Thermal spray material and its coated article excellent in high-temperature wear resistance and build-up resistance

Abstract

 A thermal spray material used to coat the roll surface which is used to carry high-temperature heat treating material within a continuous heat-treating furnace. The thermal spray material is a composite powder or a blended powder consisting of alloy particles and 5 to 50 vol. % of metal boride particles. The alloy particle contains 5 to 40 wt. % of Cr and 5 to 20 wt. % of Al, and the balance of at least one of Ni, Co and Fe.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to thermal spray material and its coated articles excellent in high-temperature wear resistance and build-up resistance which are mainly used for carrier rolls for high-temperature heat-treating materials in a continuous heat-treating furnace.

[0002] When a steel plate, for example, is continuously annealed, the plate is passed through an oxidizing or reducing atmosphere at a temperature of 600 to 1300°C, and many heat-resistant rolls are located and used as hearth rolls to support this steel plate. However, the continuous use of the rolls for many hours often causes adhered oxide scale on the steel plate or ferrous powder to adhere and deposit on the surface of each of the rolls into so-called build-up. Since the surface of the steel plate to be carried is deteriorated such as occurrence of scratches when this build-up occurs, the annealing operation sometimes may have to be immediately suspended to repair such as grinding the roll surface or replacement the roll.

[0003] Therefore numerous preventive measures against build-up on the roll surface are proposed, such as, for example, being disclosed in JP-A-58-249839, JP-A-59-70712, JP-A-59-126772 and JP-A-63-199857. As preventive measures against build-up on the hearth roll surface, it is suggested in the above-mentioned Japanese Patent Unexamined Publications to coat the roll surface by means of the thermal spraying method for use with the metallic oxide such as Al₂O₃, SiO₂, ZrO₂ and Cr₂O₃, with the carbide, for example, Cr₃C₂ or with one or more of these compounds (or ceramic materials) blended with metal such as Co, Cr, Ni, Al, Y, Mo, Zr or an alloy thereof. Although, however, the result of prevention of build-up was achieved by these methods for the time being, any of them has weak points in adhesive strength, heat-shock resistance and wear resistance of the spray film: peeling occurs, and the life of the roll is exceedingly short owing to wear.

DETAILED DESCRIPTION OF THE INVENTION

[0004] It is an object of the present to find out a coating material and coated layer which prevent build-up on the above hearth roll, and have all of excellent properties such as adhesion strength, heat-shock resistance and wear resistance.

[0005] The present invention intends to solve these problems by using thermal sprayed layer of powder containing oxidation-resistant alloy particles and metal boride particles.

[0006] To further improve the wear resistance of this sprayed layer, a part of the oxidation-resistant alloy powder and a part of metal boride powder may be replaced with oxide particles to disperse metallic oxide particles all over the thermal sprayed layer, or the dispersion of oxidation-resistant alloy may be strengthened by uniformly dispersing a very small amount of fine metallic oxide particles in the oxidation-resistant alloy powder, or precipitation hardening of oxidation-resistant alloy may be performed by additioning titanium and carbon and precipitating these as titanium carbide in the oxidation-resistant alloy.

[0007] Equipment members, such as a hearth roll or other carrier members used in a furnace, have been coated with a cermet containing 5 to 95 vol.% of the compounds of oxide (or ceramic material), etc. on the surface of base material of heat resisting cast steel, etc. by the spraying method so far.

[0008] If only the compounds are used as a coating material, obtained coating will easily peels from the base material by heat shock because its thermal coefficient of expansion is much smaller than that of the metallic base material though it is excellent in prevention of build-up. For this reason, a cermet coating containing metallic component is used, but its heat-shock resistance is still unsatisfactory. The present invention solves this problem by using metal boride which has a thermal coefficient of expansion about equal to that of metallic material and is excellent in preventing build-up.

[0009] The build-up occurs when oxide or ferrous powder formed on a steel plate adheres and deposits on the surface of the hearth roll, and the inventors have found out that metal boride is very excellent in preventing build-up. Also it is well known that the thermal coefficient of expansion of metal boride is about equal to that of metallic material.

