Method for protecting copper crucible base for vacuum consumable arc melting of refractory niobium alloy

Abstract

The invention belongs to the technical field of vacuum melting, and particularly relates to a protection method of a copper crucible base for vacuum consumable electric arc melting of a refractory niobium alloy. According to the method, the copper crucible base is simply and conveniently pretreated to increase the contact area of the copper crucible base and the alloy bottom pad, and meanwhile, specific arcing process parameters are set to form a molten pool as soon as possible and reduce the frequency of striking the copper crucible base by a secondary electric arc, so that the copper crucible base is protected. According to the protection method, the copper crucible base does not need to be frequently repaired, the contact surface of the alloy bottom pad and the copper crucible base does not need to be frequently repaired either, a thought is provided for cost reduction and efficiency improvement of production of the refractory niobium alloy cast ingot through vacuum consumable electric arc melting, and the method is low in cost, easy to implement and suitable for large-scale production. The method can prevent the electric arc from damaging the copper crucible base even under the condition that the copper crucible base has not serious defects.

Description

Technical Field

[0001] This invention belongs to the field of vacuum melting technology, and specifically relates to a method for protecting the base of a copper crucible used in vacuum consumable arc melting of refractory niobium alloys. Background Technology

[0002] Vacuum arc remelting (VAR) is currently the mainstream method for preparing base metal ingots, widely used in the production of active metals such as titanium and zirconium, as well as refractory niobium alloys. The working principle of VAR is to heat the consumable electrode (CEA) under high vacuum and high voltage conditions using a DC vacuum arc generated between the CEA and a water-cooled copper crucible base, or between the CEA and the molten pool, causing the CEA to melt. The molten droplets fall into the water-cooled copper crucible under gravity, solidifying to form a dense ingot, thus refining the alloy (e.g., degassing, inclusion removal). Due to its relatively low melting point, titanium alloys typically form a good molten pool shortly after arc ignition during VAR, entering a stable melting stage. However, for refractory niobium alloys, with their high melting point (usually above 2400℃), the high melting current prevents the formation of a molten pool in the short time after arc ignition. The arc continues to act on the copper crucible base, causing damage and leading to difficulties in ingot demolding and ingot composition contamination.

[0003] In existing technologies, to prevent arc damage to the copper crucible base during the arc initiation stage, a certain thickness of alloy base pad of the same grade is usually placed on the copper crucible base in advance. This reduces the degree of damage to the copper crucible base from the arc and avoids component contamination. However, with the accumulation of production processes, pits inevitably appear on the copper crucible base, causing the protective effect of the alloy base pad to disappear. The reason is disclosed in the patent application document CN214120754U, specifically: with repeated machining repairs of the alloy base pad and the accumulation of smelting furnaces, the number of molten nodules and cladding formed in each furnace increases. At the same time, the difficulty in demolding has long plagued the production of niobium-titanium ingots, which not only greatly affects production efficiency, but also causes fatal damage to the inner wall of the lower part of the crucible when the ingot is hammered out, thus affecting the life of the crucible. Usually, new niobium-titanium alloy base pads are prepared periodically and the lower part of the crucible is corrected to reduce various effects. This is because a damaged copper crucible base becomes uneven, creating small gaps between the alloy base and the copper crucible base. The contact surface changes from a completely flat surface to a partially convex contact, significantly reducing the contact area. This results in a large potential difference between the alloy base and the copper crucible base, generating a secondary electric arc that strikes and damages the copper crucible base. Furthermore, frequent repairs of the copper crucible base and the same grade of alloy base incur high labor, time, and economic costs, severely hindering the company's production progress. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for protecting the copper crucible base in vacuum arc melting of refractory niobium alloys. This invention involves simple pretreatment of the copper crucible base to increase the contact area between the copper crucible base and the alloy base, while simultaneously setting specific arc-starting process parameters to quickly form a molten pool, reducing the frequency of secondary arc strikes on the copper crucible base, thereby protecting it. This protection method eliminates the need for frequent repairs of the copper crucible base and the contact surfaces between the alloy base and the copper crucible base. It provides a cost-effective and efficient approach for vacuum arc melting of refractory niobium alloy ingots, and proposes a low-cost, easy-to-implement method that can prevent arc damage to the copper crucible base even when it has minor defects.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: This invention protects a method for protecting a copper crucible base used in vacuum consumable arc melting of refractory niobium alloys, comprising the following steps: Step 1: Use a wire brush to clean the oxide scale from the copper crucible base and perform a flattening treatment on the copper crucible base. After the flattening treatment, the flatness deviation of the copper crucible base should be <5mm. Without the flattening treatment, the gap between the alloy base and the crucible base will be larger, increasing the possibility of arcing between the two. Ideally, the two should be in complete contact to avoid arcing between them.

