High-yield short-process preparation method of HT700T high-temperature alloy cast ingot

Abstract

The invention discloses a high-yield short-process preparation method of an HT700T high-temperature alloy cast ingot, which comprises the following steps of: smelting raw materials into an electrode by adopting vacuum induction melting (VIM) according to the alloy component proportion, then producing the cast ingot with uniform components by adopting vacuum consumable melting (VAR), and eliminating a low-melting-point precipitated phase and component segregation of the cast ingot through high-temperature homogenization, so that the high-yield short-process preparation method of the HT700T high-temperature alloy cast ingot is realized. The method is used for subsequent bar forging. The electrode with excellent surface quality is obtained by using an ingot mold with a reasonable size structure and a proper pouring temperature in VIM smelting. In the VAR smelting process, the appropriate electrode melting rate is selected, a certain amount of arc-stabilizing magnetic field is applied, the stability of the smelting process is improved, meanwhile, an optimized heat top sealing technology is adopted to reduce the shrinkage cavity area of the head of the cast ingot, and the HT700T high-temperature alloy cast ingot which is good in component uniformity, small in segregation and high in yield is obtained. In the high-temperature homogenization process, a four-step heating system is adopted to eliminate low-melting-point phase and dendritic segregation of the cast ingot, the quality of the cast ingot is improved, and the production time is shortened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nickel-based superalloy smelting, and particularly relates to a preparation method of HT700T alloy ingot. BACKGROUND

[0002] HT700T is a Ni-Cr-Fe-based superalloy used for supercritical power station heating unit pipes, and has complex service conditions, and needs to have good metallurgical quality to ensure the service life of the material. Meanwhile, while ensuring the metallurgical quality of the material, the cost factor also needs to be considered to reduce the construction cost of the power station. At present, the smelting production of the superalloy ingot mainly has single-link process, double-link process and triple-link process, wherein the double-link process is the main choice considering the cost, production efficiency, quality and other factors. The double-link process routes are vacuum induction melting (VIM) + electroslag remelting (ESR) or vacuum induction melting (VIM) + vacuum arc remelting (VAR). Among them, the VIM smelting mainly provides the electrode with accurate composition and purity for remelting, and is the basis of various smelting process routes; the ESR smelting has good effects of removing inclusions and desulfurization; the VAR smelting can effectively remove gas and impurity elements, and obtain the ingot with good composition uniformity and good crystallization structure. The VAR smelting is widely used for smelting large-size ingots due to the good cooling condition provided by the VAR smelting, and the segregation of the ingot is small. In the VAR process, the quality of the electrode and the electrode melting rate, droplet parameters and cooling condition in the smelting process are the keys to control the metallurgical quality of the ingot. In the arc burning process, the arc may be ignited with the ingot crown of the crystallizer, and thus the large ingot crown is hit by the arc and falls into the molten pool, and thus the metallurgical defects may be caused, and therefore how to ensure the stability of the arc is a big difficulty in the VAR process.

[0003] In addition, before the ingot is forged, generally, high-temperature homogenization treatment needs to be performed to eliminate the low-melting-point phase and segregation in the ingot, so as to ensure the metallurgical quality of the subsequent alloy rod. The heating temperature and holding time setting is the key of the homogenization process. When the process setting is unreasonable, the overburning, the precipitation phase cannot be eliminated, and the segregation elimination condition is not good. SUMMARY

[0004] The present application provides a high-yield short-process preparation method of HT700T superalloy ingot, and can obtain the HT700T alloy ingot with excellent metallurgical quality, high yield and short production cycle.

[0005] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows: a high-yield short-process preparation method of HT700T superalloy ingot, comprising the following steps:

[0006] S1: pouring the HT700T alloy VIM ingot by vacuum induction melting (VIM);

[0007] S2: the ingot after step 1 is subjected to a red delivery annealing treatment, the holding temperature is 800-1100℃, and the time is 4-6h;

[0008] S3: the ingot after step 2 is subjected to treatment on the electrode surface by using an automatic grinding machine to remove the surface oxide skin, so that an electrode for vacuum arc consumable melting (VAR) is obtained;

[0009] S4: the electrode after step 3 is subjected to vacuum arc consumable melting, so that an HT700T alloy VAR ingot is obtained;

[0010] S5: the ingot after step 4 is subjected to high-temperature homogenization treatment, so that an HT700T alloy finished ingot is obtained.

