Cobalt-based alloy

Abstract

To provide a cobalt-based alloy which enables multi-layer without changing a material, has hardness lower than a Co-Cr-W-based alloy and is excellent in abrasion resistance, and has low crack sensitivity even in multi-layer construction.SOLUTION: A cobalt-based alloy has a chemical composition containing, by weight ratio 0.4 to 0.7% C, 0.5 to 2.0% Si, 1.0% or less Mn, 22.0 to 27.0% Cr, 2.0 to 3.5% W, 3.5 to 7.5% Fe, 4.0 to 6.5% Ni, less than 4.0% Mo, 5.0 to 7.5% Nb, 0.3 to 0.8% Ti, 0.1% or less other inevitable impurity, and 45 to 60% Co, and produces a first carbide structure 1 mainly containing Ti, and a second carbide structure 2 mainly containing Nb.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a cobalt-based alloy for a welding material used in build-up welding, and more particularly to a cobalt-based alloy having low crack sensitivity and excellent wear resistance.

Background Art

[0002] Conventionally, in welding materials mainly based on cobalt used for build-up welding, in order to improve wear resistance, wear resistance has been ensured by selecting a hard welding material. As a welding material having high hardness and excellent wear resistance, Co-Cr-W alloys (Stellite alloys: "Stellite" is a registered trademark, hereinafter omitted) are known. Further, Patent Document 1 discloses a high-hardness build-up alloy powder having a welding crack sensitivity equivalent to or higher than that of Co-Cr-W alloys and a wear resistance equivalent to or higher than that of Co-Cr-Mo-Si alloys.

[0003] In build-up welding, a certain height may be required. In that case, a build-up operation is performed in which the weld layer is sequentially welded multiple times to finish at a predetermined height, so-called multi-layer build-up. In the case of multi-layer build-up, in order to impart wear resistance to the surface, a cobalt-based material with low hardness is used for the bottom layer, and a construction method is used in which only up to two layers from the final layer are built up with a Stellite alloy (♯6, ♯12, ♯1, etc.).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the above construction method, there are problems such as the need to change the welding material, the importance of heat management, and the need to continuously apply heat during construction to prevent the temperature from dropping. This requires equipment such as electric furnaces and gas burners, and there are also problems such as restrictions on the construction site and equipment for construction.

[0006] In addition, when cracks occur in the weld bead, materials mainly based on cobalt tend to have a strong tendency for the cracks to spread widely, and the cracks may progress not only to the welded metal part but also to the base material. Due to high hardness, cracks are likely to occur, that is, there is a problem of high crack sensitivity. When cracks occur, the cracks must be removed over a wide range using a gouging grinder or the like. However, cobalt dust is generated during the removal process, which may have an adverse effect on the human body. Cobalt dust cannot be completely prevented even if the operator wears a protective mask.

[0007] Furthermore, although it is stated in the above Patent Document 1 that the material has good weld crack sensitivity, it still remains the same in terms of ensuring high hardness and wear resistance, and there remains the problem that cracks are likely to occur due to high hardness. Also, the evaluation is more by overlaying, and it is not shown whether it can be applied to multi-layer overlays where the crack sensitivity is likely to increase.

[0008] Therefore, the present invention has been made in view of the above problems, and an object thereof is to provide a cobalt-based alloy that enables multi-layer overlay without changing the material, has a lower hardness than Co-Cr-W-based alloys, is excellent in wear resistance, and has low crack sensitivity even in multi-layer overlay construction.

Means for Solving the Problems

[0009] To solve the above problems, the cobalt-based alloy according to the present invention has a chemical composition by weight ratio, where C is 0.4 to 0.7%, Si is 0.5 to 2.0%, Mn is 1.0% or less, Cr is 22.0 to 27.0%, W is 2.0 to 3.5%, Fe is 3.5 to 7.5%, Ni is 4.0 to 6.5%, Mo is less than 4.0%, Nb is 5.0 to 7.5%, Ti is 0.3 to 0.8%, other inevitable impurities are 0.1% or less, and Co is 45 to 60%.

