CA2239476C - Ductile embedment in brittle metal parts - Google Patents
Ductile embedment in brittle metal parts Download PDFInfo
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- CA2239476C CA2239476C CA 2239476 CA2239476A CA2239476C CA 2239476 C CA2239476 C CA 2239476C CA 2239476 CA2239476 CA 2239476 CA 2239476 A CA2239476 A CA 2239476A CA 2239476 C CA2239476 C CA 2239476C
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Abstract
A cast pulverizer grinding ring made of cast brittle, white-cast iron has increased resistance to failure by being provided with a mechanically intricate, lightweight ductile material system embedded in the brittle metal. The surface of the embedded material is coated, prior to casting, with a slurry of magnesium oxide particles in an alcohol vehicle together with a small amount of Bentonite particles (siliceous clay) and heat cured organic binder which resists attack by the molten metal and which avoids strong bonding with the solidified cast metal. The ductile embedment itself is a lightweight structure advantageously expanded metal. The intimate contact between the ductile embedment and the cast metal provides structural support while the lack of a strong bond between the ductile embedment and the cast iron allows an elastic/plastic deformation of the ductile steel forming the embedment to differ from that of the white-iron material cast therearound. The embedded ductile material holds the grinding ring together even if a through crack occurs.
Description
DUCTILE EMBEDMENT IN BRITTLE METAL PARTS
FIELD OF THE INVENTION
The present invention relates generally to coal pulverizers and, more particularly, to an improved coal pulverizer grinding ring casting and method of making same which is less susceptible to failure even if through cracks develop in the pulverizer grintling ring casting. The S present invention accomplishes this result by embedding a mechanically intricate, lightweight, ductile material system in the brittle wear metal of the pulverizer grinding ring casting during manufacture. The ductile metal is steel having a higher melting telllpeldl~e than the cast iron poured therearound so that it resists melting during the casting process. The embedded ductile material is entirely P~nc~e~ in and is in intim~te contact with the brittle metal. However, the 10 surface of the embedded ductile material is coated, prior to casting, with an impervious substance which resists attack by the molten metal and which avoids strong bonding with the solidified cast brittle metal. The intim~te contact between the embedded ductile m~teri~l and the cast iron provides structural support while the lack of a strong bond between the ductile material and the cast iron allows an elastic/plastic deformation of the ductile material to differ 15 from that of the cast iron m~ter~l . By this construction, if the cast iron pulverizer rintling ring cracks, the embedded ductile material will hold the grinding ring together even if a through crack propagates through the entire casting.
BACKGROUND OF THE INVENTION
Pulverizers have been used in industrial and utility settings for many years. Fig. I is a perspective sectional view of a type EL pulverizer manufactured by The Babcock & Wilcox Company (B&W). Fig. 2 is a sectional side view of a lower portion of the pulverizer of Fig. 1.
This EL pulverizer, generally designated 10, is primarily used to crush coal to a desired fineness so that it may be burned in steam generator apparatus to produce steam. Raw coal is provided into the mill 10 via raw coal infeed 12. The coal falls by gravity onto the surface of a lower rotating grinding ring 14. The lower grinding ring 14 is driven by a yoke 16 connected to a gearbox 18 which, in turn, is typically driven by a heavy-duty electric motor (not shown).
10 Located above the lower grinding ring 14 is a stationary upper grin~ling ring 20 which is spring-loaded by means of several single coil loading springs 22 to create grinding pressure. A set of balls 24 is placed between the upper stationary grinding ring 20 and the lower rotating grinding ring 14 and the incoming raw coal is crushed between the balls 24 and the lower rotating grinding ring 14. The area where the coal is pulverized is referred to as the grinding zone, and 15 during operation the incoming raw coal being fed to the mill 10 is mixed with partially ground coal already present on the lower rotating grin~ling ring 14 to form a circulating load in the mill 10. Hot primary air is provided into the mill at primary air inlet 26 to both dry the pulverized coal and transport it out of the mill 10 through classifier means 28 and out of pulverized coal outlets 30. The hot primary air causes the coal to circulate in between the grinding elements 20 (i.e., the lower and upper grinding rings 14, 20, respectively, and the balls 24) where it is pulverized to a greater degree in each pass therethrough. As the pulverized coal becomes fine enough to be picked up by the incoming primary air, it is transported upwardly through the classifier means 28 which is set so that coal of a desired fineness is sepal~led from the stream of pulverized coal entering the classifier 28 and caITied out through the pulverized coal outlets 25 30 by the hot primary air. Oversize pulverized coal material which has not achieved the desired fineness level is returned to the grinding zone.
Pulverizers and their grinding elements operate in a severe environment, due to the combination of the intense grinding pressure applied to the grinding elements and the high temperature incoming primary air, which can be 600~F or more. Extremely wear resistant 30 materials, typically white cast irons, are used for these grinding elements. Through suitable alloy additions, such as chromium, the cast iron wear properties can be enhanced. Particular examples of such wear alloys include Ni-Hard IV, developed by Inco, and Elverite I and VAM-20~ developed by B&W. One might generically refer to these alloys as Cr-Mo cast wear irons or white-irons and their chemical compositions can generally be defined according to ASTM
5 Standard A-532.
Generally, enhancing the wear resistance of these materials is done at a sacrifice with respect to other mechanical properties, particularly their ductility and fracture toughn~ss The same wear elements that achieve enhanced durability in such grinding service are typically quite brittle, and subject to failures, such as when the pulverizer grinding ring cracks.
