1.72%, Al 0.25%, Si 0.02%, Mn 0.01%, P 0.006%, S 0.0006%, Co 0.02%, Mo 0.01%, B 0.004%, Cu 0.02%, Ca 0.004%, N 0.005%, O 0.002%, and the balance Fe.

[0037] The specific method for preparing the alloy was as follow.

[0038] Vacuum induction melting (VIM): according to the requirements of composition of the designed alloy, recycled material accounting for 50% of the elements needed for alloy of unit weight, was weighed, and fresh metals accounting for the rest of the weight were weighed. A 12-ton VIM furnace was adopted, with an upper melting temperature limit being 1550 C. After melting down, the composition of molten steel was detected, and the molten steel was controlled to contain approximately 3.02 wt % Nb, approximately 1.80 wt % Ti, and approximately 0.30 wt % Al by adding fresh metals. According to the amount of the added fresh metal, refining was carried out under electromagnetic stirring at 1350 C. for 15-30 min, and tapping was conducted at a temperature of 1400 C. After pouring the molten steel, furnace cooling was carried out for 4 h and demoulding was carried out to obtain one VIM ingot with a diameter of 820 mm and a weight of 12 tons which was directly annealed. An annealing furnace was pre-heated to 600 C., heated to 800 C. at a rate of 5 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 1 C./h and held at the temperature for 10 h, and then air-cooled.

[0039] The annealed VIM ingot was polished and flattened at the head and the tail to prepare an electroslag electrode.

[0040] Electroslag remelting (ESR): A crystallizer with a diameter of 1100 mm was adopted and a quaternary (CaF.sub.2CaOAl.sub.2O.sub.3TiO2) slag system specifically having a composition of, by weight, 60% CaF.sub.2, 10% CaO, 25% Al.sub.2O.sub.3, and 10% TiO2 was adopted. A steady-state melting rate was 15 kg/min. During smelting, Ar at 0.2 bar pressure was introduced for protection, so as to prevent the molten steel from contacting with oxygen and nitrogen in the air. Before exchanging the electrode, when the weight of the remained first electrode was 800 kg, the melting rate was increased by 1.05 kg/min to 25 kg/min on the basis of the steady-state melting rate by adjusting input power and a voltage and was held until the electrode exchanging began. Smelting parameters during exchanging the electrode remained the same as those before exchanging the electrode, and the time for exchanging did not exceed 2 min. After exchanging the electrode, when 100 kg of a second electrode was melted, the melting rate was decreased by 0.50 kg/min to 10 kg/min by adjusting the input power and the voltage, and hot topping was carried out when the weight of the remained second electrode was 200 kg.

[0041] After ESR, an ESR ingot was water cooled in a water-cooled crystallizer in the furnace for 4 h, and within 0.5 h after demoulding, transferred to an annealing furnace for stress-relief annealing. The annealing furnace was pre-heated to 300 C. and held at the temperature for 12 h, heated to 600 C. at a rate of 5 C./h and held at the temperature for 4 h, heated to 800 C. at a rate of 5 C./h and held at the temperature for 5 h, cooled to 600 C. at a rate of 5 C./h and held at the temperature for 12 h, and then air-cooled.

[0042] Diffusion annealing ESR ingot at high temperature: The ESR ingot was loaded into the furnace at the temperature of 550 C. or below and held at the temperature for 4 h, heated to 800 C. at a rate of 10 C./h, heated to 1050 C. at the rate of 5 C./h and held at the temperature for 4 h, heated to 1150 C. at the rate of 5 C./h and held at the temperature for 24 h, cooled down to 800 C. at the rate of 5 C./h and held at the temperature for 32 h, and then air cooled.

[0043] Forging the electrode: The ESR ingot with a diameter of 1100 mm was heated to 1100 C. for 4 h before forging. A free forging was performed on a 3500-ton high-speed forging press by stretching the ingot in one direction, in which reduction per pass in one direction was controlled to be 25 mm, and the final forging temperature was 850 C. Finally, the ESR ingot was polished and flattened at the head and the tail to provide a primary consumable electrode with a diameter of 820 mm for use in a first VAR.

