STRUCTURES

[0001] This application claims priority to US Provisional Application 61/264461 field on November 25, 2009.

TECHNICAL FIELD

[0002] The present invention relates to mechanical engineering of refractory metal to improve strength thereof at a high temperature. In particular, the present invention relates to surface mechanical engineering of refractory metal sheet materials such as precious metal sheet materials to improve mechanical strength thereof at a high temperature. The present invention is useful, e.g., in fabricating refractory metal structures operating at a high temperature such as molten glass melting, conditioning, delivery and forming apparatuses.

BACKGROUND

[0003] Precious metals alloys, such as platinum and platinum-rhodium have long been used as the "glass contact" material for high quality melting, conditioning, delivery and forming systems. These and other precious metals are used because of their relatively high inert behavior in regards to contamination of molten glass with defects such as gaseous and solid inclusions as well as their strength at glass system

temperatures. This inert behavior and strength comes at a price, the high cost of the precious metal. Because of the high cost of the precious metals, there has always been an incentive to reduce the total amount of precious metals in a given glass melting system. The easiest way to reduce the quantity of metal in a system is to thin the metal. From a glass contact defect generation standpoint, this is not an issue. From a metal strength standpoint, this can present significant issue. For structures that are self supporting, the stress on the metal is inversely proportional to its thickness. For example, reducing the metal thickness by 50% will double the stress on the metal. The impact of this increased stress on the structural integrity and sag of this thinned down structure is compounded because the sag or creep rate is proportional to the stress raised to the first to fifth power, depending on the stress regime of the precious metals. If the system is operating in the regime where creep rate is proportional to stress to the fifth power, a doubling of the stress will result in a creep rate 32 times higher for a 50% decrease in metal thickness. [0004] The problem of sag or creep of thin structures is a fairly common issue over a number of industries, not just the glass industry. This issue has been addressed in a number of ways, other than simply thickening the structure walls. One of the most common ways is corrugation or ribbing. This is commonly seen in metal cans, like coffee cans, that have ribbing around their periphery to stiffen them. There are other more exotic structures that are also forms of ribbing, but in a multi-dimensional way. These complex structures increase the moment of inert of a structure, without increasing metal thickness. However, applying ribbing to structures requires specialized metal forming equipment to allow the rib formation in the metal.

[0005] Therefore, there exists a need for a simple, effective method for creating creep -resistant metal structures, especially those operating at high temperatures, such as those used in a glass melting, conditioning, delivery and forming system.

[0006] The present invention satisfies this and other needs.

SUMMARY

[0007] Several aspects of the present invention are disclosed herein. It is to be understood that these aspects may or may not overlap with one another. Thus, part of one aspect may fall within the scope of another aspect, and vice versa. Unless indicated to the contrary in the context, the differing aspects shall be considered as overlapping with each other in scope.

[0008] Each aspect is illustrated by a number of embodiments, which, in turn, can include one or more specific embodiments. It is to be understood that the embodiments may or may not overlap with each other. Thus, part of one embodiment, or specific embodiments thereof, may or may not fall within the ambit of another embodiment, or specific embodiments thereof, and vice versa. Unless indicated to the contrary in the context, the differing embodiments shall be considered as overlapping with each other in scope.

[0009] According to a first aspect of the present disclosure, provided is a method for making a metal structure comprising a metal sheet having a first major surface and a second major surface, comprising the following steps:

(i) selectively grit-blasting a plurality of parts of the first major surface of the metal sheet at a temperature lower than the annealing temperature of the metal sheet to impart a stress in the metal sheet; (ii) subjecting the metal sheet to a heat treatment at a temperature not lower than T(anneal) - 400°C so that the stress relaxes and the metal sheet deforms, such that the deformed metal sheet exhibits a higher strength in a desired direction than before step (i) is carried out.