Current methods to prevent catastrophic failure of such white-iron coal pulverizer rings require the installation of a tight-fit external band to the outer periphery of the ring. This band is inten~le~ to m~int~in ~lignm~nt and structural integrity if the grin(1ing ring fractures. Applying the external band requires very careful fitting if it is to remain tight in service. Such fitting may require accurate m~chining of the hard-iron part to facilitate shrink-fit of a machined band, or 15 it may require weldjoining of segments which draw tight as the weld cools. Provisions for shrink fitting of accurately machined parts places some restrictions upon the geometry of the ring outer periphery so that a pre-heated band can be slipped over the m~ehined diameter.
Welding in place requires prehe~ting the band segments to a controlled temperature, as well as careful control ofthe weld seam to assure proper shrink as the band and the weld cool. For both 20 methods of band in~t~ tion, the surface where the band is applied must be substantially cylindrical. One perceived advantage of the external banding method is the effect of preload as a strengthening benefit for the brittle material. However, even superficial analysis shows that bands of practical si~ are very small in cross section compared to the rings upon which they are mounted, so that any strengthening effect of induced compressive stress is very small. The 25 primary purpose of the tight fit is to prevent the band from coming loose in operation, with a secondary effect being to hold closed such fractures as may develop.
The susceptibility of white cast iron pulverizer grinding elements to cracking and failure (due to their brittle nature) has been recognized by others. Sabel (U.S. Patent Nos. 3,473,746 and 3,612,421 ) discloses grinding wear element c~ting~ made of hard, brittle metal which are 30 reinforced or retained by rods or rings of a different metal which has great tensile strength and resistance to fracture. The wear alloy is cast about the ret~inin~ or stabilizing structure, which is also provided with a suitable coating to prevent it from being sintered to or joined directly to the metal of the wearing part. If the casting wear body breaks or cracks, since there is no clearance between the coating of the inner structure and the outer abutting means of the outer 5 structure, cracking and breaking of the outer structure does not result in separation of the outer structure. Even if the brittle casting cracks, (l~m~ging separation of the casting along these cracks does not occur.
The approach disclosed in the aforementioned Sabel patents employs a few massiveembedded elements. Further, over this fairly massive reinforcing structure there is provided an 10intervening layer which ranges in thickness from 0.5 to 5 mm or 0.02 to 0.2 in. This is a very thick layer which serves as an insulator as much as a defluxing substance to prevent bonding.
A potential problem with this approach is that the relative movement cited by Sabel during the solidification and cooling of the cast metal would tend to destroy the structural integrity of the intervening layer as needed to transmit forces from the cast metal to the embedded metal.
15The problem of grinding ring element failure still exists and a solution to this problem would be welcomed by the industry.
SUMMARY OF THE INVENTION
In contrast to the aforementioned banding approach, and the approach which employed 20 a few massive embedded elements in the grinding element c~ting~, the present invention involves embedding a mechanically intricate, lightweight, ductile material system (which provides many small lig~m~.nt~ of ductile material) in the cast iron forming the wearing portion of the pulverizer grinding ring. The ~ felled form of the ductile embedment itself is expanded metal, but other forms such as woven wire could equally serve the same purpose. Because the 25 ductile embedment is not massive, it quickly reaches melt temperature and must be protected from fusion with the melt. A thin coating is applied to prevent this fusion, the coating compri~ing an alcohol slurry of a ceramic and a binder. The coating is quite thin and does not present a major interruption to the path of force tr~ncmi~sion between the cast material and the ductile embe-lm~nt It is vitally ilnpol~ll that the embedment not present a significant heat sink 30 which would produce early local solidification of the cast metal. Early solidification would very likely cause improper flow of metal into the embedment and would produce internal defects during further solidification and cooling of the cast metal.
As Sabel may have anticipated, but did not state, fusion between the cast metal and the embedment must be avoided to prevent the development of a path for crack propagation between 5 the brittle cast metal and the ductile embedment. However, Sabel's stated concern was more with accommodating relative movement during solidification of the molten metal.
The present invention is significantly different from the aforementioned patents in the following respects. First, the present invention employs many, light-section members of embedded structure. Contrast this with the few, massive load bearing members of the embedded 10 structure of Sabel. Further, the present invention employs a thin protective layer which allows the embedment to quickly reach molten metal temperature, in contrast to the thick protective layer employed in the patents which insulates the embedment. As a result, the present invention results in there being little temperature difference between the ductile embedment and the cast metal during solidification, in contrast to the approach of Sabel in which significant temperature 15 differences occur between the embedment and cast metal during solidification. As a result, the ductile embedments of the present invention do not produce any significant chilling effect on the molten cast metal, in contrast to the potential significant effect that the massive load bearing members of the Sabel embedments may have on the cast metal. Finally, the present invention's employment of many, light-section members of embedded structure with a thin coating layer 20 results in an excellent mechanical connection between the ductile embedment and the cast metal.
In contrast, it is submitted that there is a relatively doubtful integrity of the force tr~n~mi~sion path through the thick ~lolecti~e coating layer on the massive load bearing members used in the approach disclosed in Sabel.