[0010] Also, the cobalt-based alloy according to the present invention may be characterized in that the chemical composition is by weight ratio, where C is 0.4 to 0.7%, Si is 0.5 to 2.0%, Mn is 1.0% or less, Cr is 22.0 to 27.0%, W is 2.0 to 3.5%, Fe is 3.5 to 7.5%, Ni is 4.0 to 6.5%, Nb is 5.0 to 7.5%, Ti is 0.3 to 0.8%, other inevitable impurities are 0.1% or less, and Co is 45 to 60%.

Advantages of the Invention

[0011] According to the cobalt-based alloy of the present invention, multi-layer build-up welding can be performed without changing the material. Also, the built-up welded part has a lower hardness and better wear resistance than a Co-Cr-W-based alloy. And the cobalt-based alloy according to the present invention has low crack sensitivity even in multi-layer build-up welding, that is, it is difficult to crack.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0013] Hereinafter, the cobalt-based alloy according to the present invention will be described in detail.

[0014] The cobalt-based alloy according to an embodiment of the present invention is a welding material suitable for build-up welding when performing multi-layer build-up on an object with a complex shape, etc. Its chemical composition, by weight ratio, is C: 0.4 to 0.7%, Si: 0.5 to 2.0%, Mn: 1.0% or less, Cr: 22.0 to 27.0%, W: 2.0 to 3.5%, Fe: 3.5 to 7.5%, Ni: 4.0 to 6.5%, Mo: less than 4.0%, Nb: 5.0 to 7.5%, Ti: 0.3 to 0.8%, other inevitable impurities: 0.1% or less, and Co: 45 to 60%.

[0015] This welding material of the cobalt-based alloy has lower hardness than conventional Stellite alloys but is excellent in wear resistance. Therefore, it is possible to reduce the concern about the occurrence of cracks in build-up construction. As a result, rework is not required, there is no hindrance to the process, which not only leads to cost reduction, but also ensures the safety of workers. In addition, there is no need to worry about restrictions on the use of equipment such as construction sites and electric furnaces, or to be overly sensitive to heat management. Incidentally, this welding material of the cobalt-based alloy is suitable for multi-layer build-up, but can also be used for build-up welding on objects that require wear resistance not limited to multi-layer build-up. Also, in a construction method with a large heat source such as the plasma arc welding method, preheating is not required, and construction can be performed without preheating.

[0016] (Base area ratio) The base area ratio of the cobalt-based alloy of this embodiment and a conventional Stellite alloy was measured and compared using an image (×500) of a scanning electron microscope. Hereinafter, the results of using Stellite alloy #6 and Stellite alloy #21, which are frequently used conventional Stellite alloys, as the comparison targets are shown.

[0017] As a result, in the powder alloy of the cobalt-based alloy of this embodiment, the base area ratio differed depending on the Ti addition amount and was 85.5 to 90%, whereas the base area ratio of Stellite alloy #6 was 77% and that of Stellite alloy #21 was 93%. Thus, the cobalt-based alloy of this embodiment maintains a base area ratio intermediate between those of the conventional Stellite alloy #6 and Stellite alloy #21, and the structure in which the area of the base portion occupies the majority. When the cobalt-based alloy of this embodiment was measured according to the Ti addition amount, the base area ratio was 90% at Ti 0.3% and 85.5% at Ti 0.8%, showing that the area ratio decreases as the Ti addition amount increases, and the area ratio occupied by the base portion decreases as the Ti addition amount increases.

[0018] (Precipitated carbide shape) For the cobalt-based alloy of this embodiment, the shape of the precipitated carbide was observed using a scanning electron microscope. Fig. 1 is a diagram showing the crystallization and precipitation state of the metal structure and is a photograph (×5000) taken with a scanning electron microscope. The first carbide structure 1 is a carbide structure mainly composed of Ti. The second carbide structure 2 is a carbide structure mainly composed of Nb. In the cobalt-based alloy of this embodiment, by adding a predetermined amount of Ti to the Co-Cr alloy, a carbide structure mainly composed of Nb, Mo, etc. is formed in a granular shape with Ti as the nucleus, and it becomes a stronger composite carbide and spreads to the base portion and the precipitated portion.

