WO1985000837A1

WO1985000837A1 - Turbine components having increased life cycle and method

Info

Publication number: WO1985000837A1
Application number: PCT/US1984/001289
Authority: WO
Inventors: Eugene V. Clark; George K. Sievers
Original assignee: Clark Eugene V; Sievers George K
Priority date: 1983-08-15
Filing date: 1984-08-15
Publication date: 1985-02-28
Also published as: EP0152471A1; AU3391184A; EP0152471A4

Abstract

Method and apparatus whereby turbine components are increased in erosion resistance, and turbine apparatus increased in cycle life thereby, by forming an intermetallic compound of the base metal, iron, cobalt or nickel, and boron, and a polyboride-forming element such as chromium or a refractory element: tungsten, tantalum, molybdenum or niobium at the fluid directing surfaces (16, 18) of the component (10), and enriching the surface relatively in chromium where the base alloy is an iron-chromium alloy such as a stainless steel.

Description

TURBINE COMPONENTS HAVING INCREASED LIFE CYCLE AND METHOD

Background Of The Invention

A large number of steam turbines are in use in the United States and throughout the world for power generation. Typically these turbines have multiple, finely engineered and costly sets of closely matched stationary nozzle blocks and opposed rotors, typically fabricated of superalloys, and shaped to pass steam at supercritical temperatures and high velocities directionally to impart rotation to a generator shaft coupled to the rotors. Erosion of the fluid-directing turbine components, e.g. by continual exfoliant impingement during fluid passage, reshapes nozzles and fluid paths, reducing efficiency of the turbine and eventually eroding the fluid-directing surface supporting substrates. Thus, it is common to remove from service and overhaul such turbines periodically. Currently, for many turbines the life cycle between regular overhauls is two years. This invention has to do with increasing the life cycle of turbines and like rotating machinery and apparatus through the substantial elimination, over greatly extended cycles of operation, of a prominent cause of premature failure in both rotating and stationary components: erosion of the fluid directing surfaces. More particularly, the invention involves reducing the tendency of fluid-directing surfaces of

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^ IPO ,Λ»j turbine and other rotating machinery components to develop micr ocracks which development has been a prominent factor in lessening erosion resistance of turbine component surfaces. The invention thereby enables these component surfaces to have increased erosion resistance in erosive fluid environments. The life cycle of the turbine structures, heretofore limited by the erosion of the fluid directing surfaces, is thus increased with enormous savings in downtime and overhaul costs. For example, the invention can provide at least a doubling of the time between overhauls of power generating steam turbines, while maintaining peak efficiency levels over a longer time within the extended cycle. Moreover, severe deterioration of fluid surface supporting substrates caused by misdirection of fluids by eroded surfaces when the overhaul cycle is too extended is substantially prevented for greatly larger periods and increased savings in capital expenditure for repair or replacement is realized. Accordingly, the invention provides improvements in erosion resistance of rotating machinery components, such as vanes and nozzles, improved rotating and stationary turbine and like rotating machinery components, and methods therefor. More particularly, the invention provides improvements in fluid directing surfaces of rotating machinery components, such as vanes and nozzles having fluid directing surfaces formed of iron base metal alloy, for improved erosion resistance, by both increasing their hardness with boriding and reducing the depletion of iron in the subsurface layer and the incidence of microcracking which has heretofore lessened the life cycle of rotating machinery components. In another aspect, particular stainless steel alloys which are popular as metals for steam power turbines are improved by application of diffusion alloys in accordance with the invention. A widely used alloy for such turbines is 400 Series Stainless Steel. A typical steel will contain iron and about 12% by weight chromium. Higher chromium contents are counter indicated because of a commensurate loss in ductility or increased brittleness. In the turbine environment, it is necessary to resist failure through impact and thus brittle steels are not useful and chromium contents have been kept in the 12% by weight range. As noted below, diffusion alloying such steels with boron tends to deplete the structure portion immediately subjacent the diffusion region of iron because of the high affinity of iron for boron. The result is that iron borides are formed but at the expense of the subsurface layer iron content, so that a good diffusion layer but with poor bonding support to the structure is realized. It has been discovered that increased chromium content in the diffusion layer will block the depletion of iron as well as form increased chromium borides, so that a good diffusion layer is realized with an intact subjacent layer which provides good bonding support for the surface layer. Advantageously, the increased chromium content is selectively kept at the diffusion surface layer and thus the balance of the structure is not increased in chromium and keeps its superior non-brittle character. The differential increase in chromium content between the surface layer in particular and the structure generally enables obtaining iron, chromium borides and the desirable surface properties they provide while at the same time preserving the integrity of subsurface crystal structure .