[0006] Step 2: Place the alloy base pad of the same grade on the copper crucible base, adjust the placement angle, and ensure that the maximum gap between the alloy base pad and the copper crucible base does not exceed 5mm; according to production experience, if the gap exceeds 5mm, the possibility of arcing between the alloy base pad and the copper crucible base increases.

[0007] Step 3: Using niobium alloy ingots as consumable electrodes, consumable electrodes as cathodes, and copper crucibles as anodes, the furnace is loaded, welded, and inspected. Subsequently, the furnace is sealed, evacuated, and the leakage rate is measured. Under the condition of vacuum degree ≤0.133Pa, the arc ignition process parameters are set to enable rapid arc ignition and the establishment of a molten pool in the copper crucible. After the molten pool is established, the alloy base pad of the same grade and the crucible base will not ignite, protecting the copper crucible base from secondary arc damage, and then entering the stable smelting stage.

[0008] The arc initiation process parameters are as follows: The melting voltage is set to 35.0V~45.0V. This voltage is based on production experience; melting stability is high within this range. Excessive melting voltage increases the possibility of arc breakage, while insufficient melting voltage easily leads to short circuits and adhesion. First, maintain a low current of 2.0kA~5.0kA for 1min~3min. This step is understood as preheating with a small current to raise the temperature of the vacuum consumable arc furnace, consumable electrode, copper crucible, and alloy base. Then, rapidly increase the current to the maximum melting current within 2min~5min. Depending on the copper crucible specifications, the maximum melting current for Φ160mm~Φ380mm is 6kA~20kA. The purpose of this step is to quickly apply current to melt the consumable electrode and rapidly establish a molten pool, reducing the possibility of secondary arc generation. If the time is less than 2 minutes, the equipment will be under high load, causing significant damage to the power supply and components. If the time is more than 5 minutes, the molten pool will be slow to emerge, increasing the risk of impacting the crucible base. At the same time, the phenomenon of molten droplets pulling and short circuits on the consumable electrode will also be aggravated. After that, the arc stabilizing current is adjusted to 1.0A~6.0A, and then the arc stabilizing current is reversed, changing the direction of the current in the arc stabilizing coil. The magnetic field is reversed at the same time, thereby realizing the forward and reverse rotation of the metal molten pool, increasing the uniformity of element mixing, similar to the operation of a washing machine. The reversal frequency is 10s / time~30s / time.

[0009] After the melting process is completed and the crucible is removed from the furnace, the bottom surface of the copper crucible base and the same grade alloy base pad are inspected and treated to prepare for use in the next vacuum self-consuming arc melting process.

[0010] After the melting process is completed and the product is removed from the furnace, the lower end face of the alloy base of the same grade is treated. Visually, there should be no copper metal adhering to it. The treatment method depends on the specific situation. For example, if there is slight copper adhesion, grinding is sufficient. If there is severe adhesion, machining and leveling are required.

[0011] Preferably, the thickness of the alloy base pad is 30mm to 80mm; if the alloy base pad is too thin, it will not provide protection, and if it is too thick, it is unnecessary.

[0012] Preferably, the distance between the lower end of the consumable electrode and the alloy base is 20mm~50mm.

[0013] Preferably, the copper crucible has a size of Φ160mm~Φ380mm.