[0044] First VAR: A crystallizer with a diameter of 920 mm was adopted, the first VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 3.5 kg/min; helium cooling was initiated when 800 kg of the primary consumable electrode was melted; a current was reduced and the melting rate was adjusted to 3.0 kg/min when the weight of the remained primary consumable electrode was 1500 kg; and hot topping controlled by the current was initiated when the weight of the remained primary consumable electrode was 200 kg. The steel ingot was polished and flattened at the head and the tail to provide a first VAR ingot with a diameter of 900 mm for use in a second VAR.

[0045] Second VAR: A crystallizer with a diameter of 1050 mm was adopted, the second VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 4.0 kg/min;

[0046] helium cooling was initiated when 1000 kg of the first VAR ingot was melted; the current was reduced and the melting rate was adjusted to 3.0 kg/min when the weight of the remained first VAR ingot was 2000 kg; and hot topping controlled by the current was carried out when the weight of the remained first VAR ingot was 250 kg. After the second VAR, the steel ingot was vacuum-cooled for 3 h and transferred within 2 h to the annealing furnace for stress-relief annealing, which prevents the steel ingot from cracking after demoulding. The annealing furnace was pre-heated to 300 C. and held at the temperature for 4 h, heated to 800 C. at a rate of 5 C./h and held at the temperature for 5 h, cooled to 550 C. at the a of 5 C./h and held at the temperature for 5 h, and then air-cooled.

[0047] Test result: The trial-prepared Alloy 706 VAR ingot with the diameter of 1050 mm and the weight of 15.5 tons is free from hot cracking or metallurgical defects such as black and white freckles. The composition of the head and tail of the steel ingot is tested, and the result shows that there is no apparent burning loss of Al and Ti at the head and the tail, with 0.27% Al element at the head, 0.24% Al at the tail, 1.68% Ti at the head and 1.78% Ti at the tail. The second VAR ingot with the diameter of 1050 mm is diffusion annealed by homogenizing at high temperature, then polished and forged to prepare a bar. Upon non-destructive examination, it shows that there is no abnormal signal at the site where the electrode is exchanged, indicating that the second VAR can effectively address metallurgical quality defects suffered at the site of the Alloy 706 where the electrode is exchanged.

EXAMPLE 2

[0048] This example is used to illustrate a method for preparing Alloy 706 (a VAR ingot with a diameter of 1050 mm).

[0049] The target Alloy 706 had a composition of (by mass percentage):

C 0.011%, Cr 16.2%, Ni 42.5%, Nb 2.80%, Ti 1.57%, Al 0.15%, Si 0.02%, Mn 0.02%, P 0.008%, S 0.0004%, Co 0.01%, Mo 0.02%, B 0.003%, Cu 0.05%, Ca 0.001%, N 0.0045%, O 0.0025%, and the balance Fe.

[0050] The specific method for preparing the alloy was as follow.

[0051] Vacuum induction melting (VIM): According to the requirements of composition of the designed alloy, recycled material accounting for 50% of the elements needed for alloy of unit weight was weighed, and fresh metal accounting for the rest of the weight was weighed. A 12-ton VIM furnace was adopted, with an upper melting temperature limit being 1550 C. After melting down, the composition of molten steel was detected, and the molten steel was controlled to contain approximately 2.90 wt % Nb, approximately 1.62 wt % Ti, and approximately 0.20 wt % Al by adding fresh metals. Refining was carried out under electromagnetic stirring at 1480 C. for 40 min, and tapping was conducted at a temperature of 1500 C. The molten steel was poured in two steps, followed by furnace cooling for 4 h and demoulding to obtain two VIM ingots with a diameter of 820 mm and a weight of 12 tons, which were directly annealed. An annealing furnace was pre-heated to 650 C., heated to 900 C. at a rate of 25 C./h and held at the temperature for 24 h, cooled to 700 C. at a rate of 15 C./h and held at the temperature for 10 h, and then air-cooled.

[0052] The annealed VIM ingots were polished and flattened at the head and the tails to prepare electroslag electrodes.