Summary of the Invention It is^" accordingly a major objective of the invention to improve turbine and like apparatus components to increase their life cycles particularly in steam power generation uses. It another object to increase the erosion resistance of fluid directing surfaces of components of turbines and like apparatus. It is a further object to modify the composition of the fluid directing surfaces of components of turbines and the like to form inter etallic compounds reducing the propensity of the surface to form microcracks. It is still another object to provide fluid directing surfaces on components of turbines and like apparatus which are erosion resistant through a substantial reduction in incidence of microcracking, and novel fluid directing structures comprising nozzle blocks and rotors having diffusion alloy surfaces. Another object is to provide method for the formation of fluid directing surfaces which are resistant to microcracking and highly erosion resistant therefore. Yet another object is to improve boride diffusion alloys with iron base alloys containing above about 10% by weight chromium, a polyboride- former, but not so much as to exhibit undue brittleness for turbine use, by the distributive incorporation selectively of base metal, boron, chromium, and polyboride-forming refractory elements in the fluid-directing surface layer of the component. A further object is the improvement of boride diffusion alloys which are iron, cobalt or nickel base alloys, by distributive incorporation of such base metals boron and polyboride-forming refractory elements other than chromium, such as tantalum, molybedenum, tungsten and niobium in the borided surface layer of the component. These and other objects to become apparent hereinafter are realized in the method of manufacturing a fluid-directing structure for turbines and like apparatus, which includes forming the structure of an alloy consisting essentially of iron base metal and chromium, the chromium being present in an amount within a predetermined range between about 10% by weight and that amount tending to make the structure too brittle for turbine use, and diffusion alloying the surface layer of the structure with boron, the boron diffusion alloying having a tendency to deplete iron atoms from the alloy in the structure portion subjacent the diffusion alloy surface layer for reaction with boron to the detriment of bonding of the resulting diffusion alloy surface layer to the structure, the iron and boron diffusion alloy surface layer being subject to premature erosion deterioration responsive to incidence of microcracking in the diffusion alloy whereby life cycles for the structure in erosive fluid environments are reduced of the improvement comprising pre-diffusing additional chromium selectively into the structure surface and differentially with respect to the remainder of the structure so as to not increase the amount of chromium in the structure inward of the surface layer beyond an amount within the

OMPI ^JVATIO^ predetermined range, and thereafter forming integrally with the iron and boron diffusion alloy and distributively within the surface layer an intermetallic compound of iron, boron and chromium effective to reduce the incidence of microcracking, whereby erosion-caused premature failure of the surface layer is prevented and the life cycle of the structures increased without adversely affecting the usefulness of the structure for turbine use. In particular embodiments, the present method also includes pre-dif fusing additional chromium in an amount blocking depletion of iron from the portion of the structure subjacent the diffusion alloy surface layer in an amount detracting from the bonding of the surface layer to the structure; pre-diffusing the additional chromium to a depth of 0.001 to 0.0005 inch; the pr e-dif f us ion of chromium being carried out at a temperature of about 1900°F; the pre-diffusing of chromium being carried out for a period of about 4 hours at such temperature; diffusing boron to a depth of about 0.002 to 0.0005 inch; the diffusion of boron being carried out at a temperature of about 1725°F; and, the diffusion of boron being carried out for about 8 hours. In these and other embodiments, the method further includes diffusing a polyboride-forming refractory element into the surface layer in an amount effective against premature erosive deterioration due to microcracking in the layer , wherein the polyboride-forming refractory element is selected from tantalum, tungsten, molybdenum or niobium, and the intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds. In a particularly prefered embodiment, there is provided a method of increasing the life cycle of an iron base, chromium-containing alloy, fluid-directing structure for turbines or the like having a surface layer comprising a diffusion alloy of boron and iron, which includes differentially increasing the chromium content of the surface layer relative to the structure generally, and thereafter diffusing boron into the relatively increased chromium content surface layer to form integrally with the boron and iron diffusion alloy an intermetallic compound of the base metal, boron and chr nium in an amount effective to reduce the incidence of microcracking, whereby erosion caused premature failure of the surface layers is prevented and the life cycle of the structure increased. In this and like embodiments, there is also included increasing the chromium content by pre-diffusing additional chromium into the surface layer in an amount blocking boron diffusion caused depletion of iron from the portion of the structure subjacent the diffusion alloy surface layer in an amount detrimental to the bonding of the surface layer to the structure portion, e.g. by pre-diffusing the additional chromium at a time and temperature sufficient to provide a diffused chromium depth of about 0.001 to 0.0005 inch. As in other embodiments, this embodiment may also include diffusing a polyboride-forming refractory element into the surface layer in an amount effective to reduce microcracking in the layer, wherein the polyboride-forming refractory element is tantalum, tungsten, molybdenum or niobium, and