[0014] Preferably, the oxide scale on the copper crucible base is cleaned until the copper's original color is revealed.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a method for protecting the copper crucible base during vacuum arc melting of refractory niobium alloys, comprising the following steps: cleaning the oxide scale from the copper crucible base and flattening the copper crucible base; placing an alloy base pad of the same grade on the copper crucible base, adjusting the placement angle to ensure the gap between the base and the crucible base is within an appropriate range, increasing the contact area between the copper crucible base and the alloy base pad, thus reducing the generation of secondary arcs after they come into contact; sealing the furnace, evacuating, and measuring the leakage rate; after meeting the vacuum requirements, setting specific arc ignition process parameters, and rapidly igniting the arc to establish a molten pool, reducing the frequency of arc strikes on the copper crucible base, and then transitioning to a stable melting stage. This invention, through pretreatment and rapid arc ignition technology, effectively addresses the issue of arc damage to the copper crucible base during the melting of refractory niobium alloys, reduces the frequency of copper crucible maintenance, and is simple and easy to operate, which helps reduce enterprise production costs and improve production efficiency.

[0016] 2. The protection method provided by this invention enables furnace smelting by treating the copper crucible base. Furthermore, by using a rapid arc-starting process, a molten pool is quickly established and a stable smelting state is reached, reducing the risk of arc strikes on the copper crucible base and effectively protecting it. This lowers the probability of smelting accidents. In this case, the arc strikes the consumable electrode. Consumable melting involves the arc acting on the cathode (consumable electrode) to melt it, forming molten droplets that fall into the crucible to crystallize and form an ingot. The arc cathode has higher energy, hence the consumable electrode is the cathode. Moreover, it eliminates the need to frequently return the copper crucible base to the equipment manufacturer for repair, improving production efficiency and significantly reducing enterprise production costs. Attached Figure Description

[0017] Figure 1 The images show a comparison of the lower surface of the alloy base before and after melting in Embodiment 1 of the present invention. The left image is a physical image of the lower surface of the gold alloy base before melting, and the right image is a physical image of the lower surface of the gold alloy base after melting.

[0018] Figure 2 The images show a comparison of the copper crucible base before and after smelting in Embodiment 2 of the present invention. The left image is a physical image of the copper crucible base before smelting, and the right image is a physical image of the copper crucible base after smelting.

[0019] Figure 3 The diagram shows the structure of a vacuum arc furnace for melting refractory niobium alloys. (a) shows the structure inside the vacuum arc furnace before the molten pool is reached, and (b) shows the structure inside the vacuum arc furnace after the molten pool is reached.

[0020] Explanation of reference numerals in the attached figures: 1-Copper crucible; 2-Alloy base; 3-Consumable electrode; 4-Auxiliary electrode; 5-Power supply; 6-Molten pool. Detailed Implementation

[0021] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0022] In the existing technology, a consumable electrode is used as the cathode and a copper crucible is used as the anode. The electric arc is located between the two and in contact with them. The electric arc melts the consumable electrode by contacting it, forming molten droplets. However, if the electric arc is in contact with the copper crucible for a long time, it will damage the crucible. In the melting of low-melting-point titanium alloys, no alloy base is required. A full molten pool forms quickly after arc ignition, preventing the arc from continuously contacting the copper crucible base. Furthermore, the current is relatively small, so even if the arc hits the copper crucible, the impact is minimal in the short term. However, the present invention aims to melt refractory niobium alloys. Niobium alloys have a high melting point, requiring a large melting current and a long time to establish a molten pool. Without an alloy base, the arc would act on the copper crucible base for an extended period, potentially damaging it. Using an alloy base transfers the arc between the consumable electrode and the base. However, in actual production, the surface between the crucible base and the alloy base is inevitably uneven, leading to a delayed molten pool formation and the risk of secondary arc ignition. This invention addresses this problem by increasing flatness through pretreatment to reduce the possibility of secondary arc ignition and by using rapid arc ignition parameters to quickly establish a molten pool, thus reducing the likelihood and duration of direct arc action on the crucible base without a molten pool.