[0053] Electroslag remelting (ESR): A crystallizer with a diameter of 1100 mm was adopted and a quaternary (CaF.sub.2CaOAl.sub.2O.sub.3TiO.sub.2) slag system specifically having a composition of, by weight, 64% CaF.sub.2, 15% CaO, 15% Al.sub.2O.sub.3, and 6% TiO.sub.2 was adopted. A steady-state melting rate was 10 kg/min. During smelting, Ar at 0.2 bar pressure was introduced for protection, which prevents the molten steel from contacting with oxygen and nitrogen in the air. Before exchanging the electrode, when the weight of the remained first electrode was 500 kg, the melting rate was increased by 0.55 kg/min to 15 kg/min on the basis of the steady-state melting rate by adjusting input power and a voltage and was held until the electrode exchanging began. Smelting parameters during exchanging the electrode remained the same as those before exchanging the electrode, and the time for exchanging did not exceed 2 min. After exchanging the electrode, when 200 kg of a second electrode was melted, the melting rate was decreased by a slope of 0.75 kg/min to 10 kg/min by adjusting the input power and the voltage, and hot topping was carried out when the weight of the remained second electrode was 500 kg.

[0054] After ESR, an ESR ingot was water cooled in a water-cooled crystallizer in the furnace for 4 h, and within 0.5 h after demoulding, transferred to an annealing furnace for stress-relief annealing. The annealing furnace was pre-heated to 450 C. and held at the temperature for 24 h, heated to 650 C. at a rate of 15 C./h and held at the temperature for 4 h, heated to 950 C. at a rate of 25 C./h and held at the temperature for 12 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air-cooled.

[0055] Diffusion annealing ESR ingot at high temperature: The ESR ingot was loaded into the furnace at the temperature of 550 C. or below and held at the temperature for 4 h, heated to 950 C. at a rate of 10 C./h, heated to 1100 C. at the rate of 15 C./h and held at the temperature for 12 h, heated to 1190 C. at a rate of 20 C./h and held at the temperature for 48 h, cooled down to 850 C. at the rate of 15 C./h and held at the temperature for 24 h, and then air cooled.

[0056] Forging the electrode: The ESR ingot with the diameter of 1100 mm was heated to 1150 C. for 10 h before forging. A free forging was performed on a 3500-ton high-speed forging press by stretching in one direction, in which reduction per pass in one direction was controlled to be 25 mm, and the final forging temperature was 900 C. Finally, the ESR ingot was polished and flattened at the head and the tail to provide a primary consumable electrode with a diameter of 820 mm for use in a first VAR.

[0057] First VAR: A crystallizer with a diameter of 920 mm was adopted, the first VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 5.5 kg/min; helium cooling was initiated when 1000 kg of the primary consumable electrode is melted; a current was reduced and the melting rate was adjusted to 4.0 kg/min when the weight of the remained primary consumable electrode is 2000 kg; and hot topping controlled by the current was carried out when the weight of the remained primary consumable electrode is 500 kg. The steel ingot was polished and flattened at the head and the tail to provide a first VAR ingot with a diameter of 900 mm for use in a second VAR.

[0058] Second VAR: A crystallizer with a diameter of 1050 mm was adopted, the second VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 6.5 kg/min; helium cooling was initiated when 1500 kg of the first VAR ingot is melted;, the current was reduced and the melting rate was adjusted to 6.0 kg/min when the weight of the remained first VAR ingot is 2500 kg; and hot topping controlled by the current was carried out when the weight of the remained first VAR ingot is 800 kg. After the second VAR, the steel ingot was vacuum-cooled for 3 h and transferred to the annealing furnace for stress-relief annealing within 2 h after demoulding, which prevents the cracking of the steel ingot after demoulding. The annealing furnace was pre-heated to 450 C. and held at the temperature for 8 h for achieving homogeneous temperature distribution, heated to 850 C. at a rate of 10 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air-cooled.

[0059] During the second VAR, when the smelting proceeded to the site where the electrode is exchanged, fluctuation of the following key melting parameters occurred: fluctuation of current exceeded 1000 A, fluctuation of voltage exceeded 2V, and fluctuation of melting rate exceeded 0.6 kg/min. This indicated that, the second VAR during smelting was still unstable, and the metallurgical defects at the site where the electrode is exchanged were transferred to the second VAR ingot, therefore a third VAR was required.