[0019] (Base hardness) The cobalt-based alloy of this embodiment was compared with a conventional Stellite alloy in terms of the hardness of the base. The microstructures of the cobalt-based alloy of this embodiment and the conventional Stellite alloy were measured for hardness at 100 points at 5-μm intervals in a nanoindentation test, and only the hardness of the base, which occupies the majority of the structure, was averaged for 50 points each, and the hardness comparison was made. Note that in accordance with ISO14577, the hardness unit is expressed in HIT / GPa, and the higher the numerical value, the harder it is.

[0020] The cobalt-based alloy of this embodiment was measured for hardness in the range of Ti addition amount of 0.3 to 0.8%, and it was 6.0 to 6.4 HIT / GPa. Stellite alloy #21 was 6.8 HIT / GPa, and Stellite alloy #6 was 8.8 HIT / GPa. It can be seen that the base of the cobalt-based alloy of this embodiment has a lower hardness than Stellite alloy #21, and it can be said that it has an effective influence on the crack sensitivity.

[0021] (Precipitated carbide hardness) The cobalt-based alloy of this embodiment was compared with a conventional Stellite alloy in terms of the hardness of the precipitated carbide. In the nanoindentation hardness test, the measured value of the precipitated carbide was 20 to 26 HIT / GPa in terms of the hardness in the range of Ti addition amount of 0.3 to 0.8% in the cobalt-based alloy of this embodiment. With the increase in the Ti addition amount, the precipitated carbide tends to become harder. Stellite alloy #21 was 18.4 HIT / GPa, and Stellite alloy #6 was 20 HIT / GPa. The precipitated carbides of the cobalt-based alloy of this embodiment have resulted in the presence of precipitated carbides harder than Stellite alloy #6, and it can be said that they contribute to the improvement of wear resistance.

[0022] (Effect of Ti addition on wear resistance) Ti is an important element related to the wear resistance, which is a characteristic of the cobalt-based alloy of this embodiment. Even with a trace addition amount of 0.1%, Ti has a greater influence on the wear resistance than 0% with no addition. A test on the wear resistance when Ti was added in the range of 0.1 to 1.0% was carried out. Figure 2 is a diagram showing the test results regarding the wear resistance with the Ti addition amount.

[0023] When measured at an addition amount of 0.1 to 0.2%, a significant decrease in hardness was confirmed depending on the measurement position. It is considered that the solid solution form collapsed and the compounds contained in the base part were attracted to Ti, leading to a decrease in hardness. From the results shown in Fig. 2, it can be said that the wear resistance gradually improves as the addition amount of Ti increases.

[0024] (High-temperature tensile test) A high-temperature tensile test was carried out and compared between the cobalt-based alloy of this embodiment and a conventional Stellite alloy. Fig. 3 is a diagram showing the results of a comparative test regarding high-temperature tensile (elongation), and Fig. 4 is a diagram showing the results of a comparative test regarding high-temperature tensile (yield strength).

[0025] As shown in Fig. 3 and Fig. 4, it can be understood that the cobalt-based alloy of this embodiment is excellent in yield strength as a result. Since the yield strength is superior to that of conventional Co-based materials and the elastic deformation region is wide, it can be said that the cobalt-based alloy of this embodiment has low crack sensitivity and is an alloy that can be multi-layered.

[0026] (Wear resistance) Regarding the cobalt-based alloy of this embodiment, when the addition amount of Ti was 0.5%, the wear amount was set to "1", and a comparison of wear resistance with the current Stellite alloy material specified by JIS was carried out by a rubber wheel test. Fig. 5 is a diagram graphically showing the comparison results of wear resistance with a conventional cobalt-based alloy. The conditions of the rubber wheel test were as follows: load: 8.8 kg, rotation speed: 3000 rotations, rotational speed: 120 rpm, wheel dimensions: Φ250 mm × 15 mm, No. 6 emery sand used: HV900 - 1000, and the average value of the wear amount measured with two pieces each. As shown in Fig. 5, it was found that the wear resistance of the powder alloy of the cobalt-based alloy of this embodiment can obtain wear resistance higher than that of Stellite alloy #6 and Stellite alloy #12.

