MXPA98008784A - Superficial alloy for high temperature alloys - Google Patents

Abstract

A surface alloy component is provided which comprises a base alloy with a diffusion barrier layer enriched in silicon and chromium being provided adjacent thereto. An enriched collection layer is created adjacent to said diffusion barrier and contains silicon and chromium and optionally titanium or aluminum. A reactive gas treatment could be used to generate a protective layer available on the outer surface of said component

Description

SUPERFICIAL ALLOY FOR HIGH TEMPERATURE ALLOYS.

BACKGROUND OF THE INVENTION (i) Field of the Invention The present invention relates to coating systems for the generation of protective surface alloys for high temperature metal alloy products. More specifically, the coating systems generate surface alloys having functionally controlled microstructures to impart predetermined beneficial properties to said alloy products including improved coking resistance, carburization strength and product longevity. (ii) Description of the Related Art Stainless steels are a group of alloys based on iron, nickel and chromium as the main constituents, with additives that may include carbon, niobium, titanium, molybdenum, manganese and silicon to achieve specific structures and properties. The main types are known as martensí, ferritic, double and REF .: 028523 austenitic. Austenitic stainless steel is used in general when high effort and high resistance to corrosion is required. A group of such steels is collectively known as high temperature alloys (HTAs) and is used in industrial processes that operate at elevated temperatures generally above 650 ° C and extend up to the temperature limits of ferrous metallurgy at approximately 1150 ° C. The main austenitic alloys used have a chromium, nickel and iron composition in the range of 18 to 38% by weight of chromium, 18 to 48% by weight of nickel, the remainder of iron and alloy additives.

The mass composition of HTAs is built with respect to physical properties such as resistance to stretching and stress, and chemical properties of the surface such as corrosion resistance. Corrosion takes many forms depending on the operating environment and includes carburization, oxidation and sulfidation. The protection of the composition of the alloy is often provided by the surface that is enriched in chromium oxide. The specific compositions of the alloys used represent an optimization of physical properties (mass) and chemical properties (surface). The ability to apply the chemical properties of the surface through a surface alloy and the physical properties through the mass composition, would provide great opportunities to improve the development of the materials in many industrial environments of important services.

The surface alloy can be carried out using a variety of coating processes to release the correct combination of materials to the surface of the component at an appropriate speed. These materials would need to be alloyed with the volume matrix in a controlled manner that results in a microstructure capable of providing the pre-constructed or desired benefits. This would require control of the relative interdifusion of all constituents and the evolution of the global phase. Once formed, the surface alloy can be activated or reactivated, as required, by a reactive gaseous thermal treatment. Because surface alloy and surface activation require considerable mobility of atomic constituents, that is, temperatures greater than 700 ° C, HTA products can benefit most of the process due to their designed operating capacity at elevated temperatures. The procedure can also be used on products designed for lower operating temperatures, but may require a subsequent thermal treatment after surface alloying and activation to restore physical properties.

Surface alloys or coating systems can be developed to provide a full range of benefits to the end user, starting with a commercial base alloy chemical composition and adapting the coating system to establish the specific development requirements. Some of the properties that can be developed in such systems include: corrosion resistance of higher hot gases (carburization, oxidation, sulfidation); controlled catalytic activity; and resistance to hot erosion.

Two metal oxides are mainly used to protect the alloys at high temperatures, specifically eromia and alumina, or a mixture of the two. The compositions of stainless steels for high temperature use are adapted to provide a balance between good mechanical properties and good resistance to oxidation and corrosion. Compositions that can provide an alumina layer are favored when good resistance to oxidation is required, while compositions capable of forming a chromia layer are selected for resistance to hot corrosive conditions. Unfortunately, the addition of high levels of aluminum and chromium to the composition of the alloy is not compatible with the retention of good mechanical properties and coatings containing aluminum and / or chromium are normally applied over the composition of the alloy to provide the desired surface oxide.

One of the most important industrial processes from a material perspective is the production of olefins such as ethylene by pyrolysis of a hydrocarbon stream (thermal fractionation). The feed stream of hydrocarbons such as ethane, propane, butane or naphtha, is mixed with the stream and passes through a top-feeding furnace made of welded pipes and fittings. The upper feed furnace is heated on the outer wall and heat is conducted to the surface of the inner wall leading to pyrolysis of the hydrocarbon feed to produce the desired product mixture. An undesirable side effect of the process is the accumulation of coke (carbon) on the surface of the internal wall of the kiln. There are two main types of coke: catalytic coke (or filamentous coke) that grows in long threads when promoted by a catalyst such as nickel or iron, and amorphous coke which forms in the gas phase and leaves the gas stream. In the thermal fractionation of light feeding, the catalytic coke can have 80 to 90% of the deposit and provides a large surface area for the collection of amorphous coke.