[0011] Preferably, the pouring temperature of the VIM ingot in S1 is 1490-1550℃, the ingot mold used for pouring has a nominal diameter of 320mm, a length of 4m-4.4m, an inner cavity taper of less than 0.8°, a requirement of inner roundness of less than 4mm, and a maximum deviation of the center line from the vertical axis of less than 2mm / 1000mm.

[0012] Preferably, in S4, the melting speed+droplet control is adopted in the VAR stable melting stage, the melting speed is 2.2-5.0kg / min, and the droplet is 3-12s -1 ; the helium pressure in the stable stage is 400-900Pa; a stable arc magnetic field is applied in the stable melting stage, the magnetic field strength is 2-10G, and the switching period is 10-20s; the nominal diameter of the VAR crystallizer is 406mm.

[0013] Preferably, in S4, the current+voltage control is adopted in the hot seal top melting stage, the current is reduced to 1.2kA from the last state in the stable melting stage at a rate of 0.02-0.03kA / min and is maintained for 20-40min;

[0014] Preferably, in S4, the high-temperature homogenization treatment adopts a four-step ladder heating system, the ingot loading temperature is ≤600℃, the first step heating temperature is 600-900℃, and the holding time is ≥60min; the second step heating temperature is 1100℃, and the holding time is 120-240min; the third step heating temperature is 1160℃, and the holding time is 1500-1800min; the second step to the third step adopts a gradual heating mode, and the heating time is not less than 600min; the fourth step heating temperature is 1180℃, and the holding time is 1200-3000min; the third step to the fourth step adopts a gradual heating mode, and the heating time is not less than 100min.

[0015] Compared with the prior art, the present application has the following beneficial effects:

[0016] The application develops a process route of HT700T alloy ingot, and the HT700T alloy is smelted by using the process, so that the ingot with short production flow, stable and controllable production process and high finished product rate can be obtained, the production cycle of the ingot is saved, and the production cost is reduced. Meanwhile, the metallurgical quality of the ingot after high-temperature homogenization is good, the structure is not overburned, the corresponding metallurgical defects in the finished rod are avoided, and the quality of the product is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a VIM ingot surface photo of example 1 of the application;

[0018] Figure 2 It is a VAR ingot head shrinkage photo of example 1 of the application;

[0019] Figure 3 It is an ingot structure photo after high-temperature homogenization of example 1 of the application;

[0020] Figure 4 It is a VIM ingot surface photo of comparative example 1 of the application;

[0021] Figure 5 It is a VAR ingot head shrinkage photo of comparative example 1 of the application;

[0022] Figure 6 It is an ingot structure photo after high-temperature homogenization of comparative example 1 of the application. DETAILED DESCRIPTION

[0023] The application will be further described in detail below in combination with the drawings and examples. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative work belong to the protection scope of the application.

[0024] As shown in the drawings, Figures 1-6 The application provides a high-finished product rate short-flow preparation method of HT700T high-temperature alloy ingot, and the specific scheme is as follows:

[0025] S1: pouring a HT700T alloy VIM ingot by vacuum induction melting (VIM);

[0026] S2: performing red delivery annealing treatment on the ingot obtained after step 1, the holding temperature is 800-1100 ℃, and the time is 4-6 h;

[0027] S3: performing treatment on the surface of the ingot after step 2 by using an automatic grinding machine to remove the surface oxide skin, so as to obtain an electrode that can be used for vacuum arc consumable melting (VAR);

[0028] S4: Vacuum arc consumable melting the electrode after step 3 to obtain a HT700T alloy VAR ingot;

[0029] S5: High temperature homogenization treatment of the ingot after step 4 to obtain a HT700T alloy finished ingot.

[0030] The application will be further described in detail in connection with specific examples and comparative examples.

[0031] Example 1

[0032] The HT700T alloy ingot was prepared according to the following method:

[0033] (1) Vacuum induction melting (VIM): The high-temperature alloy raw materials were melted in an 8-ton vacuum induction furnace. The main component ranges of the alloy are shown in Table 1.

[0034] Table 1 Main component ranges of HT700T alloy

[0035]

[0036] The pouring temperature of the VIM ingot was 1530℃, the ingot mold used for pouring had a nominal diameter of 320mm, a length of 4m, no taper in the inner cavity, a requirement of ≤3mm for the roundness, and a maximum deviation of the center line from the vertical axis of less than 2mm / 1000mm.

