BASED SUPERALLOYS

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to United States Provisional Patent Application No. 62/312,252, filed on March 23, 2016, the entire contents of which are hereby incorporated by reference.

BACKGROUND

Field of the Invention

[0002] This invention relates to methods of treating unrefined tungstic acid to produce a semi-refined anhydrous tungsten trioxide. The anhydrous tungsten trioxide can be used as an input raw material for the manufacture of both tungsten bearing steels and nickel based superalloys.

Description of Related Art

[0003] The inclusion of tungsten in metallurgical applications has been widely recognized since the mid 1800's. Tungsten was regarded as one of the first alloying elements used to improve steel properties such as hardness and wear resistance. When added to steel, tungsten forms tungsten carbides or complexes with other elements such as chromium, molybdenum, vanadium, or iron. This carbide formation increases the amount of undissolved and excess carbide in the hardened steel, causing precipitation of fine or very fine grained carbides evenly distributed in the steel matrix. The formation of stable carbides can increase the hardness of the steel, particularly at high temperatures.

[0004] Initially, tungsten-alloyed steels were made by the simple addition of pure tungsten powder or a purified tungsten ore to the steel melt. The more current practice involves adding tungsten to the steel melt in the form of tungsten scrap, as a tungsten-rich prealloy known as ferro-tungsten, or as natural or synthetic scheelite ore concentrates. The tungsten scrap, ferro-tungsten, or scheelite concentrates can be added directly to the arc furnace to produce a base melt which can then be further refined, such as in an argon oxygen decarburization (AOD) or vacuum oxygen decarburization (VOD) converter. Standard ferro- tungsten is available in different grades having varying carbon content and the tungsten amount is typically between 75 and 80 wt%. Scheelite is a calcium tungstate material having the chemical formula CaW0 ₄ . Scheelite can be subject to extraction techniques to recover a concentrated amount of tungsten trioxide (WO3). [0001] Despite the advantages of including tungsten in steel making and other metallurgical applications, very little attention has been given to developing new ways to introduce tungsten into the alloying process.

SUMMARY

[0005] Unrefined, hydrated tungstic acid is roasted in an oxidizing atmosphere to reduce the water and sulfur content. The roasting process produces a semi-refined anhydrous tungsten trioxide. Once roasted, this material can be melted with an iron base or nickel base material to form iron-tungsten or nickel-tungsten alloys of varying tungsten concentrations.

[0006] According to one aspect, described is a process of treating unrefined tungstic acid to produce anhydrous tungsten trioxide through roasting of the unrefined tungstic acid at a roasting temperature and for a roasting time period sufficient to remove the moisture from the unrefined tungstic acid.

[0007] In some non-limiting embodiments, the roasting temperature is between 300°C and 1200°C, such as between 300°C and 900°C.

[0008] In some non-limiting embodiments, the roasting time period is between 1 hour and 6 hours, such as between 1 hour and 5 hours.

[0009] In one non-limiting embodiment, the roasting temperature is between 600°C and 900°C and the roasting time period is between 3 hours and 5 hours.

[00010] In some non-limiting embodiments, a sulfur content of the anhydrous tungsten trioxide is reduced to 0.03 wt% or less, such as between 0.01 wt% and 0.03 wt , through the roasting process.

[00011] In some non-limiting embodiments, roasting the unrefined tungstic acid reduces a sulfur content of the unrefined tungstic acid by greater than 95 wt%.

[00012] In some non-limiting embodiments, roasting the unrefined tungstic acid reduces the weight of the unrefined tungstic acid by between 20 and 70 wt%.

[00013] In some non-limiting embodiments, the process can further include sintering the anhydrous tungsten trioxide at a temperature of 1600°C or more for between 2 hours and 4 hours.

[00014] According to another aspect, described is an anhydrous tungsten trioxide material prepared by roasting unrefined tungstic acid according to the process described above.

[00015] According to yet another aspect, described is a process of producing a tungsten- bearing material. The process includes melting a base material in the presence of an anhydrous tungsten trioxide material prepared according to the process described above. In some non-limiting embodiments, the base material is an iron base material. In other non- limiting embodiments, the base material is a nickel base material.

[00016] These and other aspects of the present invention will be apparent from the detailed description of the preferred embodiments provided below.

DETAILED DESCRIPTION

[00017] As used herein, all numbers expressing dimensions, physical characteristics, percentages, and the like, used in the specification and claims are to be understood as being modified in all instances by the term "about."

[00018] Described is a process of producing alloy grade tungsten for use in tungsten bearing steels and nickel based superalloys.