The coke can act as a thermal insulator, requiring a continuous increase in the temperature of the external wall of the tube to maintain constant feeding. A point is reached when the accumulated coke is so large that the surface temperature of the tube can no longer be increased and the upper feed furnace is taken independently to remove the coke by combustion (decoking). The decoking operation typically lasts for 24 to 96 hours and is necessary once every 10 to 90 days for light-weight ovens and considerably larger for operations in heavy-duty ovens. During a decoking period, there is no commercial production that represents a greater economic loss. Additionally, the decoking process degrades the tubes at an accelerated rate, leading to a shorter period of time. In addition to the inefficiencies introduced to the operation, the formation of coke also leads to accelerated carburization, other forms of corrosion and erosion of the inner wall of the tube. Carburization results from the diffusion of carbon in the steel forming brittle carbide phases. This process leads to volume expansion and brittle formation results in the loss of strength and the possible initiation of thermal fractionation. With the increase in carburization, the ability of the alloy to provide some resistance to coking through the formation of a chromium base layer deteriorates. At normal operating temperatures, half the wall thickness of some steel tube alloys can be carburized as little as two years of service. Typical tube lifetimes are in the range of 3 to 6 years.

It has been shown that aluminized steels, steels coated with silica, and steel surfaces enriched in manganese oxides or chromium oxides are beneficial in reducing the formation of catalytic coke. Alonizing ™ or aluminization involves the diffusion of aluminum into the surface alloy by packed cementation, a chemical vapor deposition technique. The coating is functional to form a NiAl type compound and provides an aluminum layer which is effective in reducing the formation of catalytic coke and protecting against oxidation or other forms of corrosion. The coating is not stable at temperatures such as those used in ethylene furnaces, and is also brittle, exhibiting a tendency to spallation or diffusion in the matrix of the base alloy. In general, packaged cementation is limited to the deposition of only a single element, the co-deposition of other elements, for example chromium and silicon, being extremely difficult. Commercially, it is limited in general to the deposition of only a few elements, mainly aluminum. Some work has been done on the co-deposition of two elements, for example chromium and silicon, but the process is extremely difficult and of limited commercial utility. Another method for the application of coatings by diffusion of aluminum to an alloy substrate is set forth in U.S. Pat. 5,403,629 published by P. Adam et al. This patent details a process for the vapor deposition of a metal interlayer on the surface of a metallic component, for example by sputtering. An aluminum diffusion coating is subsequently deposited on the interlayer.

Alternative diffusion coatings have also been explored. In an article in "Processing and Properties" titled "The Effect of Time at Temperature on Silicon-Titanium Diffusion Coating on IN738 Base Alloy" by M.C. Meelu and M.H. Lorretto, the evaluation of a Si-Ti coating is exposed, which had been applied by means of packed cementation at high temperatures for prolonged periods of time.

Damagingly, however, to date no coatings have been developed which, in the context of hydrocarbon processing at temperatures in the range of 850 to 1100 ° C, have been found effective in reducing or eliminating the deposition of catalytic coke or for provide improved carburization resistance during a commercially viable operating life. A greater difficulty in seeking an effective coating is the propensity of many applied coatings that fall to adhere to the alloy substrate of the pipe under the high temperature operating conditions specified in the hydrocarbon pyrolysis furnaces. Additionally, the coatings lack the necessary strength to any or all of the thermal stability, thermal shock, hot erosion, carburization, oxidation and sulfidation. A product commercially available for the production of olefins by pyrolysis of the hydrocarbon stream must be capable of providing the necessary coking and carburizing resistance during an extended operating life while exhibiting thermal stability, hot erosion resistance and resistance to thermal shock.

Description of the invention It is therefore a principal objective of the present invention to impart beneficial properties to the HTAs through the surface allowing to eliminate or substantially reduce the catalytic formation of coke on the internal surfaces of pipes, pipes, fittings and other auxiliary furnace devices used for the production of olefins by pyrolysis of the hydrocarbon stream or in the production of other hydrocarbon-based products.