[0010] For the metal boride used for the present invention, any of the following may be used: chrome boride, zirconium boride, titanium boride, molybdenum boride, niobium boride, tantalum boride, tungsten boride and hafnium boride. Generally, the atmosphere in a continuous refining annealing line, in which hearth rolls and other carrier members are used, is under oxidizing free atmosphere mainly containing nitrogen and hydrogen, but the above-mentioned carrier members are often exposed to oxidizing atmosphere during temperature of the furnace are rising or lowering when the operation suspends for repair, etc. Accordingly since especially chrome-, zirconium- and titanium-boride are excellent in oxidation resistance, respectively, among the above metal borides, these are suitable for the surface coating material for hearth rolls and so on.

[0011] The surface coating layer for steel manufacturing process equipment members such as hearth rolls is damaged by the wear besides the heat shock mentioned above. The metal boride used for the present invention is also excellent in wear resistance because it has a very high hardness. Since, however, it is deficient in toughness owing to high hardness, there are problems with relation to shock resistance and adhesive strength in case of a load being rapidly applied to the coating. Therefore the sprayed coating layer used for the present invention should be a cermet composed of metal boride and alloy. That is, the content of the metal boride is limited to 5 to 50 vol.%; the cermet is inferior in wear resistance and a property of preventing build-up in case of less than 5 vol.% of the metal boride, and is inferior in shock resistance, adhesive strength and heat-shock resistance over in case of more than 50 vol.% of the metal boride. Ten to 20 vol.% of the metal boride is most desirable. Such cermet coating layer is generally obtained by spraying blended powder consisting of each ingredient powders. But composite powder constituted by the monolithic combination of individual particles or each of ingredients is desirable from a standpoint of uniformity of a coating layer.

[0012] The composite powder of the present invention can be prepared by a so-called mechanical alloying method, in which metal powders of the respective components are blended and stirred by the use of a stirring machine at a high speed with a high energy for a certain period of time, whereby a composite powder alloyed mechanically containing the respective component particles as mechanically bonded therein can be obtained. In other words, each of particles of the composite powder is a composite particle alloyed mechanically and consists of every components. The mechanical alloying method is a high-energy milling techniques as described in U.S. Patent Nos. 3,591,362 and 2,740,210.

[0013] As mentioned above, the steel manufacturing process roll used at high temperature such as hearth rolls, may be exposed to some oxidizing atmosphere. Therefore the oxidation resistance is always required and important. The oxidation resistance is given through alloy portions, and the amount of the alloy portions in cermet used for the present invention should be equal to or more than the content of metal boride. Further, it is naturally required for the alloy portions to have a property of preventing build-up. To provide metallic material with oxidation resistance, it becomes necessary to form a thin, close oxide protection coating for prevention of oxygen diffusion on the surface. For the protection coat for prevention of oxygen diffusion, chromium oxide or aluminum oxide is excellent, and chromium and aluminum should be always contained in the alloy of iron, nickel or cobalt. Chromium forms a protective coating of chromium oxide below 800°C, and this chromium effect is not sufficient when its chromium content in the alloy is 5 wt.% or less. When cromium content in the alloy is more than 40 wt.%, the alloy portions as a whole become brittle. More desirably, Cr content in the alloy is 17 to 27 wt.%. Aluminum accelerates forming of a protective coating of aluminum oxide at 800°C or more, and forming of a protective coating of chromium oxide at 800°C or less. When aluminum content in the alloy is 5 wt.% or less, this aluminum effect is not sufficient, but when the content exceeds 20 wt.%, the alloy portions becomes remarkably brittle like in case of chromium. More desirably, aluminum content in the alloy is 5 to 15 wt.%. When the entire alloy layer becomes brittle, the heat-shock resistance and adhesion strength of the sprayed coating layer becomes remarkably brittle. A protective coating of chromium oxide or aluminum oxide is excellent in preventing build-up.

[0014] For the alloy in the cermet coating layer used in the present invention, any one excellent in oxidation resistance thus will do, and therefore rare-earth metal such as yttrium or silicon, etc. may be contained in the alloy besides the above-mentioned chromium and aluminum to improve the oxidation resistance. The content of rare-earth metal or silicon should be less than 2 wt.% in the alloy. The desirable oxidation resistant alloy in this invention contains essentially of about from 15 to 40 wt.% of Cr, from 5 to 20 wt.% of Al and the balance of Ni or Co. More preferably, the alloy consists essentially of about from 17 to 27 wt.% of Cr, from 5 to 15 wt.% of Al, less than 2 wt.% of Si, less than 2 wt.% of Y, less than 2 wt.% of Y₂O₃ and the balance of Ni or Co. The alloy powder is generally produced from a molten alloy of a specified composition by an inert gas atomizing method and so on, and it may be produced by alloying each metal components powder by a mechanical alloying process.