[0060] Diffusion annealing second VAR ingot at high temperature: The second VAR ingot was loaded into the furnace at the temperature of 550 C. or below and held at the temperature for 4 h, heated to 950 C. at a rate of 10 C./h, heated to 1100 C. at a rate of 15 C./h and held at the temperature for 12 h, heated to 1190 C. at a rate of 20 C./h and held at the temperature for 48 h, cooled down to 850 C. at the a of 15 C./h and held at the temperature for 24 h, and then air cooled.

[0061] Forging the electrode: The second VAR ingot with the diameter of 1050 mm was heated to 1150 C. for 10 h before forging. A free forging was performed on the 3500-ton high-speed forging press by stretching in one direction, in which reduction per pass in one direction was controlled to be 25 mm, and the final forging temperature was 900 C. Finally, the steel ingot was polished and flattened at the head and the tail to provide a tertiary consumable electrode with a diameter of 820 mm for use in a third VAR.

[0062] Third VAR: A crystallizer with a diameter of 1050 mm was adopted, the third VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 6.5 kg/min; helium cooling was initiated when 1500 kg of the VAR ingot is melted; the current was reduced and the melting rate was adjusted to 6.0 kg/min when the weight of the remained VAR ingot was 2500 kg; and hot topping controlled by the current was carried out when the weight of the remained VAR ingot was 800 kg. After the third VAR, the steel ingot was vacuum-cooled for 3 h and transferred to the annealing furnace for stress-relief annealing within 2 h, which prevents the steel ingot from cracking after demoulding. The annealing furnace was pre-heated to 450 C. and held at the temperature for 8 h for achieving homogeneous temperature distribution, heated to 850 C. at a rate of 10 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air-cooled.

[0063] Test result: The trial-prepared Alloy 706 VAR ingot with the diameter of 1050 mm and the weight of 16.2 tons is free from hot cracking or metallurgical defects such as black and white freckles. The composition at the head and tail of the steel ingot is tested, and the result shows that the there is no apparent burning loss of Al and Ti at the head and the tail, with 0.17% Al element at the head, 0.12% Al at the tail, 1.65% Ti at the head and 1.47% Ti at the tail. The third VAR ingot with the diameter of 1050 mm is diffusion annealed by homogenizing at high temperature, then polished and forged to prepare a bar. Upon non-destructive examination, it shows that there is no abnormal signal at the site where the electrode is exchanged, indicating that the third VAR can effectively address the metallurgical quality defects suffered at the site of the Alloy 706 where the electrode is exchanged.

EXAMPLE 3

[0064] This example is used to illustrate a method for preparing Alloy 718 (a consumable ingot with a diameter of 1050 mm).

[0065] The target Alloy 718 had a composition of (by weight):

C 0.015%, Cr 18.5%, Ni 53.5%, Nb 5.05%, Ti 0.92%, Al 0.55%, Si 0.04%, Mn 0.05%, P 0.006%, S 0.0008%, Co 0.02%, Mo 2.95%, B 0.004%, Cu 0.05%, Ca 0.001%, N 0.0048%, O 0.0024%, and the balance Fe.

[0066] The specific method for preparing the alloy was as follow.

[0067] Vacuum induction melting (VIM): According to the requirements of composition of the designed alloy, recycled material accounting for 40% of the elements needed for alloy of unit weight was weighed, and fresh metals accounting for the rest of the weight were weighed. A 12-ton VIM furnace was adopted, with an upper melting temperature limit being 1550 C. After melting down, the composition of molten steel was detected, and the molten steel was controlled to contain approximately 5.08 wt % Nb, approximately 0.97 wt % Ti, and approximately 0.60 wt % Al by adding fresh metals. Refining was carried out under electromagnetic stirring at 1480 C. for 40min, and tapping was conducted at a temperature of 1500 C. The molten steel was poured in two steps, followed by furnace cooling for 6 h and demoulding to obtain two VIM ingots with a diameter of 820 mm and a weight of 12 tons which were directly annealed. An annealing furnace was pre-heated to 800 C., heated to 1000 C. at a rate of 45 C./h and held at the temperature for 32 h, cooled to 780 C. at a rate of 35 C./h and held at the temperature for 32 h, and then air-cooled.

[0068] The annealed VIM ingots were polished and flattened at the head and the tail to prepare electroslag electrodes.

