GB2204595A - Metal refining process - Google Patents

Abstract

The process comprises melting nickel, cobalt or iron with alloy metal constituents, the starting ingredients having relatively high carbon contents for the initial melt down, so as initially to reduce the oxygen and nitrogen contamination levels down to very low limits and avoid scull formation, and subsequently adding oxygen in a controlled manner to the molten metal to reduce the level of carbon to a low level whilst avoiding the formation of difficult-to-remove, very high melting point metal oxides which may otherwise accrete onto the refractory wall of the crucible as scull in conventional processes.

Description

METAL REFINING PROCESS This invention relates to an improved process of metal alloy refining and particularly to an improved process for producing low-oxygen, low nitrogen and low-carbon content alloys from at least partly contaminated source materials.

A main use of the process is in the refining of so-called "superalloys" which contain nickel or cobalt as a major base metal component, and possibly also contain further base metal components such as iron (and/or cobalt) or nickel, respectively.

There is a constantly increasing demand for such "superalloys" to very high purity specifications. With such very high purity specifications there are however major practical problems arising from, on the one hand, extended processing times and, on the other hand, self-defeating effects whereby a process for reducing one impurity to the lowest possible limits involves the introduction or increase in level of other impurities.

In general, increased purity is achieved in processes based on the use of vacuum induction furnaces, by the use of the highest possible vacuum (lowest possible pressure) and extended processing times over many hours to achieve sufficient refining of undesired contaminants such as oxides and nitrides. In order to minimize these, therefore, every possible step is normally taken to select, on the one hand, metal and alloy starting ingredients of the highest purity (lowest possible carbon content etc.) and, on the other hand, to exclude such components, (carbon, oxygen and nitrogen) from the body of metal during processing. In practice a significant amount of scull in the form of high melting point metal compounds such as nickel oxide, chromium oxide, and chromium nitrides, is formed in solution and gives rise to accretion onto the crucible refactory wall.Thereafter removal of this scull, which has an adverse influence on the further processing of the main body of metal, is particularly difficult and requires a high temperature to bring about a reaction in the scull at limited carbon levels and at very low carbon levels the scull is not reacted even at such high temperatures.

The present invention provides, in accordance with one of its several aspects, a process for producing a low-oxygen, low-nitrogen, low-carbon content alloy having a base metal component selected from nickel, cobalt and iron together with alloy metal components, said components including materials contaminated with oxygen and/or nitrogen-containing compounds, comprising the steps of (a) melting down the metal components in a furnace, (b) adding carbon to the melt to reduce the oxygen and nitrogen levels thereof and (c) adding further oxygen to reduce the carbon to a low level.

Advantageously, the alloy metal components include reactive and non-reactive components, non-reactive components being present at step (a) above, there being further provided a step (d) comprising adding the reactive metal components to the body of metal.

The present invention provides in accordance with a further one of its several aspects a process for producing a low-oxygen, low-nitrogen, low-carbon content alloy consisting essentially of at least one base metal component, selected from nickel, cobalt and iron, and at least one of; one or more non-reactive alloy metal component, and one or more reactive metal component from at least partly oxygen-and/or nitrogen-contaminated sources, as defined herein, which process comprises the steps of (a) providing a body of metal consisting essentially of at least a portion of the base metal component and any said non-reactive alloy metal component, but substantially free of any said reactive alloy metal component, (b) melting down the body of metal under substantially sub-atmospheric pressure into a pool of molten metal, and providing the body of metal with a carbon supply in a quantity in excess of the final carbon content of the alloy by a limited amount, (c) allowing at least part of said carbon supply to react with the oxygen content of said body of metal so as substantially to cause said oxygen content to react with carbon and flush nitrogen contamination down to low levels, (d) providing to the reduced-oxygen content body of metal a limited oxygen supply, not greater than that required to react with the carbon content thereof, and allowing said oxygen supply to react with at least part of any excess carbon content of the body of metal substantially without the formation of metal oxides, and (e) adding to the reduced-oxygen, reduced-nitrogen, reduced-carbon content body of metal, any said reactive metal component and any balance of the base metal and non-reactive metal components.

In one preferred example, the process of the invention relates to the production of a superalloy containing not more than 35% of iron as a base metal component, at least one, preferably a plurality, of non-reactive metal components, and at least one reactive metal component.

In another preferred example, the process of the invention relates to the production of a master alloy consisting essentially of nickel as a major base metal component, and at least one reactive metal component, preferably selected from titanium and aluminium.