Accordingly, one aspect of the present invention is drawn to the article of manufacture 25 of a pulverizer grinding ring casting which has increased resistance to failure even if through cracks develop in the casting. The pulverizer grin~ing ring comprises a mechanically intricate, lightweight, ductile material system embedded within white cast iron forming the casting, the ductile m~teri~l system providing many small ligaments of ductile material and having a thin coating of an alcohol-based slurry of fine ceramic particles in an organic binder which prevents 30 the white cast iron from dissolving the ductile material during manufacture. However, the coating allows the ductile material to quickly reach molten metal temperature and minimi7~
relative thermal expansion, and the thickness of the coating is selected so as to provide intim~te proximity of the coated ductile m~teri~l to the solidified white cast metal so that little or no movement across through cracks developing in the casting is needed before loads applied to the grinding ring casting during operation of the pulverizer during service are assumed by the embedded ductile material.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better lln~rst~n~ling ofthe invention, its opeld~ g advantages and specific results attained by its uses, 10 reference is made to the accompanying drawings and the following description in which plefelled embodiments ofthe invention are illustrated and described.
BRIEF DESCRIPTION OF THE DRAWINGS
Ln the drawings:
Fig. 1 is a sectional, perspective view of a B&W type EL pulverizer which illustrates the major components thereof;
Fig. 2 is a sectional view of a lower portion of Fig. l;
Fig. 3 is a plan view of a lower grinding ring element used in the type EL pulverizer of Figs. 1 and 2;
Fig. 4 is a sectional view of the lower grinding ring element of Fig. 3, viewed in the direction of arrows 44, illustrating the application of a ductile embedment to the lower grinding ring according to the present invention; and Fig. S illustrates examples of standard and fl~ttened expanded metal which may be used as the ductile embedment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings generally, wherein like numerals leplesent the same orfunctionally similar elements throughout the several drawings, and to Fig. 3 in particular, there is shown a plan view of a lower rotating grinding ring element 14 used in a type EL pulverizer 30 of Figs. 1 and 2 supra. The lower grinrling ring element 14 is typically cast in one piece.
. CA 02239476 1998-06-03 Ductile embedments in hard-iron or white-iron wear parts cannot be termed reinforcement in any conventional sense. That is, these materials cannot be expected to add to the strength of the wear parts. However, being ductile, the embe~lment~ are not subject to the rapid crack propagation properties of brittle white-iron and will remain intact across a crack which otherwise encompasses an entire cross-section. This property provides a continued measure of structural integrity to allow mill operation in a normal fashion until the parts are worn out or until they can be replaced at the owner's convenience during a planned m~inten~nce outage.
Referring now to Fig. 4, there is shown a sectional view of the lower grinding ring 14 of Fig. 3, viewed in the direction of arrows 4-4, which illustrates the application of a ductile embedment 32 to a one-piece, white cast iron grinding ring for a pulverizer, according to the present invention. The ductile embedment 32 has an overall geometry which is designed to provide a continuous ductile structure in locations which are not subject to normal abrasive wear. For example, as shown in the lower grinding ring 14 for an EL pulverizer, the ductile embedment 32 takes the form of a large flat ring 34, more or less like a substantially horizontal washer, surrounded by a substantially vertical cylinder 36. Ring 34 and cylinder 36 may be mechanically or metallurgically (i.e., welded) connected to one another at P, continually or at discrete locations around the circumference of grinding ring 14. As shown, the ductile embedment 32 preferably has a T-shape cross-section. During casting, provision of course would be provided to support or suspend the ductile embedment 32 at the proper location within the mold. While the ductile embedment 32 is shown in Fig. 4 as being applied to a lower grinding ring 14, it is understood that a similar washer shape 34 could be chosen and a shorter cylinder portion 36 used placed atop the washer shape 34 (to produce an L-shape) when it is employed in an upper grinding ring 20 of a type EL pulverizer. The ductile embedment 32 is advantageously made from expanded (standard or flattened) metal; typical #6 gauge (0.200 in.
thickness) has been used although heavier gauges could be chosen. See Fig. 5 for examples of both types of expanded metal. The design of the ductile embedment 32 is particularly aimed at placing the ductile m~t~ri~l in locations which provide a continuous path for tensile forces while assuring that the embedded material is well embedded and not subject to being destroyed by normal abrasive wear. A further design requirement is to avoid excessive mass in the ductile embedment 32 which could promote the development of casting defects if the molten metal locally solidifies prematurely. Somewhat analogous to fiber reinforcement, many small reinforcement members are superior in overall effectiveness to a few massive members.
Priorto casting the white cast iron material forming the lower grintling ring 14 about the 5 ductile embedment 32, the embedment 32 is cleaned to provide an oil-free surface prior to being coated with a specialized coating 38. The coating may be brushed, sprayed or applied by dipping the ductile embedment 32 in the coating 38 material. The particular method of application of the coatmg 38 is not necessarily critical, so long as any areas which remain uncoated are only small, local discontinuities in the coating 38. Such discontinuities in the coating 38 of the ductile embedment 32 should be minimi7e-1, however, by means of effective application methods and careful handling of coated parts. The coating, schematically represented in Fig. 4 as 38, is a slurry of fine ceramic particles in an alcohol vehicle with an organic binder. Typically, a single,~thin layer is applied which provides protection from the molten metal dissolving the embedment 32, but which allows the lightweight ductile embedment 15 32 to quickly reach molten metal temperature, thereby minimi7ing relative thermal expansion.