It is another object of the present invention to increase the carburization resistance of HTAs used for pipes, pipes, fittings and auxiliary furnace devices while in service.

It is a further object of the present invention to increase the longevity of the improved development benefits derived from the surface allowing under commercial conditions to provide thermal stability, resistance to hot erosion and resistance to thermal shock.

In accordance with the present invention two different types of surface alloy structures are provided, both generated from the deposition of two coating formulations, Al-Ti-Si and Cr-Ti-Si followed by the appropriate thermal treatments.

The first type of surface alloy is generated after the application of the coating material and an appropriate heat treatment followed subsequently, the formation of a collection adjacent to the base alloy and containing the enriched elements and the elements of the base alloy such that a layer of alumina or one of cea ia. can be generated by reactive gaseous thermal treatment (surface activation), by using Al-Ti-Si and Cr-Ti-Si as the coating materials, respectively. This type of surface alloy is compatible with low temperature commercial processes that operate at less than about 850 ° C.

The second type of surface alloy is also produced using Al-Ti-Si or Cr-Ti-Si as the coating materials, however, the heat treatment cycle is such as to produce a diffusion barrier adjacent to the base alloy and an enriched collection adjacent to said diffusion barrier. The surface activation of this type of surface alloy produces a protective layer that is mainly chromia when Cr-Ti-Si is used. Both layers are highly effective in reducing or eliminating the formation of catalytic coke. This type of surface alloy is compatible with commercial processes at a high temperature of up to 1100 ° C such as olefins that are made by pyrolysis of the hydrocarbon stream.

The diffusion barrier is defined as a reactive interdiffusion layer, enriched in silicon and chromium that contains intermetallics of the elements of the base alloy and the deposited materials. Enriched collection is defined as an interdiffusion layer containing materials deposited adjacent to the diffusion barrier, if formed, or the base alloy, which is functional to maintain a protective oxide layer on the outermost surface.

In its broadest aspect, the method of the invention provides a protective surface on a base alloy containing iron, nickel and chromium comprising deposition on said base alloy of silicon and elemental titanium with at least one of titanium and chromium, and the heat treatment of said base alloy to generate a surface alloy consisting of an enriched collection containing said elements deposited on said base alloy.

More particularly, the method comprises depositing an effective amount of elemental silicon and titanium with at least one of aluminum and chromium at a temperature in the range of 300 to 1100 ° C to provide an enriched collection which contains from 4 to 30% by weight of silicon, 0 to 10% by weight of titanium, 2 to 45% by weight of chromium and optionally 4 to 15% by weight of aluminum, the remainder of iron, nickel and any of base alloy additives, and the thermal treatment of said base alloy at a temperature in the range of 600 to 1150 ° C for an effective time to provide an enriched collection having a thickness in the range of 10 to 300 μm.

In a preferred embodiment, the method of the invention which additionally comprises the thermal treatment of said base alloy at a temperature in the range of 600 to 1150 ° C for an effective time to form an intermediate diffusion barrier between the base alloy substrate and the enriched collection containing inter-metals of the deposited elements and the elements of the base alloy, said diffusion barrier preferably having a thickness of 10 to 200 μm and containing from 4 to 20% by weight of silicon, 0 to 4% by weight. titanium weight and 10 to 85% by weight of chromium, the rest of iron and nickel and any alloying additives. The protective surface is reacted with an oxidizing gas selected from at least one of oxygen, air, steam, carbon monoxide or carbon dioxide, alone, or with any of hydrogen, nitrogen or argon whereby a protective supply layer having a thickness of approximately 0.5 to 10 μm is formed on said enriched collection.

In one embodiment of the method of the present invention, aluminum or chromium is replaced by an element selected from Groups IVA, VA and VIA of the Periodic Table, or manganese; or the titanium is replaced by an element selected from Group IV of the Periodic Table capable of segregating the outermost layer to form a stable protective layer, yttrium or cerium could be added to the composition to improve the stability of the protective layer.