[0010] In certain embodiments of the first aspect of the present disclosure, in step (ii), the metal sheet is subjected to a heat treatment at a temperature not lower than T(anneal) - 300°C, in certain embodiments not lower than T(anneal) - 200°C, in certain embodiments not lower than T(anneal) - 100°C, in certain embodiments not lower than T(anneal) - 50°C, in certain embodiments not lower than T(anneal), in certain

embodiments not lower than T(anneal) + 100°C, in certain embodiments not lower than T(anneal) + 200°C, in certain embodiments not lower than T(anneal) + 300°C, in certain embodiments not lower than T(anneal) + 400°C.

[0011] In certain embodiments of the first aspect of the present disclosure, at the end of step (i), the grit-blasted parts of the first major surface comprises a first array of bands that do not intersect with one another.

[0012] In certain embodiments of the first aspect of the present disclosure, at the end of step (i), the grit-blasted parts of the first major surface comprises a second array of bands that do not intersect with one another, and the second array of bands intersect with the first array of bands to form a web pattern.

[0013] In certain embodiments of the first aspect of the present disclosure, the metal structure operates at a temperature higher than the annealing temperature of the metal sheet.

[0014] In certain embodiments of the first aspect of the present disclosure, in step (i), both the first major surface and the second major surface of the metal sheet is grit-blasted to form complementary patterns of grit-blasted areas.

[0015] In certain embodiments of the first aspect of the present disclosure, the metal structure comprises a tube comprising the metal sheet.

[0016] In certain embodiments of the first aspect of the present disclosure, the metal sheet comprises a refractory metal.

[0017] In certain embodiments of the first aspect of the present disclosure, the metal sheet comprises platinum.

[0018] In certain embodiments of the first aspect of the present disclosure, the metal structure is a component of a molten glass handling system. [0019] In certain embodiments of the first aspect of the present disclosure, in step (i), the metal sheet is formed into the structure before grit-blasting.

[0020] In certain embodiments of the first aspect of the present disclosure, in step (i), at least part of the metal sheet is grit-blasted before formed into the structure.

[0021] In certain embodiments of the first aspect of the present disclosure, after step (ii), the metal sheet is cooled down to a temperature below the annealing temperature of the metal.

[0022] In certain embodiments of the first aspect of the present disclosure, step (i) comprises:

[0023] (i-1) grit-blasting the full surface of the first major surface of the metal sheet;

[0024] (i-2) further subjecting a plurality of parts of the first major surface grit- blasted in step (i-1) to further selective grit-blasting; and subsequently

[0025] (i-3) applying an inorganic coating over at least part of first major surface of the metal sheet.

[0026] In certain embodiments of the first aspect of the present disclosure above, in step (i-3), the inorganic coating comprises at least one of Zr0 ₂ , A1 ₂ 0 ₃ , CaO, MgO, and Ti0 ₂ .

[0027] In certain embodiments of the first aspect of the present disclosure above, in step (i-3), the inorganic coating is applied by using a method selected from plasma spray coating, sputtering, and chemical vapor deposition.

[0028] In certain embodiments of the first aspect of the present disclosure, the metal sheet has a thickness of at most 2000 im, in certain embodiments at most 1500 im, in certain other embodiments at most 1200 im, in certain other embodiments at most 1000 im, in certain embodiments at most 800 im, in certain embodiments at most 500 im, in certain other embodiments at most 300 im.

[0029] In certain embodiments of the first aspect of the present disclosure, in step (i), the grit-blasting causes dents on the blasted surface having a depth of up to 500 im, in certain embodiments up to 300 im, in certain embodiments up to 200 im, in certain other embodiments up to 100 im.

[0030] In certain embodiments of the first aspect of the present disclosure, the metal sheet comprise multiple segments bonded together at a joint, and in step (i) at least part of the joint is subjected to grit blasting. [0031] In certain embodiments of the first aspect of the present disclosure, in step (i), the grit-blasting results in a stress in the metal sheet that strengthens the joint.

[0032] In certain embodiments of the first aspect of the present disclosure, in step (i), the grit-blasting results in a stress that results in a deformation that strengthens the joint at the end of step (ii).

[0033] In certain embodiments of the first aspect of the present disclosure, at the end of step (ii), a surface of the metal sheet is deformed by showing a buckle from 200 im to 500 im, in certain embodiments from 200 im to 450 im, in certain embodiments from 250 to 450 im, in certain embodiments from 300 im to 450 im, in certain embodiments from 300 im to 400 im.