[00019] The starting material for this process is unrefined hydrated tungstic acid. Tungstic acid refers to the hydrated form of tungsten trioxide and, for purposes of this disclosure, includes all hydrates thereof, including the monohydrate and di-hydrate. Hydrated tungstic acid is commercially available from a variety of sources and may be available in different physical forms.

Roasting Process:

[00020] The hydrated tungstic acid is roasted to prepare the material for addition, including by direct addition, to a melting furnace in subsequent operations. The roasting process removes moisture contained therein. Hydrated tungstic acid typically contains both free moisture and chemically combined water. Free moisture refers to the water molecules that are not chemically combined with the tungsten trioxide. Both the free moisture and the chemically combined water must be removed to produce anhydrous tungsten trioxide. Removal of the free moisture and chemically combined water will typically amount to an overall loss of weight in the hydrated tungstic acid of between 20 and 70 wt% based on the weight of the starting material due to total water removal (known as loss on ignition, or "LOI"). If sufficient water is not removed, subsequent use of the material as a feedstock during melting operations will be problematic. For example, safety issues arising from entrapped water being introduced into a bath of molten metal are a very serious concern. Additionally, the presence of moisture can cause metal yield losses to occur as the fine metal dust particles of tungsten trioxide are literally blown out of the furnace and into the flue gas stream due to rapid steam generation. Preferably, all or nearly all of the water is removed through the roasting process. [00021] In addition to water removal, the sulfur content in the input material is reduced through the roasting process. Roasting is carried out in an oxidizing atmosphere (air) and sulfur is removed in the form of gaseous sulfur dioxide. The sulfur content in the input material, which is typically in the range of 0.5 wt , is too high for most end use melting applications. Through the roasting process, the sulfur content can be reduced to less than 0.03 wt , such as between 0.01 wt and 0.03 wt , with a lower value being most preferred. An overall sulfur reduction of 90% or more, such as 95% or more, can be achieved with the process described herein.

[00022] During roasting, acid fumes are produced which should be contained, such as through the use of a gas scrubber, to prevent air pollution. Traces of acid should be removed from the roasted input material prior to melting to avoid subsequent air pollution and handling problems.

[00023] Roasting occurs at a temperature and for a time period sufficient to remove all, or substantially all, of the free and combined water and to reduce the sulfur content to a more acceptable level, consistent with the principles discussed above. In some non-limiting embodiments, roasting can occur at a temperature of between 300°C and 1200°C in an oxidizing atmosphere, such as in an unprotected atmosphere of air, with the roasting time varying based on the temperature selected. For example, the roasting time may be between one and six hours with higher roasting temperatures requiring roasting times nearer the bottom of this range. Roasting for a period longer than six hours is also possible, though sufficient moisture removal can usually be accomplished in under six hours. In other non- limiting embodiments, roasting occurs at a temperature of less than 900°C for a maximum of five hours, such as between 600°C and 900°C for between three and five hours, or about 700°C for about three hours. Roasting can occur in an industrial multi-hearth roaster, or other suitable furnace (such as a rotary kiln), operating at or near atmospheric pressure. Laboratory testing using a muffle furnace has confirmed that roasting at a temperature of less than 900°C for five hours or less in an air atmosphere can produce anhydrous tungsten trioxide with a resulting sulfur reduction greater than 95%.

[00024] Several samples of roasted input material were prepared according to the process described above wherein roasting was at 700°C for three hours. An averaged chemical analysis of the samples is shown in Table 1. The samples showed an average loss on ignition of 35%. Table 1 - Analysis of Samples of Anhydrous Tungsten Trioxide

[00025] Once the roasting process is complete, the resulting anhydrous tungsten trioxide is suitable as a direct charge material to any air melting furnace (e.g., electric arc/AOD, or Air Induction Furnace).

Melting Trials:

[00026] In order to validate this assertion, a trial experiment was devised. This trial involved the use of a small (4kg) air induction furnace with a graphite crucible. The graphite crucible was selected to provide a reducing environment during the melting process and to therefore facilitate reduction of the tungsten trioxide. Two separate trials were carried out. The first used iron as the base material and the second used nickel as the base material. However, it is envisioned that other base materials, or combinations of base materials, could be used as well.

a) Iron Base

[00027] Slitter scrap was used as a source of iron, and the roasted tungsten trioxide, prepared by roasting hydrated tungstic acid for three hours at a temperature of 700°C was added directly to the furnace, along with the slitter scrap, prior to powering up the furnace. Once power was applied, the tungsten trioxide began to melt first, followed shortly thereafter by the slitter scrap. No slag cover or flux was added to any of the melting trials.