The surface alloyed component of the invention produced by the method broadly comprises a base stainless steel alloy containing iron, nickel and chromium, and an enriched collection layer adjacent to said base alloy, containing silicon and chromium, and optionally one or more of titanium or aluminum or selected elements of the Groups IVA, VA and VIA of the Periodic Table, or manganese, cerium or yttrium, and the rest of iron, nickel and any base alloy additives; or optionally, wherein the silicon and chromium and optionally one or more of titanium or aluminum or elements selected from the Groups IVA, VA and VIA of the Periodic Table, or manganese, cerium or yttrium, have been applied to said base alloy under effective conditions to allow reactive interdiffusion between said base alloy and the deposited materials, whereby enriched collection is formed which is functional to form a protective supply layer on said outermost surface of said component. The composition of the enriched collection preferably comprises silicon in the range of 4 to 30% by weight, titanium in the range of 0 to 10% by weight, chromium in the range of 2 to 45% by weight, and optionally 4 to 15% in weight of aluminum.

The surface alloy component additionally preferably comprises a diffusion barrier layer, adjacent to said base stainless steel alloy, said diffusion barrier having a thickness in the range of 10 to 200 μm, and containing intermetallics of the deposited elements and the elements of the base alloy, whereby the diffusion barrier and the enriched collection are formed which are functional to reduce the diffusion of deterioration constituents mechanically in said base alloy and to form a protective supply layer on the outermost surface of the component . According to this embodiment, the silicon content in the diffusion barrier layer comprises silicon in the range of 4 to 20% by weight, chromium in the range of 10 to 85% by weight, and titanium in the range of 0 to 4% by weight; and said enriched harvesting composition comprises silicon in the range of 4 to 30% by weight, chromium in the range of 2 to 42% by weight, and titanium in the range of 5 to 10% by weight, and optionally aluminum in the range of between 4 to 15% by weight.

Description of the Drawings The products of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a surface alloy after coating deposition, surface alloy and surface activation; Figure 2 is a photomicrograph depicting the microstructure of a surface alloy produced in a forged alloy 20Cr-30Ni-Fe using. the coating formulation Al-Ti-Si; Figure 3 is a photomicrograph depicting the microstructure of a surface alloy produced in a molten 35Cr-45Ni-Fe alloy using the Al-Ti-Si coating formulation; Y Figure 4 is a photograph showing a treated sample (left) and an untreated sample (right) of the results of method 1 of the accelerated carburization test after 22 cycles.

Description of the Preferred Modality Having reference to the accompanying figures, a process for producing surface alloy components will now be described. Suitable base alloy compositions of the components to be surface alloyed would include austenitic stainless steels.

The coating materials would be selected from silicon and elemental titanium, with one or more of aluminum, chromium, elements selected from Groups IVA, VA and VIA of the Periodic Table, manganese, cerium or yttrium. Titanium could be replaced with another element of the VAT Group. The preferred elements would be titanium, aluminum and chromium in combination with silicon. However, satisfactory surface alloys could be prepared from chromium, titanium and silicon, in combination, or from aluminum, titanium and silicon, in combination. Additionally, an initial silicon coating could be applied followed by a coating of the blends described above to further enhance the silicon enrichment. The selected elements will depend on the required properties of the surface alloy.

For the Al-Ti-Si combination, aluminum would be in the range of 15 to 50% by weight, titanium would be in the range of 5 to 30% by weight and the rest of silicon.

For the Cr-Ti-Si combination, chromium would be in the range of 15 to 50% by weight, titanium would be in the range of 5 to 30% by weight and the rest of silicon.

The typical ranges for the average composition of the surface alloy layers formed in a forged alloy of 20Cr-30Ni-Fe using Al-Ti-Si are shown in the Table I Table I The typical ranges for the average composition of the surface alloy layers formed in a molten 35Cr-45Ni-Fe alloy (supplier B) using Al-Ti-Si are shown in Table II.

Table II It is to be noted that one of the advantages of the coating described above is that a Ni: Ti: Si ratio of 4: 2: 1 respectively is functional to form a very stable compound in conjunction with the other elements. This stable coating does not diffuse into the substrate and maintains a high content of silicon and titanium near the surface. A composition of the exemplary component would be Ni 49.0 - Fe 10.3 - Cr 3.5 - Ti 22.7 - Si 13.3 and 1.4 of other components.

Selection of a Release Method for Revolving Materials The coating materials could be released to the surface of the component by a variety of methods whose selection is based on the composition of the coating, the temperature of the deposition, the flow on the required surface, the level of spatial homogeneity needed, and the shape of the coating. component to be coated. The main coating technologies are identified later.

Thermal electrospray methods include electrospray with molten metal, electrospray with plasma, high speed acid fuel (HVOF), and Electrospray with Low Pressure Plasma (LPPS). These are generally lines of interest and are best suited for external surfaces. The use of robotic technology has greatly improved its drive power. New electronic cannon technologies have also been developed capable of coating the internal surfaces of pipe products which are greater than 100 nm in the internal diameter and in lengths exceeding 5 meters.