[0034] In certain embodiments of the first aspect of the present disclosure, at the end of step (ii), the stress imparted in step (i) relaxes substantially completely.

[0035] One or more embodiments of the present invention has one or more of the following advantages. First, mechanically reinforced metal structure can be fabricated by using the method at a relatively low cost, without the need of special equipment to deform the metal sheet. Second, the method when used for metal work having weld joints, can strengthen the joints and therefore the overall structure. Third, the induced curvature and deformation to the metal structure by using the method maintains even after stress relaxation at a temperature above the stress relief temperature, even above the annealing temperature, of the metal, rendering the metal structure mechanically robust even at a high operating temperatures. Fourth, the method of the present invention can be used to selectively improve the mechanical performance, especially sag resistance, in a given, desired direction. Fifth, the method of the present invention can be used on complex metal structures made of thin metal sheets with flexibility. In addition, complex surface patterns can be relatively easily created to result in desired distortion and reinforcement to the final structure. The metal structure can be strengthened using the method of the present invention after the formation of the overall structure, or prior to the formation of the overall structure, to improve the sag or creep-resistance in a desired direction thereof. Sixth, the present invention, when used for fabricating metal structures made of precious metals such a Pt and ^-containing alloys, can achieve significant cost reduction by utilizing smaller amount of metal for fabricating a metal structure having substantially the same mechanical performance. [0036] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.

[0037] It is to be understood that the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.

[0038] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In the accompanying drawings:

[0040] FIG. 1 is a schematic illustration of a top view of a grit-blasted thin Pt-Rh sheet formed from a flat, non-grit-blasted metal sheet by using an embodiment of the method of the present invention.

[0041] FIG. 2 is a schematic illustration of an end view of the same grit-blasted Pt- Rh sheet illustrated in FIG. 1 showing the distortion caused by grit-blasting.

[0042] FIG. 3 is a schematic illustration of a top view of a thin metal sheet formed from a flat non-grit-blasted metal sheet by using another embodiment of the method of the present invention.

[0043] FIG. 4 is a diagram showing the relationship between the sag (deviation from horizontal plane) as a function of the out-of-plane bow (distortion of the metal sheet caused by selective grit-blasting and post heat treatment) of a series of metal sheets prepared according to the method of the present invention.

DETAILED DESCRIPTION

[0044] Unless otherwise indicated, all numbers such as those expressing weight percents of ingredients, dimensions, and values for certain physical properties used in the specification and claims are to be understood as being modified in all instances by the term "about." It should also be understood that the precise numerical values used in the specification and claims form additional embodiments of the invention. Efforts have been made to ensure the accuracy of the numerical values disclosed in the Examples. Any measured numerical value, however, can inherently contain certain errors resulting from the standard deviation found in its respective measuring technique. [0045] As used herein, in describing and claiming the present invention, the use of the indefinite article "a" or "an" means "at least one," and should not be limited to "only one" unless explicitly indicated to the contrary.

[0046] For the convenience of description, in the present application, including the claims and the description, the term platinum shall be understood to include not just pure platinum, but also platinum-containing alloys such as Pt-Rh alloys.

[0047] As used herein, stress relief temperature (T(sr)) of a metal material is defined as T(anneal) - 400°C, where T(anneal) is the annealing temperature of the metal material.

