DE69111552T2 - Coated furnace roll and process for its manufacture. - Google Patents

Abstract

A thermal spray powder feed composition for forming a refractory oxide coating comprising a mixture of zirconium silicate and zirconia at least partially stabilized with a stabilizing oxide. The as-deposited coating has a chemical composition of which, by x-ray phase analysis, comprises ZrSi04 and Zr02.x where x is selected from Ca0, Y203, Mg0, Ce02, and Hf02. <IMAGE>

Description

The invention relates to a hearth roller for use in the continuous heat treatment of steel and a method for producing this roller.

This invention is particularly concerned with the problem of providing a hearth roll with a highly wear and thermal shock resistant coating suitable for heat treating a sheet of steel, stainless steel and silicon steel in a hearth (furnace). The hearth rolls guide the sheet of steel through the hearth. The temperature in the hearth can vary from about 816ºC (1500ºF) to over 1093ºC (2000ºF) depending on the type of steel, the rate of travel of the steel sheet as it passes through the furnace and the time spent in the furnace.

A major problem encountered in heat treating operations is material transfer or buildup from the steel sheet to the hearth rolls. If buildup occurs, material will build up on the hearth rolls and damage the steel sheet being processed. To prevent this problem, frequent roll replacement is required, resulting in replacement costs and lost production. This problem has become more severe in recent years as thinner sheets are used in conjunction with higher speeds and temperatures to increase productivity.

To suppress material transfer to the hearth rolls and increase wear resistance, it is desirable to coat the hearth roll with a coating that is substantially chemically inert at elevated temperatures. An interlayer of metal or core micrometal alloy is used to prevent spalling. Spalling can also be prevented by using a graded coating in which the composition of the interlayer varies gradually from 100% alloy to 100% ceramic. Unfortunately, the ceramic coatings currently available crack during thermal cycling because there is a large difference in thermal expansion between the substrate, a heat-resistant alloy, and the coating. The alloy interlayer at the interface is oxidized in the presence of oxygen at or above 1000ºC, resulting in spalling of the ceramic layer. When a graded coating is used, the alloy component of the coating is also oxidized, which in turn increases the volume of the coating. When cooling, the Coating due to excessive stresses caused by substrate shrinkage.

EP-A-230 554 discloses a corrosion and wear resistant material produced by thermal spraying using a powder which may consist of zirconium oxide and zirconium silicate. The zirconium silicate is used as a corrosion barrier to protect the zirconium oxide by first producing a composite powder wherein the zirconium oxide is buried in the zirconium silicate. The intended application is high temperature wear resistance in an oxidizing environment, typical of a gas turbine.

FR-A-1 536 493 teaches the use of a combination of zirconium oxide and zirconium silicate together with oxides whose melting point is above 1000 ºC. This combination could be used to produce the outer layer of a multilayer coating system to be used as a protective layer in the production of acetylene or the cracking of hydrocarbons.

According to the present invention, there is provided a hearth roll having a metallic surface provided with a wear-resistant, refractory oxide coating comprising particles of zirconium silicate and/or its decomposition products SiO₂ and ZrO₂ in a mixture with particles of zirconium oxide fully or partially stabilized with an oxide selected from CaO, Y₂O₃, MgO, CeO₂ and HfO₂.

According to another aspect of the present invention there is provided a method of forming a wear and thermal shock resistant refractory oxide coating on a metallic hearth roll for use in the continuous heat treatment of steel, comprising:

a powder feed composition is prepared by mixing powder particles of zirconium silicate with powder particles of zirconium oxide which is at least partially stabilized with an oxide selected from CaO, Y₂O₃, MgO, CeO₂ and HfO₂; and

the powder feed composition is thermally sprayed onto the metallic roll to form a coating composition which, when applied, comprises ZrO₂ x, where x is one of the stabilizing oxides, ZrSiO₄ and/or its decomposition products SiO₂ and ZrO₂, as well as SiO₂, according to an X-ray phase analysis.

