阅读说明：本技术 精炼含氮金属合金的工艺 (Process for refining nitrogen-containing metal alloy ) 是由 贝蒂尔·瓦尔登 于 2018-12-19 设计创作，主要内容包括：本发明涉及一种在炉中使用自耗电极的电弧重熔来精炼含氮金属合金的工艺,所述工艺包括如下步骤：提供所述金属合金的自耗电极；提供第二电极；在所述炉内提供受控气氛；在所述自耗电极和所述第二电极之间触发电弧,以熔化所述自耗电极,从而形成熔化的金属合金池；维持所述自耗电极与所述熔化的金属合金池之间的所述电弧；将所述熔化的金属合金输送到模具中,并且铸造精炼的金属合金锭,其中,提供受控气氛包括在1Pa至500Pa的Ar气压下使Ar气流动通过所述炉。(The invention relates to a process for refining a nitrogen-containing metal alloy in a furnace using arc remelting of a consumable electrode, said process comprising the steps of: providing a consumable electrode of the metal alloy; providing a second electrode; providing a controlled atmosphere within the furnace; striking an arc between the consumable electrode and the second electrode to melt the consumable electrode to form a molten metal alloy pool; maintaining the arc between the consumable electrode and the molten metal alloy pool; delivering the molten metal alloy into a mold, and casting a refined metal alloy ingot, wherein providing a controlled atmosphere comprises flowing an Ar gas through the furnace at an Ar gas pressure of 1Pa to 500 Pa.)

1. A process for refining a nitrogen-containing metal alloy in a furnace using arc remelting of a consumable electrode, said process comprising the steps of:

-providing a consumable electrode of said metal alloy;

-providing a second electrode;

-providing a controlled atmosphere within the furnace;

-striking an arc between the consumable electrode and the second electrode to melt the consumable electrode to form a molten metal alloy pool;

-maintaining the arc between the consumable electrode and the molten metal alloy pool;

-transferring the molten metal alloy into a mould and casting a refined metal alloy ingot,

wherein providing a controlled atmosphere comprises flowing an Ar gas through the furnace at an Ar gas pressure of 1Pa to 500 Pa.

2. The process of claim 1, wherein the Ar gas pressure is from 2Pa to 500 Pa.

3. The process of claim 1, wherein the Ar gas pressure is from 1Pa to 100Pa, such as from 2Pa to 50Pa, such as from 5Pa to 50 Pa.

4. A process according to any one of claims 1 to 3, wherein the electrode gap between the consumable electrode and the molten metal alloy bath is controlled such that the arc remains stable and diffuse.

5. A process according to claim 4, wherein the electrode gap is in the range of 5mm to 15mm, such as 7mm to 12mm, such as 8mm to 10 mm.

6. A process according to claim 4 or 5, comprising controlling the electrode gap by means of droplet short circuit control.

7. The process of any preceding claim, comprising forming a steady flow of Ar gas through the furnace prior to striking the arc.

8. The process of any one of the preceding claims, wherein flowing Ar gas through the furnace comprises flowing Ar gas continuously at a constant or substantially constant Ar gas pressure.

9. A process according to any one of the preceding claims, wherein the average arc voltage used to maintain the arc is in the range 20V to 25V.

10. The process according to any of the preceding claims, wherein the metal alloy is a stainless steel alloy, a superalloy or a highly alloyed steel alloy.

11. The process of any one of the preceding claims, wherein the metal alloy has a nitrogen content of at least 0.001 weight percent (wt%) to 0.20 weight percent (wt%), such as 0.025 wt% to 0.10 wt%.

Technical Field

The present invention relates to a process for refining nitrogen-containing metal alloys using arc remelting of a consumable electrode.

Background

Vacuum Arc Remelting (VAR) is a process used to refine metal alloys to obtain better creep and fatigue resistance. In the VAR process, a consumable electrode of a metal alloy to be refined is placed in a vacuum chamber of a VAR furnace, a second electrode is positioned below the consumable electrode, and an arc is struck between the electrodes. The consumable electrode thus begins to melt and a molten metal alloy pool is formed. An electric arc is maintained between the consumable electrode and the pool of molten metal alloy, the molten metal alloy is delivered to the die, and a refined metal alloy ingot is cast. US4578795 discloses examples of VAR processes and VAR furnaces.

