KR20060004696A - Composition gradient cermets and reactive heat treatment process for preparing same - Google Patents

Abstract

Cermets, particularly composition gradient cermets can be prepared starting with suitable bulk metal alloys by a reactive heat treatment process involving a reactive environment selected from the group consisting of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen and mixtures thereof.

Description

COMPOSITION GRADIENT CERMETS AND REACTIVE HEAT TREATMENT PROCESS FOR PREPARING SAME

The present invention broadly relates to summits, in particular compositional gradient summits and reactive heat treatment processes for making them.

Erosion resistant materials can be used in many applications where surfaces are subject to erosion. For example, in various chemical and petroleum environments interiors of refining process vessels that are exposed to aggressive fluids containing hard solid particles such as catalyst particles are subject to both erosion and corrosion. It is a technical goal to protect the interior of such vessels, especially against high temperature erosion and decomposition of corrosion-causing substances. Recently, heat resistant liners have been used for the inner walls of cyclones, such as those that require protection against the most severe erosion and corrosion, such as the inner cyclones of a fluid catalytic cracking unit (FCCU). The lifetime of these heat resistant liners is significantly limited by the mechanical attrition, cracking and spallation of the liner. Erosion-resistant materials at the current state of the art are chemically bonded castable alumina heat resistant materials. These castable alumina heat resistant materials are applied to surfaces in need of protection, hardened upon thermal curing, and attached to the surfaces through metal-anchor or metal-reinforced materials. It is also easily bonded to other heat resistant surfaces. The typical chemical composition of one commercially available heat resistant material is 80.0% Al ₂ O ₃ , 7.2% SiO ₂ , 1.0% Fe ₂ O ₃ , 4.8% MgO / CaO, 4.5% P ₂ O ₅ (% by weight) to be.

Ceramic-metal composites are called summits. Summits with adequate chemical stability can provide as much as 10 times higher erosion resistance than heat resistant materials known in the art. Summits are generally made using powder metallurgy techniques where metal and ceramic powders are mixed, compressed and sintered at high temperatures. Since powder metallurgy prepared summits usually have a homogeneous microstructure and uniform composition, a complex attachment method is required to adhere the summit on metal surfaces where surface corrosion resistance is required.

The compositional gradient summit is that one surface of the summit is ceramic-rich and the unexposed surface is a metal-rich summit. In a typical compositional gradient summit, there is a gradient in the concentration of the summit in the metal composition such that the concentration of the ceramic decreases with depth. Since it is compatible and easy to weld a substantially metallic object to another substantially metallic object, cost-effectively attaching the summit directly onto the metal or alloy surface using a method such as welding. In order to achieve this compositional gradient summit, it is desirable. In addition, such compositional gradient summits may also show good durability, especially under conditions where thermal fluctuations are present. However, there is a need for an effective method of making compositional gradient summits.

One object of the present invention is to provide a method for producing a summit, in particular a compositional gradient summit, via reactive heat treatment of the metal alloy.

It is another object of the present invention to provide compositional gradient summit products derived from reactive heat treatment processes.

These and other objects will be apparent from the description below.

Summary of the Invention

One embodiment includes heating a metal alloy containing at least one of chromium and titanium at a temperature in a range from about 600 ° C. to about 1150 ° C. to form a heated metal alloy; At least one member selected from the group consisting of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, and mixtures thereof for a time sufficient to provide a reacted alloy at a temperature in the range of about 600 ° C to about 1150 ° C. Exposing the heated metal alloy to a reactive environment; And cooling the reacted alloy to a temperature below about 40 ° C. to provide a compositional gradient summit material.

Another embodiment relates to compositional gradient summit products obtained from the disclosed reactive heat treatment process.

FIG. 1 shows the carbon activity of an environment based on the reaction CH _4- > C + 2H ₂ as compared to austenitic stainless steel (a _C equilibrium with Fe _{3 C} ). In addition, the carbon activity values of the gas mixtures applicable to the present invention are indicated.

FIG. 2 shows the mass gain by carbon incorporation (measurement of summit layer formation) of 304 stainless steel (74Fe: 18Cr: 8Ni (wt%)) as a function of CH ₄ content in H ₂ at 1100 ° C. for 3 hours. will be.

FIG. 3 shows the thickness variation of the surface summit structure in 304 stainless steel as a function of temperature in a 37.3 volume% CH ₄ : 62.7 volume% H ₂ environment over 3 hours.

FIG. 4 shows the thickness variation of surface summits formed on various Fe: Ni: Cr based high temperature alloys as a function of reaction time in 1100 ° C. and 37.3 volume% CH ₄ : 62.7 volume% H ₂ environment.