_. OMPI _ the intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds. Preferably the method includes diffusing first chromium and then boron sequentially from separate diffusion packs for a time and at a temperature sufficient to diffuse chromium and boron respectively into the structure surface in iron, boron, and chromium intermetallic compound forming relation; pack diffusing the chromium into the fluid-directing surface of the" structure in surface layer selectively chromium enriching relation; pack diffusing the boron into the fluid-directing surface of the structure under iron boride and chromium boride forming conditions, and if desired also diffusing a refractory element into the fluid-directing surface of the structure under forming conditions for the iron, boron, chromium and refractory element intermetallic compounds, whereby incidence of microcracking in the surface is reduced. In certain embodiments, the method further includes coating the refractory element onto the structure fluid-directing surface before diffusing into the surface, pack diffusing boron into the fluid-directing surface, the refractory element being diffused into the fluid-directing surface simultaneously with the boron diffusion, selecting as the refractory element one or more of tantalum, tungsten, molybdenum or niobium and including also applying the element as particulate in an organic liquid or as an integral single element coating to the fluid-directing surface before subjecting the surface to pack boron diffusion. Alternatively, the method contemplates pack diffusing the refractory element into the fluid-directing surface. The invention further comprises provision of a fluid-directing structure for turbines and like apparatus produced by one or more of the foregoing methods or otherwise and comprising an alloy of iron base metal and above about 10% by weight chromium, the fluid-directing structure having a fluid-contacting surface layer differentially enriched with added chromium relative to the subjacent portion of the structure and reacted with boron to form iron and chromium borides. In accordance with the invention, the fluid-directing structure typically in its subjacent structure portion is substantially free of iron depletion resultant from boron diffusion by virtue of the added chromium present in the surface layer, the additional chromium extends to a depth of about 0.001 to 0.0005 inch in the surface layer, the diffused boron extends to a depth of about 0.002 to 0.0005 inch in the surface layer, and there may additionally be present a polyboride-forming refractory element in the surface layer in an amount effective against premature erosive deterioration due to microcracking in the layer, the polyboride-forming refractory element being typically selected from tantalum, tungsten, molybdenum or niobium, whereby the intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds. In particularly preferred embodiments, the invention provides an iron base, chromium-containing alloy, fluid-directing structure for turbines or the like having a surface layer comprising a diffusion alloy of boron and iron, in which the chromium content of the

