Quick-through heat treatment process for preventing heat treatment cracking of special alloys

Abstract

The present application relates to a kind of quick heat treatment processes for preventing special alloy heat treatment crack, the process is directly powered for the large specification blank to carry out quick heat treatment, and the small specification blank is carried out quick heat treatment using salt bath method.The process described in the present application is suitable for a small number of special grades in special alloy (especially in high-temperature alloy), solve the problem of crack during annealing softening heat treatment after cold working in the production process of these high crack tendency difficult processing special alloy, so that it can be manufactured into alloy material, provide a solution for the normal manufacture of this kind of alloy material.The process can be used as pretreatment before formal heat treatment, also can be used as formal heat treatment, achieve the purposes of annealing, softening, preventing crack.The obtained alloy material has no crack, good quality, the advantage that the yield is 100%, has remarkable effect in feasibility, reliability, yield, quality and the like.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment in the production of special alloys, and relates to a rapid temperature heat treatment process to prevent cracks from occurring during the annealing softening heat treatment after cold working in the production of special alloys with high cracking tendency and difficult processing. Background Technology

[0002] Special alloys mainly include high-temperature alloys, corrosion-resistant alloys, functional metallic materials, and special stainless steels. Special alloy materials typically include bars, strips, wires, plates, strips, and tubes. In the production process from billet to finished product, hot deformation processing and continuous hot deformation processing are rarely used or purchased due to reasons such as failure to meet performance requirements, low process stability, large equipment investment, low equipment adaptability, and low equipment utilization. Cold deformation processing (such as cold drawing, cold rolling, etc.) combined with annealing softening heat treatment are the main process methods. After cold working, the billet will undergo work hardening, its strength and hardness will increase, and its plasticity will decrease significantly. After a certain point, it will lose its plastic deformation ability and cannot continue to be cold worked. Annealing softening heat treatment is necessary to restore the plasticity of the billet before it can continue to be cold worked.

[0003] However, a small number of special grades of special alloys (especially high-temperature alloys) have some problems with heat treatment. These problems manifest as follows: when the alloy undergoes annealing and softening heat treatment after cold working, if the usual heating method is used, i.e., the heating stage takes several hours or even just a few minutes (usually called slow heating, slow reaching temperature, etc.), cracks will occur in the alloy material, whether it is a box furnace for periodic heat treatment or a continuous heat treatment furnace for continuous heat treatment, and whether it is a cold charging furnace, a hot charging furnace, or a temperature-controlled charging furnace. These cracks include longitudinal cracks, diagonal cracks, and transverse cracks. In some cases, the alloy material may even completely break due to cracks. Moreover, the probability of this problem occurring is almost 100%. During cold charging in the furnace, a cracking sound can be heard as the billet cracks and bursts during the heating process. After heat treatment, the cracks are visible to the naked eye, and sometimes the alloy material even breaks completely. Similarly, during hot charging and low-temperature charging, the same cracking sound can be heard as the billet cracks and bursts during loading. This problem exists in a few special alloy grades, such as 68NiCrWCoTiAl, GH4049, and GH4099. These alloys have very high contents of alloying elements such as carbon, tungsten, molybdenum, cobalt, titanium, and aluminum, resulting in a high tendency to crack and significant difficulties in processing. This makes it virtually impossible to produce finished products from these alloys, and even if produced, their quality is highly uncertain and carries a risk of severe quality defects. This poses a significant technical obstacle to alloy material production. Therefore, simultaneously achieving annealing, softening, and crack prevention is a problem that must be solved. Research has shown that the problem can be perfectly solved by using a rapid heating method with a heating time of ≤30 seconds during the heating phase (which is the core technology of this invention). Summary of the Invention

[0004] This invention provides a rapid-temperature heat treatment process to prevent cracking during the annealing and softening heat treatment following cold working in the production of special alloys with a high tendency to crack. This process is applicable to a small number of special grades of special alloys (especially high-temperature alloys), solving the problem of cracking during the annealing and softening heat treatment following cold working in the production of these high-cracking, difficult-to-machine special alloys, enabling them to be manufactured into alloy materials and providing a solution for the normal manufacturing of such alloy materials. This process can be used as a pretreatment before formal heat treatment or as part of the formal heat treatment, simultaneously achieving multiple objectives such as annealing, softening, and preventing cracking. It has the advantages of producing crack-free alloy materials with good quality and a 100% yield.