[0069] Electroslag remelting (ESR): A crystallizer with a diameter of 1100 mm was adopted and a quaternary (CaF.sub.2CaOAl.sub.2O.sub.3TiO.sub.2) slag system having a composition of, by weight, 75% CaF.sub.2, 25% CaO, 10% Al.sub.2O.sub.3, and 1% TiO.sub.2 was adopted. A steady-state melting rate was 55 kg/min. During smelting, Ar at 0.2 bar pressure was introduced for protection, which prevents the molten steel from contacting with oxygen and nitrogen in the air. Before exchanging the electrode, when the weight of a remained first electrode was 1000 kg, the melting rate was increased by 2 kg/min to 12 kg/min on the basis of the steady-state melting rate by adjusting input power and a voltage and was held until the electrode exchanging began. Smelting parameters during exchanging the electrode remained the same as those before exchanging the electrode, and the time for exchanging did not exceed 2 min. After exchanging the electrode, when 500 kg of a second electrode was melted, the melting rate was decreased by a slope of 2 kg/min to 15 kg/min by adjusting the input power and the voltage, and hot topping was carried out when the weight of the remained second electrode was 600 kg.

[0070] After ESR, an ESR ingot was water cooled in a water-cooled crystallizer in the furnace for 4 h, and within 1 h after demoulding, transferred to an annealing furnace for stress-relief annealing. The annealing furnace was pre-heated to 550 C. and held at the temperature for 32 h, heated to 750 C. at a rate of 25 C./h and held at the temperature for 4 h, heated to 1000 C. at a rate of 30 C./h and held at the temperature for 32 h, cooled to 750 C. at the rate of 35 C./h and held at the temperature for 32 h, and then air-cooled.

[0071] Diffusion annealing ESR ingot at high temperature: The ESR ingot was loaded into the furnace at the temperature of 550 C. or below and held at the temperature for 4 h, heated to 1000 C. at a rate of 5 C./h, heated to 1150 C. at the rate of 25 C./h and held at the temperature for 32 h, heated to 1250 C. at a rate of 25 C./h and held at the temperature for 72 h, cooled down to 950 C. at a rate of 35 C./h and held at the temperature for 32 h, and then air cooled.

[0072] Forging the electrode: The ESR ingot with the diameter of 1100 mm was heated to 1150 C. for 12 h before forging. A free forging was performed on a 3500-ton high-speed forging press by stretching in one direction, in which reduction per pass in one direction was controlled to be 25 mm, and the finishing forging temperature was 1000 C. Finally, the ESR ingot was polished and flattened at the head and the tail to provide a primary consumable electrode with a diameter of 820 mm for use in a first VAR.

[0073] First VAR: A crystallizer with a diameter of 920 mm was adopted, the first VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 7.5 kg/min; helium cooling was initiated when 2000 kg of the primary consumable electrode is melted; a current was reduced and the melting rate was adjusted to 7.0 kg/min when the weight of the remained primary consumable electrode is 1000 kg; and the hot topping which is controlled by the current was carried out when the weight of the remained primary consumable electrode is 1000 kg. The steel ingot was polished and flattened at the head and at the tail to provide a first VAR ingot with a diameter of 900 mm for use in a second VAR.

[0074] Second VAR: A crystallizer with a diameter of 1050 mm was adopted, the second VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 8.5 kg/min; helium cooling was initiated when 3000 kg of the first VAR ingot is melted; the current was reduced and the melting rate was adjusted to 7.5 kg/min when the weight of the remained first VAR ingot is 5500 kg; and hot topping controlled by the current was carried out when the weight of the remained first VAR ingot is 1500 kg. After the second VAR, the steel ingot was vacuum-cooled for 3 h and transferred to the annealing furnace for stress-relief annealing within 2 h, which prevents the steel ingot from cracking after demoulding. The annealing furnace was pre-heated to 750 C. and held at the temperature for 32 h for achieving homogeneous temperature distribution, heated to 1000 C. at a rate of 50 C./h and held at the temperature for 32 h, cooled to 750 C. at the rate of 35 C./h and held at the temperature for 32 h, and then air-cooled.