It has been found that a thermally sprayed coating formed from a powder feed composition of zirconium silicate and at least partially stabilized zirconium oxide to form a hearth roll has high resistance to thermal shock during thermal cycling. The powder feed composition according to the present The invention provides a coating particularly suitable for protecting the hearth roll in a continuous heat treating arrangement for heat treating layers of steel, stainless steel or silicon steel. The powder feed composition is applied by a thermal spray technique to form a coating which, when applied, has a composition comprising zirconium oxide, silicon oxide and zirconium silicate according to X-ray phase analysis. The major component of the powder composition is partially stabilized zirconium oxide, with the zirconium silicate preferably limited to a maximum of 60 wt%. For plasma spray applications, the powder feed composition should comprise at least 65 wt% stabilized zirconium oxide, the balance being essentially zirconium silicate, whereas for detonation gun applications, the powder feed composition should comprise at least 40 wt% stabilized zirconium oxide and up to 60 wt% zirconium silicate. The zirconium oxide may be either fully or partially stabilized, although partially stabilized zirconium oxide is preferred.

In accordance with the present invention, it has also been discovered that flaking of the coating at elevated temperatures above 1150°C can be prevented by thermally spraying a metallic interlayer. The preferred interlayer is a cobalt-based metal matrix comprising Co-Cr-Al-Ta-Y and a dispersion of Al₂O₃.

The present invention is based on the discovery that a powder feedstock composition consisting essentially of a mixture of zirconium silicate and zirconium oxide, wherein the zirconium oxide stabilized with an oxide such as yttria, calcia or magnesia can be thermally sprayed to form a coating which is resistant to thermal shock and which is resistant to the buildup of steel or steel oxide from a continuous heat treating assembly. Any conventional thermal spraying technique can be used to form the coating, including detonation gun deposition and plasma spray deposition. The chemical composition of the thermally sprayed coating should consist of a mixture of at least 40 wt.% zirconium oxide (ZrO2) including a stabilizer for the zirconium oxide selected from CaO, Y2O3, MgO, CeO2 and HfO2. is selected, the remainder being zirconium silicate (ZrSiO₄) and/or its decomposition products SiO₂ and ZrO₂. The preferred percentage by weight of the component in the coating is 55 to 85% stabilized ZrO₂ and 15 to 45% ZrSiO₄ and/or its decomposition products SiO₂ and ZrO₂. The optimum percentage by weight of the component oxides in the coating is 70 to 85% stabilized ZrO₂ and 15 to 30% ZrSiO₄ and/or its decomposition products. The stabilizer should constitute between 2 and 20 wt.% of the zirconium oxide component.

The coatings are preferably applied by detonation gun deposition or plasma spray deposition. A typical detonation gun consists essentially of a water-cooled barrel several meters (feet) long and about 2.5 mm (1 inch) inside diameter. In operation, a mixture of oxygen and a fuel gas, such as acetylene, in a specific ratio (usually about 1:1) is introduced into the barrel along with a charge of coating material in powder form. Gas is then ignited and the detonation wave accelerates the powder to about 730 m/s (2400 ft/sec), heating the powder to a temperature near or above its melting point. After the powder leaves the barrel, the barrel is purged with nitrogen, preparing the system for the next detonation. The cycle is then repeated several times per second.

The detonation gun deposits a circle of coating on the substrate with each detonation. The coating circles are approximately 25 mm (1 inch) in diameter and a few micrometers (a few ten thousandths of an inch) thick. Each coating circle consists of many overlapping, microscopic, thin, lens-shaped particles or patches corresponding to the individual powder particles. The overlapping patches interlock and bond with each other and the substrate without automatically alloying at the interface. The placement of the circles during coating deposition is precisely controlled to build a smooth coating of uniform thickness and to minimize heating of the substrate.