Electrochemical and electroplating methods have good drive power for complex shapes in the range of elements that can be deposited.

Steam-based methods include packed cementation, thermal chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and physical vapor deposition (PVD). PVD methods are very diverse and include cathodic arc, sputtering (DC, RF, magnetron) and electron beam evaporation.

Other coating methods include hydrosol gel and fluidized bed processes with the former capable of deliberating a wide range of coating materials to the components in a simple and complex manner.

The hybrid methods combine more than one of the above methods to ensure that the constructed surface alloy can be generated from the released constituent materials, for example, CVD, followed by PVD, or electrochemical followed by PVD.

Each of the above methods has capabilities and limitations that define its applicability for the improvement of the development of the required component. The key to the release requirements of any method considered for a given coating formulation are the geometry of the component to be coated, the driving power of the method, the rate of deposition and the uniformity of deposition.

All the above methods can be used for the release of the coating materials to the external surfaces of a wide range of component geometries, each with well defined drive power. The preferred methods for releasing a wide range of coating materials to the internal surfaces of the complex shaped parts are the PVD methods. This is due to the flexibility in the selection of the material consumed (coating), and the ability to assemble the consumable coating within the complex shaped part. An example in the coating of tubular products is given by J.S. Sheward entitled "The Coating of Internal Surfaces by PVD Techniques" published at the Meeting of the 19th International Conference on Metallurgical Coatings and Thin Films, San Diego, April 6-10, 1992.

The use of ion bombardment in the magnetron is well known in the art and is detailed in the review of J.A.

Thornton and A.S. Penfold titled "Cylindrical Magnetron Sputtering "in Film Processes, Academic Press (1987) Specific examples in the patent literature including US Pat. Nos. 4, 376,025 and 4,407,713 published by B. Zega entitled" Cylindrical Cathode for Magnetically-Enhanced Sputtering "and" Cylindrical Magnetron Sputtering Cathode. and Apparatus "respectively, and US Pat. No. 5,298,137 by J. Marshall entitled" Method and Apparatus for Linear Magnetron ", claimed to improve deposition uniformity.

In this invention, the production of a surface alloy component is divided into four main stages: (a) pre-determined, to generate a clean surface compatible with vapor-based coating methods; (b) coating deposition, to release the coating materials required for the surface alloy; (c) surface alloy, to generate a specific or prefabricated microstructure and (d) Surface activation, to generate a protective layer by means of reactive gaseous treatment.

Steps (a) to (c) are required, step (d) is optional, as will be described later.

In stage (a), the pre-determined, a combination of chemical, electrochemical and mechanical methods are used to remove organic and inorganic contaminants, any oxide layer, and where the Bielby layer is present (a damaged area formed through cold work production processes). The pre-termination sequence used is defined by the mass composition, the surface composition and the geometry of the component. The totality and uniformity of the pre-termination sequence is critical to the overall quality of the coating and the surface alloy product.

For step (b), coating deposition, the preferred methods of coating the inner wall surfaces of components such as pipes, pipes and fittings are ionically bombarded (DC or RF), with or without enhancement in the magnetron, and PECVD. The selection method is carried out mainly by the composition of the coating material to be released to the surface of the component. With ion bombardment methods, the magnetron enhancement can be used to reduce the overall coating time per component. In such cases, the impact surface (or cathode) is prepared by applying the coating formulation on a support tube which has the shape of the component to be coated and a diameter smaller than that of the component. The support tube with the consumable coating material is then introduced into the component in a manner capable of releasing the coating material uniformly. The methods of applying the consumable coating on the support tube may include any of the coating methods previously listed. Thermal electrospray methods were found to be the most useful for the range of coating materials required for the processing of the components for the application of olefins processing. The magnetron improvement of the sputtering process was carried out using permanent magnets inside the support tube or by passing a high current of DC or AC through the supporting tube to generate an appropriate magnetic field. The latter method is based on electromagnetic theory which specifies that the flow of an electric current through a conductor leads to the formation of normal circular magnetic induction lines to the direction of the flow stream, for example, D. Halliday and R. Resnic, "Physics Part II" published by John Wiley & Sons, Inc. (1962). When permanent magnets are used to generate the field, the composition of the tube being supported is not important, however, when a high current is used, the tube being supported should be made of materials with low electrical resistance such as copper or aluminum. The process gas normally used is argon at pressures that are in the range of 1 to 200 mtorr, and if required, low levels of hydrogen (less than 5%) are added to provide a slightly reduced atmosphere. The temperature of the component during deposition is typically in the range of 300 to 1100 ° C.