[0048] The method of the present invention can be used to make any metal structures comprising a metal sheet. The metal structure is advantageously used at an elevated operating temperature not lower than T(sr) of the metal, even not lower than T(anneal) thereof. For example, the metal structure can be used at a temperature not lower than T(sr) + 100°C, or not lower than T(sr) + 200°C, or not lower than T(sr) + 300°C, or not lower than T(sr) + 400°C, or not lower than T(sr) + 500°C, or not lower than T(sr) + 600°C, or not lower than T(sr) + 700°C, or not lower than T(sr) + 800°C. However, the metal structure may operate at a temperature lower than T(sr) or T(anneal) as well. A particularly interesting application of the method of the present invention is for making the metal structures, especially precious metal structures, for making glass articles involving the melting, conditioning, delivery and forming of glass. Depending on the application, the metal materials can be platinum, platinum- containing alloys such as Pt- Rh alloys, iron, nickel, nickel alloys such as inconel, aluminum and alloys thereof, copper and alloys thereof, titanium and alloys thereof, chromium and alloys thereof, zinc and alloys thereof, and the like. A Pt-Rh typically has a T(anneal) about 1000°C, thus a T(sr) of about 600°C. In addition, the metal sheet does not have to have a homogeneous chemical composition. In certain embodiments, the metal sheet may be a sheet comprising a plurality of smaller sheets joined together by welding, riveting, and other physical and/or chemical means. In other embodiments, the metal sheet may comprise a plurality of layers having differing chemical compositions. For example, the metal sheet could be a zinc-plated steel sheet, a gold-plated copper sheet, a platinum-plated steel sheet, and the like. The present invention will be further illustrated in the context of a glass making system using Pt-containing materials for the metal sheet. However, one having ordinary skill in the art should readily appreciate that the present invention can be applied to other materials and metal structures. [0049] The present invention is particularly advantageous for making metal structures using metal sheets having a thickness of at most 2000 im, in certain

embodiments at most 1500 im, in certain other embodiments at most 1200 im, in certain other embodiments at most 1000 im, in certain embodiments at most 800 im, in certain embodiments at most 500 im, in certain other embodiments at most 300 im. Thin metal sheets tend to be flexible in the direction in which it has a low overall thickness.

Enhancement to the mechanical performance, especially sag and creep resistance, of a thin sheet can be significant by using the present invention, especially where the increase of overall thickness of the thickness of the sheet in the desired direction is pronounced compared to the original thickness of the metal sheet.

[0050] In certain embodiments, the method of the present invention can result in an increase of the thickness of the metal sheet in a given desired direction of at least 10%, in certain embodiments at least 20%, in certain embodiments at least 30%, in certain other embodiments at least 50%, in certain other embodiments at least 80%, in certain other embodiments at most 100%. The percentage of thickness increase (IC%) is calculated by IC% = ((T2 - Τ1) Γ1)*100%, where T2 is the overall thickness of the metal sheet at the end of step (ii), and Tl is the overall thickness of the metal sheet before step (i). The meaning of T2 and Tl is illustrated in FIG. 2. Clearly, due to the overall thickness increase, the creep-resistance of the metal sheet in the direction perpendicular to the end of the sheet is increased.

[0051] In certain embodiments of the method of the present invention, at the end of step (ii), a surface of the metal sheet is deformed by showing a buckle from 200 im to 500 im, in certain embodiments from 200 im to 450 im, in certain embodiments from 250 to 450 im, in certain embodiments from 300 im to 450 im, in certain embodiments from 300 im to 400 im. By "buckle" is meant the difference between T2 and Tl (i.e., T2 - Tl), where the meaning of T2 and Tl is given above.

[0052] In general, in the method of the present invention, to increase the sag or creep resistance of the metal sheet in a given direction, it is desired to increase the overall thickness of the cross-section of the metal sheet obtained by intersecting the metal sheet with a plane perpendicular to the given direction. Thus, for example, with respect to a metal having two major dimensions x and y perpendicular to each other in one of its major surfaces, if higher creep -resistance is desired in x direction, it is desired that the overall thickness of a cross-section of the metal sheet in z direction, when the sheet is intersected by a plane parallel to the x direction, is increased. The method of the present invention, by selectively grit-blasting part of a major surface of the metal sheet, can create desired deformation to the metal sheet which leads to the increase of overall thickness of the metal sheet in a desired direction, thereby increasing the strength in the desired direction.