The coating itself is preferably a slurry of magnesium oxide (MgO) in an alcohol vehicle together with a small amount of Bentonite particles (siliceous clay) and heat cured organic binders. The coating itself is a minimum thickness of approximately 25 to 75 microns or .001 -.003 inch (wet film). The wet coating may be ignited to promote the heat cure cycle which takes 20 approximately 10-20 seconds. The coating is of a minimum thickness to present very slight resistance to heat transfer while also providing intimate proximity of the ductile embedment's 32 surfaces to the solidified cast metal forming the lower grinding ring 14. This intimacy is n~cec~ry so that little or no movement across a possible crack in the cast metal is needed before loads are assumed by the ductile embedded structure 32. Static fracture tests have shown that 25 the ductile embedment 32 survives immersion in molten VAM-20~ and undergoes significant elongation of the individual ligaments following failure of the cast metal itself.
One commercially available coating which can be used in the present application is a proprietary product known as "Moldcote 34" supplied by the Foundry Products Division of FOSECO Tntern~tional Ltd., a Burmah Castrol Company.
The present invention replesents a significant improvement over previous approaches.
The benefits of using expanded metal for the ductile embedment 32 have been previously discussed. Further, the coating 38 composition applied thereto is different in composition, method of application, thickness and int~n-led function from the approach employed by Sabel, 5 who used a sand-resin mix into which a preheated embedment was dipped. The heat from the metal melted some ofthe resin and the sticky resin caused adherence of sand particles. Repeated dip cycles might be used to build a substantial thickness and this thickness allowed, through partial collapse of the layers, movement between the solidifying cast metal and the embedded structure as relative therrnal expansion and contraction occur. In contrast, the coating 38 10 employed in the present invention is a slurry of fine ceramic particles in an alcohol vehicle with an organic binder applied in a single, thin layer to provide protection so that the molten metal does not dissolve the ductile embedment 38, but which allows the lightweight ductile embedment to quickly reach molten metal temperature and minimi~ relative thermal expansion.
While the present invention has been described with particular application to the 15 prevention of failures in the lower grinding rings of EL pulverizers, the invention has application to the lower and upper ~rin~ling rings of E and EL mills of all sizes and possibly to the tires used in what are known as MPS pulveri~rs. Additionally, there may be potential application for the ductile embedment 32 to ring segments of such grinding rings, and such applications are included in the spirit and scope of the present invention. Accordingly, while in accordance with 20 provisions of the statutes there have been described herein specific embodiments of the invention, those skilled in the art will understand that nomin~l changes may be made in the form of the invention covered by the appended claims to enhance its use in various settings and that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope and equivalents of the following claims.
FIELD OF THE INVENTION
The present invention relates generally to coal pulverizers and, more particularly, to an improved coal pulverizer grinding ring casting and method of making same which is less susceptible to failure even if through cracks develop in the pulverizer grintling ring casting. The S present invention accomplishes this result by embedding a mechanically intricate, lightweight, ductile material system in the brittle wear metal of the pulverizer grinding ring casting during manufacture. The ductile metal is steel having a higher melting telllpeldl~e than the cast iron poured therearound so that it resists melting during the casting process. The embedded ductile material is entirely P~nc~e~ in and is in intim~te contact with the brittle metal. However, the 10 surface of the embedded ductile material is coated, prior to casting, with an impervious substance which resists attack by the molten metal and which avoids strong bonding with the solidified cast brittle metal. The intim~te contact between the embedded ductile m~teri~l and the cast iron provides structural support while the lack of a strong bond between the ductile material and the cast iron allows an elastic/plastic deformation of the ductile material to differ 15 from that of the cast iron m~ter~l . By this construction, if the cast iron pulverizer rintling ring cracks, the embedded ductile material will hold the grinding ring together even if a through crack propagates through the entire casting.
BACKGROUND OF THE INVENTION
Pulverizers have been used in industrial and utility settings for many years. Fig. I is a perspective sectional view of a type EL pulverizer manufactured by The Babcock & Wilcox Company (B&W). Fig. 2 is a sectional side view of a lower portion of the pulverizer of Fig. 1.
This EL pulverizer, generally designated 10, is primarily used to crush coal to a desired fineness so that it may be burned in steam generator apparatus to produce steam. Raw coal is provided into the mill 10 via raw coal infeed 12. The coal falls by gravity onto the surface of a lower rotating grinding ring 14. The lower grinding ring 14 is driven by a yoke 16 connected to a gearbox 18 which, in turn, is typically driven by a heavy-duty electric motor (not shown).
10 Located above the lower grinding ring 14 is a stationary upper grin~ling ring 20 which is spring-loaded by means of several single coil loading springs 22 to create grinding pressure. A set of balls 24 is placed between the upper stationary grinding ring 20 and the lower rotating grinding ring 14 and the incoming raw coal is crushed between the balls 24 and the lower rotating grinding ring 14. The area where the coal is pulverized is referred to as the grinding zone, and 15 during operation the incoming raw coal being fed to the mill 10 is mixed with partially ground coal already present on the lower rotating grin~ling ring 14 to form a circulating load in the mill 10. Hot primary air is provided into the mill at primary air inlet 26 to both dry the pulverized coal and transport it out of the mill 10 through classifier means 28 and out of pulverized coal outlets 30. The hot primary air causes the coal to circulate in between the grinding elements 20 (i.e., the lower and upper grinding rings 14, 20, respectively, and the balls 24) where it is pulverized to a greater degree in each pass therethrough. As the pulverized coal becomes fine enough to be picked up by the incoming primary air, it is transported upwardly through the classifier means 28 which is set so that coal of a desired fineness is sepal~led from the stream of pulverized coal entering the classifier 28 and caITied out through the pulverized coal outlets 25 30 by the hot primary air. Oversize pulverized coal material which has not achieved the desired fineness level is returned to the grinding zone.