FIG. 5 shows (a) surface chromium in 310 stainless steel (54Fe: 21Ni: 25Cr (% by weight)) after reactive heat treatment at 1100 ° C. for 3 hours in a CH ₃ : 62.7% by volume H ₂ environment of 37.3% by volume. (B) Cr-rich carbide [(Cr _0.6 Fe _0.4 ) ₇ C ₃ ] and Cr-exhausted steel (63Fe: 31Ni: 6Cr (wt%) to produce a carbide-metal summit structure and (b) a composite ceramic-metal two-phase structure Scanning electron microscopy showing the expanded area on the surface showing)). In this scanning electron micrograph, the Cr-rich carbide appears dark gray and appears to be depressed because the metal is etched deeper than the carbide. These figures show the final product with the summit surface, the product of the production process of the present invention.

FIG. 6 shows (a) 55Fe: 35Cr: 10Ni (% by weight) alloy, (b) 45Fe: 45Cr after reactive heat treatment at 1100 ° C. for 24 hours in a 10 vol% CH ₄ : 90 vol% H ₂ environment. An optical micrograph is shown showing the M ₇ C ₃ (M = Cr and Fe) carbide-metal summit structure on the 10 Ni (wt%) alloy and (c) 35Fe: 55Cr: 10Ni (wt%) alloy.

FIG. 7 shows a mixed TiC and M ₇ C ₃ phase on a 60Fe: 25Cr: 10Ni: 5Ti (% by weight) alloy after reactive heat treatment at 1100 ° C. for 24 hours in a 10 vol% CH ₄ : 90 vol% H ₂ environment. M = Cr and Fe) shows an optical micrograph showing the carbide-metal summit structure.

The first step of the method of making the compositionally gradient summit material comprises heating a metal alloy containing at least one of chromium and titanium at a temperature in the range of about 600 ° C. to about 1150 ° C. to form a heated metal alloy. Metal alloys containing at least one of chromium and titanium include about 12-60% chromium, 0-10% titanium and iron, nickel, cobalt, silicon, aluminum, manganese, zirconium, hafnium, vanadium, niobium, tantalum About 30 to 88% by weight of a metal selected from the group consisting of molybdenum, tungsten and mixtures thereof. In a preferred embodiment, the major mass component of the alloy is iron. Thus, stainless steels such as types 304SS, 347SS, 321SS, 310SS and the like and iron-nickel based alloys such as Incoloy 800H are particularly suitable for the process of the invention.

The second step of the process consists essentially of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen and mixtures thereof for a time sufficient to provide a reacted alloy at a temperature in the range of about 600 ° C to about 1150 ° C. Exposing the heated metal alloy to a reactive environment comprising at least one member selected from the group.

If the reactive environment is a reactive carbon environment, it is believed that the carbonization reaction occurs. While not intending to be bound by the mechanisms of the reactive heat treatment process, Applicants believe that this carbonization process is performed on chromium-rich and titanium carbide phases such as Cr ₇ C ₃ , Cr ₂₃ C ₆ , (Cr _0.6 Fe) on the alloy surface and in the alloy matrix. _0.4 ) ₇ C ₃ , (Cr _0.6 Fe _0.4 ) ₂₃ C ₆ and TiC is considered to result in the generation of summits and in particular compositional gradient carbide summits.

A reactive carbon environment is defined as an environment where the thermodynamic activity (a _C ) of carbon in the environment is greater than the thermodynamic activity of the alloy:

(a _C ) Environment> (a _C ) Metal

Suitable reactive carbon environments for the present invention include one or more of CO, CH ₄ , C ₂ H ₆ and C ₃ H ₈ . The reactive carbon environment can optionally include H ₂ . The reactive carbon environment may further comprise O ₂ , CO ₂ and H ₂ O. The following reactions [1], [2] and [3], shown below, are some of the reactions which are believed to occur under heat treatment conditions to provide reactive carbon. Carbon reacts with the metal surface to form chromium-rich and titanium-rich carbide phases.

CO + H ₂ ---> C + H ₂ O [1]

2CO ---> C + CO ₂ [2]

CH ₄ ---> C + 2H ₂ [3]

When heat treatment is performed after reaction [3], the carbon activity (a _C ) in the environment is given by Equation 1:

Where

G ⁰ is the free energy of activity, R is the gas constant, T is the temperature in Kelvin, and P is the partial pressure of the individual gas methane and hydrogen. Carbon activity as a function of (P _{CH 4} / P ² _H ² ) is shown in FIG. 1, where the preferred range of P _{CH 4} / P ² _H ² in the process of the invention is shown.