OMPI surface layer particularly is differentially increased relative to the structure generally, and locally reacted with boron in the region of increased chromium content to form integrally with the boron and iron diffusion alloy an intermetallic compound of the base metal, boron and chromium in an amount effective to reduce the incidence of microcracking, whereby erosion caused premature failure of the surface layers is prevented and the life cycle of the structure increased. In such structures there may further be present a polyboride-forming refractory element in the surface layer in an amount effective to reduce microcracking in the layer, e.g. selected from the polyboride-forming refractory elements tantalum, tungsten, molybdenum or niobium, whereby the intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds. In an additional embodiment, there is provided a fluid-directing structure for turbines and like apparatus comprising an alloy of iron base metal and chromium in an amount between about 10% by weight and an amount tending to make the structure too brittle for turbine use, the fluid-directing structure having a fluid-contacting surface layer differentially enriched with added chromium relative to the subjacent portion of the structure and reacted with boron to form iron and chromium borides against erosion at the surface of the structure, while preserving the non-brittle properties of the structure beyond the surface layer . The several polybor ide-f ormers othere than chromium mentioned above can be used in another embodiment of the invention, by the provision of a fluid-directing structure for turbines and like apparatus comprising iron, nickel or cobalt base metal and a fluid-directing surface layer thereon comprising a diffusion alloy of the structure base metal and boron, the surface layer being subject to premature erosion deterioration responsive to incidence of microcracking in the diffusion alloy reducing life cycles for the structure in erosive fluid environments; and within the surface layer a diffusion alloy of intermetallic compounds of the base metal , boron and a polyboride-forming refractory element distributed through and integral with the boron diffusion alloy in surface layer microcracking incidence-reducing amount, whereby premature erosion deterioration in the surface layer is prevented and the cycle- life of the structure in erosive environments is increased thereby. In particular embodiments, the base metal is iron, and the intermetallic compounds are iron, boron and refractory element intermetallic compounds; the base metal is cobalt, and the intermetallic compounds are cobalt, boron and refractory element intermetallic compounds; or the base metal is nickel, and the intermetallic compounds are nickel, boron and refractory element intermetallic compounds. In these and like embodiments, the refractory element is typically tantalum, and the intermetallic compounds are iron, cobalt or nickel, boron and tantalum intermetallic compounds, e.g. the base metal is iron, and the intermetallic compounds are iron, boron and tantalum intermetallic compounds, or the base metal is cobalt, and the intermetallic compounds are cobalt, boron and tantalum intermetallic compounds, or the base metal is nickel, and the intermetallic compounds are nickel, boron and tantalum intermetallic compounds. Similarly, where the refractory element is tungsten the intermetallic compounds are iron, cobalt or nickel, boron and tungsten intermetallic compounds, e.g. the base metal is iron, and the intermetallic compounds are iron, boron and tungsten intermetallic compounds, or the base metal is cobalt, and the intermetallic compounds are cobalt, boron and tungsten intermetallic compounds, or the base metal is nickel, and the intermetallic compounds are nickel, boron and tungsten intermetallic compounds, or the base metal is iron, and the intermetallic compounds are iron, boron and tungsten intermetallic compounds. Similarly where the the refractory element is molybdenum the intermetallic compounds are iron, cobalt or nickel, boron and roolybedenum intermetallic compounds, e.g. the base metal is cobalt, and the intermetallic compounds are cobalt, boron and molybdenum intermetallic compounds, or the base metal is nickel, and the intermetallic compounds are nickel, boron and molybdenum intermetallic compounds, or the base metal is iron, and the intermetallic compounds are iron, boron and molybdenum intermetallic compounds. Similarly where the refractory element is niobium, the intermetallic compounds are iron, cobalt or nickel, boron and niobium intermetallic compounds, e.g. the base metal is cobalt, and the intermetallic compounds are cobalt, boron and niobium intermetallic compounds, or the base metal is nickel, and the intermetallic compounds are nickel, boron and niobium intermetallic compounds, or the base metal is iron, and the intermetallic compounds are iron, boron and niobium intermetallic compounds. There is further provided in accordance with the invention, method of increasing the life cycle of a nickel, cobalt or iron base metal fluid-directing structure for turbines and like apparatus having a surface layer comprising a diffusion alloy of boron and the base metal having a tendency to microcrack and then erode in erosive fluid environments, which includes forming integrally with the boron and base metal diffusion alloy and distr ibutively within the surface layer an intermetallic compound of the base metal, boron and a polyboride-forming refractory element effective to reduce the incidence of microcracking, whereby erosion caused premature failure of the surface layers is prevented and the life cycle of the structures increased. In particular embodiments there is further contemplated employing a nickel base fluid-directing structure in the method, or employing a cobalt base fluid-directing structure, or employing an iron base metal fluid-directing structure. In these and like embodiments the method includes pack diffusing the boron into the fluid-directing surface of the structure under base metal-boride forming conditions; diffusing the refractory element into the fluid-directing surface of the structure under forming conditions for the boron, base metal and refractory element intermetallic compounds, whereby incidence of microcracking in the surface is reduced. In highly particular embodiments the method further includes coating the refractory element onto the structure fluid-directing surface before diffusing into the surface; pack diffusing boron into the fluid-directing surface, the refractory element being diffused into the fluid-directing surface simultaneously with the boron diffusion; and wherein the refractory element is tantalum, tungsten, molybdenum or niobium, applying the element as particulate in an organic liquid to the fluid-directing surface before subjecting the surface to pack boron diffusion, e.g. when employing tantalum as the refractory element, or employing tungsten as the refractory element, or employing molybdenum as the refractory element, or employing niobium as the refractory metal, or mixtures of two or more of such polyboride-forming refractory elements. In another embodiment, the invention includes, when the refractory element is tantalum, tungsten, molybdenum or niobium, depositing the element onto the fluid-directing surface as an integral single element coating before subjecting the surface to pack boron diffusion, and/or pack diffusing the refractory element into the fluid-directing surface, and effecting refractory ele ent pack diffusion into the fluid-directing surface before pack boron diffusion into the surface.

THE DRAWING The invention will be further described in conjunction with the attached drawing wherein: Fig. 1 is a fragmentary perspective view of a nozzle block segment of a steam turbine apparatus; Fig. 2 is a schematic view of the fluid-directing function of the nozzle block vanes; Fig. 3 is an enlarged schematic view of a pair of nozzle block vanes in fluid-directing arrangement, the vanes having been surface-alloyed in accordance with the invention as shown thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In supercritical steam power plants, it is not uncommon for the boiler tubes, particularly in the superheater and reheat sections, to exhibit heavy oxide formation. This oxide tends to break off bodily from the tube walls, the detaching being usually referred to as exfoliation, whereupon the oxide is carried in the steam and into impingement with the nozzles and blades of the turbines which are being driven by the steam to generate power. It is characteristic of the mode of impingement that numerous small contacts are made and the surface is gradually eroded, affecting adversely the contour of the blade or vane. With reference to the drawings, a fluid directing structure in the form of a nozzle block portion is shown at 10 in Fig. 1. The nozzle block 10 defines one segment of a annular set of like segments each having a hub 12 and a perimetrical flange 14. The flange 14 defines multiple arrayed directional passageways 16, defined by opposed, congruent vanes 18, See Fig. 2, into which steam is passed and from which steam is directed onto cooperating rotating parts to effect turbine shaft rotation. For purposes of the present invention both stationary and rotating fluid structures can be improved by use of the described surface alloys, but the description will be confined primarily to stationary nozzle block structures for convenience in reference. The passageways 16 are defined by the fluid directing surfaces 20, of vanes 18. See Fig. 3. The fluid directing surfaces 20 are continually impinged by small oxide impacts which in the presence of microcracking and thus in the absence of use of the present invention, will result in erosion and consequent premature failure or reduced life cycle for the component. However, in accordance with the invention, the hereinafter described diffusion alloys are formed in and on such surfaces to provide a surface layer 22 shown in Fig. 3. It is characteristic of the surface layer 22 that microcracking is reduced and longer service lives experienced through the presence in the surface layer of the described intermetallic compounds. The surface layer 22 is formed on the surfaces 10 of the vanes 18 by a variety of processes all of which diffuse the surface forming metal into the original vane surface, forming a new surface which extends inward and outward form the original surface locus. Diffusion alloying is well known and typically therein the diffusion alloy surface is formed by pack diffusion wherein the vane or like part is disposed in a diffusion pack containing the metals to be diffused, a refractory such as aluminum oxide and and a halide carrier, and heated at very high temperatures for extended periods to drive the diffusing metals into the surface. In the present invention the surface being