Pulverizers and their grinding elements operate in a severe environment, due to the combination of the intense grinding pressure applied to the grinding elements and the high temperature incoming primary air, which can be 600~F or more. Extremely wear resistant 30 materials, typically white cast irons, are used for these grinding elements. Through suitable alloy additions, such as chromium, the cast iron wear properties can be enhanced. Particular examples of such wear alloys include Ni-Hard IV, developed by Inco, and Elverite I and VAM-20~ developed by B&W. One might generically refer to these alloys as Cr-Mo cast wear irons or white-irons and their chemical compositions can generally be defined according to ASTM
5 Standard A-532.
Generally, enhancing the wear resistance of these materials is done at a sacrifice with respect to other mechanical properties, particularly their ductility and fracture toughn~ss The same wear elements that achieve enhanced durability in such grinding service are typically quite brittle, and subject to failures, such as when the pulverizer grinding ring cracks.
Current methods to prevent catastrophic failure of such white-iron coal pulverizer rings require the installation of a tight-fit external band to the outer periphery of the ring. This band is inten~le~ to m~int~in ~lignm~nt and structural integrity if the grin(1ing ring fractures. Applying the external band requires very careful fitting if it is to remain tight in service. Such fitting may require accurate m~chining of the hard-iron part to facilitate shrink-fit of a machined band, or 15 it may require weldjoining of segments which draw tight as the weld cools. Provisions for shrink fitting of accurately machined parts places some restrictions upon the geometry of the ring outer periphery so that a pre-heated band can be slipped over the m~ehined diameter.
Welding in place requires prehe~ting the band segments to a controlled temperature, as well as careful control ofthe weld seam to assure proper shrink as the band and the weld cool. For both 20 methods of band in~t~ tion, the surface where the band is applied must be substantially cylindrical. One perceived advantage of the external banding method is the effect of preload as a strengthening benefit for the brittle material. However, even superficial analysis shows that bands of practical si~ are very small in cross section compared to the rings upon which they are mounted, so that any strengthening effect of induced compressive stress is very small. The 25 primary purpose of the tight fit is to prevent the band from coming loose in operation, with a secondary effect being to hold closed such fractures as may develop.
The susceptibility of white cast iron pulverizer grinding elements to cracking and failure (due to their brittle nature) has been recognized by others. Sabel (U.S. Patent Nos. 3,473,746 and 3,612,421 ) discloses grinding wear element c~ting~ made of hard, brittle metal which are 30 reinforced or retained by rods or rings of a different metal which has great tensile strength and resistance to fracture. The wear alloy is cast about the ret~inin~ or stabilizing structure, which is also provided with a suitable coating to prevent it from being sintered to or joined directly to the metal of the wearing part. If the casting wear body breaks or cracks, since there is no clearance between the coating of the inner structure and the outer abutting means of the outer 5 structure, cracking and breaking of the outer structure does not result in separation of the outer structure. Even if the brittle casting cracks, (l~m~ging separation of the casting along these cracks does not occur.
The approach disclosed in the aforementioned Sabel patents employs a few massiveembedded elements. Further, over this fairly massive reinforcing structure there is provided an 10intervening layer which ranges in thickness from 0.5 to 5 mm or 0.02 to 0.2 in. This is a very thick layer which serves as an insulator as much as a defluxing substance to prevent bonding.
A potential problem with this approach is that the relative movement cited by Sabel during the solidification and cooling of the cast metal would tend to destroy the structural integrity of the intervening layer as needed to transmit forces from the cast metal to the embedded metal.
15The problem of grinding ring element failure still exists and a solution to this problem would be welcomed by the industry.
SUMMARY OF THE INVENTION
In contrast to the aforementioned banding approach, and the approach which employed 20 a few massive embedded elements in the grinding element c~ting~, the present invention involves embedding a mechanically intricate, lightweight, ductile material system (which provides many small lig~m~.nt~ of ductile material) in the cast iron forming the wearing portion of the pulverizer grinding ring. The ~ felled form of the ductile embedment itself is expanded metal, but other forms such as woven wire could equally serve the same purpose. Because the 25 ductile embedment is not massive, it quickly reaches melt temperature and must be protected from fusion with the melt. A thin coating is applied to prevent this fusion, the coating compri~ing an alcohol slurry of a ceramic and a binder. The coating is quite thin and does not present a major interruption to the path of force tr~ncmi~sion between the cast material and the ductile embe-lm~nt It is vitally ilnpol~ll that the embedment not present a significant heat sink 30 which would produce early local solidification of the cast metal. Early solidification would very likely cause improper flow of metal into the embedment and would produce internal defects during further solidification and cooling of the cast metal.
As Sabel may have anticipated, but did not state, fusion between the cast metal and the embedment must be avoided to prevent the development of a path for crack propagation between 5 the brittle cast metal and the ductile embedment. However, Sabel's stated concern was more with accommodating relative movement during solidification of the molten metal.