In the arc plasma spray process, an electric arc is ignited between a non-consumable electrode and a second non-consumable electrode spaced apart from the first. Gas is brought into contact with the non-consumable electrode so that it contains the arc. The gas containing the arc is constricted by a nozzle, resulting in an outflow having a high heat content. The powders used to form the coatings are introduced into the outflow nozzle and deposited on the surface to be coated. This process, described in US-A-2 858 411, produces a deposited coating that is strong, dense and adheres to the substrate. The applied coating also has irregularly shaped microscopic patches or flakes that interlock and bond to each other and to the substrate.

In general, the coating composition for the arc plasma spraying process is substantially equivalent to the corresponding composition of the starting material When a detonation gun is used to apply the feedstock, the vaporization of the constituents can result in a significantly different ratio of the constituents in the as-applied coating. Thus, when using any thermal spray process, some change in chemical composition can occur during deposition. Such changes can be compensated for by adjusting the powder composition or deposition parameters.

Due to the complex phase diagram for Zr-Si-O, the solidifying ZrSiO4 powder particles may contain ZrSiO4 as a crystallographic phase and/or ZrO2 + SiO2 as the decomposition products of the molten ZrSiO4 in separate crystallographic phases within individual patches. Thus, ZrO2 and SiO2 are closely associated with each other within each patch, which was ZrSiO4 in powder form. By "associated" is meant the extremely fine and intermingled crystalline structure of SiO2, ZrO2 and/or ZrSiO4 crystallites within the patch.

Although the coatings according to the present invention are preferably applied by detonation or plasma spray deposition, it is possible to use other thermal spray techniques, such as high velocity combustion spraying (including hypersonic jet spraying), flame spraying and so-called high velocity plasma spraying processes (including low pressure or vacuum spraying processes). As will be readily understood by those skilled in the art, other techniques can be used to deposit the coatings according to the present invention.

The thermal spray coating can be applied directly to the metal substrate. However, an intermediate layer compatible with the substrate and resistant to oxidation is preferred. An intermediate layer of a ceramic/metal alloy mixture with a cobalt-based metal matrix containing alumina is preferred. Optimum coatings are a cobalt-based alloy with dispersions of alumina as described in [US-A-4,124,737. The refractory oxide coating reacts with the preferred intermediate layer to produce an impenetrable thin layer of aluminum and/or zirconium oxide phases at the interface which prevents oxidation of the intermediate layer and provides a good bond between the intermediate layer and the ceramic layer. The impermeable layer may be less than 0.005 mm (5 µm) thick, and it is formed as a result of interdiffusion or oxidation in an environment at an elevated temperature in the presence of oxygen. A similar layer may form in an inert atmosphere as a result of the reaction between the oxide top layer and the metallic intermediate layer.

The present invention will now be described in more detail with reference to the following examples, but is in no way limited thereto.

Such an illustration of the invention in the examples (particularly Example 3) is shown in the accompanying drawing, the sole drawing showing a schematic representation of the equipment used to investigate the tendency of the as-applied coating of a hearth roll to deposit metal or metal oxides under local dynamic and static conditions.

example 1

To confirm the superiority of the coating composition according to the present invention, various powder mixtures containing yttria stabilized zirconia (ZrO2 8% Y2O3) and zirconium silicate (ZrSiO4) were mechanically mixed in the mixing ratios shown in Table I and fed to a plasma torch in a conventional manner to form a coating on a 304 stainless steel ingot. The ceramic coating was applied to one face of the 304 steel ingot, which had dimensions of 6.99 x 1.91 x 1.27 cm (23/4 x 3/4 x ½" high), and which was first coated with a detonation gun with an intermediate layer of 0.05 to 0.075 mm (50 to 75 µm) of a cobalt-based coating of 90 (Co-25Cr-7.5Al-0.8Y-10Ta) + 10% Al₂O₃. The ceramic coating was then ground to a thickness of 100 µm prior to thermal cycling.