^• OMPI ^yA . WI?0 _^Λ» diffused is an alloy of iron base metal and chromium, and various other elements which do not alter the fundamental utility of the iron-chromium alloy for turbine use. "Base metal" refers to the fact that iron is the largest single element in the surface. Other elements which can be present in the iron base alloy, in addition to the minimum amount of chromium discussed below, are carbon, typically less than 2%; manganese, typically less than 3% but possibly as high as 15% in certain ofthe 200 Series Stainless Steels; phosphorous, typically well less than 1%; sulfur, typically less than 0.1%; silicon, typically less than 3%; and nickel, typically less than 10%, but ranging as high as 35%, all the foregoing and other percentages in this application being by weight. The chromium content in such steels can range from above about 10% up to about 30%, but above about 12% ductility properties desirable in a turbine component fall off, and brittleness increases, so that unless compensated for by other elements being added, as a practical matter useful iron-chromium alloys for turbine component use have chromium content in the 10 to 12% range, possibly up to 15% in particular cases. It has been heretofore unsuspected that such chromium containing alloys, containing low amounts of chromium, could be successfully diffusion alloyed with boron without so depleting the surface layer subjacent portion of iron that the final surface would be inadequately bonded to the structure. In USP 3,029,162, to Samuels et al, the patentees disclose the use of various metals, including chromium, silicon, aluminum, vanadium, titanium, zirconium. molybdenum, tungsten, columbium and tantalum, diffused into various substrates: iron, nickel, cobalt, molybdenum, tungsten, titanium, zirconium, copper and alloys thereof in advance of diffusing boron into the substrates. Samuels does not use iron chromium alloys, nor distinguish, nor apparently realize that the mere presence of chromium is not enough since a low chromium alloy (10-12%) will not block iron depletion during boriding, nor give a useful turbine component, unless the surface layer of the component structure is selectively increased in chromium content over and above that amount present in the structure generally. Nor that such increased amounts of chromium will have an adverse affect on the component performance if present generally throughout the component rather than differentially present at the outer surface as taught herein. In addition to the selective diffusion of chromium into the surface layer, in advance of boronizing, in iron depletion blocking amount, and for the purpose of eliminating or minimizing microcracking tending to develop upon erosion exposure, the present invention contemplates further incorporation of a polyboride-forming refractory element e.g. from a typical boronizing pack to which the refractory element has been added. While not wishing to be bound to any particular theory of operation, it has been observed that the rate of microcracking, i.e. the incidence of microcracking in the diffused surfaces is reduced as a result of the cod if fusion of the polyboride-forming refractory element into the part surface, and that this reduced incidence is accompanied by increased resistance to erosion from oxide impingements in long term service testing. In carrying out the present method, the parts are typically subjected to pack diffusion under first chromizing and then, separately under boronizing conditions, e.g. in a chromizing pack having the weight composition 10 to 25% finely divided chromium, 90 to 75% finely divided aluminum oxide, and a halide carrier such as the halide noted below at 0.02 to 0.2%, at an elevated temperature such as 1900 F. for 4 to 6 hours, and after cleaning , treating in a boronizing pack having the weight composition 0.1, preferably 5 to 10% amorphous boron, 0.01 to 0.1 ammonium halide, such as NH.F, and the balance aluminum oxide. Before effecting boron diffusion, preferably for 5 to 20 hours at 1600° to 1750 F., the polyboride-forming refractory element may be added to the pack or preferably applied as a thin coating to the part surface to be diffused, in an amount enabling polyboride formation with the refractory and base metal under the diffusion conditions, and then diffusion is carried out. The refractory metals useful herein include tantalum, tungsten, molybdenum, and niobium. These metals form polyboride compounds within the surface of the superalloy and with the base metal and added boron. The amount of refractory metal used is not narrowly critical, but should be sufficient to obtain polyborides of the base metal and boron distributively across and within the part surface layer, and not greatly in excess of that needed to form such polyborides. "Distributed" herein and its cognitives such as distributively, in reference to chromium or refractory polyboride forming element compounds with base metal and boron herein refers to the presence of the these intermetallic compounds in substantially all portions of the surface layer subject to microcracking, but not necessarily uniformly in depth or concentration from one portion to another. The intermetallic compounds referred to are additionally "integral" with the surface layer, by which is meant that the compounds of boron, base metal and refractory element are a part of the crystal lattices constituting the surface layer.