The present invention is significantly different from the aforementioned patents in the following respects. First, the present invention employs many, light-section members of embedded structure. Contrast this with the few, massive load bearing members of the embedded 10 structure of Sabel. Further, the present invention employs a thin protective layer which allows the embedment to quickly reach molten metal temperature, in contrast to the thick protective layer employed in the patents which insulates the embedment. As a result, the present invention results in there being little temperature difference between the ductile embedment and the cast metal during solidification, in contrast to the approach of Sabel in which significant temperature 15 differences occur between the embedment and cast metal during solidification. As a result, the ductile embedments of the present invention do not produce any significant chilling effect on the molten cast metal, in contrast to the potential significant effect that the massive load bearing members of the Sabel embedments may have on the cast metal. Finally, the present invention's employment of many, light-section members of embedded structure with a thin coating layer 20 results in an excellent mechanical connection between the ductile embedment and the cast metal.
In contrast, it is submitted that there is a relatively doubtful integrity of the force tr~n~mi~sion path through the thick ~lolecti~e coating layer on the massive load bearing members used in the approach disclosed in Sabel.
Accordingly, one aspect of the present invention is drawn to the article of manufacture 25 of a pulverizer grinding ring casting which has increased resistance to failure even if through cracks develop in the casting. The pulverizer grin~ing ring comprises a mechanically intricate, lightweight, ductile material system embedded within white cast iron forming the casting, the ductile m~teri~l system providing many small ligaments of ductile material and having a thin coating of an alcohol-based slurry of fine ceramic particles in an organic binder which prevents 30 the white cast iron from dissolving the ductile material during manufacture. However, the coating allows the ductile material to quickly reach molten metal temperature and minimi7~
relative thermal expansion, and the thickness of the coating is selected so as to provide intim~te proximity of the coated ductile m~teri~l to the solidified white cast metal so that little or no movement across through cracks developing in the casting is needed before loads applied to the grinding ring casting during operation of the pulverizer during service are assumed by the embedded ductile material.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better lln~rst~n~ling ofthe invention, its opeld~ g advantages and specific results attained by its uses, 10 reference is made to the accompanying drawings and the following description in which plefelled embodiments ofthe invention are illustrated and described.
BRIEF DESCRIPTION OF THE DRAWINGS
Ln the drawings:
Fig. 1 is a sectional, perspective view of a B&W type EL pulverizer which illustrates the major components thereof;
Fig. 2 is a sectional view of a lower portion of Fig. l;
Fig. 3 is a plan view of a lower grinding ring element used in the type EL pulverizer of Figs. 1 and 2;
Fig. 4 is a sectional view of the lower grinding ring element of Fig. 3, viewed in the direction of arrows 44, illustrating the application of a ductile embedment to the lower grinding ring according to the present invention; and Fig. S illustrates examples of standard and fl~ttened expanded metal which may be used as the ductile embedment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings generally, wherein like numerals leplesent the same orfunctionally similar elements throughout the several drawings, and to Fig. 3 in particular, there is shown a plan view of a lower rotating grinding ring element 14 used in a type EL pulverizer 30 of Figs. 1 and 2 supra. The lower grinrling ring element 14 is typically cast in one piece.
. CA 02239476 1998-06-03 Ductile embedments in hard-iron or white-iron wear parts cannot be termed reinforcement in any conventional sense. That is, these materials cannot be expected to add to the strength of the wear parts. However, being ductile, the embe~lment~ are not subject to the rapid crack propagation properties of brittle white-iron and will remain intact across a crack which otherwise encompasses an entire cross-section. This property provides a continued measure of structural integrity to allow mill operation in a normal fashion until the parts are worn out or until they can be replaced at the owner's convenience during a planned m~inten~nce outage.
Referring now to Fig. 4, there is shown a sectional view of the lower grinding ring 14 of Fig. 3, viewed in the direction of arrows 4-4, which illustrates the application of a ductile embedment 32 to a one-piece, white cast iron grinding ring for a pulverizer, according to the present invention. The ductile embedment 32 has an overall geometry which is designed to provide a continuous ductile structure in locations which are not subject to normal abrasive wear. For example, as shown in the lower grinding ring 14 for an EL pulverizer, the ductile embedment 32 takes the form of a large flat ring 34, more or less like a substantially horizontal washer, surrounded by a substantially vertical cylinder 36. Ring 34 and cylinder 36 may be mechanically or metallurgically (i.e., welded) connected to one another at P, continually or at discrete locations around the circumference of grinding ring 14. As shown, the ductile embedment 32 preferably has a T-shape cross-section. During casting, provision of course would be provided to support or suspend the ductile embedment 32 at the proper location within the mold. While the ductile embedment 32 is shown in Fig. 4 as being applied to a lower grinding ring 14, it is understood that a similar washer shape 34 could be chosen and a shorter cylinder portion 36 used placed atop the washer shape 34 (to produce an L-shape) when it is employed in an upper grinding ring 20 of a type EL pulverizer. The ductile embedment 32 is advantageously made from expanded (standard or flattened) metal; typical #6 gauge (0.200 in.
thickness) has been used although heavier gauges could be chosen. See Fig. 5 for examples of both types of expanded metal. The design of the ductile embedment 32 is particularly aimed at placing the ductile m~t~ri~l in locations which provide a continuous path for tensile forces while assuring that the embedded material is well embedded and not subject to being destroyed by normal abrasive wear. A further design requirement is to avoid excessive mass in the ductile embedment 32 which could promote the development of casting defects if the molten metal locally solidifies prematurely. Somewhat analogous to fiber reinforcement, many small reinforcement members are superior in overall effectiveness to a few massive members.