In particular, VARs are used to refine metal alloys to be used in e.g. aerospace applications, or in the oil and gas industry, such as stainless steel alloys, super alloys based on iron (Fe), cobalt (Co) or nickel (Ni), and highly alloyed steel alloys. In the VAR process, non-metallic inclusions as well as harmful and volatile elements may be removed from the metal alloy. However, in the VAR process, volatile elements that have a beneficial effect on the metal alloy may also volatilize and be lost due to the low pressure inside the VAR furnace. For example, the nitrogen (N) containing amount of the metal alloy is generally reduced during the VAR process. In many cases, it is not possible to increase the nitrogen content in the metal alloy of the consumable electrode without exceeding the nitrogen solubility of the metal alloy, thereby forming bubbles. The bubbles negatively affect the VAR process by causing vibrations and unstable vacuum pressure within the furnace chamber.

For some alloys, arc remelting is performed in a furnace at Ar gas pressure of, for example, 30kPa to reduce nitrogen loss. However, it has been found that this can lead to unstable arcing and a large amount of vibration, which limits the size of ingots that can be produced.

Disclosure of Invention

It is a primary object of the present invention to provide a process for refining nitrogen-containing metal alloys using arc remelting of a consumable electrode which overcomes, at least in some aspects, the above disadvantages. In particular, it is an object to provide a process by which the reduction of the nitrogen content of a metal alloy can be reduced, so that a refined metal alloy can obtain a nitrogen content close to that of the metal alloy before refining.

At least the first object is achieved by a process according to claim 1. Advantageous embodiments of the process are disclosed in the dependent claims.

By flowing Ar gas through the furnace at a relatively low pressure of 1Pa to 500Pa, volatilization of nitrogen (N) is prevented, and thus reduction of N in the metal alloy during the arc remelting process is reduced. The refined metal alloy may obtain an N content close to the N content of the unrefined metal alloy of the consumable electrode.

The argon (Ar) pressure, as well as other process parameters such as the arc voltage and electrode gap between the consumable electrode and the molten metal alloy bath, should be such that a stable and diffuse arc is maintained between the consumable electrode and the molten metal alloy bath. The Ar gas pressure should be low enough so that no plasma is generated. The plasma may cause the arc to narrow and thereby become quiescent, resulting in poor melting of the consumable electrode and increased nitrogen volatilization. By keeping the Ar gas pressure low enough, the arc can scan the consumable electrode surface quickly, making the melting process easier to control.

According to one embodiment, Ar gas pressure (P)_Ar) > 2 Pa. According to another embodiment, P_ArNot less than 5 Pa. According to another embodiment, P_ArNot less than 10 Pa. According to another embodiment, P_Ar> 20Pa, according to a further embodiment, P_ArIs more than or equal to 50 Pa. The presence of sufficient Ar gas pressure will ensure that the technical effect of the target is achieved, i.e. significantPreventing the N in the metal alloy from volatilizing.

As mentioned above, the Ar gas pressure should not be too high. According to one embodiment, P_ArLess than or equal to 500 Pa. According to one embodiment, P_ArLess than or equal to 400 Pa. According to one embodiment, P_ArLess than or equal to 300 Pa. According to one embodiment, P_Ar≤200Pa。

According to one embodiment, the Ar gas pressure is 2Pa to 500 Pa. According to one embodiment, the Ar gas pressure is 1Pa to 100 Pa. According to another embodiment, the Ar gas pressure is from 2Pa to 50Pa, according to yet another embodiment, the Ar gas pressure is from 5Pa to 50 Pa.

The electrode gap may preferably be in the range of 5mm to 15mm, more preferably 7mm to 12mm, even more preferably 8mm to 10 mm.

The average arc voltage for maintaining the arc may be in the range of 20V to 25V.

According to one embodiment, the process includes controlling the electrode gap by means of drop-short control. In this context, droplet short circuit control refers to a process of controlling the electrode gap by maintaining a droplet short circuit set point (i.e., droplet short circuit frequency or droplet short circuit period). Droplet short circuit control helps control the electrode gap. For example, the droplet short-circuiting frequency may be set to 0.5s^-1To 10s^-1E.g. 1s^-1To 4s^-1. Alternatively, the electrode gap may be controlled using voltage control, i.e. by maintaining a voltage set point.

According to one embodiment, the method includes forming a steady flow of Ar gas through the furnace prior to striking the arc. In this regard, stabilizing may refer to fluctuating only within a defined Ar gas pressure range or a predefined sub-range of Ar gas pressure. This will improve the conditions for triggering and maintaining a stable and diffuse arc and for obtaining a stable melting rate.