EXAMPLE 1 An iron base alloy (410 Stainless) containing 11.5 to 13.5 per cent chromium, 1 per cent silicon and manganese and smaller amounts of carbon, phosphorous, and sulfur formed into a nozzle block vane assembly was diffusion alloy surfaced in accordance with invention by cleaning the part of oxides, oils, and other foreign material, packing the part in a conventional chromizing pack containing finely divided chromium metal 10 to 25%, finely divided aluminum oxide, 90 to 75%, and ammonium halide at 0.02 to 0.2%, sealing in a suitable vessel, and heating in a furnace at 1900°F. for 4 to 6 hours. The part was then cooled and cleaned. Depth of diffusion of chromium was in the range of 0.0005 to 0.001 inch. The surface layer was enriched in chromium to about 1.5 to 2.5 times the content before diffusion, but only in the immediate surface layer, without altering the chromium content in the part generally. The chromized part was next packed in a boronizing pack containing 5 to 10% amorphous boron, but otherwise like the chromizing pack, and heating in a sealed vessel for about 8 hours at 1700 to 1750°F. The depth of iron and chromium boride surface layer was 0.0005 to 0.002 inch. The boronizing step can be modified by first coating a thin layer of finely divided tantalum onto the parts in an organic binder to effect refractory diffusion in addition to chromium and boron diffusion. The resulting nozzle blocks when installed in a power plant turbine assembly can run for two years with marked absence of erosion traceable to microcracking, or brittleness failures. The absence of substantial erosion of the parts, particularly at the fluid directing surface edges avoids misdirecting of steam which exacerbates over time, increasing erosion at the opposing surface, and reduced efficiency for the turbine. The additional fuel cost for the less efficient operation itself mandates use of the diffusion surfaced parts described herein, even apart from the greatly extended life cycle realizable from reduced overhaul needs.

EXAMPLE 2 Example 1 is duplicated using tungsten, molybdenum or niobium as the refractory metal. Results are equivalent. The above mentioned objects of the invention are thus realized including improved turbine and like apparatus component life cycles particularly in steam power generation uses, increased erosion resistance of fluid directing surfaces of components of turbines and like apparatus, formation of intermetallic compounds reducing the propensity of the such component surfaces to form microcracks. In addition novel fluid-directing structures comprising nozzle blocks and rotors having diffusion alloy surfaces have been provided, as well as methods for the formation of fluid directing surfaces which are well-bonded to a strong substrate substantially free of iron depletion despite boron diffusion, resistant to microcracking and highly erosion resistant, by the distributive incorporation of intermetallic compounds of base metal, boron, chromium and polyboride-forming refractory elements in the borided surface layer of the component. While not wishing to be bound to any particular theory of the invention, the addition of chromium to a chromium alloy with iron apparently lowers the tendency of the layer to crack, and also reduces the tendency of the boron to react preferentially with the iron in the subjacent portion of the part creating a subsurface layer depleted of iron. This depletion zone has a tendency to be brittle and not very corrosion resistant, limiting the utility of even the best of diffusion alloy layers formed atop thereof. The superior results of the present system over mere boron-chromium alloys formed without enhancing the chromium content, are believed attributable to the extensive reaction with boron permitted herein to give a thicker layer of the desired boron-chromium-iron intermetallic, while not creating the iron-depleted, brittle and poor corrosion resistant subjacent zone or layer as has heretofore been a concomitant of boronizing low chromium iron alloys.

. _O Pl , , IPO - _>j In other embodiments of the present invention the surface being diffused is typically a superalloy of cobalt, nickel or iron base metal, that is cobalt, nickel or iron, respectively, is the largest single element in the surface. Into such surface, in accordance with the invention , there is diffused boron and a polyboride-forming refractory element as noted above other than chromium, e.g. from a typical boronizing pack to which the refractory element has been added. While not wishing to be bound to any particular theory of operation, it has been observed that the rate of microcracking, i.e. the incidence of microcracking in the diffused surfaces is reduced as a result of the cod if fusion of the polyboride-forming refractory element into the part surface, and that this reduced incidence is accompanied by increased resistance to erosion from oxide impingements in long term service testing. In carrying out this method, the parts are typically subjected to pack diffusion under boronizing conditions, e.g. in a boronizing pack having the weight composition 0.1 to 10% amorphous boron, 0.01 to 0.1 ammonium halide, such as NH.F, and the balance aluminum oxide. Before effecting diffusion, preferably for 5 to 20 hours at 1600° to 1750°F. , the polyboride-forming refractory element is added to the pack or preferably applied as a thin coating to the part surface to be diffused, in an amount enabling polyboride formation with the refractory and base metal under the diffusion conditions, and then diffusion is carried out. The refractory metals useful in this embodiment include tantalum, tungsten, molybdenum, and niobium.