[0015] As mentioned above, the material of the present invention is mainly composed of cermet of oxidation-resistant alloy containing metal boride. To further improve the wear resistance as far as the heat-shock resistance and adhesive strength of the sprayed coating layer are not deteriorated, the following hard oxide particles can be contained: aluminum oxide, chromium oxide, titanium oxide, silicon dioxide, zirconium oxide, magnesium oxide, yttrium oxide, a rare-earth oxide, etc. In the present invention, a part of oxidation-resistant alloy particles and a part of metal boride particles may be replaced with the above-mentioned oxide powder to disperse oxide particles all over the sprayed coating layer. Its content is limited to 50 vol.% or less of the cermet components consisting of oxidation-resistant alloy particles and metal boride particles not to deteriorate the heat-shock resistance and adhesive strength.

[0016] On the other hand, to further improve the wear resistance in any method other than such a dispersion method for oxide particles, the oxidation-resistant alloy may be hardened by using a method not to deteriorate the toughness. This can be performed by uniformly dispersing a very small amount of fine oxide particles of aluminum oxide, chromium oxide, titanium dioxide, silicon dioxide, zirconium oxide, magnesium oxide, yttrium oxide, a rare-earth oxide or the like in the oxidation-resistant alloy to strengthen the alloy matrix. The content of the fine oxide particles in the oxidation-resistant alloy is limited to 2 vol.% or less, and when this value is exceeded, the oxidation-resistant alloy becomes brittle, resulting in deteriorated heat-shock resistance and adhesive strength of the sprayed coating layer.

[0017] In addition, it is also possible to contain an alloying element which improves the wear resistance of the oxidation-resistant alloy without deteriorating oxidation resistance and toughness of the alloy. The present inventors have found out that titanium carbide is excellent in this effect. That is, to precipitate titanium carbide, it is possible to harden the oxidation-resistant alloy without deteriorating oxidation resistance and build-up preventing effect of the oxidation-resistant alloy. To harden the alloy, it is effective to contain 10 wt.% or less of titanium and 5 wt.% or less of carbon in the alloy. Addition of more than these values allows the alloy to becomes brittle, and deteriorates the heat-shock resistance and adhesive strength of the sprayed coating layer.

[0018] Such a thermal spray material composition of the present invention is used by coating the base material surface of equipment members such as hearth rolls by using a detonation spraying method, a supersonic flame spraying method, or general spraying techniques such as a plasma spraying method. Since, however, thermal spray material composition of the present invention contains heat decomposable substance such as metal boride, the coat formed by using the detonation spraying method or supersonic flame spraying method is excellent in various characteristics, and especially the coat formed by the detonation spraying method is most desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 is an illustration for a build-up testing method used in the embodiment of the present invention, and

[0020] Fig. 2 is an illustration of the said adhesion evaluation testing method.

Examples

[0021] The preparing method for thermal spray material composition used for the present invention is first described.

[0022] The thermal spray material of the present invention is mainly composed of metal boride and oxidation-resistant alloy, and may be contain oxide as required. The material may be sole blended powder of each constituent powder, or may be composite powder in which individual particles are composed of the each component particles. The metal boride powder, oxidation-resistant mixed powder, oxide powder used in this embodiment, and carbide powder used for the comparative specimen are described below.

[0023] Metal boride powder ..... A

[0024] Zirconium boride powder (ZrB₂), 30 to 5 µm in particle diameter, being composed of 19 wt.% B and the balance of Zr.

[0025] Metal boride powder ..... B

[0026] Chrome boride powder (CrB₂), 30 to 5 µm in particle diameter, being composed of 29 wt.% B and the balance of Cr.

[0027] Metal boride powder ..... C

[0028] Titanium boride powder (TiB₂), 30 to 5 µm in particle diameter, being composed of 31 wt% B and the balance of Ti.