[0037] (2) Red delivery annealing treatment was performed after the ingot was discharged from the furnace, with a holding temperature of 1000℃ and a time of 6h.

[0038] (3) After cooling and discharging, the VIM ingot was treated by an automatic grinding machine to process the electrode surface until no visible oxide skin was left, thereby obtaining an electrode that can be used for vacuum arc consumable melting (VAR).

[0039] (4) Vacuum arc consumable melting was divided into three stages: arc starting stage, stable stage and hot sealing stage. The nominal diameter of the VAR crystallizer was 406mm. The arc starting stage time was 60min, and the maximum current setting value was 7kA. The stable stage used melt rate-droplet control. The stable stage process parameters were: melt rate 3.2kg / min, droplet 6.0s, helium pressure setting value 800Pa, magnetic field strength 5G, and switching period 20s. The hot sealing melting stage used current + voltage control, and the hot sealing starting weight was 140kg. After entering the hot sealing stage, the current decreased to 1.2kA at a rate of 0.026kA / min and was maintained for 40min. -1

[0040] ​(5) The high-temperature homogenization treatment adopts a four-step stepwise heating system, the ingot loading temperature is less than or equal to 600 ℃, the first step heating temperature is 850 ℃, the holding time is 60 min; the second step heating temperature is 1100 ℃, the holding time is 180 min; the third step heating temperature is 1160 ℃, the holding time is 1800 min; the second step to the third step adopts a gradual heating mode, the heating time is 600 min; the fourth step heating temperature is 1180 ℃, the holding time is 2000 min, the third step to the fourth step adopts a gradual heating mode, the heating time is 150 min.

[0041] The statistics of the smelting results of the ingot in Example 1 are shown in Table 2.

[0042]

[0043]

[0044] Table 2 smelting results of the HT700T alloy ingot in Example 1

[0045] Figure 1 The surface quality of the VIM ingot prepared in the embodiment is shown, and it can be seen from the figure that the surface is smooth, and there are no obvious surface defects such as cracks and cold shut, and a small amount of polishing of the surface oxide skin can be used, which significantly improves the yield and production efficiency. Figure 2 It is a photo of the VAR ingot head after longitudinal cutting of the ingot head, and it can be seen that there is no obvious hole, and the loose part is close to the upper surface shallow area, and the ingot can be used for subsequent forging without sawing the head after homogenization. Figure 3 It is a photo of the ingot after high-temperature homogenization of Example 1 of the present application (after chemical etching), and it can be seen that the structure is normal, the as-cast dendritic morphology is eliminated, and there is no overburning hole.

[0046] Comparative Example 1

[0047] The HT700T alloy ingot is prepared according to the following method:

[0048] (1) Vacuum induction melting (VIM): The high-temperature alloy raw materials are smelted in an 8-ton vacuum induction furnace. The main component range of the alloy is shown in Table 1.

[0049] The pouring temperature of the VIM ingot is 1480 ℃, the ingot mold used for pouring has a nominal diameter of 350 mm, a length of 4 m, a inner cavity taper of 1°, a inner roundness requirement of less than 5 mm, and a maximum deviation of the center line from the vertical axis of less than 5 mm / 1000 mm.

[0050] (2) The ingot is discharged after being cooled in the induction furnace for 5 hours.

[0051] (3) After the VIM ingot is discharged, the electrode surface is treated by using an automatic grinding machine to grind the electrode surface to be free of visible oxide skin, so as to obtain an electrode that can be used for vacuum arc consumable melting (VAR).

[0052] (4) The vacuum arc consumable melting is divided into three stages: an arc starting stage, a stable stage and a hot sealing stage. The nominal diameter of the VAR crystallizer is 406 mm. The arc starting stage time is 60 min, and the maximum current set value is 7 kA. The stable stage adopts a melting rate-droplet control. The stable stage process parameters are: a melting rate of 3.85 kg / min, a droplet of 2.0 s -1 , a helium gas pressure set value of 500 Pa, and no magnetic field is applied. The hot sealing melting stage adopts a current + voltage control, and the hot sealing starting weight is 120 kg. After entering the hot sealing stage, the current is reduced to 1.2 kA at a rate of 0.03 kA / min and is maintained for 20 min.

