Cobalt-chromium alloy resistant to high-speed / self-coupled sliding wear

Abstract

To provide a forgeable cobalt-based alloy which has extraordinary resistance to high-speed / self-coupled sliding wear.SOLUTION: This alloy contains about 0.83 wt.% of nickel, about 0.125 wt.% of nitrogen, about 26.85 wt.% of chromium, about 4.58 wt.% of molybdenum, about 2.33 wt.% of tungsten, about 2.97 wt.% of iron, about 0.84 wt.% of manganese, about 0.27 wt.% of silicon, about 0.065 wt.% of carbon, and about 0.11 wt.% of aluminum, with the balance cobalt plus impurities.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to cobalt-based alloys having corrosion resistance and wear resistance.

Background Art

[0002] One of the main causes of wear of metal materials is the sliding between metals, especially when there is no lubricant. At low speeds and high loads, a phenomenon called galling may occur, which can cause a large amount of material to move from one surface to the other surface, and seizure may occur. This is considered to be due to the atomic bonding of the two surfaces and / or the crack propagation deep into one or both surfaces. If attention is not paid to the material selection, high-torque nuts and bolts are affected by this form of wear. For example, austenitic stainless steel is particularly prone to this type of wear when self-bonded (i.e., both surfaces are made of the same material).

[0003] At the other end of the sliding wear spectrum, high-speed and low-load metal sliding systems tend to cause various wear mechanisms. These include (work-hardened) asperity cutting, fatigue near the surface (i.e., cracks due to repeated stress), and (when the surface temperature is high enough and the environment contains oxygen), the growth and shear of oxides on the surface of the metal material. Unlubricated bearings tend to have this form of wear, i.e., a tendency for high-speed sliding.

[0004] Cobalt-chromium alloys are the most resistant to these forms of wear. Furthermore, cobalt-chromium alloys are resistant to very corrosive liquids and can withstand much more high-temperature gas environments than steel.

[0005] One of the key factors in the slip-wear resistance of cobalt-chromium alloys is their ability to structurally transform from one atomic form to another, namely from metastable face-centered cubic (FCC) to hexagonal close-packed (HCP), under the action of mechanical stress over an industrially useful temperature range (determined by the content of other elements in the alloy). This occurs within the structure through the coalescence of stacking faults (metastable FCC phases with low stacking fault energy) and the formation of HCP platelets. This platelet formation not only absorbs energy, but the platelets also limit plastic deformation, resulting in a very high work hardening rate. These materials exhibit planar sliding, thereby increasing their resistance to fatigue and limiting fracture to the outer surface region.

[0006] Chromium is an HCP stabilizer in cobalt and its alloys. Chromium increases the propulsion for transformation and the temperature range of transformation. Similarly, molybdenum and tungsten (with chromium, resistance to corrosive liquids is significantly improved) are HCP stabilizers. Nickel, iron, and carbon (within a limited solubility range) are FCC stabilizers in cobalt and its alloys. These decrease the propulsion for transformation and the temperature range of transformation.

[0007] Pure cobalt exhibits an HCP structure at temperatures below approximately 420°C. At higher temperatures (up to its melting point), pure cobalt exhibits an FCC structure. As explained, the addition of elements helps to increase (e.g., in the case of chromium, molybdenum, and tungsten) or decrease (e.g., in the case of nickel, iron, and carbon) this transformation temperature (TT). Transformation is not easily achieved by thermal means (the transformation is slow). Therefore, cobalt-chromium alloys tend to exhibit a metastable FCC structure up to TT at room temperature when cooling from their molten state or after solution treatment at high temperatures. On the other hand, transformation readily occurs under the action of mechanical stress up to TT through the aforementioned process.

[0008] The most relevant prior art to this invention is Patent Document 1 (March 26, 1991), by Paul Crook, Aziz I. Asphahani, and Steven J. Matthews. A commercial embodiment of this patent is known as ULTIMET® alloy. Patent Document 1 discloses a cobalt-based alloy containing significant amounts of chromium, nickel, iron, molybdenum, tungsten, silicon, manganese, carbon, and nitrogen. This patent discloses an unexpected advantage of carbon in the alloy with respect to both cavitation erosion resistance and corrosion resistance (enhanced by the presence of similar levels of nitrogen). Furthermore, this patent reveals that the effect of nickel on cavitation erosion was not strong over a content range of at least 5.3–9.8 wt%. The experimental forged materials used in Crook et al.'s discovery were prepared by vacuum induction melting, electro-slag remelting, hot forging and hot rolling (to sheets and plates), and subsequent solution treatment. Interestingly, the maximum nitrogen content of 0.12 wt% was attributed to the fact that a higher level of 0.19 wt% caused cracking problems during the forging process. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] U.S. Patent No. 5,002,731 [Overview of the project]