The coating samples were heated in air to 1150ºC to 1200ºC and held at that temperature for a minimum of six hours and then air cooled. After five cycles, the samples were water quenched in the sixth cycle. If no flaking or partial flaking occurred, the coating was deemed acceptable. The results are shown in Table I below. Table I Powder mixture % Coating No. Test result Flaking after water quenching Hardness No Partial flaking* Flaked

* Powder A: Yttria stabilized zirconia (ZrO₂ 8% Y₂O₃)

* Powder B: Zirconium silicate (ZrSiO₄)

* Partial flaking indicates lifting of the ceramic layer at the edges.

From Table I, it can be seen that no spalling occurred in plasma torch applications when a powder starting composition containing greater than 65 wt.% zirconia was used, with greater than about 75 wt.% zirconia being optimal.

Example 2

Similar tests were conducted using a powder feed composition of calcium oxide stabilized zirconia (ZrO2 5CAO) and zirconium silicate in a mechanical mix, with the various powder ratios designated 8 through 12 shown in Table II below. The powder feed composition was applied by a detonation gun over a preferred interlayer of a cobalt-based metal matrix (Co-25Cr-7.5Al-10Ta-0.08Y) + (30% Al2O3). Wear performance, thermal shock resistance and anti-seizing were optimal for coatings made with mix number 8. Table II *Coating composition, wt.% Coating Water quenching from 900 ºC Cycles Cycle 50% exfoliated

* Coating composition: Based on the concentration of metallic elements measured by electron microprobe analyzer

As set forth in Examples 3, 4 and 5, further tests were conducted to confirm the superiority of a detonation gun applied coating formed on a hearth roll from a thermal spray powder feed composition comprising zirconium silicate and partially stabilized zirconia (ZrO2 5CaO) in terms of its anti-scaling properties compared to detonation gun applied coatings of conventional composition, namely (Co-25cr-7.5Al-0.8Y-10Ta) + 10% Al2O3, a coating of a feed composition of ZrSiO4 and bare steel. It should be noted that although a detonation gun deposited coating of (Co-25cr-10Ta-7.5Al-0.8Y) + 10% Al2O3 powder is a preferred intermediate layer for the ceramic coatings of this invention, it has itself been used as a hearth coating in the past and is referred to as coating number 13 in Table III for comparison purposes.

Example 3

The following experiment was conducted to prevent sticking.

Evaluation Method: A semi-circular roller simulator was used as shown in FIG. 1. The rotating piece 6 is rotated about a pivot point 7 by means of reciprocating arms 8 and 9. Two coated samples 10, 12 were mounted under the roller to simulate a reciprocating sliding movement over the samples and a static response under load. Powdered Fe₂O₃ and/or Fe powder 14 is placed on the sample 10 to make an evaluation of the dynamic setting and powdered Fe₂O₃ or Fe powder 16 is placed on the sample 12 to make an evaluation of the static attachment. The test measures the tendency of Fe or Fe₂O₃ to adhere to surfaces A, B or C, respectively, under a load of 8.5 kg.

Test conditions

Temperature: 900 ºC

Duration: 4 hours

Atmosphere: Nitrogen 98% + Hydrogen 2%

Preparation source: Fe and Fe&sub2;O&sub3;

Anti-Attachment Index: This is determined by the sum of the non-attachment points (AP) for surfaces A, B and C based on Table III below. Table III Rating Description Good Poor No adhesion Adhered material can be removed by 6 kg/cm² air flow Adhered material can be removed by rubbing with hand Adhered material cannot be removed by the above methods Results AP Point Powder Total Coating Total Uncoated Steel **

* Coating 14 was made with a powder consisting only of zirconium silicate (ZrSiO₄).

** Source material for reference: refractory cast steel

Example No. 4

Test: Wear resistance at elevated temperatures

Equipment: High temperature ball on disc wear test setup

Test conditions:

Temperature: 900 ºC

Atmosphere: N₂ 98% + H₂ 2%

Ball size: 9.252 mm diameter

Weight: 2kg

Sliding speed: 0.2 m/s

Track: 35 mm diameter

Total gliding length: 720 m

Ball: High carbon chrome bearing steel SUJ2

Disc: coated sample

Assessment of wear rate

The normalized wear rate is calculated using the following procedure.

a. Wear volume is calculated by measuring the cross-sectional area of a groove using a profilometer printout and multiplying it by the circumference of the groove.

b. The wear volume is then divided by the applied load and the total sliding length. Results Coating Wear rate mm²/kg uncoated steel

Example No. 5

Thermal shock test:

1000 ºC - 15 minutes heating from ambient temperature to 1000 ºC, quenching with water for 15 minutes - 1 cycle.