[0027] As described above, the cobalt-based alloy of the present embodiment has a low crack sensitivity due to the low hardness of the base part, and is formed as granular precipitated carbides mainly composed of Nb, Mo, etc. with Ti as the core, and becomes a stronger composite carbide and spreads to the base part and the precipitated part. By such a metal structure being scattered throughout, it has wear resistance superior to that of a comparison material such as Stellite alloy #6. Also, the hardness is 350 to 400 in terms of Vickers hardness, which is lower than that of Stellite alloy #6. Therefore, it can be said that the cobalt-based alloy of the present embodiment has characteristics different from those of conventional cobalt-based alloys that improve wear resistance by increasing hardness.

[0028] Next, the chemical composition of the welding material of the cobalt-based alloy of the present embodiment will be described. Note that the range values represent the chemical composition in % by weight ratio.

[0029] Co (cobalt) is a main component that forms and constitutes the base part of the cobalt-based alloy of the present embodiment. This component can obtain the effects of improving strength, heat resistance, wear resistance, and toughness, but if it is less than 45%, heat resistance and toughness will decrease, and if it exceeds 60%, wear resistance cannot be obtained. Therefore, it is preferably added in the range of 45 to 60%.

[0030] Cr (chromium) is a constituent element of the primary solid solution like Co, contributes to high wear resistance, and has the effect of improving strength and toughness. Based on the implementation test results, the addition of this component is preferably in the range of 22 to 27%.

[0031] Nb (niobium) can obtain the effects of improving wear resistance, refining crystals, and improving toughness. Also, it crystallizes and precipitates together with Mo so as to surround Ti, and is an element that plays a major role in the formation of composite carbides. If the addition amount of this component is low, it will be refined and affect wear resistance, and if a large amount is added, the fluidity of the molten metal will deteriorate and affect weldability. Therefore, it is preferably added in the range of 5.0 to 8.0%.

[0032] Ni (Nickel) is an element that has a great effect of enhancing the toughness of the substrate and can also achieve the effect of improving impact resistance. The addition amount of this component ranges from 4.0% to 6.5% based on the consideration of the test results.

[0033] Mo (Molybdenum), together with Nb, mainly crystallizes and precipitates in the precipitation part and plays a role in improving hardness and wear resistance. Although an improvement in wear resistance can be expected by increasing the addition amount, in the implementation test, when the addition amount exceeds 4% and multi-layer stacking (three or more layers) is performed, cracks occur. XRD was used to add Mo in 1% units and measure the peak shift position. When it exceeds 4%, the peak shift tilts to the low-angle side, and it is considered that the lattice structure is in a swollen state, which is speculated to be one of the factors for crack generation in the above test results. Also, in the nano-indentation hardness test, it was confirmed that adding Mo has the effect of stabilizing the hardness of the base part regardless of the increase or decrease in the Ti addition amount. Figure 6 is a diagram showing the results of the nano-indentation hardness test. Based on the above implementation test, the effect of stabilizing the hardness of the base part, and the results of XRD, in the chemical composition weight ratio of the cobalt-based alloy of this embodiment, it is preferable that the addition amount of Mo is less than 4%.

[0034] It should be noted that Mo may not be added to the cobalt-based alloy of this embodiment. For example, when there is a limit to the filling rate such as in flux-cored wire, there may be an option not to add Mo. Even in this case, the wear resistance will be slightly inferior compared to the case where Mo is added, but it can exhibit wear resistance superior to that of conventional Stellite alloys.

[0035] FIG. 7 is a diagram showing the comparison results of the wear resistance with a conventional cobalt-based alloy. It is a graph comparing the wear degrees of a flux cored wire Φ1.6 mm made of the components obtained by removing Mo from the cobalt-based alloy of the present embodiment, a wire made of a conventional Stellite alloy #21, and a wire made of Stellite alloy #6. The wear amount of the one obtained by removing Mo from the cobalt-based alloy of the present embodiment is represented as "1". From FIG. 7, it can be seen that even when Mo is not added to the cobalt-based alloy of the present embodiment, it has better wear resistance than the conventional Stellite alloy.

[0036] W (tungsten) is dissolved in the base part to strengthen the base part, and crystallizes and precipitates as a complex carbide together with Ti, Nb, and Mo, improving the wear resistance of the weld metal. If the content of this component is less than the lower limit value of 2.0%, the wear resistance will decrease. If it exceeds the upper limit value of 3.5%, the characteristics of the cobalt-based alloy of the present embodiment, which has low crack sensitivity, cannot be obtained. Therefore, from the actual test results, it is preferable to add this component in the range of 2.0 to 3.5%.