[0010] We have discovered that a forged low-nickel derivative of a commercial example of the alloy disclosed in Patent Document 1 exhibits exceptional resistance to high-speed sliding wear when self-bonded, compared to the original material and materials with intermediate nickel content. The effect of nickel is nonlinear and therefore contradicts current theories of cobalt-based alloy metallurgy and tribology (i.e., the science of wear). This exceptional property was achieved with a nickel content of approximately 0.83 wt% and a nitrogen content of approximately 0.125 wt%. Surprisingly, this alloy was suitable for forging (as taught in Patent Document 1). The other elements in this exceptional material and their approximate content are 26.85 wt% chromium, 4.58 wt% molybdenum, 2.33 wt% tungsten, 2.97 wt% iron, 0.84 wt% manganese, 0.27 wt% silicon, 0.065 wt% carbon, and 0.11 wt% aluminum, with cobalt making up the remainder. During the industrial production of this material, it undergoes normal compositional variations. Experiments have shown that when the nitrogen content is approximately doubled (0.254 wt%), the exceptional self-bonding resistance to high-speed sliding wear is lost. Due to its high chromium content and high molybdenum and tungsten content, this alloy should have high resistance to aqueous corrosion. [Brief explanation of the drawing]

[0011] [Figure 1] This graph shows the volume loss of the test alloy under three different high-speed / self-bonding sliding conditions. [Modes for carrying out the invention]

[0012] The forged experimental alloys associated with this discovery were produced by vacuum induction melting (VIM) followed by electroslag remelting (ESR) to generate ingots of material suitable for hot working. Prior to hot working (i.e., hot forging and hot rolling), the ingots were homogenized at 1204°C / 2200°F. Based on previous experience with this class of alloys, a hot working start temperature of 1204°C / 2200°F was used for all experimental alloys. Annealing tests showed that a solution treatment temperature of 1121°C / 2050°F, followed by rapid cooling / quenching (to create a metastable FCC solid solution structure at room temperature), was suitable for this class of material. Wear test samples were prepared using 12.7 mm thick hot-rolled (followed by annealing) plates.

[0013] To evaluate the high-speed sliding properties of each experimental material, the materials were subjected to the pin-on-disk test described in ASTM standard G99. This involves using a cylindrical pin with a convex or hemispherical cap, which is pressed against a flat, circular, rotating disk (the axis of the cylindrical pin is perpendicular to the plane of the disk).

[0014] For each alloy, three tests were performed (one with a load of 10N, one with a load of 20N, and one with a load of 30N) using one disc bolted to a motor-driven platen and three different pins (positioned on the disc at different contact radii for testing). In all cases, the pins and disc were made of the same alloy (i.e., the materials were self-bonded). The rotation speed of the disc was adjusted to compensate for the varying contact radii, allowing for a constant sliding speed of 1.885 m / s throughout the project. The duration of all tests was 1,200 seconds.

[0015] The cylindrical pin had a diameter of 6 mm and therefore a cap radius of 3 mm. The disc had a diameter of 165 mm and a thickness of 8 mm. [Table 1]

[0016] Table 1 shows the compositions of the cobalt-based experimental alloys related to this discovery. Alloy A was manufactured to match the target composition of the ULTIMET® alloy (i.e., the example in Patent Document 1). However, chemical analysis of the ingot revealed that this experimental alloy had a nitrogen content exceeding the maximum value described in the claims of Patent Document 1. This may be due to a higher-than-expected nitrogen recovery rate during the melting process. This same "higher-than-expected" recovery was also encountered with other experimental alloys. Even more surprisingly, alloy D, with a target nitrogen content of 0.20 wt% (however, the analyzed content was 0.254 wt%), was suitable for forging without cracking. This contradicted the teachings of Patent Document 1.

[0017] It should be noted that the analyzed chromium content of alloy D was higher than that of the other experimental alloys. This was not intentional (i.e., the target chromium content for all experimental alloys was 26 wt%). The reason for this high chromium content analyzed in alloy D is a higher-than-expected recovery rate, and it is presumed that chromium and nitrogen are related in this respect (chromium nitride is one of the furnace charge materials).

[0018] The objective of alloys B and C was to determine the effect of reducing the nickel content of ULTIMET® alloys under high-speed sliding wear conditions. Alloy C has a relatively low nickel content, while alloy B has an intermediate content. The objective of alloy D was to establish the effect of increasing nitrogen levels at a low nickel content.

[0019] All other elements within this alloy system were kept constant at the levels proposed in Patent Document 1. This is because it has been proven that these elements are effective in providing corrosion resistance, weldability, and manufacturability in the industry over 25 years. Chromium, molybdenum, and tungsten (enhanced by nitrogen) are involved in the high corrosion resistance of the alloys in this system. Iron is considered to bring benefits to manufacturability because many possible furnace charge materials, especially in cobalt-based alloys, contain some iron. Manganese is considered beneficial for removing sulfur during the melting of cobalt-based alloys. These levels of silicon and carbon result in excellent welding ability, particularly an ideal level of surface tension in the molten weld pool. Aluminum was not an element included in Patent Document 1 but is essential at low levels (up to 0.25 wt%) when the material is compositionally refined by AOD (argon oxygen decarburization) during air melting.