[0023] The technical solution of the present invention will be further illustrated below with specific examples: Example 1 A method for protecting the base of a copper crucible used in vacuum consumable arc melting of refractory niobium alloys, taking the melting of a Φ160mm refractory niobium alloy ingot as an example, includes the following steps: S1. Use a wire brush to clean the oxide scale on the copper crucible base until the surface reveals the natural color of copper, and then perform surface flattening treatment on the copper crucible base. The flatness deviation of the copper crucible base after treatment is less than 5mm.

[0024] S2. Prepare a 30mm thick alloy base pad of the same grade and place it on the copper crucible base. After adjusting the placement angle, the maximum gap between the alloy base pad and the copper crucible base should be less than 5mm.

[0025] S3. Using the purchased niobium alloy ingot as the consumable electrode, with the consumable electrode as the cathode and a copper crucible as the anode, the process involves loading, welding, and inspection. The raising and lowering of the consumable electrode is achieved through in-furnace welding and mechanical connection. The movable component is a motor-driven electrode rod. First, the electrode rod and auxiliary electrode are physically clamped together. Then, the auxiliary electrode and consumable electrode are welded together in the furnace, thus achieving the raising and lowering of the consumable electrode. An alloy base and arc-starting chips are placed inside the copper crucible. One end of the consumable electrode is placed inside the copper crucible. The consumable electrode-auxiliary electrode-electrode is connected to one pole of the power supply, and the alloy base-copper crucible is connected to the other pole. The furnace is then sealed, a vacuum is drawn, and the leakage rate is measured to ensure a vacuum degree ≤ 0.133 Pa. Before arc starting, the distance between the lower end of the consumable electrode and the alloy base is maintained at 50 mm. The arc is then initiated. The movement of the electrode rod during the melting process depends on the voltage (after the self-consumable electrode melts and is lost, the distance between the self-consumable electrode and the anode increases, and the voltage increases accordingly. In order to match the voltage range, the system presses down the electrode rod). First, the arc initiation process parameter is set to 2.0kA and held for 1 minute. Then, the maximum melting current is increased within 2 minutes. The maximum melting current is in a slightly fluctuating state, between 6A and 8A. At this time, the melting voltage is also in a fluctuating state, between 35.0V and 45.0V (because the niobium alloy has a high viscosity, the molten droplets are easy to form wires during the melting process. The continuous wire forming reduces the distance between the cathode and the anode, and the voltage will decrease, resulting in a continuous voltage fluctuation). Then, the arc stabilization current is adjusted to 1.0A, and the arc stabilization current commutation frequency is 10s / time. The arc is quickly initiated to establish a molten pool, and then the process enters the stable melting stage.

[0026] S4. After the melting process is completed and the crucible is removed from the furnace, inspect and treat the bottom surface of the copper crucible base and the alloy base of the same grade. After treatment, visually inspect for any copper metal adhesion, so that it can be used for secondary vacuum consumable arc melting in the next furnace. Figure 1 As shown, a comparison of the morphology of the alloy base pad of the same grade before and after melting is given. It can be seen that it is basically the same as before melting, which proves that the electric arc exists for a short time in this part. The solution of this embodiment plays a role in protecting the alloy base pad.

[0027] Example 2 A method for protecting the base of a copper crucible used in vacuum arc melting of refractory niobium alloys, taking the melting of a Φ380mm refractory niobium alloy ingot as an example, includes the following steps: S1. Use a wire brush to clean the oxide scale on the copper crucible base until the surface reveals the natural color of copper, and then perform surface flattening treatment on the copper crucible base. The flatness deviation of the copper crucible base after treatment is less than 5mm.

[0028] S2. Prepare an 80mm thick alloy base pad of the same grade and place it on the copper crucible base. After adjusting the placement angle, the maximum gap between the alloy base pad and the copper crucible base should be less than 5mm.