OMPI

V& ^Y IPO

RJ?W ^■> *~$ ' These metals form polyboride compounds within the surface of the superalloy and with the base metal and added boron. The amount of refractory metal used is not narrowly critical, but should be sufficient to obtain polyborides of the base metal and boron distr ibutively across and within the part surface layer, and not greatly in excess of that needed to form such polyborides. "Distributed" herein and its cognitives such as distr ibutively, in reference to polyboride forming refractory element compounds with base metal and boron herein refers to the presence of the these intermetallic compounds in substantially all portions of the surface layer subject to microcracking, but not necessarily uniformly in depth or concentration from one portion to another. The intermetallic compounds referred to are additionally "integral" with the surface layer, by which is meant that the compounds of boron, base metal and refractory element are a part of the crystal lattices constituting the surface layer.

EXAMPLE 3 An iron, nickel or cobalt base alloy nozzle block vane assembly were diffusion alloy surfaced in accordance with invention by coating a thin layer of finely divided tantalum onto the vanes in an organic binder, the vanes packed with a boronizing pack containing 0.1 to 10% boron and heated at 1600° to 1750°F. for 5 to 20 hours. The resulting nozzle blocks were installed in a power plant turbine assembly and run for two years along with control, untreated nozzle blocks. At the end of the testing period, the typical two-year cycle for overhaul of power plant turbines, inspection of the nozzle blocks showed essentially no erosive wear on the vanes at the edges, convex or cancave surfaces. In contrast, the untreated control blocks suffered substantial erosion of the vanes, particularly at the fluid directing surface edges. The consequence of such erosion is a misdirecting of the steam which exacerbates over time, increasing erosion at the opposing surface, and reduced efficiency for the turbine. The additional fuel cost for the less efficient operation itself mandates use of the diffusion surfaced vanes described herein, even apart from the greatly extended life cycle realizable from reduced overhaul needs.

EXAMPLE 4 Example 3 is duplicated using tungsten, molybdenum or niobium as the refractory metal. Results are equivalent. The above mentioned objects of the invention are thus realized including improved turbine and like apparatus component life cycles particularly in steam power generation uses, increased erosion resistance of fluid directing surfaces of components of turbines and like apparatus, formation of intermetallic compounds reducing the propensity of the such component surfaces to form microcracks. in addition novel fluid-directing structures comprising nozzle blocks and rotors having diffusion alloy surfaces have been provided, as well as methods for the formation of fluid directing surfaces which are resistant to microcracking and highly erosion resistant, by the distributive incorporation of intermetallic compounds of base metal, boron and chromium or other polyboride-forming refractory elements above-mentioned in the bonded surface layer of the component .

Claims

1. A fluid-directing structure for turbines and like apparatus comprising iron, nickel or cobalt base metal and a fluid-contacting surface layer thereon comprising a diffusion alloy of the structure base metal and boron, said surface layer being subject to premature erosion de erio ation responsive to incidence of microcracking in the diffusion alloy reducing life cycles for said structure in erosive fluid environments; and within said surface layer a diffusion alloy of intermetallic compounds of said base metal, boron and a polyboride-forming refractory element distributed through and integral with said boron diffusion alloy in surface layer microcracking incidence-reducing amount, whereby premature erosion deterioration in said surface layer is prevented and the cycle life of said structure in erosive environments is increased.

2. Fluid-directing structure according to claim 1, in which said base metal is iron, nickel or cobalt, said refractory element is tantalum, and said intermetallic compounds are iron, boron and tantalum intermetallic compounds.

3. Fluid-directing structure according to claim 1, in which said refractory element is tungsten and said intermetallic compounds are iron, cobalt or nickel, boron and tungsten intermetallic compounds. 4. Fluid-directing structure according to claim 1 , in which said refractory element is molybdenum and said intermetallic compounds are iron, cobalt or nickel, boron and molybedenum intermetallic compounds.

5. Fluid-directing structure according to claim 1, in which said refractory element is niobium and said intermetallic compounds are iron, cobalt or nickel, boron and niobium intermetallic compounds.

6. Method of increasing the life cycle of a nickel, cobalt or iron base metal fluid-directing structure for turbines and like apparatus having a surface layer comprising a diffusion alloy of boron and the base metal having a tendency to microcrack and then erode in erosive fluid environmen s, which includes forming integrally with said boron and base metal diffusion alloy and distributively within said surface layer an intermetallic compound of said base metal, boron and a polyboride-forming refractory element effective to reduce the incidence of microcracking, whereby erosion caused premature failure of said surface layers is prevented and the life cycle of said structures increased.