[0005] The technical solution of the process described in this invention is as follows:

[0006] A rapid-temperature heat treatment process to prevent heat treatment cracks in special alloys includes the following steps:

[0007] 1) Clean large-sized billets;

[0008] Clean the surface of the billet to remove lubricant, oil, mud, and other contaminants to prevent chemical penetration. If it is determined that the surface contaminants will not affect the billet, cleaning is not necessary. Appropriate methods such as tap water cleaning, alkaline water cleaning, or acid cleaning can be used; no special requirements apply.

[0009] 2) Unwinding the billet;

[0010] 3) Direct electric annealing softening heat treatment of large-size billets;

[0011] Electrodes are connected to both ends of the heat treatment section of the billet, and direct electric heating is applied to rapidly raise the temperature of the billet to a holding temperature of 900-1100℃ within ≤30 seconds. The holding time at this temperature is 30-60 seconds.

[0012] 4) Remove the current applied to the billet, air cool to room temperature, and roll up the billet to complete the first direct energized annealing softening heat treatment.

[0013] 5) Perform the first cold working on the billet after the first direct electric annealing softening heat treatment to reduce the cross-sectional area of ​​the billet, that is, reduce the diameter, thickness, etc.

[0014] Repeat steps 1) to 5) to perform a second, third, and other cycles of "direct electric annealing softening heat treatment + cold working" on large-sized billets until small-sized billets are obtained.

[0015] When annealing and softening heat treatment is also required for small-sized billets, the following steps are included:

[0016] 1) Clean small-sized billets;

[0017] 2) Loosen the billet as much as possible to facilitate the entry of the heat treatment medium during the salt bath annealing softening heat treatment;

[0018] 3) Salt bath annealing softening heat treatment of small-sized billets;

[0019] Prepare a salt bath heat treatment furnace. The furnace should contain a sufficient amount of heat treatment medium to ensure adequate heat capacity. Once the heat treatment medium in the salt bath heat treatment furnace reaches the required holding temperature, place the billet in the material basket and then quickly immerse the billet in the heat treatment medium at a holding temperature of 900-1100℃ within ≤5 seconds. Hold the billet at this holding temperature for 60-180 seconds. After that, remove the billet and air cool it to room temperature.

[0020] 4) Clean the billet by washing away the salt heat treatment medium adhering to it in water. Then roll the billet into a bundle to complete the first salt bath annealing and softening heat treatment.

[0021] 5) Perform the first cold working on the billet after the first salt bath annealing softening heat treatment to reduce the cross-sectional area of ​​the billet, that is, reduce the diameter, thickness, etc.

[0022] Repeat steps 1) to 5) to perform a second, third, and other cycles of "salt bath annealing softening heat treatment + cold working" on small-sized billets until the finished alloy material is obtained.

[0023] The cross-sectional area of ​​the large-size billet is >6.8mm². 2 .

[0024] The cross-sectional area of ​​the small-sized billet is ≤6.8mm². 2 .

[0025] The power source for direct power supply is either direct current (DC) or alternating current (AC).

[0026] The electrical parameters, such as current and voltage, required to quickly bring the billet to the required insulation temperature and maintain that temperature by directly energizing it should change accordingly with the changes in the cross-sectional area and length of the billet. They need to be determined based on the billet specifications (such as wire diameter, sheet / strip thickness and width), length, etc.

[0027] The direct energizing annealing softening heat treatment method is based on an alloy billet with a diameter D1 of φ9mm and a length L1 of 16m, where the applied voltage V1 is 72V and the current I1 is 800A. When the cross-sectional area and length of the billet change, the applied voltage V2 remains unchanged at 72V. The direct energizing current I2 is estimated according to the formula I1×((D2÷D1)^2×(L1÷L2))^0.5, and a ±15% correction is required to determine the current, voltage, and other electrical parameters required to quickly reach and maintain the required holding temperature of the billet.

[0028] The salt bath annealing and softening heat treatment is performed in a salt bath heat treatment furnace, and the heat treatment medium is BaCl2 and NaCl.

[0029] The weight ratio of BaCl2 to NaCl is 85:15.

[0030] The volume of the heat treatment medium is ≥100 times the net volume of the billet being treated.