[0029] S3. Using the purchased niobium alloy ingot as the consumable electrode, with the consumable electrode as the cathode and a copper crucible as the anode, the process involves loading, welding, and inspection. The raising and lowering of the consumable electrode is achieved through in-furnace welding and mechanical connection. The movable component is a motor-driven electrode rod. First, the electrode rod and auxiliary electrode are physically clamped together. Then, the auxiliary electrode and consumable electrode are welded together in the furnace, thus achieving the raising and lowering of the consumable electrode. An alloy base and arc-starting chips are placed inside the copper crucible. One end of the consumable electrode is placed inside the copper crucible. The consumable electrode-auxiliary electrode-electrode is connected to one pole of the power supply, and the alloy base-copper crucible is connected to the other pole. The furnace is then sealed, a vacuum is drawn, and the leakage rate is measured to ensure a vacuum degree ≤ 0.133 Pa. Before arc starting, the distance between the lower end of the consumable electrode and the alloy base is maintained at 50 mm. Then, the arc is initiated. The movement of the electrode rod during the melting process depends on the voltage (after the self-consumable electrode melts and is lost, the distance between the self-consumable electrode and the anode increases, and the voltage increases accordingly. In order to match the voltage range, the system presses down the electrode rod). First, the arc initiation process parameter is set to 5kA and held for 3 minutes. Then, the maximum melting current is increased within 5 minutes. The maximum melting current is in a slightly fluctuating state, between 19kA and 20kA. At this time, the melting voltage is also in a fluctuating state, between 35.0V and 45.0V (because the niobium alloy has a high viscosity, the molten droplets are easy to form wires during the melting process. The continuous wire forming reduces the distance between the cathode and the anode, and the voltage will decrease, resulting in a continuous voltage fluctuation). Then, the arc stabilization current is adjusted to 6.0A, and the arc stabilization current commutation frequency is 30s / time. The arc is quickly initiated to establish a molten pool, and then the process enters the stable melting stage.

[0030] S4. After the melting process is completed and the crucible is removed from the furnace, inspect and treat the bottom surface of the copper crucible base and the alloy base of the same grade. After treatment, visually inspect for any copper metal adhesion, so that it can be used for secondary vacuum consumable arc melting in the next furnace. Figure 2 As shown, a comparison of the morphology of the copper crucible base before and after melting is presented. It can be seen that it is basically the same as before melting, which proves that the electric arc exists in this part for a short time. This scheme plays a role in protecting the copper crucible base during vacuum self-consuming arc melting.

[0031] Example 3 A method for protecting the base of a copper crucible used in vacuum consumable arc melting of refractory niobium alloys, taking the melting of a Φ220mm refractory niobium alloy ingot as an example, includes the following steps: S1. Use a wire brush to clean the oxide scale on the copper crucible base until the surface reveals the natural color of copper, and then perform surface flattening treatment on the copper crucible base. The flatness deviation of the copper crucible base after treatment is less than 5mm.

[0032] S2. Prepare an 80mm thick alloy base pad of the same grade and place it on the copper crucible base. After adjusting the placement angle, the maximum gap between the alloy base pad and the copper crucible base should be less than 5mm.

[0033] S3. Using the purchased niobium alloy ingot as the consumable electrode, with the consumable electrode as the cathode and a copper crucible as the anode, the process involves loading, welding, and inspection. The raising and lowering of the consumable electrode is achieved through in-furnace welding and mechanical connection. The movable component is a motor-driven electrode rod. First, the electrode rod and auxiliary electrode are physically clamped together. Then, the auxiliary electrode and consumable electrode are welded together in the furnace, thus achieving the raising and lowering of the consumable electrode. An alloy base and arc-starting chips are placed inside the copper crucible. One end of the consumable electrode is placed inside the copper crucible. The consumable electrode-auxiliary electrode-electrode rod is connected to one pole of the power supply, and the alloy base-copper crucible is connected to the other pole. The furnace is then sealed, a vacuum is drawn, and the leakage rate is measured to ensure a vacuum degree ≤ 0.133 Pa. Before arc starting, the distance between the lower end of the consumable electrode and the alloy base is maintained at 20 mm. Then, the arc is initiated. The movement of the electrode rod during the melting process depends on the voltage (after the self-consumable electrode melts and is lost, the distance between the self-consumable electrode and the anode increases, and the voltage increases accordingly. In order to match the voltage range, the system presses down the electrode rod). First, the arc initiation process parameter is set to 3kA and held for 2 minutes. Then, the maximum melting current is increased within 4 minutes. The maximum melting current is in a slightly fluctuating state, between 15kA and 16kA. At this time, the melting voltage is also in a fluctuating state, between 35.0V and 45.0V (because the niobium alloy has a high viscosity, the molten droplets are easy to form wires during the melting process. The continuous wire forming reduces the distance between the cathode and the anode, and the voltage will decrease, resulting in a continuous voltage fluctuation). Then, the arc stabilization current is adjusted to 3.0A, and the arc stabilization current commutation frequency is 20s / time. The arc is quickly initiated to establish a molten pool, and then the process enters the stable melting stage.