[00028] Input data and results from three separate trials are summarized in Table 2. Differing concentrations of tungsten were produced to replicate different commercial alloys and to evaluate yield performance and ease of melting. Tungsten content of the input material (anhydrous tungsten trioxide) was analyzed at 75.6 wt% tungsten. Iron content of the slitter scrap was 99.6%. These values were used to determine the actual weight of tungsten and iron used in each trial. The resulting ingots produced from each trial were weighed and the material was analyzed to determine the output weight, and hence the final material yields. Table 2 - Input Data and Results from Melting Trials Using Iron Base Material

b) Nickel Base

[00029] A series of similar experiments were conducted using high purity nickel cathode (99.8% Ni). Input data and results are shown in Table 3.

Table 3 - Input Data and Results from Melting Trials Using Nickel Base Material Trial #1 Trial #2

Input weight WO3 (g) 267 397

Input weight Nickel Cathode (g) 1880 605

Calculated input weight Tungsten (g) 202 300

Calculated input weight Nickel (g) 1876 604

Output weight Ingot (g) 2087 868

Output weight Slag (g) 8 28

Analysis Ingot wt%

W 8.92 26.8

Ni 89.0 69.0

C 1.86 2.86

P 001 .004

s 001 001

Mo .14 .47

Analysis Slag wt%

W 53.7 68.2

Ni 35.4 20.9

C 1.09 .523

Calculated Ingot Output weight Tungsten (g) 186 233

Calculated Ingot output weight Nickel (g) 1857 599

Ingot Yield % Tungsten 92.1 77.7

Ingot Yield % Nickel 99.0 99.0

Slag % of total output weight .3 3.1

Melt time (minutes) 19 27

[00030] Several conclusions were reached from the above-described experiments. For one, it was observed that carbon pick up occurred from the interaction between the graphite crucible (used for melting) and the molten bath. In a real world situation this would not be a problem as the linings used in melting furnaces are typically refractories containing no carbon. Carbon additions can be made prior to or during melting to achieve the desired carbon end point. [00031] In addition, it was observed that a tungsten content of the ingot in the 6 to 9 wt% range produced excellent yields (90 wt% range). This tungsten content and yield range covers the vast majority of tungsten bearing steels and superalloys in commercial use today. Both melt times and slag percentage increase as tungsten content is increased above this range, causing tungsten yields to deteriorate.

[00032] It was also observed that the tungsten trioxide melted before both the iron and nickel base materials. This provides an advantage over traditional sources of tungsten, such as tungsten/ferro-tungsten solids. Due to the low melting point of tungsten trioxide (approx. 1473°C), the tungsten will rapidly diffuse in its molten state. Tungsten, a high temperature refractory metal, is notoriously difficult to dissolve in a molten metal bath when present as a solid, such as a tungsten/ferro-tungsten solid. In conventional processes, if the addition of solid tungsten or ferro-tungsten is not carefully sized, it can often be found as an undissolved solid at the end of a melt. The use of tungsten trioxide according to this disclosure will allow melters to reduce melting cycle time and ensure yields are not compromised due to any residual undissolved material. To the extent certain users prefer a more regularly sized feedstock, the powdered tungsten trioxide produced from the roasting process described above can be pressed into a briquette (similar to a standard charcoal briquette) or pelletized, which process can be aided by including one or more commercially available binder agents.

[00033] In the experiments described above, the anhydrous tungsten trioxide prepared according to the roasting process was directly added to the furnace. Initial laboratory testing has been performed which indicates additional processing of the anhydrous tungsten trioxide may prove beneficial. For example, once the roasting process is complete, the anhydrous tungsten trioxide may be subjected to a sintering process in which a high temperature reduction furnace, such as one operating at a temperature of 1600°C or more, is used to lower the oxygen content of the anhydrous tungsten trioxide and form a metallic diffusion bond between the fine particles of reduced tungsten. In one non-limiting embodiment, the sintering can be accomplished by subjecting the anhydrous tungsten trioxide to a temperature of 1600°C for between two and four hours. The diffusion bond that occurs between the fine particles of metallic tungsten may be sufficient to form a strong, sturdy porous "cake" with the strength of the cake varying depending on the extent of the oxygen reduction and the diffusion bonding. If sufficiently strong, the cake may act as a "briquette," similar to a charcoal briquette, which can be added to the furnace.

[00034] As described, a process has been developed to produce semi-refined tungsten trioxide from unrefined tungstic acid. Roasting practices have been developed to (a) reduce sulfur levels by at least 95% and (b) remove free moisture and chemically combined water. This produces a product which can be used as a raw material input for melting tungsten bearing steels and nickel based superalloys, among other potential uses.

[00035] Although various embodiments have been described in detail for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that the disclosure is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements. For example, it is to be understood that this disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.