[0031] The principle of the process described in this invention:

[0032] After cold working, the billet will undergo work hardening, which increases its strength and hardness, but significantly reduces its plasticity. After a certain point, it will lose its plastic deformation ability and can no longer be cold worked. It must be annealed and softened by heat treatment to restore its plasticity before it can be cold worked again.

[0033] After cold working, the billet is in a state of high stress, but this stress is lower than its tensile strength, so no cracks occur. When conventional heat treatment is used, during the heating process of the billet, the heat in the air, the heat treatment medium in the heat treatment furnace, is conducted from the outer surface of the billet to the inside, and both the outer surface and the inside gradually heat up. However, this process is relatively slow, and there is a temperature difference between the outer surface and the inside, which creates a stress difference between the inside and the outside. The newly generated stress is superimposed on the high stress already present due to cold working, and exceeds the tensile strength of the billet, thus causing cracks.

[0034] The direct energizing method described in this invention has two advantages. First, when the billet is heated, the heating is simultaneous and uniform, both internally and externally. The temperature rises almost simultaneously and uniformly from the outer surface to the interior, resulting in virtually no new stress difference. This stress does not superimpose with the high stress already present due to cold working and does not exceed the tensile strength of the billet, thus preventing cracking. Second, even if new stress superimposes with the high stress already present due to cold working, leading to a tendency for crack initiation, the billet reaches the holding temperature very quickly, simultaneously, and uniformly. Upon reaching this holding temperature, the billet immediately enters a state of low annealing stress, loses elasticity, and softens, preventing new stress superposition. In other words, the speed at which the billet enters a state of low annealing stress, loses elasticity, and softens far exceeds the speed at which cracks initiate. This is equivalent to "fastest is better than slow," meaning cracks do not have time to form and there is no subsequent crack propagation or cracking.

[0035] The salt bath method described in this invention involves heat conduction from the outer surface of the billet to the interior, creating a stress difference between the external and internal surfaces, resulting in new stress superposition and a tendency for crack initiation. However, during the heat treatment process, the billet is rapidly immersed in the heat treatment medium, which conducts heat to the billet. The heat treatment medium has high heat conduction efficiency, sufficient instantaneous heat capacity, and sufficient total heat capacity to continuously supply heat and maintain the temperature. This allows the billet to reach the holding temperature almost simultaneously and as a whole, both inside and outside. Upon reaching this holding temperature, the billet immediately enters a state of annealing low stress, loses elasticity, and softens, preventing new stress superposition. In other words, the speed at which the billet enters a state of annealing low stress, loses elasticity, and softens far exceeds the speed at which cracks initiate. This is equivalent to the principle of "fastest is better than slowest," meaning that cracks do not have time to form, and there is no subsequent crack propagation or cracking.

[0036] The process described in this invention uses a direct electric current method for large-size billets and a salt bath method for small-size billets. The two methods can be used separately or in combination.

[0037] Table 1 shows a comparison of the main characteristics of the process described in this invention with conventional heat treatment processes. This comparison is based solely on the premise that the material being heat-treated is a special alloy with a high tendency to crack and difficult to machine.

[0038] Table 1 Comparison of the process described in this invention with conventional heat treatment processes

[0039]

[0040]

[0041] The process described in this invention is applicable to a small number of special grades of special alloys (especially high-temperature alloys). It prevents cracks from forming during the annealing softening heat treatment after cold working in the production of these high-cracking-prone and difficult-to-machine special alloys. It is a fast-temperature heat treatment process that can be applied to the mass production of products and has unique advantages in terms of feasibility, reliability, yield, and quality.

[0042] Beneficial effects of the present invention

[0043] The process described in this invention solves the problem of cracking during the annealing and softening heat treatment after cold working in the production of special alloys with high cracking tendency and difficulty in machining. It achieves multiple objectives, including annealing, softening, and preventing cracking, enabling these special alloys to be produced normally from a point where they are practically impossible to manufacture (even if they are, their quality is highly uncertain and there is a risk of serious quality defects). The yield and quality are guaranteed, achieving a breakthrough in the manufacturing of this type of alloy and providing a solution for the normal manufacturing of such alloys.

[0044] The process described in this invention is mainly applicable to special alloys that are difficult to machine due to high cracking tendency, especially a small number of special grades of high-temperature alloys, such as 68NiCrWCoTiAl alloy, GH4049 alloy, GH4099 alloy, etc. It can be applied to the mass production of products. The resulting alloys are free of cracks, have good quality, and have a 100% yield. It has the advantages of feasibility, reliability, high yield, and good quality.