When a mixture of methane and hydrogen is used to provide a reactive carbon environment, the methane content in the gas mixture of methane and hydrogen is from about 1% to about 99% by volume, preferably from about 2% by volume to about It may range from 45% by volume. This indicates that the mass gain by carbon incorporation (measurement of the summit layer formation) of 304 stainless steel (74Fe: 18Cr: 8Ni (wt%)) exposed for 3 hours at 1100 ° C. is fluttered as a function of the CH ₄ content in H ₂ . 2 is shown. Preferred methane content in the gas mixture of methane and hydrogen corresponds to the high plateau region of the curve. In this region, the reaction time for obtaining a summit of a certain thickness is shorter. Gas mixtures with a methane content in the gas mixture of methane and hydrogen of greater than 45% by volume can also be used. However, in this range, deposition of solid carbon on the alloy surface can occur as shown by the sharp increase in the mass gain of FIG. 2.

When a mixture of CO and hydrogen is used to provide a reactive carbon environment, the CO content in the gas mixture of CO and hydrogen is from about 0.1% to about 5% by volume, preferably from about 0.1% to about 2% by volume. It can be a range.

If the reactive environment is a reactive nitrogen environment, it is believed that the nitriding reaction occurs. While not wishing to be bound by the mechanisms of the reactive heat treatment process, Applicants note that this nitriding process results in precipitation of chromium-rich and titanium nitride phases, such as Cr ₂ N and TiN, on the alloy surface and in the alloy matrix, resulting in summit and In particular, it is considered to produce a compositional gradient nitride summit.

A reactive nitrogen environment is defined as an environment in which the thermodynamic activity (a _N ) of nitrogen in the environment is greater than the thermodynamic activity of the alloy:

(a _N ) Environment> (a _N ) Metal

Since molecular nitrogen is relatively inert from the nitriding point of the alloy, an ammonia-containing atmosphere is preferred. Ammonia is metastable and dissociates into molecules N ₂ and H ₂ upon heating to elevated temperature. Preferred compositions of the reactive nitrogen environment include one or more of air, ammonia and nitrogen. The composition may further comprise H ₂ , He and Ar. Alloys containing elements such as Cr and Ti, which have a strong chemical affinity for nitrogen in this reactive nitrogen environment in the range of 600 ° C. to 1150 ° C., undergo rapid nitriding reactions. In order to increase nitrogen absorption by the alloy, the molecule NH ₃ is preferably dissociated on the alloy surface, thus allowing dissociated atomic nitrogen to dissolve at the surface and diffuse into the bulk of the metal alloy. Similar to the carbonization process, nitriding results in the formation of internal nitrides and surface nitrides in the matrix and at grain boundaries near the alloy surface.

When a mixture of ammonia and hydrogen is used to provide a reactive nitrogen environment, the ammonia content in the gas mixture of ammonia and hydrogen may be from about 1% to about 99% by volume, preferably from about 2% to about 70% by volume. It can be a range.

The preferred temperature range for achieving conversion of metal alloys containing chromium, titanium and mixtures thereof to nitride summits is in the range of about 600 ° C to about 1150 ° C.

When the reactive environment comprises a mixture of reactive carbon and reactive nitrogen, a mixed composition gradient summit is produced that includes carbide, nitride, carbonitride, and mixtures thereof. When the reactive environment is a reactive carbon and reactive nitrogen environment, it is believed that carbonitridation reactions occur. While not intending to be bound by the mechanisms of the reactive heat treatment process, Applicants note that this carbonitriding process results in precipitation of chromium-rich and titanium carbonitride phases, such as Cr ₂ CN and TiN, on the alloy surface and in the alloy matrix, And in particular for producing compositional gradient carbonitride summits.

A reactive carbon and nitrogen environment is defined as an environment in which the thermodynamic activities (a _C ) and (a _N ) of carbon and nitrogen in the environment are greater than the thermodynamic activity of the alloy. Preferred compositions of the reactive carbon and nitrogen environment include at least one of ammonia and nitrogen and at least one of CO, CH ₄ , C ₂ H ₆ and C ₃ H ₈ . The composition may further comprise H ₂ , He and Ar. Alloys containing elements such as Cr and Ti, which have a strong chemical affinity for carbon and nitrogen in these reactive carbon and nitrogen environments at temperatures in the range of 600 ° C. to 1150 ° C., undergo rapid carbonitriding reactions. Similar to the carbonization or nitriding process, carbonitriding results in the formation of internal carbonitride and surface carbonitride in the matrix and at grain boundaries near the alloy surface.

If the reactive environment is a reactive boron environment, the boridation reaction is believed to occur. While not wishing to be bound by the mechanisms of the reactive heat treatment process, Applicants note that this boration process results in precipitation of chromium-rich and titanium boride phases, such as Cr ₂ B and TiB ₂ , on the alloy surface and in the alloy matrix, And in particular for generating compositional gradient boride summits.