[0053] In certain embodiments of the method of the present invention, at the end of step (i), the grit-blasted parts of the first major surface of the metal sheet form discrete arrays, such as discrete stripes, circles, squares, hexagons, and the like. FIG. 1 shows a schematic top view in plane xoy of a first major surface of a Pt sheet 101 having subjected to selective grit-blasting that results in alternating stripes of 103 that are grit-blasted, and alternating stripes of 105 that are not grit-blasted. All grit-blasted and non-grit-blasted stripes extend in the direction of the x axis. FIG. 3 shows a schematic top view in a plane xoy of a major surface of a metal sheet 301 having subjected to selective grit-blasting that results in a plurality of hexagonal grit-blasted parts 303 intertwined with non-grit-blasted areas 305.

[0054] Grit-blasting involves impinging a surface with a stream of solid particles (grits) such as sand, metal beads, plastic beads, and the like. Grit-blasting has been used in metal engineering to clean metal surfaces, prepare surfaces for coatings and to impart artistic and/or functional surface pattern features. The kinetic energy of the individual grits impacting the metal surface causes dents on the metal surface, which subject the metal surface to a compressive stress. It is known that the compressive stress distribution on the metal sheet surface can cause the metal sheet to deform. In certain embodiments, in step (i), the grit-blasting causes dents on the blasted surface having a depth of up to 500 zm, in certain embodiments up to 300 zm, in certain embodiments up to 200 zm, in certain other embodiments up to 100 im.

[0055] However, traditionally, after selective grit-blasting, the metal sheet is maintained at a temperature below the annealing temperature of the metal, such that the compressive stress and the resulting enhanced mechanical performance is preserved. It has been perceived that, if the metal is allowed to anneal and relax at an elevated temperature, such as at not lower than T(sr) thereof, including but not limited to not lower than T(sr) + 100°C, not lower than T(sr) + 200°C, not lower than T(sr) + 300°C, not lower than T(anneal), not lower than T(sr) + 500°C, not lower than T(sr) + 600°C, not lower than T(sr) + 700°C, not lower than T(sr) + 800°C, even not lower than T(sr) + 900°C, for a sufficient period of time, the surface compressive stress will decrease or fade away, resulting in reduction and/or complete elimination of the mechanical performance gain.

[0056] The present invention is based on, in part, the discovery that selectively grit- blasted metal sheet can deform to form structures that withstand a high temperature above the annealing temperature of the metal with enhanced mechanical performance, especially enhanced sag and creep resistance. By choosing the correct grit-blasting patterns, creep- resistance in desired direction was improved, even if the metal sheet was heated to a temperature significantly higher than the annealing temperature of the metal.

[0057] Grit-blasting equipment is widely available on the market. These equipment can be used in the method of the present invention, with or without modification, to engineer a given metal sheet. Grit materials, grit speed, metal temperature, grit dwell time at various locations, are all variables that can be controlled and adjusted to achieve selective grit-blasting of multiple parts of the first major surface of the metal sheet to form surface features having differing depth, width and length. Depending on the application and the desired effect, a manual grit-blaster or an automatic grit-blaster controlled by a microprocessor may be used.

[0058] Desired grit-blast pattern can be formed by, inter alia, the use of a pattern mask and/or controlled movement of the grit-blaster relative to the surface of the metal sheet to be blasted. Simple masks made of plastic, rubber or wood may be used to allow blasting grits to pass to impinge on selected areas of the surface only. Programmed or programmable grit-blasting machine can be advantageously used, with or without the assistance of a mask, to create the desired patterns.

[0059] In general, linear blast area patterns lead to anisotropic stress distribution on the surface, which can result in preferential deformation, buckling and reinforcement in a given direction. For example, for the metal sheet illustrated in FIG. 1, at the end of step (i), buckling in the y direction can be observed. At the end of step (ii), the deformation is manifested by an end view of the sheet schematically shown in FIG. 2. As can be seen as a result of steps (i) and (ii), the overall thickness of the sheet in the z direction (i.e., peak- to-valley distance in the yoz cross-section) increased, given that the metal sheet was essentially flat before step (i) was carried out. Therefore, if preferential buckling in a given direction of the sheet is desired, linear blast patterns comprising multiple stripes that do not intersect may be desired. [0060] On the other hand, if the blast area is substantially evenly distributed in the surface, resulting in substantially the same pattern in both x and y directions, the metal sheet will distort and deform at the end of steps (i) and (ii) in both x and y directions, leading to reinforcement of the sheet in both directions. For example, the complex pattern of FIG. 3 results in deformation of the sheet in both x and y directions at the end of step (ii). Other patterns such as intersecting stripes can be used to achieve similar effect.