[0045] This process can be used as a pretreatment before formal heat treatment, or it can be used as part of the formal heat treatment. Detailed Implementation

[0046] Requirements for the process described in this invention:

[0047] 1) Requirements for unwinding and rewinding:

[0048] Unwinding and rewinding can be performed using unwinding and rewinding devices.

[0049] During or after the direct electric annealing softening heat treatment of billets, the billets should be coiled into a suitable diameter while they are still red-hot to facilitate subsequent solution treatment in the furnace.

[0050] Once the billet temperature drops to room temperature, cold bending and other cold working operations should not be performed. Bending and other operations should be performed when the billet is in a hot, red-hot state; otherwise, new internal stress will be generated.

[0051] 2) Requirements for direct electrical annealing softening heat treatment:

[0052] a) It is essential to ensure that the heating stage can reach the required temperature quickly, that is, the billet must reach the required holding temperature within ≤15 to 30 seconds.

[0053] b) The required holding temperature after heating the billet can be within a relatively wide range, specifically between 250°C lower than the solution treatment temperature corresponding to the alloy and the solution treatment temperature itself. Typically, 900-1100°C is sufficient. The achieved holding temperature is measured directly on the billet using an infrared thermometer.

[0054] c) After the billet reaches the heat preservation temperature, it should be maintained at the heat preservation temperature for 30 to 60 seconds, until the billet loses the elasticity after cold working and softens. The specific judgment method is that the billet will show phenomena such as hanging due to its own weight, falling over due to its own weight, collapsing due to its own weight, and not twisting when rolled.

[0055] d) The electrical parameters such as current and voltage required to quickly bring the billet to the required insulation temperature and maintain that temperature by directly energizing it should change accordingly with the changes in the cross-sectional area and length of the billet. They need to be determined based on the billet specifications (such as wire diameter, sheet / strip thickness and width, etc.) and length.

[0056] e) The power supply required for direct power connection can be either DC or AC.

[0057] f) Direct energized annealing and softening heat treatment of billets can be performed in a periodic or continuous manner. In the periodic manner, the billet is divided into several segments of equal length, and electrodes are connected to both ends of each segment. After the first segment's annealing and softening heat treatment is completed, the second segment is performed, and so on. In the continuous manner, the billet passes continuously between two conductive wheels (electrodes) spaced appropriately apart in a continuous energized annealing apparatus, maintaining contact between the billet and the conductive wheels. The billet between the two conductive wheels undergoes annealing and softening heat treatment. During the direct energization process, it is crucial to ensure good contact between the billet and the conductive head or wheel. The operating procedure of "energizing only after good contact with the conductive head or wheel, and disconnecting from the conductive head or wheel only after de-energizing" must be strictly followed to avoid arcing, weld spatter, and new defects due to poor contact.

[0058] g) During direct electric annealing and softening heat treatment, the billet coils must not touch each other, otherwise a short circuit will occur.

[0059] h) It should be equipped with necessary direct power supply and electrical parameter control and adjustment devices such as current and voltage.

[0060] i) The direct current method is generally not limited in terms of applicable specifications, and the cross-sectional area of ​​the billet can be large or small. However, small billets are prone to problems such as arcing damage at the conductive contact points affecting quality, difficulty in temperature control leading to overheating and melting, cumbersome operation, and relatively low efficiency. The actual use effect is poor. Therefore, it is only used for rapid temperature heat treatment in the stage of large billets. For rapid temperature heat treatment in the stage of small billets, the salt bath method is required.

[0061] 3) Requirements for salt bath annealing and softening heat treatment:

[0062] a) A salt bath heat treatment furnace (a high-temperature salt bath heat treatment furnace used in the tool manufacturing industry can be used) is adopted, wherein the heat treatment medium contains two kinds of salts, BaCl2 and NaCl, and the weight ratio of BaCl2 to NaCl is 85:15.

[0063] b) The volume of the heat treatment medium in the salt bath heat treatment furnace is 100 times or more the net volume of the billet being treated, so as to ensure that the heat treatment medium has sufficient heat capacity to achieve rapid temperature rise and maintain the holding temperature.