The sample substrate material was in the form of a thick plate with dimensions of 50 50 10 mm with one 50 50 side coated. The sample substrate was 304 stainless steel.

Resistance to thermal shock was evaluated by counting the number of cycles the coating can withstand without flaking. Sample Number of cycles No flaking after cycles

As can be seen from the tables and all of the above examples, the optimum powder starting mixture will necessarily vary depending on the stabilizer and thermal spray technique used. The as-applied coating should preferably contain at least about 40 wt.% stabilized zirconia and up to 60 wt.% zirconium silicate. Regardless of the crystalline structure, the preferred concentration of the component oxides in the coating should contain between about 55 and about 85 wt.% stabilized ZrO2 and 15 to 45% ZrSiO4 and/or its decomposition products SiO2 and ZrO2, with 70 to 85 wt.% stabilized ZrO2 being optimum. The concentration of the stabilizer in the as-applied coating composition should preferably be between 2 and 20 wt.% of the zirconia component.

Claims (10)

1. Hearth roll for use in the continuous heat treatment of steel, comprising a roll having a metallic surface provided with a wear-resistant, refractory oxide coating comprising particles of zirconium silicate and/or its decomposition products SiO₂ and ZrO₂ in a mixture with particles of zirconia which is fully or partially stabilized with an oxide selected from CaO, Y₂O₃, MgO, CeO₂ and HfO₂. 2. A roller according to claim 1, wherein the coating composition comprises at least 40 wt% of the stabilized zirconium oxide and up to 60 wt% of zirconium silicate and/or its decomposition products SiO₂ and ZrO₂. 3. A roller according to claim 2, wherein the particles of stabilized zirconia constitute the major component of the coating composition. 4. A process for forming a wear and thermal shock resistant refractory oxide coating on a metallic hearth roll for use in the continuous heat treatment of steel, which comprises: a powder feed composition is prepared by mixing powder particles of zirconium silicate with powder particles of zirconium oxide which is at least partially stabilized with an oxide selected from CaO, Y₂O₃, MgO, CeO₂ and HfO₂; and the powder feed composition is thermally sprayed onto the metallic roll to form a coating composition which, when applied, comprises according to an X-ray phase analysis ZrO₂ x, where x is one of the stabilizing oxides; ZrSiO₄ and/or its decomposition products SiO₂ and ZrO₂, as well as SiO₂. 5. A process according to claim 4, wherein the powder particle composition comprises at least 40 wt.% stabilized zirconium oxide and up to 60 wt.% zirconium silicate and/or its decomposition products SiO₂ and ZrO₂. 6. A method according to claim 4 or 5, wherein an intermediate layer of a ceramic/metal alloy is applied to the metallic hearth roller before the powder feed composition is thermally sprayed on. 7. The method of claim 6, wherein the ceramic/metal alloy interlayer is a cobalt-based metal matrix comprising Co-Cr-Al-Ta-Y and Al₂O₃. 8. A method according to claim 4 or 7, wherein the coating in the applied state is formed by feeding the powder feed composition through a plasma torch, the powder feed composition comprising at least 65% by weight of the partially stabilized zirconium oxide and up to 35% by weight of zirconium silicate. 9. A method according to claim 4 or 7, wherein the coating in the applied state is formed by feeding the powder feed composition via a detonation gun, the powder feed composition comprising at least 40 wt.% of the partially stabilized zirconium oxide and up to 60 wt.% zirconium silicate. 10. The method of claim 9, wherein the powder feed composition comprises at least 50 wt.% partially stabilized zirconia.