[0029] Oxide powder ..... D

[0030] Aluminum oxide powder (Al₂O₃): 25 to 5 µm in particle diameter

[0031] Oxide powder ..... E

[0032] Chromium oxide powder (Cr₂O₃): 25 to 5 µm in particle diameter

[0033] Oxide powder ..... F

[0034] Zirconia powder stabilized by yttria (92 wt.% ZrO₂, 8 wt.% Y₂O₃): 25 to 5 µm in particle diameter

[0035] Oxide powder ..... G

[0036] Spinel powder (Al₂O₃·MgO; 76 wt.% Al₂O₃, 24 wt.% MgO): 25 to 5 µm in particle diameter

[0037] Carbide powder ..... H

[0038] Chrome carbide powder (Cr₃C₂), 45 to 10 µm in particle diameter, being composed of 13 wt.% carbon and the balance of chromium.

[0039] Carbide powder ..... I

[0040] Titanium carbide powder (TiC), 45 to 10 µm in particle diameter, being composed of 20 wt.% carbon and the balance of Ti.

[0041] Carbide powder ..... J

[0042] Tungsten carbide powder (WC), 45 to 10 µm in particle diameter, being composed of 6 wt.% carbon and the balance of W.

[0043] Oxidation-resistant alloy powder ..... K

[0044] Alloy powder, 20 µm or less in particle diameter, obtained by means of the inert Gas atomizing method, being composed of 20 wt.% Cr, 7 wt.% Al and the balance of Ni.

[0045] Oxidation-resistant alloy powder ..... L

[0046] Alloy powder, 20 µm or less in particle diameter, obtained by means of the inert gas atomizing method, being composed of 25 wt.% Cr, 10 wt.% Al, 0.5 wt.% yttrium and the balance of Ni.

[0047] Oxidation-resistant alloy powder ..... M

[0048] Alloy powder, 20 µm or less in particle diameter, obtained by means of the mechanical alloying method, being composed of 20 wt.% Cr, 8 wt.% Al, 0.8 wt.% yttrium, 1.5 wt.% Si and the balance of Co. For the mechanical alloying at Attriter Model MA-l, produced by Mitsui-Miike K.K. was used. Co powder (10 µm or less in particle diameter), Cr powder (150 µm or less in particle diameter), Al-40% Si alloy powder (45 µm or less in particle diameter), and Co-40% Y alloy powder (200 µm or less in particle diameter) were blended by weight of 2 kg at a specified ratio, and 30 c.c. of ethyl alcohol was added in argon atmosphere for grinding and stirring for 40 hours. Thereafter, after annealing at 1150°C in an atmospheric pressure of 10⁻³ torr for 30 hours, powder with a given particle size was obtained by repeating the grinding and classifying.

[0049] Oxidation-resistant alloy powder ..... N

[0050] Alloy powder, 20 µm or less in particle diameter, obtained by means of the mechanical alloying method, being composed of 25 wt.% Cr, 12 wt.% Al, 0.5 wt.% yttrium, 1.2 wt.% Si, 0.2 wt.% Y₂O₃ and the balance of Co. For the mechanical alloying, Attriter Model MA-l, produced by Mitsui-Miike K.K. was used. Co powder (10 µm or less in particle diameter), Cr powder (150 µm or less in particle diameter), Al-40 wt.% Si alloy powder (45 µm or less in particle diameter), Co-40 wt.% yttrium alloy powder (200 µm or less in particle diameter) and Y₂O₃ powder (0.1 µm in average (or mean) particle diameter) were blended by weight of 2 kg at a given ratio, and 30 c.c. of ethyl alcohol was added in argon atmosphere for grinding and stirring for 40 hours. Thereafter, after annealing at 1150°C in an atmospheric pressure of 10⁻³ torr for 30 hours, powder with a given particle size was obtained by repeating the grinding and classification.

[0051] Oxidation-resistant alloy powder ..... O

[0052] Alloy powder, 20 µm or less in particle diameter, obtained by means of the mechanical alloying method, being composed of 20 wt.% Cr, 10 wt.% Al, 0.5 wt.% yttrium, 1.5 wt.% Si, 0.5 wt.% Al₂O₃ and the balance of Ni. For the mechanical alloying, Attriter Model MA-l, produced by Mitsui-Miike K.K. was used. Ni powder (10 µm or less in particle diameter), Cr powder (150 µm or less in particle diameter), Al-40 wt.% Si alloy powder (45 µm or less in particle diameter), Ni-40 wt.% Y alloy powder (200 µm or less in particle diameter) and Al₂O₃ powder (0.05 µm in average (or mean) particle diameter) were blended by weight of 2 kg at a given ratio, and 30 c.c. of ethyl alcohol was added in argon atmosphere for grinding and stirring for 40 hours. Thereafter, after annealing at 1150°C in an atmospheric pressure of 10⁻³ torr for 30 hours, powder with a given particle size was obtained by repeating the grinding and classifying.