A reactive boron environment is defined as an environment in which the thermodynamic activity (a _B ) of boron in the environment is greater than the thermodynamic activity of the alloy. Preferred compositions of the reactive boron environment include, for example, one or more of diborane (B ₂ H ₆ ), BCl ₃ and BF ₃ . The composition may further comprise H ₂ , He and Ar. Alloys containing elements such as Cr and Ti, which have a strong chemical affinity for boron in such reactive boron at temperatures ranging from 600 ° C. to 1150 ° C., undergo rapid boration reactions. Similar to the carbonization or nitriding process, boration results in the formation of internal borides and surface borides in the matrix and at grain boundaries near the alloy surface.

When the reactive environment is a reactive oxygen environment, it is believed that the oxidation reaction occurs. While not wishing to be bound by the mechanisms of the reactive heat treatment process, Applicants believe that this oxidation process is performed on chromium-rich and titanium oxide phases such as (Cr, Fe) ₂ O ₃ , Cr ₂ O ₃ and on the alloy surface and in the alloy matrix. It is believed to result in precipitation of TiO ₂ , resulting in summits and in particular compositional gradient oxide summits.

The reactive oxygen environment is defined as an environment in which the partial pressure of oxygen in the environment is larger than the oxygen partial pressure in equilibrium with the oxide. Preferred compositions of the reactive oxygen environment include one or more of air, oxygen and CO ₂ . The composition may further comprise H ₂ , He and Ar. Alloys containing elements such as Cr and Ti, which have a strong chemical affinity for oxygen in this reactive oxygen at temperatures in the range of 600 ° C. to 1150 ° C., undergo rapid oxidation reactions. Similar to the carbonization or nitriding process, oxidation results in the formation of internal oxides and surface oxides in the matrix and at grain boundaries near the alloy surface.

The third step of the production process is the cooling of the reacted alloy. The cooling step may include various cooling rates and / or intermediate temperatures maintained below about 40 ° C. prior to cooling. In one embodiment, the cooling step comprises cooling the reacted alloy at a rate ranging from 0.5 ° C./sec to 25 ° C./sec. In another embodiment, the cooling step comprises cooling the reacted alloy to a temperature in the range of 500 ° C. to 100 ° C., for a time of 5 minutes to 10 hours at any temperature in the range of 500 ° C. to 100 ° C. Maintaining the temperature and then cooling to a rate in the range of from 0.5 ° C./sec to 25 ° C./sec to less than about 40 ° C. Applicants believe that this preferred cooling profile has manufacturing methods and product advantages.

The exposure time (period during which the heated alloy is exposed to the reactive environment) varies from about 1 hour to 800 hours, to achieve various thicknesses of carbide, nitride, carbonitride, boride or oxide summit on the metal alloy surface. Can be. Examples of carbide summits are that the thickness of the surface carbide summits formed on various Fe: Ni: Cr high temperature alloys is a function of exposure time at 1100 ° C. in a CH ₂ : 62.7 volume% H ₂ environment at 37.3 volume% 4 is shown. As such, this example shows that the methods of the present invention can be used to obtain carbide thicknesses of any thickness that produce compositional gradient carbide summits. Alternatively, in this method of manufacture, it is also possible to use the entire bulk of chromium, titanium or mixtures of chromium and titanium comprising the alloy to completely convert to a compositional gradient summit where a gradient occurs at the entire thickness of the bulk alloy.

The thickness of the summit layer can be controlled by the composition of the reaction environment, the temperature and the exposure time. Exposure time can be measured experimentally as shown in FIG. 4 for the carbide summit. In thinner layers the exposure time will be smaller and in thicker layers the exposure time will be larger. Typical exposure times for carbide summits may range from about 1 hour to about 500 hours, preferably from about 5 hours to about 300 hours, more preferably from about 10 hours to about 200 hours. As such, exposure time and temperature are two variables that can provide the thickness of the desired summit and the desired compositional gradient summit. In nitride summits, typical exposure times may range from about 1 hour to about 800 hours, preferably from about 5 hours to about 500 hours, more preferably from about 10 hours to about 300 hours. As such, exposure time and temperature are two variables that can provide the thickness of the desired nitride summit and the desired compositional gradient nitride summit.

The thickness of a typical layer or summit structure can range from about 100 microns or more to the thickness at which the metal alloy can act, preferably from about 5 mm to about 30 mm, more preferably from about 5 mm to about 20 mm. The layer thickness can be measured by electron microscopy known to those skilled in the art of electron microscopy.

The invention is also applicable to articles composed of chromium-rich or titanium-rich carbides, nitrides, carbonitrides, borides and oxides in combination with chromium and titanium containing metal alloys.