[0064] c) It is essential to ensure rapid temperature reach, meaning that all billets in the basket must be completely immersed in the heat treatment medium in the salt bath heat treatment furnace within ≤5 seconds, so that all billets can quickly reach the required holding temperature.

[0065] d) The required holding temperature after the billet is heated can be within a relatively wide range, which is between 250°C lower than the solution treatment temperature corresponding to the alloy and the solution treatment temperature. Usually, 900-1100°C can meet the requirements. The achieved holding temperature is detected by thermocouples in the heat treatment medium in the salt bath heat treatment furnace.

[0066] e) After the billet reaches the holding temperature, it should be kept in the heat treatment medium in the salt bath heat treatment furnace for 60 to 180 seconds to make the billet lose the elasticity after cold working and soften.

[0067] f) The salt bath method is applicable only to blanks with a cross-sectional area ≤ 6.8 mm². 2 Small-sized billets (e.g., wires with a diameter ≤ φ2.94mm) are not suitable for large-sized billets (because it is difficult to ensure that the core of large-sized billets reaches the required temperature quickly).

[0068] Example 1

[0069] Taking 68NiCrWCoTiAl alloy as an example

[0070] 1. For large-format billets, annealing and softening heat treatment and cold working are performed using the direct electric current method for annealing and softening heat treatment:

[0071] Take a 68NiCrWCoTiAl alloy billet with a diameter of φ9mm and a length of 16m, rinse it clean with tap water, and then unwind it. Use a periodic process, connect electrodes to both ends of the section to be heat-treated and apply current. Reach the required holding temperature within 20-28 seconds, the holding temperature is 1050℃, and the holding time is 58-60 seconds. During this period, the direct current is 800A and the voltage is 72V. Use a DC power supply to successfully achieve annealing and softening heat treatment. Then, remove the applied current and rewind. Finally, cold draw it into an alloy billet with a diameter of φ6.8mm.

[0072] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ6.8mm and a length of 20m, during which a direct current of 550A and a voltage of 72V are applied; then cold draw it into an alloy billet with a diameter of φ5.2mm.

[0073] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ5.2mm and a length of 25m, during which a direct current of 360A and a voltage of 72V are applied; then cold draw it into an alloy billet with a diameter of φ4mm.

[0074] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ4mm and a length of 30m, during which a direct current of 250A and a voltage of 72V are applied; then cold draw it into an alloy billet with a diameter of φ2.8m to make it a small-sized billet.

[0075] 2. If annealing and softening heat treatment and cold working are to be performed on small-sized billets, the salt bath method should be used for annealing and softening heat treatment:

[0076] Take an alloy billet with a diameter of φ2.8mm, rinse it clean with tap water, and then unwind the coil. Prepare a salt bath heat treatment furnace with sufficient heat treatment medium. The volume of the heat treatment medium should be approximately 160 times the net volume of the billet to be treated, and the weight ratio of BaCl2 to NaCl in the heat treatment medium should be 85:15. When the heat treatment medium reaches the required holding temperature of 1100℃, place the billet in a basket and quickly immerse it in the heat treatment medium within about 3 seconds. Hold the billet at this temperature for about 150 seconds, then remove the billet from the heat treatment medium and allow it to cool naturally in the air. Then rinse the billet clean with tap water and then coil it up, thus completing the salt bath annealing and softening heat treatment. Finally, cold draw it into an alloy billet with a diameter of φ2.1mm.

[0077] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ2.1mm; then cold draw it into an alloy billet with a diameter of φ1.6mm.

[0078] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ1.6mm; then cold draw it into a cold-deformed alloy material with a diameter of φ1.2mm to obtain the finished alloy product.

[0079] Results: The success rate of annealing and softening heat treatment increased from ≤20% to almost 100%, the yield of annealing and softening heat treatment increased from ≤10% to almost 100%, the quality of the alloy material was good, there were no cracks, and it fully met the needs of scientific research and production. The effect was very good.

[0080] Example 2

[0081] Taking GH4049 alloy as an example

[0082] For large-format billets, annealing and softening heat treatment and cold working are performed using the direct electric current method for annealing and softening heat treatment.

[0083] Take a GH4049 alloy billet with a diameter of φ10mm and a length of 18m, rinse it clean with tap water, and then unwind it. Use a periodic process, connect electrodes to both ends of the heat treatment section and apply current, reach the required holding temperature of 1100℃ within 20-28 seconds, and maintain the holding time for 58-60 seconds. During this period, the direct current is 850A and the voltage is 72V, using a DC power supply to successfully achieve annealing and softening heat treatment. Then, remove the applied current and rewind the billet. Finally, cold draw it into an alloy billet with a diameter of φ7.8mm.