According to one embodiment, flowing the Ar gas through the furnace comprises continuously flowing the Ar gas at a constant or substantially constant Ar gas pressure. In this context, "substantially constant" is intended to mean that the Ar gas pressure is not allowed to deviate more than ± 10% from the desired Ar gas pressure value. By maintaining a constant or substantially constant Ar gas pressure during melting, oscillations that may lead to unstable arcs are prevented.

The metal alloy may be a stainless steel alloy, a superalloy based on iron (Fe), cobalt (Co) or nickel (Ni), or a highly alloyed steel alloy. In particular, the metal alloy may be a metal alloy having a nitrogen content of at least 0.001 to 0.20 weight percent (wt%), preferably 0.025 to 0.10 wt%. The process is particularly useful for metal alloys having nitrogen dissolved in the metal alloy because dissolved nitrogen is more likely to dissipate during VAR than nitrogen incorporated in the metal nitride.

Other advantages and advantageous features of the invention will appear from the detailed description below.

Drawings

Embodiments of the invention will be further described, by way of example, with reference to the accompanying drawings, in which

Fig. 1 is a flow chart illustrating a process according to an embodiment of the present invention.

Detailed Description

A process for refining a nitrogen-containing metal alloy in a furnace using arc remelting of a consumable electrode according to an embodiment of the invention is schematically illustrated in the flow chart in fig. 1. The method comprises the following steps:

a: providing a consumable electrode of a metal alloy;

b: providing a second electrode;

c: providing a controlled atmosphere within the furnace comprising flowing an Ar gas through the furnace at an Ar gas pressure of 1Pa to 500 Pa;

d: striking an arc between the consumable electrode and the second electrode to melt the consumable electrode to form a molten metal alloy pool;

e: maintaining an arc between the consumable electrode and the molten metal alloy pool; and

f: the molten metal alloy is delivered into a mold, and a refined metal alloy ingot is cast.

The consumable electrode consisting of the metal alloy to be refined may, for example, have a stainless steel alloy, a superalloy based on iron (Fe), cobalt (Co) or nickel (Ni), or a highly alloyed steel alloy. The metal alloy may have a nitrogen content of at least 0.001 weight percent (wt%) to 0.20 wt%, such as 0.025 wt% to 0.10 wt%. The consumable electrode may be cylindrical.

The consumable electrode is located in a cooling crucible, such as a water-cooled crucible surrounded by a water jacket, in the furnace chamber of the VAR furnace. The inner diameter of the crucible is greater than the diameter of the consumable electrode. The drive mechanism is used for controlling the position of the consumable electrode in the furnace and for lowering the consumable electrode as the consumable electrode is melted.

According to one embodiment, the second electrode may comprise the same metal alloy as the consumable electrode, but according to another embodiment, the second electrode may be formed of a different metal alloy, since the portion of the formed ingot containing the metal alloy from the second electrode may be easily separated from the rest of the ingot from which the metal alloy was refined. The second electrode is located below the consumable electrode in the cooled crucible. A gap is formed between the electrodes, which can be controlled using a drive mechanism.

The Ar gas pressure may be as low as 1Pa, but according to other embodiments, the Ar gas pressure may be at least 2Pa or at least 5 Pa. The Ar gas pressure may be up to 500Pa, but may also be limited to a maximum of 100Pa or 50 Pa. Ar gas may be introduced into the furnace at a location above the second electrode such that when the arc is struck, the Ar gas flows over the molten metal alloy pool. Preferably, a stable Ar gas pressure is established before the arc is struck. Preferably, the Ar gas pressure is maintained constant or substantially constant by continuously flowing Ar gas over the molten metal alloy pool during the arc remelting process, thereby helping to keep the arc stable.

The arc may be triggered by passing a current through a consumable electrode. A negative voltage is applied to the consumable electrode while maintaining the second electrode at ground potential. The voltage, current and/or electrode gap can be controlled to maintain a stable diffuse arc. According to one embodiment, the electrode gap is controlled by means of droplet short-circuit control, i.e. based on a desired detected droplet short-circuit rate. Such droplet short circuit control is described in, for example, US 4578795.

The cooled crucible, on which the electrode is placed, forms a mold in which the molten metal alloy solidifies, so that an ingot is cast. Thus, the cast ingot has a larger diameter than the consumable electrode.