In the reactive heat treatment process of the present invention, as shown in FIG. 4, a compositional gradient summit having erosion resistance superior to that of untreated alloys containing chromium, titanium and mixtures thereof is formed. This is because the erosion resistance of the alloy improves as the summit layer develops to provide hardening. In the present invention, certain amounts of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, which diffuse from the individual reaction environment to metal alloys containing chromium, titanium, and mixtures thereof are used to create the compositional gradient summit. Some of the alloys containing chromium, titanium and mixtures thereof that have not been converted to summits remain unchanged and retain their physical properties prior to treatment in accordance with the present invention. This compositional gradient structure is particularly desirable when it is desired to use welding as a method of attachment of carbide summits to the surface. In addition, the compositional gradient summit can have good thermal expansion matching with the underlying metallic substrate with good durability under thermal fluctuations. Thus, the summit layer provides erosion resistance while maintaining physical properties on the adhesion and mechanical reliability of the alloy.

Compositional gradient summits prepared by the process of the invention can be used at temperatures in the range of 300 ° C. to 800 ° C. to protect any steel or any other alloy surface exposed to severe erosion and wear. Some non-limiting examples of such uses include protective lining, lining tiles for fluid-solid separation cyclones, wear plates, nozzles and grid hole inserts, turbines, such as in cyclones of fluid catalytic cracking units used in the refining industry. Components that are placed on the blade and the eroded flow stream. In such use, the compositional gradient summits produced by the manufacturing methods of the present invention provide a combination of erosion resistance and toughness as well as optimization of thermal stress in the composition. Compared with conventional summits made through powder metallurgy techniques, this provides better thermal expansion matching to the base steel as well as adhesion through conventional welding techniques. It is also possible to provide an excellent way to protect the turbine blades from both oxidation and erosion.

Another embodiment of the invention comprises heating a metal alloy containing at least one of chromium and titanium at a temperature in the range of about 600 ° C. to about 1150 ° C. to form a heated metal alloy; At least one member selected from the group consisting of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, and mixtures thereof for a time sufficient to provide a reacted alloy at a temperature in the range of about 600 ° C to about 1150 ° C. Exposing the heated metal alloy to a reactive environment; And cooling the reacted alloy to a temperature below about 40 [deg.] C ..

The manufacturing method of the present invention can be applied to any surface. For example, at any temperature the inner surface of any chemical or petroleum processing reactor consisting of a metal selected from the group consisting essentially of chromium, titanium and mixtures thereof is heated to a temperature in the range of about 600 ° C to about 1150 ° C, Exposure to a reactive environment selected from the group consisting essentially of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, and mixtures thereof for a time sufficient to provide a reacted interior surface at a temperature in a range from about 600 ° C. to about 1150 ° C. Can be. Upon cooling to a temperature below about 40 ° C., the compositional gradient summit material forms on the inner surface of the reactor. The inner surface of the reactor including the compositional gradient summit may exhibit enhanced erosion resistance. One non-limiting example of such use is a cyclone separator of a fluid catalytic cracking unit in oil refining.

As another example, the surface of any object, such as a blade of a turbine, consists of a metal selected from the group consisting essentially of chromium, titanium and mixtures thereof at a predetermined temperature, and at a temperature in the range of about 600 ° C to about 1150 ° C. After being heated, selected from the group consisting essentially of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, and mixtures thereof for a time sufficient to provide a heat treated object at a temperature in the range of about 600 ° C. to about 1150 ° C. Exposure to reactive environments. Upon cooling to a temperature below about 40 ° C., the compositional gradient summit material forms on the surface of the object exposed to the reactive environment.

Summit compositions prepared by the process of the present invention have enhanced erosion and corrosion properties. Erosion rates were measured by the High Temperature Erosion and Wear Test (HEAT) described in the Examples section of this specification. The erosion rate of the gradient summit produced by the process of the present invention is less than 1.0 × 10 ⁻⁶ cc / (1 g of SiC erosion agent). Corrosion rates were determined by thermogravimetric analysis (TGA) analysis described in the Examples section of this specification. The corrosion rate of the gradient summit prepared by the process of the invention is less than 1.0 × ^{10 −10} g ² / cm ⁴ sec.

The summit composition prepared by the process of the invention has a fracture toughness greater than about 3 MPa · m ^1/2 , preferably greater than about 5 MPa · m ^1/2 , more preferably greater than about 10 MPa · m ^1/2. . Fracture toughness is the ability to resist crack propagation in materials under constant loading conditions. Fracture toughness is defined as the critical stress intensity factor at which cracks propagate in an unstable manner in the material. It is desirable to measure the fracture toughness according to the fracture mechanics theory using loads in a three-point bend geometry with pre-crack on the tensile side of the bend sample. The summit of the present invention may be attached to the metal surface by mechanical means or by welding.

The following non-limiting examples are included to further illustrate the present invention.

Example 1 Reactive Heat Treatment of Commercial Alloys

Reactive heat treatments were performed on selected chromium-containing commercially available alloys 304SS, 310SS, Heynes HR120 and Inconel 353MA. Nominal composition is provided below.