It has very surprisingly been found that particularly low carbon, nitrogen and oxygen content levels can be achieved by deliberately selecting starting ingredients with relatively high carbon contents for the initial melt down, contrary to all established practice, so as initially to reduce the oxygen and nitrogen contarination levels down to very low limits and avoid scull formation, and subsequently deliberately adding oxygen in a controlled manner, again contrary to normal practice, to the molten metal to reduce the level of carbon to a low level whilst avoiding the formation of difficult-to-remove, very high melting point metal oxides which may otherwise accrete onto the refractory wall of the crucible as scull in conventional processes.

The carbon content may be in any suitable form of which more than one may be present, including elemental carbon, a solid carbon compound such as a carbide of one or more of the base metal and non-reactive alloy metal components in the initial body of metal, and a fluid carbon compound such as a hydrocarbon, e.g. methane, propane, butane, or other lower alkane. In general the carbon should be provided in an amount slightly, e.g.

0,18, in excess of that required to react with all the oxygen present, including that from any oxygen and water that nay leak into the furnace, as well as the oxides present in the raw materials.

The oxygen supply may also be in any suitable form, including one or more of the following gaseous oxygen, a fluid oxygen-containing compound such as steam, and an oxide compound such as an oxide of one or more of the remaining metal components to be added to the body of the metal e.g. of at least part of the balance of the base metal component, e.g. NiO or FeO, or at least part of the balance of the non-reactive alloy metal components, e.g.

oxides of chromium, niobium, molybdenum, tungsten, tantallum and vanadum. Water (steam) provides a particularly economic source, and gaseous oxygen a very convenient alternative source for the oxygen supply in the process of the present invention.

In general the oxygen supply is controlled so that the oxygen (molecular or compound form) is introduced to the free surface of the molten body of metal at a controlled rate such that there is obtained a more or less controlled 'carbon boil' with conversion of the carbon content to carbon monoxide and flushing out of nitrogen content and also other 'tramp elements' which are relatively volatile at superalloy melting temperatures, e.g. Sb, Pb and Sn.

With the process of the invention the levels of carbon, oxygen and nitrogen content can be reduced to the lowest levels, e.g. lOppm of oxygen, 5ppm of nitrogen, with 0.02% of carbon, in considerably less time, e.g.

minutes rather than hours, than is required to achieve the normally lowest levels attainable with conventional processes. Moreover significantly lower levels of these can be achieved with the present invention than with conventional processes.

In addition it will be recognised that the process of the invention allows the use of a much wider range of starting materials, especially in relation to the non-reactive alloy metal components which can be used in significantly less pure scrap forms, e.g. containing significant levels of carbide e.g. up to 10% of carbon, than it has heretofore been practical to use.

The process of the invention may be used in the processing of various superalloys. Generally these contain at least 50% nickel, from 0 to 30% iron, and from O to 30% cobalt as base metal components; one or more of W, Th, Nb, Ta, V, B, Mn, Cr, Hf, and Mo as non-reactive alloy metal components and one or more of Ti, Al, and Zr, as reactive alloy metal components. Particular superalloys that may be mentioned in this connection include: Alloy 713C consisting essentially of nickel with C, 0.135; Mn, 0.07%; Si, 0.05%; S, 0.005%; P, 0.009%; Cr, 13.5%; Fe, 0.458: Co, 0.228; Mo, 4.3%; Ti, 0.7%; Al, 6.0%; B, 0.018; Zr, 0.08%; Nb(+Ta), 2.28; N, Sppm; O, Sppm.

Advantageously only part, e.g. half or less, of the main (nickel) base metal component is introduced at first into the body of metal for initial melt down with the non-reactive metal components, since nickel is readily available in very high purity, the balance being introduced after reduction of the carbon, oxygen and nitrogen content to sufficiently low levels such that dilution of the molten body of metal by the added high purity nickel, thereby further reducing these levels brings them down to within the required limits. In this way the concentrations of carbon, oxygen and nitrogen in the initial melt down are maximised thereby making removal of these contaminants easier.The late addition of the nickel balance also has the further advantage of cooling down the molten metal body, thereby, on the one hand, facilitating temperature control as required by supplying additional power to the body of metal to melt and stir in the added metal, and, on the other hand, accelerating processing by reducing the time required for cooling down the molten body of metal at the end of processing.