[0075] Test result: The trial-prepared Alloy 718 VAR ingot with the diameter of 1050 mm and the weight of 15 tons is free from hot cracking or metallurgical defects such as black and white freckles. The composition of the steel ingot at the head and the tail is tested, and the result shows that there is no apparent burning loss of Al and Ti at the head and the tail, with 0.60% Al element at the head, 0.48% Al at the tail, 0.87% Ti at the head and 0.98% Ti at the tail. The second VAR ingot with the diameter of 1050 mm is diffusion annealed by homogenizing at high temperature, then polished and forged to prepare a bar. Upon non-destructive examination, it shows that there is no abnormal signal at the site where the electrode is exchanged, indicating that the second VAR can effectively address the metallurgical quality defects suffered at the site of the Alloy 718 where the electrode is exchanged.

Comparative Example 1

[0076] Comparative Example 1 is used to illustrate a method for preparing Alloy 706 (a consumable ingot with a diameter of 920 mm) by adopting a triple-melt process.

[0077] The composition of the target Alloy 706 is the same as that of Alloy 706 in Example one (by mass percentage):

C 0.018%, Cr 15.8%, Ni 41.5%, Nb 3.01%, Ti 1.72%, Al 0.25%, Si 0.02%, Mn 0.01%, P 0.006%, S 0.0006%, Co 0.02%, Mo 0.01%, B 0.004%, Cu 0.02%, Ca 0.004%, N 0.005%, O 0.002%, and the balance Fe.

[0078] The specific method for preparing the alloy was as follow.

[0079] Vacuum induction melting (VIM): According to the requirements of composition of the designed alloy, recycled material accounting for 40% of the elements needed for alloy of unit weight was weighed, and fresh metals accounting for the rest of the weight were weighed. A 12-ton VIM furnace was adopted to obtain two VIM ingots with a diameter of 820 mm having a weight of 12 tons, in which an upper melting temperature limit was 1550 C. After melting down, the composition of molten steel was detected, and the molten steel was controlled to contain approximately 3.10 wt % Nb, approximately 1.82 wt % Ti, and approximately 0.35 wt % Al by adding fresh metals. Refining was carried out under electromagnetic stirring at 1480 C. for 40 min, and tapping was conducted at a temperature of 1500 C. After pouring, furnace cooling was carried out for 4 h and demoulding was carried out to obtain VIM ingots which were directly annealed. The annealing furnace was pre-heated to 650 C., heated to 900 C. at a rate of 25 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 10 h, and then air-cooled.

[0080] The annealed VIM ingots were polished and flattened at the head and the tail to prepare electroslag electrodes.

[0081] Electroslag remelting (ESR): A crystallizer with a diameter of 1100 mm was adopted and a tertiary (CaF.sub.2CaOAl.sub.2O.sub.3) slag system having a composition of 70% CaF.sub.2, 15% CaO, and 15% Al.sub.2O.sub.3 was adopted. A steady-state melting rate was 10 kg/min. During smelting, Ar at 0.2 bar pressure was introduced for protection, which prevents the molten steel from contacting with oxygen and nitrogen in the air. Before exchanging the electrode, when the weight of a remained first electrode was 600 kg, the melting rate was increased by 0.55 kg/min to 15 kg/min on the basis of the steady-state melting rate by adjusting input power and a voltage and was held until the electrode exchanging began. Smelting parameters during exchanging the electrode were the same as those before exchanging the electrode, and the time for exchanging did not exceed 2 min. After exchanging the electrode, when 200 kg of a second electrode was melted, the melting rate was decreased by 0.75 kg/min to 10 kg/min by adjusting the input power and the voltage, and hot topping was carried out when the weight of the remained second electrode was 500 kg.

[0082] After ESR, an ESR ingot was water cooled in a water-cooled crystallizer in the furnace for 4 h, and within 0.5 h after demoulding, transferred to an annealing furnace for stress-relief annealing. The annealing furnace was pre-heated to 450 C. and held at the temperature for 24 h, heated to 650 C. at a rate of 15 C./h and held at the temperature for 4 h, heated to 950 C. at a rate of 25 C./h and held at the temperature for 12 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air-cooled.