[0053] (5) The high-temperature homogenization treatment adopts a three-step temperature rising system. The ingot loading temperature is ≤600 ℃, the first step heating temperature is 850 ℃, and the holding time is 60 min; the second step heating temperature is 1160 ℃, and the holding time is 1500 min; the second step to the third step adopts a gradual heating mode, and the heating time is 120 min; the third step heating temperature is 1195 ℃, and the holding time is 5000 min; the third step to the fourth step adopts a gradual heating mode, and the heating time is 200 min.

[0054] The ingot related melting results in the comparative example 1 are shown in Table 2. The material loss, the VAR process ingot crown falling frequency, and the high-temperature homogenization first to last stage heating time are all increased compared with the embodiment, which will cause the production cost to increase, the production efficiency to decrease, and the metallurgical defect risk to increase.

[0055] Table 2 Melting results of HT700T alloy ingot in comparative example 1

[0056]

[0057] Figure 4 The VIM ingot surface quality prepared in the present comparative example is shown in the figure. As can be seen from the figure, the surface is rough, and there are surface defects such as cracks and cold shut, and a large amount of grinding is required to remove the surface oxide skin for VAR melting. Figure 5 The photo of the longitudinal section of the VAR ingot head of the present comparative example is shown. It can be seen that there are obvious large-size holes, and a certain amount of sawing is required before it can be used for subsequent forging, otherwise serious cracking problem will occur during forging. Figure 5 The ingot microstructure after high-temperature homogenization of the present comparative example 1 is shown. It can be seen that overburning holes appear, which indicates that the ingot segregation is relatively serious, and the homogenization system is unreasonable, causing the low-melting-point phase to melt, and thus the holes appear.

[0058] It should be pointed out finally that the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some of the technical features, as long as the modifications, equivalent replacements or improvements are within the spirit and principle of the present application, and should be included in the protection scope of the present application.

Claims

1. A high-yield, short-process method for preparing HT700T high-temperature alloy ingots, characterized in that: The following steps are involved: S1: Vacuum induction melting (VIM) casting of HT700T alloy VIM ingot; S2: The ingot obtained in step 1 is subjected to hot annealing treatment at a holding temperature of 800-1100° C. for 4-6 hours; S3: The ingot after step 2 is processed by an automatic grinder to remove the surface oxide scale to obtain an electrode that can be used for vacuum arc consumable melting (VAR); S4: performing vacuum arc consumable melting on the electrode after step 3 to obtain HT700T alloy VAR ingot; S5: performing a high-temperature homogenization treatment on the ingot after step 4 to obtain a finished HT700T alloy ingot.

2. The high-yield, short-process method for preparing HT700T high-temperature alloy ingots according to claim 1, characterized in that: The pouring temperature of the VIM ingot in S1 is 1490-1550°C, the nominal diameter of the ingot mold used for pouring is Φ320mm, the length is 4m-4.4m, the inner cavity taper is less than 0.8°, the inner roundness is required to be less than 4mm, and the maximum deviation between the center line and the vertical axis is less than 2mm / 1000mm.

3. The high-yield, short-process method for preparing HT700T high-temperature alloy ingots according to claim 1, characterized in that: In the S4, the VAR stable melting stage adopts melting rate + droplet control, the melting rate is 2.2-5.0 kg / min, the droplet is 3-12s -1 ; The helium pressure in the stable stage is 400~900Pa; a stable arc magnetic field is applied in the stable melting stage, the magnetic field strength is 5~20G, and the switching cycle is 10~20s; the nominal diameter of the VAR crystallizer is Φ406mm.

4. The high-yield, short-process method for preparing HT700T high-temperature alloy ingots according to claim 1, characterized in that: In S4, the hot capping melting stage adopts current + voltage control, and the current is reduced from the last state of the stable melting stage to 1.2 kA at a rate of 0.02 to 0.03 kA / min and maintained for 20 to 40 minutes.

5. The high-yield, short-process method for preparing HT700T high-temperature alloy ingots according to claim 1, characterized in that: In the S4, the high-temperature homogenization treatment adopts a four-step stepped heating system, the ingot charging temperature is ≤600°C, the first step heating temperature is 600-900°C, and the holding time is ≥60min; the second step heating temperature is 1100°C, and the holding time is 120-240min; the third step heating temperature is 1160°C, and the holding time is 1500-1800min; the second step to the third step adopts a gradual heating method, and the heating time is not less than 600min; the fourth step heating temperature is 1180°C, and the holding time is 1200-3000min, and the third step to the fourth step adopts a gradual heating method, and the heating time is not less than 100min.

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