Priorto casting the white cast iron material forming the lower grintling ring 14 about the 5 ductile embedment 32, the embedment 32 is cleaned to provide an oil-free surface prior to being coated with a specialized coating 38. The coating may be brushed, sprayed or applied by dipping the ductile embedment 32 in the coating 38 material. The particular method of application of the coatmg 38 is not necessarily critical, so long as any areas which remain uncoated are only small, local discontinuities in the coating 38. Such discontinuities in the coating 38 of the ductile embedment 32 should be minimi7e-1, however, by means of effective application methods and careful handling of coated parts. The coating, schematically represented in Fig. 4 as 38, is a slurry of fine ceramic particles in an alcohol vehicle with an organic binder. Typically, a single,~thin layer is applied which provides protection from the molten metal dissolving the embedment 32, but which allows the lightweight ductile embedment 15 32 to quickly reach molten metal temperature, thereby minimi7ing relative thermal expansion.
The coating itself is preferably a slurry of magnesium oxide (MgO) in an alcohol vehicle together with a small amount of Bentonite particles (siliceous clay) and heat cured organic binders. The coating itself is a minimum thickness of approximately 25 to 75 microns or .001 -.003 inch (wet film). The wet coating may be ignited to promote the heat cure cycle which takes 20 approximately 10-20 seconds. The coating is of a minimum thickness to present very slight resistance to heat transfer while also providing intimate proximity of the ductile embedment's 32 surfaces to the solidified cast metal forming the lower grinding ring 14. This intimacy is n~cec~ry so that little or no movement across a possible crack in the cast metal is needed before loads are assumed by the ductile embedded structure 32. Static fracture tests have shown that 25 the ductile embedment 32 survives immersion in molten VAM-20~ and undergoes significant elongation of the individual ligaments following failure of the cast metal itself.
One commercially available coating which can be used in the present application is a proprietary product known as "Moldcote 34" supplied by the Foundry Products Division of FOSECO Tntern~tional Ltd., a Burmah Castrol Company.
The present invention replesents a significant improvement over previous approaches.
The benefits of using expanded metal for the ductile embedment 32 have been previously discussed. Further, the coating 38 composition applied thereto is different in composition, method of application, thickness and int~n-led function from the approach employed by Sabel, 5 who used a sand-resin mix into which a preheated embedment was dipped. The heat from the metal melted some ofthe resin and the sticky resin caused adherence of sand particles. Repeated dip cycles might be used to build a substantial thickness and this thickness allowed, through partial collapse of the layers, movement between the solidifying cast metal and the embedded structure as relative therrnal expansion and contraction occur. In contrast, the coating 38 10 employed in the present invention is a slurry of fine ceramic particles in an alcohol vehicle with an organic binder applied in a single, thin layer to provide protection so that the molten metal does not dissolve the ductile embedment 38, but which allows the lightweight ductile embedment to quickly reach molten metal temperature and minimi~ relative thermal expansion.
While the present invention has been described with particular application to the 15 prevention of failures in the lower grinding rings of EL pulverizers, the invention has application to the lower and upper ~rin~ling rings of E and EL mills of all sizes and possibly to the tires used in what are known as MPS pulveri~rs. Additionally, there may be potential application for the ductile embedment 32 to ring segments of such grinding rings, and such applications are included in the spirit and scope of the present invention. Accordingly, while in accordance with 20 provisions of the statutes there have been described herein specific embodiments of the invention, those skilled in the art will understand that nomin~l changes may be made in the form of the invention covered by the appended claims to enhance its use in various settings and that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope and equivalents of the following claims.
Claims (6)
1. A pulverizes grinding ring casting, comprising:
a mechanically intricate, lightweight, ductile material system embedded within white cast iron forming the casting, the ductile material system comprising one of standard or flattened expanded metal providing many small ligaments of ductile material and having a thin coating of an alcohol-based slurry of fine ceramic particles in an organic binder which prevents the white cast iron from dissolving the ductile material during manufacture, but which allows the ductile material to quickly reach molten metal temperature and minimize relative thermal expansion, the thickness of the coating being selected within a range of 0.001 inch to 0.003 inch so as to provide intimate proximity of the coated ductile material to the solidified white cast metal so that little or no movement across through cracks developing in the casting is needed before loads applied to the grinding ring casting during operation of the pulverizes during service are assumed by the embedded ductile material.
a mechanically intricate, lightweight, ductile material system embedded within white cast iron forming the casting, the ductile material system comprising one of standard or flattened expanded metal providing many small ligaments of ductile material and having a thin coating of an alcohol-based slurry of fine ceramic particles in an organic binder which prevents the white cast iron from dissolving the ductile material during manufacture, but which allows the ductile material to quickly reach molten metal temperature and minimize relative thermal expansion, the thickness of the coating being selected within a range of 0.001 inch to 0.003 inch so as to provide intimate proximity of the coated ductile material to the solidified white cast metal so that little or no movement across through cracks developing in the casting is needed before loads applied to the grinding ring casting during operation of the pulverizes during service are assumed by the embedded ductile material.