[0037] Ti (titanium) is the most important element in the components of the cobalt-based alloy of the present embodiment. It forms granular precipitated carbides mainly composed of Nb, Mo, etc. with Ti as the core, becoming stronger complex carbides, granular in the base part and the precipitated part, and scattered throughout the metal structure. This plays a major role in improving the wear resistance. As described above, even with an addition of 0.1%, the wear resistance is significantly improved. However, in terms of hardness, when the addition is 0% and 0.1%, the Vickers hardness decreases by about HV50, and unstable values are confirmed depending on the position where the hardness is measured.

[0038] As part of this cause investigation, the Ti appearance area ratio for different Ti additions was measured with image processing software using SEM images at n number 2. As shown in Fig. 8, according to the measurement results, for the 0.1% addition and 0.2% addition, it is less than 0.1 - 0.2%, and the area ratio cannot be ensured with respect to the addition amount. For additions of 0.3% or more, it is stable in the range of 0.4 - 0.5%, and the area ratio can be ensured. From this, it can be said that when the addition is less than 0.3%, the number of strong complex carbides generated is small, and there is a possibility of uneven wear resistance. Therefore, from the perspective of ensuring the stability of the number of refined carbides from the viewpoint of the microstructure, the minimum addition amount of Ti is set to 0.3%.

[0039] Also, in the actual build-up test, when the Ti addition amount is 1.0%, the flow of the molten pool is poor, resulting in an overlapping bead, which not only impairs the weldability but may also affect the crack sensitivity. As a result of measuring the peak position by XRD, when it is 0.8% or more, the peak shift is inclined to a low angle, and it is presumed that the swelling of the lattice structure affects the improvement of crack sensitivity. Also, when observing the microstructure, when the addition is 1.0% or more, Ti does not disperse but forms lumps, and the crack sensitivity increases. From the above, the addition of Ti in the cobalt-based alloy of this embodiment is preferably in the range of 0.3 - 0.8%.

[0040] C (carbon) is effective for strength, hardness, and wear resistance and solid-solves in the base part for strengthening. When the content of this component is less than 0.4%, it leads to a decrease in the hardness of the base part, insufficient carbide formation, and impaired wear resistance and hardness. On the other hand, when it is 0.8% or more, the crack sensitivity improves, and sufficient crack sensitivity cannot be obtained for the cobalt-based alloy of this embodiment. Based on the results in the actual test, the range of this component is 0.4 - 0.7%.

[0041] Si (silicon) mainly exists in the eutectic part and forms silicides. Also, by lowering the melting point of the alloy, the molten area spreads easily and the weldability improves. However, if too much is added and it exceeds 2.0% by weight ratio, the weld metal becomes brittle. Therefore, it is preferable to add this component in the range of 0.5 - 2.0% or less.

[0042] Mn (Manganese) easily dissolves in iron and has the effect of increasing the tensile strength and yield point of steel. Since wear resistance and strength decrease when the content increases, the upper limit is set at 1.0%.

[0043] Fe (Iron) has the effect of improving the lubricity and fluidity of the molten metal of the alloy powder. If the addition amount is too large, it will affect wear resistance and corrosion resistance, so the range is set at 3.5 - 10%.

[0044] Hereinafter, examples of the cobalt-based alloy of the present embodiment will be described.

[0045] (First Example) The sample was prepared as follows.

[0046] The cobalt-based alloy powder of the first example is a mixed powder prepared by mixing the cobalt-based alloy powder of the above embodiment with a particle size of -212 / 63 μm.

[0047] For the comparative examples, JIS standard cobalt-based atomized powder of the same particle size was used. Comparative example 1 was made using Stellite alloy #21, comparative example 2 was made using Stellite #12, and comparative example 3 was made using Stellite #6.

[0048] The base material, welding conditions, etc. are as follows.