[0084] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ7.8mm and a length of 23m, during which a direct current of 590A and a voltage of 72V are applied; then cold draw it into an alloy billet with a diameter of φ6mm.

[0085] Repeat the above steps to anneal and soften the alloy billet with a diameter of φ6mm and a length of 28m, during which a current of 410A and a voltage of 72V are directly applied; then cold draw it into a cold-deformed alloy material with a diameter of φ4.5mm to obtain the finished alloy product.

[0086] Results: The success rate of annealing and softening heat treatment increased from ≤30% to almost 100%, the yield of annealing and softening heat treatment increased from ≤20% to almost 100%, the quality of the alloy material was good, there were no cracks, and it fully met the needs of scientific research and production. The effect was very good.

Claims

1. A rapid-temperature heat treatment process to prevent heat treatment cracks in special alloys, characterized in that, Includes the following steps: 1) Clean large-sized billets; 2) Unwinding the billet; 3) Direct electric annealing softening heat treatment of large-sized billets; Electrodes are connected to both ends of the section where the billet is to be heat-treated, and the billet is heated by direct electricity to rapidly raise the temperature to a holding temperature of 900-1100℃ within ≤30 seconds. The holding time at this temperature is 30-60 seconds. 4) Remove the current applied to the billet, air cool to room temperature, and roll up the billet to complete the first direct energized annealing softening heat treatment; 5) Perform the first cold working on the billet after the first direct electric annealing softening heat treatment to reduce the cross-sectional area of ​​the billet; Repeat steps 1) to 5) until small-sized blanks are obtained; The cross-sectional area of ​​the large-size billet is >6.8mm². 2 ; The cross-sectional area of ​​the small-sized billet is ≤6.8mm². 2 .

2. The process according to claim 1, characterized in that, When annealing and softening heat treatment is also required for small-sized billets, the following steps are included: 1) Clean small-sized billets; 2) Loose coils of billet; 3) Salt bath annealing softening heat treatment of small-sized billets; Place the billet in the material frame and immerse it rapidly in the heat treatment medium at a holding temperature of 900-1100℃ within ≤5 seconds. The holding time at this temperature is 60-180 seconds. Then remove the billet and air cool it to room temperature. 4) Clean the billet, then bundle and roll it up to complete the first salt bath annealing and softening heat treatment; 5) Perform a first cold working on the billet after the first salt bath annealing softening heat treatment to reduce the cross-sectional area of ​​the billet; Repeat steps 1) to 5) until the alloy material is obtained.

3. The process according to claim 1 or 2, characterized in that, Reducing the cross-sectional area of ​​the billet includes reducing its diameter and thickness.

4. The process according to claim 1, characterized in that: The power source for direct power supply is either direct current or alternating current; The electrical parameters of current and voltage required to quickly bring the billet to the required insulation temperature and maintain that temperature by directly energizing it should change accordingly with the changes in the cross-sectional area and length of the billet, and need to be determined based on the billet specifications and length.

5. According to the process described in claim 4, the specifications of the billet include the diameter of the wire, the thickness of the strip, and the width.

6. The process according to claim 1, characterized in that: The direct energizing annealing softening heat treatment method is based on an alloy billet with a diameter D1 of φ9mm and a length L1 of 16m, where the applied voltage V1 is 72V and the applied current I1 is 800A. When the cross-sectional area and length of the billet change, the applied voltage V2 remains unchanged at 72V. The direct energizing current I2 is calculated using the formula I1 × ((D2 ÷ D1)). 2 ×(L1÷L2)) 0.5 Estimate, and make a ±15% correction, to determine the electrical parameters of current and voltage required to quickly bring the billet to the required insulation temperature and maintain that temperature.

7. The process according to claim 2, characterized in that: The salt bath annealing and softening heat treatment is performed in a salt bath heat treatment furnace, and the heat treatment medium is BaCl2 and NaCl.

8. The process according to claim 7, characterized in that: The weight ratio of BaCl2 to NaCl is 85:

15.

9. The process according to claim 2, characterized in that: The volume of the heat treatment medium is ≥100 times the net volume of the billet being treated.

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