[0083] Diffusion annealing ESR ingot at high temperature: The ESR ingot was loaded into the furnace at the temperature of 550 C. or below and held at the temperature for 4 h, heated to 950 C. at a rate of 10 C./h, heated to 1100 C. at the rate of 15 C./h and held at the temperature for 12 h, heated to 1190 C. at a rate of 20 C./h and held at the temperature for 48 h, cooled down to 850 C. at the rate of 15 C./h and held at the temperature for 24 h, and then air cooled.

[0084] Forging the electrode: The ESR ingot with the diameter of 1100 mm was heated to 1150 C. for 10 h Before forging. A free forging was performed on a 3500-ton high-speed forging press was adopted for stretching in one direction, in which reduction per pass in one direction was controlled to be 25 mm, and a finishing forging temperature was 900 C. Finally, the ESR ingot was polished and flattened at the head and the tail to prepare a consumable electrode with a diameter of 820 mm.

[0085] VAR: A crystallizer with a diameter of 920 mm was adopted, the VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 5.5 kg/min; helium cooling was initiated when 1000 kg of the consumable electrode is melted; a current was reduced and the melting rate was adjusted to 4.0 kg/min when the weight of the remained consumable electrode is 2000 kg; and the hot topping controlled by the current was carried out when the weight of the remained consumable electrode is 500 kg. After the VAR, the steel ingot was vacuum-cooled for 3 h and transferred to the annealing furnace for stress-relief annealing within 2 h after demoulding, which prevents the steel ingot from cracking after demoulding. The annealing furnace was pre-heated to 450 C. and held at the temperature for 8 h for keeping homogeneous, heated to 850 C. at a rate of 10 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air-cooled.

[0086] Test result: The Alloy 706 VAR ingot with the diameter of 920 mm and the weight of 15 tons prepared by the triple-melt process is free from hot cracking. The composition the steel ingot at the head and the tail is tested, and the result shows that there is apparent burning loss of Al and Ti at the head and the tail, with 0.29% Al element at the head, 0.19% Al at the tail, 1.62% Ti at the head and 1.80% Ti at the tail. The second VAR ingot with the diameter of 1050 mm is diffusion annealed by homogenizing at high temperature, then polished and forged to prepare a bar. Upon non-destructive examination, it shows that there is abnormal signal at the site where the electrode is exchanged, and obvious black freckles are found at the site where the electrode is exchanged by dissection.

Comparative Example 2: Consumable Ingot of Alloy 706 Having a Diameter of 1050 mm

[0087] Comparative Example 2 is used to illustrate a method for preparing Alloy 706 by adopting a triple-melt process (a VAR ingot with a diameter of 1050 mm).

[0088] The composition of the desired Alloy 706 is the same as that of Alloy 706 in example two (by mass percentage):

C 0.011%, Cr 16.2%, Ni 42.5%, Nb 2.80%, Ti 1.57%, Al 0.15%, Si 0.02%, Mn 0.02%, P 0.008%, S 0.0004%, Co 0.01%, Mo 0.02%, B 0.003%, Cu 0.05%, Ca 0.001%, N 0.0045%, O 0.0025%, and the balance Fe.

[0089] The specific method for preparing the alloy was as follow.

[0090] Vacuum induction melting (VIM): According to the requirements of composition of the designed alloy, recycled material, accounting for 50% of the elements needed for alloy of unit weight, was weighed, and fresh metals, accounting for the rest of the weight, were weighed. A 12-ton VIM furnace was adopted to obtain two VIM ingots with a diameter of 820 mm having a weight of 12 tons, in which an upper melting temperature limit was 1550 C. After melting down, the composition of molten steel was detected, and by adding fresh metals, the molten steel was controlled to contain approximately 3.10 wt % Nb, approximately 1.72 wt % Ti, and approximately 0.30 wt % Al. Refining was carried out under electromagnetic stirring at 1480 C. for 40 min, and tapping was conducted at a temperature of 1500 C. After pouring, furnace cooling was carried out for 4 h and demoulding was carried out to obtain VIM ingots which are directly annealed. An annealing furnace was pre-heated to 650 C., heated to 900 C. at a rate of 25 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 10 h, and then air-cooled.

[0091] The annealed VIM ingots were polished and flattened at the heads and the tails to prepare electroslag electrodes.