In general the initial melt down processing stage can be carried out under conditions similar to those used in conventional vacuum induction furnace processing, save that the processing time required to reduce oxygen and nitrogen content to low levels by use of oxygen sources will be substantially shorter. Thus processing will generally be carried out at a temperature of from 1500 to 1650eC, preferably from 1550 to 15700C at a pressure of below 1 torr, preferably below 0.5 torr. Advantageously the pressure and the oxygen supply rate are progressively reduced over the processing period, for example from 0.5 to 0.05 torr. over a period from 10 to 60 minutes.It will of course be appreciated that conditions will require to be varied according to the desired alloy composition, degree of contamination of the starting materials, etc. in general suitable conditions can be determined more or less readily by simple trial and error, e.g. with the aid of elemental analysis of samples at periodic intervals during processing.

As noted hereinbefore the decarbonization of the initial melt down product is effected by controlled oxygen supply to the surface of the melt or body of molten metal whereby carbon is burnt off preferentially at the surface as carbon monoxide or carbon dioxide without the formation of metal oxides. Advantageously the oxygen supply is controlled so that as the carbon content of the melt is progressively reduced1 so the oxygen supply rate is correspondingly reduced, the vacuum desirably also being progressively increased.

A further problem that can arise in superalloy production is that of contamination of the final product by oxides and nitrides introduced in the routinely available reactive metal (Ti, Al) sources. It has now been found though that this problem can be overcome by the use of previously prepared reactive metal-nickel alloys which can be obtained in relatively pure form including those reduced and containing small amounts of Mg as a final deoxidiser. Filtration of aluminiumn (alone) could be achieved by casting through a carbon fibre filter as could also such filtration of previously prepared alloys which are saturated with carbon or do not have a major solubility of carbon at their melting point.

Particularly preferred reactive metal alloys may be obtained and used in the process of the invention, consist essentially of from 10 to 90% Ti and from 90 to 10% Ni, e.g. 76% Ti and 24% Ni; and from 98 to 2% Al and from 2 to 98% Ni e.g. 16% Al and 84% Ni, and 80% Al and 20% Ni.

Naturally the present invention extends to alloys produced by the process of the invention.

Further preferred features and advantages of the invention will appear from the following detailed examples given by way of illustration only. In the examples and elsewhere in the specification all percentage compositions are given on a wt/wt basis unless otherwise indicated.

EXAMPLE 1 - Production of INCO713C Alloy A - Initial Melt down The following base melt of non-reactive metals is placed in a crucible of one tonne capacity inside a vacuum induction melting furnace (available from Consarc Engineering, Bellshill, Scotland) with in-leakage of water and air at less than 3 litres per hour.

Cr - 135kg, as metallic chromium, chromium oxide, chromium carbide, Ni-Cr alloy, and high-carbon (above 0.028 C) Ni-Cr alloy.

Mo - 43kg, as metallic molybdenum, scrap metal molybdenum, molybdenum oxide, molybdenum carbide, Ni-Mo alloy and high carbon (over 0.2%C) Ni-Mo alloy.

Ni - 365kg, as metallic nickel and including amounts contained in the above Cr and Mo alloys.

C - 3kg as carbon, plus the necessary carbon to reduce the metal oxides, less the carbon contained in carbides of Mo and Cr or carbon contained in Ni-Cr alloy and Ni-Mo alloy.

The pressure (vacuum) is adjusted by the pumps and a hydrocarbon (propane) gas flow is directed onto the free surface of the metal body in the crucible at a rate of 20 litres per minute at a pressure of 0.1. Pressure up to 1 torr may be used depending upon the number and size vacuum pumps which are used.

The hydrocarbon gas flow is continued until the melt down is completed and then for a further ten minutes at 15700C or until the oxygen and nitrogen level is less than Sppm as determined by a sample analysis using gas analysis equipment. Alternatively the deoxidising gas flow period could be determined by statistical analysis of the process using known procedures.

B - Decarbonization An oxygen gas flow is then directed onto the free surface of the melt, to convert the carbon therein to carbon dioxide, at a rate of 60 litres of oxygen per minute providing a carbon level reduction rate of about 30 grams of carbon per minute until the total remaining carbon content in the bath is 500 grams. During this procedure the temperature is maintained at 15700C and the pressure (vacuum) between 0.1 and 1 torr depending upon the number and size of vacuum pumps used. The oxide level at this stage will not exceed 15ppm with a nitride level at or below 3ppm.