[0020] During the production of Alloy C, which is the alloy of the present invention, the compositional variations expected based on previous experience with such materials are as follows. Nickel: ±0.375 wt% (or 0.3754 wt% including rounding error). Nitrogen: ±0.02 wt% (or 0.024 wt% including rounding error). Chromium: ±1.5 wt% (or 1.54 wt% including rounding error). Molybdenum: ±0.5 wt% (or 0.54 wt% including rounding error). Tungsten: ±0.5 wt% (or 0.54 wt% including rounding error). Iron: ±1.0 wt% (or 1.04 wt% including rounding error). Manganese: ±0.25 wt% (or 0.254 wt% including rounding error). Silicon: ±0.2 wt% (or 0.24 wt% including rounding error). Carbon: ±0.02 wt% (or 0.024 wt% including rounding error). Aluminum: ±0.075 wt% (or 0.0754 wt% including rounding error).

[0021] These values were obtained from the quality system melting specifications of the ULTIMET® alloy, except for nickel (the content of which is much lower in alloy C than in the ULTIMET® alloy). For nickel, melting specifications related to a cobalt-based material with low nickel content (HAYNES® 6B alloy) were used.

[0022] If all compositional dispersions (including rounding errors) expected during industrial production of alloy C are at the lower limit of the range, alloy E (an example within the scope described in the claims) in the following table is created. On the other hand, if all compositional dispersions (including rounding errors) expected during industrial production of alloy C are at the upper limit of the range, alloy F (an example within the scope described in the claims) in the following table is created.

[0023] Based on the experimental data generated for alloys A, B, C, and D, these two examples (i.e., alloy E and alloy F) are expected to share an outstanding resistance to high-speed / self-lubricating sliding wear of alloy C, which is the core composition, and should exhibit high corrosion resistance due to their high chromium content and the large amounts of molybdenum and tungsten present. [Table 2]

[0024] As recommended by ASTM standard G99 for a pin-on-disk test that mainly induces wear of the pin (rather than the disk), the following formula was used to provide an overall assessment of damage (volume loss) based on laser analysis of the surface. [Equation]

[0025] The values generated for each of the four experimental alloys at each of the three loads are shown in Table 3. The values are compared graphically in Figure 1. [Table 3]

[0026] Upon examining these data, it can be seen that alloy C, the material described in the claims, is approximately seven times more resistant to high-speed sliding wear (self-bonding) than alloy A at a 10N load, approximately 4.4 times more resistant at a 20N load, and approximately 2.9 times more resistant at a 30N load. Alloy C also exhibited a significantly lower coefficient of friction than alloy A during these high-speed sliding tests. For example, the continuous readings of the coefficient of friction for alloy C at a 20N load varied between approximately 0.2 and 0.3, while the coefficient of friction for alloy A at the same load varied between approximately 0.4 and 0.5. All of these results were completely unexpected, especially since the results for alloy B (with a moderate nickel content) were not far off from those for alloy A.

[0027] The alloy of the present invention can be formed into forged products such as plates, sheets, bars, tubes, wires, and billets. This alloy can also be used in casting, welding, welding overlays, and powder products.

[0028] While specific current preferred embodiments of the alloy of the present invention have been described, it should be understood that the present invention is not limited thereto and can be embodied in various ways within the scope of the following claims.

Claims

1. A cobalt-based alloy resistant to high speed / self-bonded sliding wear comprising 0.83 wt.% nickel, 0.125 wt.% nitrogen, 26.85 wt.% chromium, 4.58 wt.% molybdenum, 2.33 wt.% tungsten, 2.97 wt.% iron, 0.84 wt.% manganese, 0.27 wt.% silicon, 0.065 wt.% carbon, and 0.11 wt.% aluminum, with the balance cobalt plus impurities, The alloy is a cobalt-based alloy having a coefficient of friction between 0.2 and 0.3 at a load of 20 N when tested using the pin-on-disk method of ASTM standard G99.

2. 10. The alloy of claim 1, wherein the alloy is in a form selected from the group consisting of plate, sheet, bar, tube, wire, billet, casting, weldment, weld overlay, and powder product.

3. 0.4546 to 1.2054 wt. % nickel, 0.101 to 0.149 wt. % nitrogen, 25.31 to 28.39 wt. % chromium, 4.04 to 5.12 wt. % molybdenum, 1.79 to 2.87 wt. % tungsten, 1.93 to 4.01 wt. % iron, 0.586 to 1.094 wt. % manganese, 0.03 to 0.51 wt. % silicon, 0.041 to 0.089 wt. % carbon, and 0.0346 to 0.1854 wt. % aluminum, and Impurity-added residual cobalt A cobalt-based alloy resistant to high speed / self-bonded sliding wear comprising: The alloy is a cobalt-based alloy having a coefficient of friction between 0.2 and 0.3 at a load of 20 N when tested using the pin-on-disk method of ASTM standard G99.

4. 4. The alloy of claim 3, wherein the alloy is in a form selected from the group consisting of plate, sheet, bar, tube, wire, billet, casting, weldment, weld overlay, and powder product.

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