The sample had a rectangular geometry with dimensions of about 1.25 cm x 1.25 cm x 1 cm. The sample surface was polished with 600 grit SiC finish and ultrasonically cleaned in acetone. The procedure used in the present invention is intended to establish the kinetics of carbonization of selected alloys in a purely carbonized environment (CH ₄ -H ₂ ), measured thermogravimetrically in a Cahn 1000 thermogravimetric unit. will be. Irradiation was carried out in the temperature range of 800 ° C to about 1160 ° C. The coupon was heated to a temperature of 1100 ° C. in a hydrogen environment in a vertical quartz reactor tube and held at that temperature for about 5 minutes. At this time, and changing the configuration, 37.3% by volume of CH ₄ -62.7% by volume of H ₂ in. After 3 hours from exposure, the metal sample was cooled by lowering the furnace surrounding the quartz reactor. After bringing the sample to room temperature, the surface microstructures were examined by scanning electron microscopy. "Bal" means the balance of the metal in the component composition.

FIG. 5A shows a 400 micron thick chromium carbide-metal summit layer, 310 stainless steels (54Fe: 21Ni: 25Cr) after reaction heat treatment at 3100 vol% CH ₄ : 62.7 vol% H ₂ environment for 3 hours at 1100 ° C. FIG. (% By weight)) on the surface. Summit microstructures show that Cr-rich carbide [(Cr _0.6 Fe _0.4 ) ₇ C ₃ ] and Cr-exhausted steel (63Fe: 31Ni: 6Cr (wt%)) produce composite ceramic-metal two-phase structures An enlarged view is shown in FIG. 5B. Cr-rich means that the metal Cr is in a higher proportion by weight than the other component metals containing M with 54Fe: 21Ni: 25Cr (wt%). In these scanning electron micrographs, Cr-rich carbides appear dark gray, and because the metal is etched deeper than the carbides, they appear markedly depressed. These figures show the final product with the summit surface made in accordance with the present invention. Changing the exposure duration in the carbon gas environment changes the thickness of the cementite layer. This is shown in the graph of FIG.

Example 2 Reactive Heat Treatment of Commercial Alloys

The chromium containing alloys described above were each heat treated in a tube furnace for 24 hours at 1100 ° C. in a 10 volume% CH ₄ : 90 volume% H ₂ environment. The sample was heated to a temperature of 1100 ° C. in a hydrogen environment and held at that temperature for about 5 minutes. After 24 hours from exposure, the alloy sample was cooled. After the sample reached room temperature (25 ° C.), the surface microstructure and thickness of the summit layers formed on the various alloy surfaces were examined by cross-sectional scanning electron microscopy. The chemical composition of the M ₇ C ₃ carbide phase and the Cr-exhausted binder phase was investigated by semi-quantitative energy dispersive x-ray spectroscopy. The tendency for Fe and Ni to partition between the metal matrix and the carbide precipitate is expected to be different. The thickness of the summit layer, the Cr and Fe content on the M ₇ C ₃ carbide and the composition on the Cr-exhausted metal matrix in the summit layer are summarized below.

Example 3 Reactive Heat Treatment of Custom-Made Alloys

Alloys containing different concentrations of Fe, Ni, Cr and Ti were prepared by arc melting. The arc-melted alloy buttons were annealed at 1100 ° C. overnight in an inert argon atmosphere and furnace-cooled to room temperature. A cubic sample of about 1.25 cm x about 1.25 cm x about 0.75 cm was cut from the button. Sample faces were polished with a 600-grit finish and washed in acetone. Samples were exposed to 10 volume% CH ₄ : 90 volume% H ₂ gas environment at 1100 ° C. for 24 hours.

Detailed electron microscopy and chemical analysis of the alloy after exposure showed that a specific alloy composition in the Fe—Ni—Cr system produced a summit structure with M ₇ C ₃ carbide and metal phase. The thickness of the summit layer, the Cr and Fe content on the M ₇ C ₃ carbide and the composition on the Cr-exhausted metal matrix in the summit layer are summarized in Table 3.

Unlike the examples of commercially available alloys selected, a relatively thick summit layer was obtained, and the Cr concentration in the metal matrix phase formed in the Fe—Ni—Cr system was relatively rich. Higher Cr concentrations in the metal phase promote oxidation resistance at higher temperatures. The optical microscopy image shown in FIG. 6 shows M ₇ C ₃ (M = Cr and Fe) carbide in the surface area after reactive heat treatment at 1100 ° C. for 24 hours at 10 vol% CH ₄ : 90 vol% H ₂ . Shows the size and shape of the metal summit structure.

An alloy of 60Fe: 25Cr: 10Ni: 5Ti (% by weight) composition produced a summit structure with mixed TiC and M ₇ C ₃ carbide and metal phases. The thickness of the summit layer, the Cr and Fe content on the M ₇ C ₃ carbide and the composition on the Cr-exhausted metal matrix in the summit layer are summarized in Table 3. The optical microscopy image shown in FIG. 7 shows mixed TiC and M ₇ C ₃ (M = Cr) in the surface area after reactive heat treatment at 1100 ° C. for 24 hours at 10 vol% CH ₄ : 90 vol% H ₂ . And Fe) the size and shape of the carbide-metal summit structure.