For step (c), the surface alloy may be initiated in part or carried out parallel to this operation by depositing at sufficiently high temperatures greater than 600 ° C with well-defined time-temperature and flow profiles, or it may be carried out due to at the end of the deposition in the temperature range of 600 to 1150 ° C.

In step (d), surface activation is considered optional in that the inactivated surface alloy can provide many intended benefits, including resistance to coking at some level. However, appropriate or complete activation can also increase the overall resistance to coking by means of the formation of a higher outer layer. The activation can also be carried out as part of the production process, or with the surface alloyed component in service. The last one is useful in the regeneration of the protective layer if it is consumed (eroded) or damaged. When the activation is carried out as part of the production process, it can be initiated during the surface alloying stage, or after its completion. The process is carried out by reactive gaseous thermal treatment in the temperature range of 600 to 1100 ° C.

The product and process of the invention will now be described with reference to the following non-limiting examples.

EXAMPLE 1 This example demonstrates the resistance to coking of tubes treated against untreated tubes.

A laboratory-scale unit was used to quantify the rate of coking in the inner wall of a tube by running the pyrolysis process for 2 to 4 hours or until the tube was completely covered with coke, whichever came first. The test piece was typically 12 to 16 mm in external diameter and 450 to 550 mm in length. The tube was installed in the unit and the temperature of the process gas was monitored during its total length to establish an appropriate temperature profile. The ethane feed was introduced at a steady state ratio of 0.3: 1 current: hydrocarbon. The contact time used was found in the range of 100 to 150 msec and the thermal fractionation temperature was approximately 915 ° C. The sulfur level in the gas stream was approximately 25 to 30 ppm. The product stream was analyzed with a gas chromatograph to quantify the product mix, yield levels and conversion. At the end of the run the coke was burned and quantified to calculate an average coking rate. After decoking, the run was typically repeated at least once.

The results for 6 treated tubes are reported in Table III, identifying the coating materials used for the treatment and the surface of the inner wall of the tube being tested for coking resistance. Quartz is used as a reference that represents a highly inert surface without catalytic activity. The formation and collection of the amorphous coke from the gas phase is considered independent of the catalytic coke formed on the surface of the tube and can count up to 1 g / min, depending on the collection area (surface area or roughness) on the surface of the tube. Therefore, a surface without catalytic activity is expected to exhibit a coking rate of 0 to 1 mg / min due simply to the collection of amorphous coke. Differences within this range are not considered important and attributable to differences in surface roughness. The runs of the reference metal tube are also shown with their test results taken from a database of the test unit. The 20Cr-30Ni-Fe reference metal alloy is considered the lowest alloy used in the manufacture of olefins and exhibits the highest coking rate of 8 to 9 mg / min. With such a coking speed, the test tube is completely covered (coked) in less than 2 hours. The larger alloys tested (richer in Cr and Ni) provide an improvement with a reduction in the coking rate from 4 to 5 mg / min.

The results show that the treated metal tubes perform as good as the quartz reference tubes. The remaining objective, as described above, is in the production of a surface alloy exhibiting excellent coke resistance, while also exhibiting the other properties required for commercial viability p. ex. , (resistance to carburization, thermal stability, resistance to hot erosion and resistance to thermal shock).

T III: Results of the Pyrolysis Test of Treated and Untreated Pipes EXAMPLE II This example is included to demonstrate the lack of carburization following accelerated carburization and aging tests.

Two accelerated carburization test methods have been used to evaluate the carburization resistance. The first method (Accelerated Carburization Method 1) comprises a cycle of duration of -24 h and consists of the pyrolysis of ethane at 870 ° C for 6 to 8 hours to deposit charcoal on the surface of the test piece, followed by a rinsing 8 hours at 1100 ° C in an atmosphere of 70% hydrogen and 30% carbon monoxide to diffuse the carbon deposited in the test piece, and finally, a coke combustion at 870 ° C using vapor / air mixtures and for 5 to 8 hours. Under these conditions, the forged tube of the 20Cr-30Ni-Fe alloy composition with 6 mm wall thickness is typically carburized by means of the wall thickness medium after 15 to 16 cycles. This level of carburization is normally observed at the end of the service cycle of the products of the tube in commercial ovens and can therefore be considered to represent a life time of the tube.