7. In the method of manufacturing a fluid-directing structure for turbines and like apparatus, which includes forming said structure of an alloy consisting essentially of iron base metal and chromium, said chromium being present in an amount within a predetermined range between about 10% by weight and 2 that amount tending to make said structure too brittle

3 for turbine use, and diffusion alloying the surface layer

4 of said structure with boron, said boron diffusion

5 alloying having a tendency to deplete iron atoms from the

6 alloy in the structure portion subjacent the diffusion

7 alloy surface layer for reaction with boron to the

8 detriment of bonding of the resulting diffusion alloy

9 surface layer to the structure, said iron and boron

10 diffusion alloy surface layer being subject to premature

11 erosion deterioration responsive to incidence of

12 microcracking in the diffusion alloy whereby life cycles

13 for the structure in erosive fluid environments are

14 reduced; the improvement comprising pre-diffusing

15 additional chromium selectively into said structure

16 surface and differentially with respect to the remainder

17 of said structure so as to not increase the amount of

18 chromium in said structure inward of said surface layer

19 beyond an amount within said predetermined range, and

20 thereafter forming integrally with said iron and boron

21 diffusion alloy and distributively within said surface

22 layer an intermetallic compound of iron, boron and

23 chromium effective to reduce the incidence of

24 microcracking, whereby erosion-caused premature failure 2.5 of said surface layer is prevented and the life cycle of

26 said structures increased without adversely affecting the

27 usefulness of said structure for turbine use. 28

29 8. The method according to claim 7, including

30 also pre-diffusing additional chromium in an amount

31 blocking depletion of iron from said portion of said 2 structure subjacent said diffusion alloy surface layer in an amount detracting from the bonding of said surface layer to said structure.

9. The method according to claim 7, including pre-diffusing said additional chromium to a depth of about 0.001 to 0.0005 inch.

10. The method according to claim 7, in which said pre-dif fusion of chromium is carried out at a temperature of about 1900°F.

11. The method according to claim 7, including diffusing boron to a depth of about 0.002 to 0.0005 inch. diffusion of boron

9. The method according to claim 7, including also diffusing a polyboride-forming refractory element into said surface layer in an amount effective against premature erosive deterioration due to microcracking in said layer.

10. The method according to claim 7, in which said polyboride-forming refractory element is tantalum, tungsten, molybdenum or niobium, and said intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds.

11. The method according to claim 7, including also diffusing said refractory element into the fluid-directing surface of said structure under forming conditions for said boron, base metal and refractory 2 element intermetallic compounds, whereby incidence of

3 microcracking in said surface is reduced. 4

5 12. The method according to claim 11, including

6 also coating said refractory element onto the structure

7 fluid-directing surface before diffusing into said

8 surface. 9

10 13. The method according to claim 11, including

11 also pack diffusing boron into said fluid-directing

12 surface, said refractory element being diffused into said

13 fluid-directing surface simultaneously with said boron 1.4 diffusion.

15

16 14. The method according to claim 11 in which

17 said refractory element is tantalum, tungsten, molybdenum

18 or niobium and including also applying said element as

19 particulate in an organic liquid to said fluid-directing

20 surface before subjecting said surface to pack boron

21 diffusion. 22 3 15. A fluid-directing structure for turbines and 4 like apparatus comprising an alloy of iron base metal and 5 above about 10% by weight chromium, said fluid-directing 6 structure having a fluid-contacting surface layer 7 differentially enriched with added chromium relative to 8 the subjacent portion of the structure and reacted with 9 boron to form iron and chromium borides. 0 1 16. The fluid-directing structure according to 2 claim 15, in which said subjacent structure portion is substantially free of iron depletion resultant from boron diffusion by virtue of the added chromium present in said surface layer.

17. The fluid-directing structure according to claim 15, in which said additional chromium extends to a depth of about 0.001 to 0.0005 inch in said surface layer .

18. The fluid-directing structure according to claim 15, in which said diffused boron extends to a depth of about 0.002 to 0.0005 inch in said surface layer.

19. The fluid-directing structure according to clai 15 , in which there is also present a polyboride-forming refractory element in said surface layer in an amount effective against premature erosive deterioration due to microcracking in said layer.

20. The fluid-directing structure according to claim 19, in which said polyboride-forming refractory element is tantalum, tungsten, molybdenum or niobium, and said intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds.

21. An iron base, chromium-containing alloy, fluid-directing structure for turbines or the like having a surface layer comprising a diffusion alloy of boron and iron, in which the chromium content of said surface layer is differentially increased relative to said structure generally, and locally reacted with boron in the region of increased chromium content to form integrally with said boron and iron diffusion alloy an intermetallic compound of said base metal, boron and chromium in an amount effective to reduce the incidence of microcracking, whereby erosion caused premature failure of said surface layers is prevented and the life cycle of said structure increased.

22. The fluid-directing structure according to claim 21, in which there is further present a polyboride-forming refractory element into said surface layer in an amount effective to reduce microcracking in said layer.

23. Fluid-directing structure according to claim 22, in which said polyboride-forming refractory element is tantalum, tungsten, molybdenum or niobium, and said intermetallic compounds are iron, boron, chromium and refractory element intermetallic compounds.

SNATIO^