[0092] Electroslag remelting (ESR): A crystallizer with a diameter of 1100 mm was adopted and a quaternary slag system specifically having a composition of, by weight, 64% CaF.sub.2, 15% CaO, 15% Al.sub.2O.sub.3, and 6% TiO.sub.2 was adopted. A steady-state melting rate was 10 kg/min. During smelting, Ar at 0.2 bar pressure was introduced for protection, which prevents the molten steel from contacting with oxygen and nitrogen in the air. Before exchanging the electrode, when the weight of a remained first electrode was 600 kg, the melting rate was increased by 0.55 kg/min to 15 kg/min on the basis of the steady-state melting rate by adjusting input power and a voltage and was held until the electrode exchanging began. Smelting parameters during exchanging the electrode were the same as those before exchanging the electrode, and the time for exchanging did not exceed 2 min. After exchanging the electrode, when a second electrode was melted 200 kg, the melting rate was decreased by a slope of 0.75 kg/min to 10 kg/min by adjusting the input power and the voltage, and hot topping was carried out when the weight of the remained second electrode was 500 kg.

[0093] After ESR, an ESR ingot was water cooled in a water-cooled crystallizer in the furnace for 4 h, and within 0.5 h after demoulding, transferred to an annealing furnace for stress-relief annealing. The annealing furnace was pre-heated to 450 C. and held at the temperature for 24 h, heated to 650 C. at a rate of 15 C./h and held at the temperature for 4 h, heated to 950 C. at a rate of 25 C./h and held at the temperature for 12 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air-cooled.

[0094] Diffusion annealing ESR ingot at high temperature: The ESR ingot was loaded into the furnace at the temperature of 550 C. or below and held at the temperature for 4 h, heated to 950 C. at a rate of 10 C./h, heated to 1100 C. at the rate of 15 C./h and held at the temperature for 12 h, heated to 1190 C. at a rate of 20 C./h and held at the temperature for 48 h, cooled down to 850 C. at the rate of 15 C./h and held at the temperature for 24 h, and then air cooled.

[0095] Forging the electrode: The ESR ingot with the diameter of 1100 mm was heated to 1150 C. for 10 h before forging. A free forging was performed on a 3500-ton high-speed forging press was adopted by stretching in one direction, in which reduction per pass in one direction was controlled to be 25 mm, and a finishing forging temperature was 900 C. Finally, the ESR ingot was polished and flattened at the head and the tail to provide a consumable electrode with a diameter of 820 mm.

[0096] VAR: A crystallizer with a diameter of 1050 mm was adopted, the VAR was controlled by the melting rate, and a steady-state melting rate was controlled to be 6.0 kg/min; helium cooling was initiated when 1500 kg of the consumable electrode is melted; a current was reduced and the melting rate was adjusted to 5.0 kg/min when the weight of the remained consumable electrode is 2500 kg; and the hot topping which is controlled by the current was carried out when the weight of the remained consumable electrode is 800 kg. After the VAR, the steel ingot was vacuum-cooled for 3 h and transferred to the annealing furnace for stress-relief annealing within 2 h, which prevents the steel ingot from cracking after demoulding. The annealing furnace was pre-heated to 450 C. and held at the temperature for 8 h for keeping homogeneous, heated to 850 C. at a rate of 10 C./h and held at the temperature for 24 h, cooled to 600 C. at a rate of 15 C./h and held at the temperature for 12 h, and then air cooled.

[0097] Test result: The Alloy 706 VAR ingot with the diameter of 1050 mm having the weight of 15.8 tons prepared by the triple-melt process is free from hot cracking. The composition of the steel ingot at the head and tail is tested, and the result shows that there is no apparent buring loss of Al and Ti at the head and the tail, with 0.16% Al element at the head, 0.12% Al at the tail, 1.65% Ti at the head and 1.50% Ti at the tail. The second VAR ingot with the diameter of 1050 mm is diffusion annealed by homogenizing at high temperature, then polished and forged to prepare a bar. Upon non-destructive examination, it shows that there is abnormal signal at the site where the electrode is exchanged, and obvious black freckles are found at the site where the electrode is exchanged by dissection.

[0098] What is provided above is merely some preferred embodiments of this application. The scope of this application is not limited by the above embodiments. Therefore, some improvements and modifications can be made by those skilled in the art without departing from the principle of this application, and should be considered to fall within the scope of this application.