A total of 9 surface alloys have been tested using the above procedure. All surface alloys passed the absolute test with minimal or no carburization. Figure 4 shows one of the treated tubes (sample on the left) showing excellent resistance to carburization together with an untreated tube after 22 cycles.

The second test method (Accelerated Carburization Method 2) used to assess that carburization resistance is more serious than Method 1 in that a carbon layer thickness is initially painted on the surface of the test piece, followed by a hot rinse at 1100 ° C in an atmosphere of 70% hydrogen and 30% carbon monoxide for 16 hours. The sample is removed from the test unit, the additional carbon is repainted and the cycle repeats. Three such cycles are sufficient to completely carburize the 6 mm wall thickness of the untreated tubes of the 20Cr-30Ni-Fe forged composition. The test is considered more severe than Method 1 due to the longer duration of the rinsing portion of the cycle, and because the test does not allow the surface to recover in any way with a protective layer. Surface alloys considered commercially avail have passed this test. The test is intended to provide a relative range.

EXAMPLE III This example is included to demonstrate the superior hot erosion resistance of the treated alloys.

The resistance to hot erosion is carried out to evaluate the adhesion layer and the erosion rates of the surface alloyed components. The tube segments are heated to 850 ° C and exposed to air. The erosive particles are propelled towards the test surface at a predefined speed and impact angle. The weight loss of the sample is quantified for a fixed particle charge (total dosage).

A total of five surface alloys - combinations of base alloys have been tested. In all cases, as shown in T IV, the weight loss measurements show that the erosion resistance of the surface alloyed components is 2 to 8 times that of the untreated samples. Al-Ti-Si systems in a cast alloy exhibit the lowest erosion rate of the tested systems.

T IV: Results of the Hot Erosion Test Base alloy Weight Loss Materials (mg) Coating used for Alloy collision 30 ° Superficial collision 90 ° 20Cr-30Ni-Fe forged Cr-Ti-Si (sample A) 8.9 7.4 (sample B) 13.9 10.7 none (reference) 45.3 57.8 35Cr-45Ni-Fe Al-Ti-Si 4.9 (melted, Cr-Ti-Si 4.2 supplier A) none (reference) 9.8 35Cr-45Ni-Fe Al-Ti-Si 1.2 (melted, Cr-Ti-Si 2.2 supplier B) none (reference) 9.3 EXAMPLE IV This example is included to demonstrate the thermal stability of the treated alloys.

The thermal stability test is carried out to ensure the survival of a surface alloy at the operating temperatures of commercial ovens. The test coupons are heated in an inert atmosphere at various temperatures in the range of 900 to 1150 ° C for up to 200 hours at each temperature. Any change in the structure or composition is quantified and used to project the maximum operating temperature for a given surface alloy.

The results for the 35Cr-45Ni-Fe cast alloy from supplier B indicate that the Al-Ti-Si and Cr-Ti-Si systems can be operated at temperatures up to 1100 ° C. A temperature of up to 1125 ° C can be used for the Cr-Ti-Si system but could lead to a slow deterioration of the Al-Ti-Si system. The Cr-Ti-Si system begins to deteriorate at temperatures exceeding 1150 ° C. Olefin processing plants generally use a maximum exterior tube wall temperature of 1100 ° C, and in most cases operate below 1050 ° C.

EXAMPLE V This example is included to demonstrate the thermal shock resistance of the surface alloy parts.

The thermal shock resistance test is used to evaluate the ability of the surface alloy to withstand the interruption of the emergency furnace in service when large temperature changes could occur for a very short time. The test equipment evaluates the tube segments fed with gas from the outer surface of the wall to a steady state temperature of 950 to 1000 ° C for 15 minutes followed by rapid cooling to about 100 ° C or less in about 15 minutes. A test sample experiences a minimum of 100 such cycles and is then characterized.

Both Al-Ti-Si and Cr-Ti-Si systems passed this test without deterioration. The systems in the 20Cr-30Ni-Fe forged tube alloy were tested for a total of 300 cycles without observed deterioration. The untreated reference samples in all cases exhibited severe chromium loss after 100 cycles.

It will be understood, of course, that modifications can be made in the embodiments of the invention illustrated and described herein without departing from the scope and view of the invention as defined by the appended claims.