[0053] Oxidation-resistant alloy powder ..... P

[0054] Alloy powder, 20 µm or less in particle diameter, obtained by means of the inert gas atomizing method, being composed of 25 wt.% Cr, 10 wt.% Al, 0.5 wt.% yttrium, 1 wt.% Si, 5 wt.% Ti, 2 wt.% carbon and the balance of Co.

[0055] All the thermal spray materials of the embodiments of the present invention were processed in a form of composite powder for use, and the composite powder was prepared by mixing these individual powder for a given ratio shown in Table 1, and charging 2 Kg into Attriter Model MA-l produced by Mitsui-Miike K.K., and then by using the mechanical alloying method after mixing and grinding in argon atmosphere for 3 hours. For the stirring granulation, an ordinary stirring mixer was used, and 2 wt.% of polyvinyl alcohol was added to the ground product for stirring for about 10 to 30 minutes. The thus obtained composite powder was adjusted to 45 to 10 µm in particle diameter by repeating grinding and classifying after drying at 150°C in the air for two hours, and thereby, each of composite powders shown in Table 1 was obtained. Table 1 shows composition (by vol.%) of each powder used in that test.

[0056] It it described below how to prepare sprayed coating specimens used in each test. For thermal spray, the detonation spraying method or plasma spraying method was used under the following conditions.


The characteristics of the sprayed coating layer were evaluated on build-up test (MN value), adhesive strength, heardness of coating at a cross section and heat-shock test. A purpose of the tests and testing methods are described below.

Build-up Test:

[0057] To evaluate the build-up resistance performance, a build-up test was conducted as a deposit lay-up test. Fig. 1 shows a schematic illustration of the build-up testing method. Iron oxide 2 (as build source Fe₃O₄) was located between two stainless steel plates (JIS SUS316, 30 x 50 x 5 mm) (spray coated specimens 1 with sprayed coating on each of surfaces A, B and C, and was allowed to reciprocate while applying a fixed carried load of 8.5 Kg using a roll 3 having semicircle-shape at a lateral cross section perpendicular to the roll axis. After testing, at a testing temperature of 850°C in testing atmosphere of 95% N₂-5% H₂ for four hours, the surface of the steel plates was evaluated with the following score.

score 3:

When the plate is turned around to a vertical position, the build-up source falls.

score 2:

The build-up source falls by compressed air at 6 Kg/cm².

score 1:

The build-up source falls by rubbing with hand.

score 0:

The build-up source does not fall by the above means.

Adhesive strength Evaluation Test:

[0058] To evaluate mechanical adhesive strength such as impact peeling resistance of the spray coated layer, an adhesive strength evaluation test was conducted by using the pin tester method shown in Fig. 2. After spray coating (see reference numeral 6 in Fig. 2), in a film thickness of about 500 µm on an end of sleeve 5 of 20 mm diameter, having a central through-bore in which a tapered pin 4 is inserted, the pin 4 was pulled out against a frame 7, using a tensile testing machine to determine the rupture load per unit rupture area.

Coating Hardness at a Cross Section:

[0059] To evaluate wear resistance for each of sprayed coating layers, a hardness test for each of the layers at a cross section was conducted. A micro Vickers hardness tester in the market was used, and a load of 300 gram was applied to each of the layers in order to determine Vickers hardness.

Heat-Shock Test:

[0060] To evaluate the heat-shock resistance of the sprayed coating layer, a heat-shock test was conducted. A specimen was prepared by spray coating in film thickness of 200 µm on a stainless steel plate (JIS SUS316; 50 x 50 x 10 mm) base material. After heating the specimen to a temperature of 1000°C and holding the same at the temperature within an electric oven for 20 minutes, the speciment was taken out, and was water-quenched for observing the surface. The heating and water-quenching operations were repeated 30 times at maximum. For the evaluation, the number of times for repetition until the film being peeled was used. These test results are summarized in Table 2. As can be seen from the Table 2, the coating layer of the present invention is more excellent in all of the build-up resistance, adhesive strength, cross section hardness and heat-shock resistance than conventional comparative specimens.