C - Niobium Addition 22 kg of Nb is added as an alloy of Ni:Nb containing carbon and magnesium with the ratio of Ni:Nb being approximately 52%Nb:48%N with less than l0ppm of oxygen and nitrogen, while maintaining a propane gas flow of 20 litre/min directed at the surface of the melt. The propane gas flow is used in this and in the following process stages as a protective non-deleterious gas supply as described in our earlier European Patent specification No. 0127430.

D - Titanium Addition 7 kg of Ti is then added as an alloy of Ni and Ti containing carbon and magnesium with an Ni:Ti ratio of 25% Ni:75% Ti, while maintaining propane gas flow of 20 litre/min directed at the surface of the melt, the nitrogen and oxygen content of the NiTi alloy containing less than l0ppm of oxygen and nitrogen.

E - Aluminium Addition 60 kg of Al is then added as an alloy of Ni and Al containing magnesium with an Ni:Al ratio of 88% Ni : 12% Al and maintaining a propane gas flow as before.

F - Minor Components and Trim Additions Additions of B, Zr and any other metals required to adjust the alloy composition to the required final specification are made again under the propane gas flow.

G - Casting The final melt is then cast into forms while still maintaining the propane gas flow into the chamber.

Claims (11)

1. A process for producing a low-oxygen, low-nitrogen, low-carbon content alloy having a base metal component selected from nickel, cobalt and iron together with alloy metal components, said components including materials contaminated with oxygen and/or nitrogen-containing compounds, comprising the steps of (a) melting down the metal components in a furnace, (b) adding carbon to the melt to reduce the oxygen and nitrogen levels thereof and (c) adding further oxygen to reduce the carbon to a low level.

2. A process as claimed in claim 1, wherein the alloy metal components include reactive and non-reactive components, non-reactive components being present at step (a) above, there being further provided a step (d) comprising adding the reactive metal components to the body of metal.

3. A process for producing a low-oxygen, low-nitrogen, low-carbon content alloy consisting essentially of at least one base metal component, selected from nickel, cobalt and iron, and at least one of; one or more non-reactive alloy metal component, and one or more reactive metal component from at least partly oxygen-and/or nitrogen-contaminated sources, as defined herein, which process comprises the steps of (a) providing a body of metal consisting essentially of at least a portion of the base metal component and any said non-reactive alloy metal component, but substantially free of any said reactive alloy metal component, (b) melting down the body of metal under substantially sub-atmospheric pressure into a pool of molten metal, and providing the body of metal with a carbon supply in a quantity in excess of the final carbon content of the alloy by a limited amount, (c) allowing at least part of said carbon supply to react with the oxygen content of said body of metal so as substantially to cause said oxygen content to react with carbon and flush nitrogen contamination down to low levels, (d) providing to the reduced-oxygen content body of metal a limited oxygen supply, not greater than that required to react with the carbon content thereof, and allowing said oxygen supply to react with at least part of any excess carbon content of the body of metal substantially without the formation of metal oxides, and (e) adding to the reduced-oxygen, reduced-nitrogen, reduced-carbon content body of metal, any said reactive metal component and any balance of the base metal and non-reactive metal components.

4. A process as claimed in any one of the previous claims wherein the carbon is added in a form selected from elemental carbon, a solid carbide of the base metal, a solid carbide of the non-reactive alloy metal, and a fluid carbon compound.

5. A process as claimed-in any one of the preceding claims wherein the carbon added is provided in an amount in the region of 0.1% by weight in excess of the stoichiometric requirement with respect to reactive oxygen/oxides present.

6. A process as claimed in any one of the preceding claims, wherein the oxygen is added in a form selected from gaseous oxygen, a fluid oxygen containing compound and an oxide of one of said metal compounds.

7. A process as claimed in any one of the preceding claims wherein where the base metal component is nickel, no more than approximately half of the nickel is introduced into the initial melt, the balance being added after the reduction of the carbon level.

8. A process as claimed in any one of the preceding claims wherein the operating temperature of the melt is between 1500 and 16500C at a pressure no greater than 1 torr.

9. A process as claimed in claim 9, wherein the temperature is between 15500 and 15700C at a pressure no greater than 0.5 torr.

10. A process as claimed in any one of claims 2 to 9, wherein the reactive alloy metal components contain titanium, nickel and aluminium.

11. A process for producing a low-oxygen, low-nitrogen, low-carbon content alloy having a base metal component comprising nickel cobalt or iron and alloy metal components, substantially as hereinbefore described with reference to the example.