Example 4 Erosion Test

Reactive heat treatments were performed on a commercially available 310SS to prepare samples for high temperature erosion and wear tests (HEAT). The 310SS sample had a rectangular geometry with dimensions of about 2.0 inches by 2.0 inches by 0.5 inches. One sample was subjected to reactive heat treatment in a tube furnace for 138 hours at 1100 ° C. in an environment of 10 vol% CH ₄ : 90 vol% H ₂ and named C310SS1100. The other samples were reactive heat treated in a tube furnace for 96 hours at 1150 ° C. in an environment of 10 vol% CH ₄ : 90 vol% H ₂ and named C310SS1150.

The erosion rate was measured as the volume of summed, heat resistant, or comparative material removed per unit mass of erosive particles of defined average size and shape incorporated into the gas stream, and the unit was cc / g (eg, <0.001 cc / 1000 g of SiC). Important limited erosion test conditions are erosion material and size distribution, velocity, mass flux, impact angle of the erosion agent as well as erosion test temperature and chemical environment.

High temperature erosion and abrasion test (HEAT) measured the erosion loss of the summit. The carrier gas and the atmosphere (preferably air) that simulate the desired use were heated to the same temperature. The HEAT test was preferably operated as follows. In a preferred operation of the HEAT test, the summit sample blocks (C310SS1100 and C310SS1150) of about 2 inches ² and about 0.5 inches thick were weighed with an accuracy of +/- 0.01 mg. The center of one side of the block was treated with 1200 g / min of SiC particles incorporated in the air jet coming from the riser tube having a 0.5 inch diameter, the ends of the riser tube 1 inch away from the target disk. The 58 μm angled SiC particles used as erosion agents were 220 grit # 1 Grade Black Silicon Carbide (UK Abrasives, Inc., Northbrook, Ill.). The erosion velocity impinging on the summit target is 45.7 m / sec (150 ft / sec), and the impingement angle of the gas erosion stream against the target is 45 ° between the major axis of the riser tube and the surface of the sample disc. It was +/- 5 degrees, Preferably it is 45 degrees +/- 2 degrees. Carrier gas was air in all tests. The erosion test in the HEAT unit was conducted at 732 ° C. (1350 ° F.) for 7 hours. After the test, the summit specimens were reweighed with an accuracy of +/- 0.01 mg to determine the weight loss. The erosion rate was equal to the volume of material removed per unit mass of erosion particles incorporated in the gas stream, and is in cc / g. The improvement in Table 4 is the reduction in weight loss due to erosion compared to the value of 1.0 for the baseline RESCOBOND ^™ AA-22S (Pittsburgh Lesco Products, Inc.). Typically AA-22S comprises at least 80.0% Al ₂ O ₃ , 7.2% SiO ₂ , 1.0% Fe ₂ O ₃ , 4.8% MgO / CaO, 4.5% P ₂ O ₅ (% by weight). . Micrographs of the eroded surface were collected by electron microscopy to determine the damage mechanism. Table 4 summarizes the erosion losses of selected summits measured by HEAT.

The HEAT test measures very aggressive erosive particle. More typical particles are softer, resulting in lower erosion rates. For example, FCCU catalysts are based on alumina silicates that are typically softer than alumina, which is typically much softer than SiC.

Example 5 : Corrosion Test

Each summ of Example 4 was subjected to an oxidation test. The procedure used is as follows:

1) Sample summits about 10 mm ² and about 1 mm thick were polished with 600 grit diamond finish and washed in acetone.

2) The specimens were then exposed to 100 cc / min air at 800 ° C. on a thermogravimetric analyzer (TGA).

3) Step (2) was carried out at 800 ° C. for 65 hours.

4) After 65 hours, the specimen was cooled to ambient temperature.

5) The thickness of the oxide scale was measured by cross-sectional microscopic examination of the corrosion surface.

The thickness of the oxide scale ranged from about 0.5 μm to about 1.5 μm. The summit composition showed a corrosion rate of less than about 1 × ^{10 −11} g ² / cm ⁴ · s or an average oxide scale of less than 30 μm when treated with 100 cc / min air at 800 ° C. for at least 65 hours. They show good corrosion resistance.