[0061] As will be apparent from the above, the spray coated layer using the composition of the present invention is excellent in all of the build-up resistance, adhesion, heat-shock resistance and wear resistance, and its life is hardly affected by peeling and wear. Therefore it is very useful to extend the life of steel manufacturing process parts at high temperature such as hearth rolls, and to improve the quality of steel plates which are carried by the rolls.

Claims

1. A thermal spray material which is a composite powder or a blended powder containing metal boride particles and alloy particles wherein:
   the metal boride particles constitute 5 to 50 vol. % of the thermal spray material, and each of the alloy particles contains 5 to 40 wt. %, advantageously from 15 to 40 wt. %, of chromium, 5 to 20 wt. % of aluminum, and the balance at least one of nickel, cobalt and iron, and incidental impurities.

2. A thermal spray material according to claim 1 wherein: the composite powder or the blended powder further contains at least one metal oxide selected from a group consisting of aluminum oxide, chromium oxide, titanium oxide, silicon oxide, zirconium oxide, magnesium oxide, yttrium oxide, and a rare-earth oxide, in an amount of up to 50 vol. %, advantageously from 5 to 50 vol. %, of the entire thermal spray material.

3. A thermal spray material according to claim 1 wherein the thermal spray material is a composite powder alloyed mechanically comprising 5 to 50 vol. % chromium boride and an alloy containing 5 to 40 wt. %, advantageously from 15 to 40 wt. %, of chromium, 5 to 20 wt. % of aluminium and the balance of nickel, and incidental impurities.

4. A thermal spray material according to claim 1 wherein the thermal spray material is a composite powder alloyed mechanically comprising 5 to 50 vol. % chromium boride and an alloy containing 5 to 40 wt. %, advantageously from 15 to 40 wt. %, of chromium, 5 to 20 wt. % of aluminum, less than 2.0 wt. % of silicon, less than 2.0 wt. % of yttrium and the balance of cobalt, and incidental impurities.

5. A thermal spray material which is a composite powder or a blended powder containing metal boride particles and alloy particles wherein:
   the metal boride particles constitute 5 to 50 vol. % of the thermal spray material, and each of the alloy particles contains 5 to 40 wt. %, advantageously from 15 to 40 wt. %, of chromium and 5 to 20 wt. % of aluminum, 10 wt. % or less of titanium,
   5 wt. % or less of carbon, and the balance at least one of nickel, cobalt and iron, and incidental impurities.

6. A thermal spray material according to claim 5 wherein:
   the composite powder or the blended powder further contains at least one metal oxide selected from a group consisting of aluminum oxide, chromium oxide, titanium oxide, silicon oxide, zirconium oxide, magnesium oxide, yttrium oxide, and a rare-earth oxide, in an amount of up to 50 vol. %, advantageously from 5 to 50 vol. %, of the entire thermal spray material.

7. A thermal spray material according to claim 1 or 5, wherein the metal boride particles are particles of at least one of chromium boride, zirconium boride or titanium boride.

8. A thermal spray material according to any one of claims 1 to 7, wherein each of the alloy particles contains, in the form of a uniform dispersion, 2 vol. % or less of at least one oxide selected from a group consisting of aluminium oxide, chromium oxide, titanium oxide, silicon oxide, zirconium oxide, magnesium oxide, yttrium oxide, and a rare-earth oxide.

9. A coated article which comprises a coating layer formed by spraying the thermal spray material of any one of claims 1 to 8 by using the detonation spraying method, or plasma spraying method.

10. A thermal spray material according to claim 8 wherein the thermal spray material is composite powder alloyed mechanically comprising 5 to 50 vol. % chromium boride and an alloy containing 5 to 40 wt. %, advantageously from 15 to 40 wt. %, of chromium, 5 to 20 wt. % of aluminum, less than 2.0 wt. % of silicon, less than 2.0 wt. % of yttrium, less than 2.0 wt. % of yttrium oxide and the balance of cobalt, and incidental impurities.

11. A hearth roll which has a coating layer formed by spraying the thermal spray material of any one of claims 1 to 10.

Drawing