2. The pulverizes grinding ring casting according to claim 1, wherein the thin coating comprises an alcohol-based slurry of magnesium oxide in alcohol together with a small amount of Bentonite particles and heat cured organic binder.
3. The pulverizes grinding ring casting according to claim 1, wherein the mechanically intricate, lightweight, ductile material system embedded within the white cast iron matrix has a T-shape in cross section, and is provided around an entire circumference of the pulverizes grinding ring casting.
4. The pulverizes grinding ring casting according to claim 1, wherein the expanded metal is approximately No. 6 gauge and is embedded within the white cast iron forming the casting at locations not subject to normal abrasive wear.
5. The pulverizes grinding ring casting according to claim 1, wherein the mechanically intricate, lightweight, ductile material system embedded within the white cast iron comprises a large, washer-shaped flat ring surrounded by a cylinder which extends substantially around a circumference of the pulverizer grinding ring casting.
6. A pulverizer grinding ring casting, comprising:
a mechanically intricate, lightweight, ductile material system embedded within white cast iron forming the casting, the ductile material system embedded within the white cast iron forming the casting at locations not subject to normal abrasive wear having a T-shape in cross-section and comprising one of standard or flattened expanded metal providing many small ligaments of ductile material and having a thin coating of an alcohol-based slurry of fine ceramic particles in an organic binder which prevents the white cast iron from dissolving the ductile material during manufacture, but which allows the ductile material to quickly reach molten metal temperature and minimize relative thermal expansion, the thickness of the coating being selected so as to provide intimate proximity of the coated ductile material to the solidified white cast metal so that little or no movement across through cracks developing in the casting is needed before loads applied to the grinding ring casting during operation of the pulverizer during service are assumed by the embedded ductile material.
a mechanically intricate, lightweight, ductile material system embedded within white cast iron forming the casting, the ductile material system embedded within the white cast iron forming the casting at locations not subject to normal abrasive wear having a T-shape in cross-section and comprising one of standard or flattened expanded metal providing many small ligaments of ductile material and having a thin coating of an alcohol-based slurry of fine ceramic particles in an organic binder which prevents the white cast iron from dissolving the ductile material during manufacture, but which allows the ductile material to quickly reach molten metal temperature and minimize relative thermal expansion, the thickness of the coating being selected so as to provide intimate proximity of the coated ductile material to the solidified white cast metal so that little or no movement across through cracks developing in the casting is needed before loads applied to the grinding ring casting during operation of the pulverizer during service are assumed by the embedded ductile material.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86932297A | 1997-06-05 | 1997-06-05 | |
| US08/869,322 | 1997-06-05 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2239476A1 CA2239476A1 (en) | 1998-12-05 |
| CA2239476C true CA2239476C (en) | 2003-03-18 |
Family
ID=25353340
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2239476 Expired - Fee Related CA2239476C (en) | 1997-06-05 | 1998-06-03 | Ductile embedment in brittle metal parts |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA2239476C (en) |
| ES (1) | ES2158746B1 (en) |
| TW (1) | TW391899B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102858457A (en) * | 2010-04-23 | 2013-01-02 | Fl史密斯公司 | Wearable surface for a device configured for material comminution |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| RU2719148C1 (en) * | 2016-04-08 | 2020-04-17 | Метсо Свиден Аб | Crusher containing replaceable protective armor |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB888007A (en) * | 1957-05-24 | 1962-01-24 | Nordberg Manufacturing Co | Improvements in or relating to crushers, grinders and like machines |
| US3473746A (en) * | 1966-01-12 | 1969-10-21 | Erik Arne Sabel | Wearing parts for crushers |
| GB1173241A (en) * | 1967-01-09 | 1969-12-03 | Exchem Holdings | Improvements relating to Gyratory Crushers |
| US3503564A (en) * | 1967-08-24 | 1970-03-31 | Nordberg Manufacturing Co | Bowl for crushers and the like |
| US3587987A (en) * | 1969-03-07 | 1971-06-28 | Nordberg Manufacturing Co | Segmented crusher liner |
| US3612421A (en) * | 1969-07-22 | 1971-10-12 | Erik Arne Sabel | Wearing parts for crushers |
| EP0070338A1 (en) * | 1981-07-21 | 1983-01-26 | Mazel (1980) Limited | Improvements in or relating to crushing, material-reducing and like machines |
| JP2820890B2 (en) * | 1994-06-06 | 1998-11-05 | 株式会社神戸製鋼所 | Mantle or concave for swirling crusher |
-
1998
- 1998-04-06 ES ES9800736A patent/ES2158746B1/en not_active Expired - Fee Related
- 1998-05-04 TW TW87106855A patent/TW391899B/en not_active IP Right Cessation
- 1998-06-03 CA CA 2239476 patent/CA2239476C/en not_active Expired - Fee Related
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102858457A (en) * | 2010-04-23 | 2013-01-02 | Fl史密斯公司 | Wearable surface for a device configured for material comminution |
Also Published As
| Publication number | Publication date |
|---|---|
| ES2158746B1 (en) | 2002-03-01 |
| TW391899B (en) | 2000-06-01 |
| CA2239476A1 (en) | 1998-12-05 |
| ES2158746A1 (en) | 2001-09-01 |
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