[0034] S4. After the melting is completed and the crucible is removed from the furnace, the bottom surface of the copper crucible base and the bottom surface of the same grade alloy pad shall be inspected and treated. After treatment, there shall be no copper metal adhering to the surface, so that it can be used for the next vacuum self-consuming arc melting in the furnace.

[0035] Figure 3 The electrode rod is omitted. The longer upper part is the auxiliary electrode 4 + consumable electrode 3. The gray part at the bottom is the alloy base pad 2 of the same grade, and the orange part is the copper crucible 1. The consumable electrode 4-auxiliary electrode 3-electrode rod are connected to one pole of the power supply 5, and the alloy base pad 2-copper crucible 1 are connected to the other pole of the power supply 5. (a) The figure shows that the arc continuously strikes the alloy base pad 2 before it leaves the molten pool. However, due to the gap between the alloy base pad 2 and the base of the copper crucible 1, a secondary arc will be generated between the copper crucible 1 and the alloy base pad 2, which will damage the base of the copper crucible 1. Therefore, on the one hand, a simple pretreatment is adopted to make it more flat, increase the contact between the alloy base pad 2 and the base of the copper crucible 1, and improve the conductivity and make it less likely to start an arc. On the other hand, as shown in Figure (b), by improving the arc starting process parameters, the molten pool 6 is quickly established. In this way, the circuit can connect the molten metal surface with the wall of the copper crucible 1, thereby reducing the probability of the arc striking the base of the copper crucible 1.

[0036] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for protecting the base of a copper crucible used in vacuum consumable arc melting of refractory niobium alloys, characterized in that, Includes the following steps: Clean the oxide scale off the copper crucible base and level it. Place the alloy base pad of the same grade on the copper crucible base, and make sure that the maximum gap between the alloy base pad and the copper crucible base does not exceed 5mm. Using niobium alloy ingots as consumable electrodes, consumable electrodes as cathodes, and copper crucibles as anodes, the furnace was loaded, welded, and inspected. Then, under vacuum conditions of ≤0.133Pa, the arc ignition process parameters were set to enable rapid arc ignition and the establishment of a molten pool in the copper crucible, which then transitioned to a stable smelting stage. The arc ignition process parameters are as follows: after setting the melting voltage to 35.0V~45.0V, maintain it at 2.0kA~5.0kA for 1min~3min, then increase it to the maximum melting current within 2min~5min. The maximum melting current is 6kA~20kA. After that, adjust the arc stabilization current to 1.0A~6.0A.

2. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 1, characterized in that, The flatness deviation of the copper crucible base after leveling treatment is <5mm.

3. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 1, characterized in that, The thickness of the alloy base pad is 30mm~80mm.

4. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 1, characterized in that, After adjusting the arc-stabilizing current, the arc-stabilizing current is reversed to change the direction of the current and magnetic field, thereby achieving forward and reverse rotation of the molten pool.

5. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 4, characterized in that, The frequency of arc current commutation is 10s / time to 30s / time.

6. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 1, characterized in that, The distance between the lower end of the consumable electrode and the alloy base is 20mm~50mm.

7. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 1, characterized in that, The specifications of the copper crucible are Φ160~Φ380mm.

8. The method for protecting the copper crucible base for vacuum consumable arc melting of refractory niobium alloys according to claim 1, characterized in that, Clean the oxide scale off the base of the copper crucible until the copper's original color is revealed.

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