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates.

Having described the invention as above, the content of the following is claimed as property.

Claims (14)

1. A method of providing a protective surface on a base alloy containing iron, nickel and chromium, characterized in that it comprises: deposition on said base alloy of silicon and elemental titanium with at least one of aluminum and chromium in the amount of 5 to 30% in weight of titanium, 15 to 50% by weight of aluminum or chromium and the rest of silicon, and the heat treatment of the base alloy to generate a surface alloy at a temperature in the range of 300 to 1100 ° C consisting of a collection enriched containing said elements deposited in said base alloy in the amount of 4 to 30% by weight of silicon, 0 to 10% by weight of titanium, 2 to 45% by weight of chromium and optionally 4 to 15% by weight of aluminum , the rest of iron, nickel and any base alloy additive.

2. A method as claimed in claim 1, characterized in that the heat treatment of the base alloy at a temperature in the range of 600 to 1150 ° C for an effective time to provide an enriched collection having a thickness in the range of 10 to 300 μm.

3. A method as claimed in claim 1, characterized in that it additionally comprises the heat treatment of the base alloy at a temperature in the range of 600 to 1150 ° C for an effective time to form an intermediate diffusion barrier between the base alloy substrate and the enriched collection containing intermetallics of the deposited elements and the elements of the base alloy in the amount of 4 to 20% by weight of silicon, 0 to 4% by weight of titanium, 10 to 85% by weight of chromium and 0 to 5% by weight of aluminum, the rest of iron and nickel and any alloy additive.

4. A method as claimed in claim 3, characterized in that the diffusion barrier has a thickness in the range of about 10 to 200 μm.

5. A method as claimed in claim 1, characterized in that the reaction of the protective surface with an oxidizing gas in which a protective supply layer is formed over the enriched collection.

6. A method as claimed in claim 5, characterized in that the reactive gas is at least one of oxygen, air, steam, carbon monoxide or carbon dioxide, alone, or with either hydrogen, nitrogen or argon.

7. A method as claimed in claim 6, characterized in that the protective layer contains chromium and aluminum and has a thickness of about 0.5 to 10 μm.

8. A method as claimed in claim 1 or 2, characterized in that the replacement of aluminum or chromium with an element selected from Groups IVA, VA and VIA of the Periodic Table, or manganese, capable of segregating to the outermost surface to form a stable protective layer.

9. A method as claimed in claim 1, 2 or 3, characterized in that the replacement of titanium with an element selected from Group IV of the Periodic Table.

10. A method as claimed in claim 5, 6 or 7, characterized in that additionally yttrium or cerium is added to improve the stability of the protective layer.

11. A surface alloy component, characterized in that it comprises: an alloy base of stainless steel containing iron, nickel and chromium and the enriched collection adjacent to the base alloy, said enriched collection 3 has a thickness in the range of 10 to 300 μm, and contains silicon and chromium, and optionally one or more of titanium and aluminum, in the amount of 4 to 30% by weight of silicon, 2 to 45% by weight of chromium, 0 to 10% by weight of titanium and optionally 4 to 15% by weight weight of aluminum, the rest of iron, nickel and any base alloy element, whereby the enriched harvesting is formed which is functional to form a protective supply layer on the outermost surface of the component.

12. A surface alloy component as claimed in claim 11, characterized in that it additionally comprises a diffusion barrier enriched in silicon and chromium, adjacent to the base alloy of stainless steel and between the base alloy and the enriched collection, the diffusion barrier has a Thickness in the range of 10 to 200 μm; whereby the diffusion barrier and the enriched collection are functional to reduce the diffusion of deterioration constituents mechanically in the base alloy and to form a protective supply layer on the outermost surface of the component.

13. A surface alloy component as set forth in claim 12, characterized in that the content of silicon in the diffusion barrier layer is in the range of 4 to 20% by weight, said chromium content being in the range of 10 to 85% by weight, the titanium content is in the range of 0 to 4% by weight, and the aluminum content is in the range of 0 to 15% by weight; and the composition of the enriched cathode comprises silicon in the range of 4 to 30% by weight, chromium in the range of 2 to 42% by weight and titanium in the range between 5 to 10% by weight and optionally aluminum in the range of 4 to 15% by weight.

14. A surface alloyed component as claimed in claim 11, 12 or 13, characterized in that the alloyed surface component is an internally coated pipe, pipe or accessory.