[0049] Base material: SS400 19t×100w×200L Restrained by welding on a 50t×450w×300L SS400 material (Welding Conditions) Current 180A, Voltage 22 - 23V, Welding speed 80mm / min, Powder amount 18g / min (Build-up Conditions) Preheat 150°C, No post-heat, Weaving width 12mm, Bead length 150mm First layer: 6 passes, Second layer: 5 passes, Third layer: 4 passes Gas used: Argon gas Plasma gas 3L / min, Carrier gas 4L / min, Shielding gas 20L / min Tungsten electrode used: Φ3.2mm Next, for the Vickers hardness, the surface of the final layer with build-up welding was cut into a 30 mm dimension, the surface part was polished with waterproof paper #1200, 10 random points were measured with a load of 20 kg, and the average value of the 10 points was calculated.

[0050] Also, the penetrant inspection test was carried out according to JIS standard Z2343 after the welding was completed and the temperature returned to room temperature as follows.

[0051] Chemical pretreatment (cleaning solution) was performed, immersed in the penetrant for 5 minutes or more, and then the development solution was used for the implementation judgment. When cracks are confirmed regardless of the number, it is evaluated as "×".

[0052] Furthermore, the wear amount test (rubber wheel test) was carried out in the following manner (similar to Fig. 5).

[0053] Load: 8.8 kg Rotation speed: 3000 rotations Rotation velocity: 120 rpm Wheel dimension: Φ250 mm × 15 mm The average value of the wear amounts with 2 pieces each of No. 6 silicon carbide with HV900 - 1000 was measured. In accordance with Stellite alloy #6, a wear amount of 0.8 g or less was evaluated as "〇", and those exceeding 0.8 g of the wear amount were evaluated as "×".

[0054] The results of the first embodiment are shown in Table 1.

[0055]

Table 1

[0056] From Table 1, in the first embodiment, in all of Examples 1 - 4, the wear resistance was ensured to be equal to or higher than Stellite #6, and the hardness resulted in low hardness. (Second Embodiment) The cobalt-based alloy powder of the second embodiment is different from the first embodiment in that it is a component obtained by removing Mo from the cobalt-based alloy of the above embodiment (with 0.6% Ti added).

[0057] A flux cored wire with a diameter of Φ1.6mm was fabricated using this component, and wear test comparisons were conducted using conventional Stellite alloy #21 and Stellite alloy #6 as comparison materials.

[0058] The base material is the same as in the first embodiment, and the welding conditions are as follows.

[0059] (Build-up conditions) MIG welding: Welding current: 270A, Voltage: 27V, Preheat: 150°C, Interpass temperature: about 200°C, Weaving width: 15mm, Bead length: 150mm, 5-layer build-up Gas used: Argon gas 20L / min For the two types of comparison materials, construction was carried out under conventional construction conditions.

[0060] When the wear amount test (rubber wheel test) was carried out in the same manner as in the first embodiment, it was found that the wear resistance of the flux core wire also ensured more than that of Stellite #6.

[0061] As described above, the cobalt-based alloy according to the present invention has been described based on the embodiments, but the present invention is not limited thereto, and various design changes can be made within the scope that can achieve the object of the present invention and does not deviate from the gist of the invention, and all of them are included in the scope of the present invention.

Industrial Applicability

[0062] The cobalt-based alloy according to the present invention is suitable as a welding material for build-up welding with excellent wear resistance when performing multi-layer build-up on an object with a complex shape.

Explanation of Symbols

[0063] 1 First carbide structure 2 Second carbide structure

Claims

1. The chemical composition is by weight ratio, C is 0.4 to 0.7%, Si is 0.5 to 2.0%, Mn is 1.0% or less, Cr is 22.0 to 27.0%, W is 2.0 to 3.5%, Fe is 3.5 to 7.5%, Ni is 4.0 to 6.5%, Mo is less than 4.0%, Nb is 5.0 to 7.5%, Ti is 0.3 to 0.8%, other inevitable impurities are 0.1% or less, and Co is 45 to 60% characterized by a cobalt-based alloy.

2. The chemical composition is by weight ratio, C is 0.4 to 0.7%, Si is 0.5 to 2.0%, Mn is 1.0% or less, Cr is 22.0 to 27.0%, W is 2.0 to 3.5%, Fe is 3.5 to 7.5%, Ni is 4.0 to 6.5%, Nb is 5.0 to 7.5%, Ti is 0.3 to 0.8%, other inevitable impurities are 0.1% or less, and Co is 45 to 60% characterized by a cobalt-based alloy.

Images