Claims (26)

Heating the metal alloy containing at least one of chromium and titanium at a temperature in the range of about 600 ° C. to about 1150 ° C. to form a heated metal alloy; At least one member selected from the group consisting of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, and mixtures thereof for a time sufficient to provide a reacted alloy at a temperature in the range of about 600 ° C to about 1150 ° C. Exposing the heated metal alloy to a reactive environment; And Cooling the reacted alloy to a temperature below about 40 ° C. to provide a compositional gradient summit material, A method of making a compositional gradient summit material. The method of claim 1, The metal alloy is 12 to 60% by weight of chromium, 0 to 10% by weight of titanium and iron, nickel, cobalt, silicon, aluminum, manganese, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten and mixtures thereof 30 to 88% by weight of a metal selected from the group consisting of. The method of claim 1, The metal alloy comprises 12 to 60 wt% chromium, 0 to 10 wt% titanium and 30 to 88 wt% stainless steel. The method of claim 1, Wherein said reactive environment is a reactive carbon environment comprising at least one of CO, CH ₄ , C ₂ H ₆ and C ₃ H ₈ . The method of claim 1, Wherein said exposing step is carried out for a time of about 1 hour to 800 hours. The method of claim 4, wherein Wherein said exposing step is carried out for a period of time providing a reacted alloy comprising precipitated chromium-rich carbide, titanium carbide and a mixture of chromium-rich and titanium carbide. The method of claim 6, Wherein the chromium-rich carbide comprises Cr ₇ C ₃ , Cr ₂₃ C ₆ , (Cr _0.6 Fe _0.4 ) ₇ C ₃ , (Cr _0.6 Fe _0.4 ) ₂₃ C _6, and mixtures thereof. The method of claim 6, The titanium carbide comprises TiC. The method of claim 1, Wherein said exposing step is carried out for a time sufficient to form a reacted alloy layer of thickness of about 100 microns to about 30 mm on the surface of the metal alloy or in the bulk matrix. The method of claim 1, Wherein said exposing step is carried out for a time such that the reacted alloy is thick including the entire depth of said metal alloy. The method of claim 1, Wherein said cooling step comprises cooling the reacted alloy at a rate in the range of 0.5 ° C./sec to 25 ° C./sec. The method of claim 1, Cooling the alloy to which the cooling step is reacted to a temperature in the range of 500 ° C. to 100 ° C., maintaining the temperature for a time of 5 minutes to 10 hours at any temperature in the range of 500 ° C. to 100 ° C. And thereafter cooling to less than about 40 ° C. at a rate in the range of from 0.5 ° C./sec to 25 ° C./sec. The method of claim 1, Wherein said reactive nitrogen environment comprises at least one of air, ammonia and nitrogen. The method of claim 13, Wherein said exposing step is carried out for a period of time providing a reacted alloy comprising precipitated chromium-rich nitride, titanium nitride and a mixture of chromium-rich and titanium nitride. The method of claim 14, And the chromium-rich nitride comprises Cr ₂ N. The method of claim 14, The titanium nitride comprises TiN. The method of claim 1, Wherein said reactive carbon and nitrogen environment comprises at least one of ammonia and nitrogen and at least one of CO, CH ₄ , C ₂ H ₆ and C ₃ H ₈ . The method of claim 1, Wherein said reactive boron environment comprises at least one of B ₂ H ₆ , BCl ₃ and BF ₃ . The method of claim 1, Wherein said reactive oxygen environment comprises at least one of air, CO ₂ and oxygen. Heating a metal alloy containing at least one of chromium, titanium, and mixtures thereof at a temperature in the range of about 600 ° C. to about 1150 ° C. to form a heated metal alloy; At least one member selected from the group consisting of reactive carbon, reactive nitrogen, reactive boron, reactive oxygen, and mixtures thereof for a time sufficient to provide a reacted alloy at a temperature in the range of about 600 ° C to about 1150 ° C. Exposing the heated metal alloy to a reactive environment; And Cooling the reacted alloy to a temperature below about 40 ° C. A composition gradient summit product prepared by a manufacturing method comprising a. 20. A compositional gradient summit product having a fracture toughness of greater than about 3 MPa m ^1/2 prepared by the process of any one of claims 1-19. 20. A compositional gradient summit product having an erosion rate of less than about 1.0 × 10 ⁻⁶ cc / (1 g of SiC erosion agent) prepared by the process of any one of claims 1-19. A corrosion rate of less than about 1 × ^{10 −10} g ² / cm ⁴ · s when treated with 100 cc / min of air for at least 65 hours at 800 ° C., prepared by the process of any one of claims 1 to 22 or Compositional gradient summit products having an average oxide scale of less than 150 μm in thickness. 24. A method of protecting a metal surface exposed to erosive materials at a temperature in the range of 850 ° C or less, comprising providing the summit composition according to any of claims 20 to 23 to the metal surface. 24. A method of protecting a metal surface exposed to erosive material at a temperature in the range of 300 ° C to 850 ° C, comprising providing a summation composition according to any of claims 20 to 23. The method of claim 24, Wherein said surface comprises an inner surface of a fluid-solid separation cyclone.

Images