[0061] Grit-blasting of a single major surface of the metal sheet can be sufficient in many cases to cause the desired amount of distortion at the end of step (ii). In certain embodiments, in order to maximize the distortion, and hence the resulting strengthening to the sheet, it is desirable to grit-blast both major surfaces in such a way that the distortion caused by the blasting on both surface complement and reinforce each other.

[0062] Step (i) of grit-blasting can be carried out before or after the metal sheet is formed into the metal structure of desired structure and geometry. For example, a flat sheet may be first subjected to grit-blasting, and the deformed sheet can then be engineered into a desired two- or three-dimensional structure such a tube. Alternatively, a flat sheet may be formed into a desired two- or three-dimensional structure such as a tube first, then subjected to the step (i) of grit-blasting.

[0063] Likewise, step (ii) of heat treating the metal sheet can be carried out before or after the metal sheet is formed into the metal structure of desired shape and geometry. For example, a flat sheet can be first grit-blasted, then subjected to step (ii) of heated treatment to allow it to deform, and subsequently formed into a desired two- or three- dimensional structure or geometry such as a tube. Alternatively, a flat sheet can be first grit-blasted, then formed into a desired two-or three-dimensional structure such as a tube, and then subjected to step (ii) of heat treatment to deform into the final structure and shape.

[0064] Step (ii) of heat treatment has the characteristic that it is carried out at a temperature sufficient to relax the stress induced to the metal sheet as a result of step (i). Thus the heat treatment temperature in step (ii) is not lower than T(sr) defined above, such as not lower than T(sr) + 100°C, not lower than T(sr) + 200°C, not lower than T(sr) + 300°C, not lower than T(anneal), not lower than T(sr) + 500°C, not lower than T(sr) + 600°C, not lower than T(sr) + 700°C, not lower than T(sr) + 800°C, even not lower than T(sr) + 900°C. By "close to the annealing temperature" is meant the temperature is not lower than T(anneal) - 50°C, where T(anneal) is the annealing temperature of the metal. As is understood by one having ordinary skill in the art, the lower the temperature of the heat treatment, the longer it takes for the stress to relax completely and for the distortion to stabilize. For a metal structure made using the method of the present invention that operates at a temperature higher than or close to the metal annealing point, the step (ii) of heat treatment may be carried out, in part or in whole, during the operation of the metal structure. For example, the platinum glass melting, conditioning, delivery and handling system typically operates at a temperature close to or higher than the annealing point of platinum. Therefore, for these metal structures, step (ii) may be carried out during the operation of the glass melting process, or before the glass melting process is initiated.

[0065] In certain embodiments, it is desirable to coat a surface, especially a surface exposed to ambient atmosphere, of the metal structure, to impart desirable properties thereto, such as oxidation resistance, water resistance, corrosion resistance, and the like. The coating can be organic, inorganic, or a combination or mixture of both. For example, molten glass delivery and handling devices are typically coated with a layer of refractory oxide, such as Zr0 ₂ on the external surface to improve oxidation resistance thereto.

When a coating is desired, it is advantageous to grit-blast the full surface of the metal structure to improve the adhesion of the coating thereto, advantageously non- discriminately, followed by preferential, selective grit-blasting of step (i), and

subsequently a step of applying the inorganic or organic coating to the surface, in part or in whole. The final coating step may be carried out prior to or after the step (ii) of heat treatment. In certain embodiments, it is advantageous to carry out step (ii) prior to the application of the inorganic coating, especially where the coefficient of thermal expansion of the coating and the metal sheet being coated differ significantly. This is because in step (ii), the deformation of the metal structure may cause the delamination of a pre- applied coating. Refractory inorganic coating materials that can be applied include, but are not limited to, Zr0 ₂ , A1 ₂ 0 ₃ , CaO, MgO, Ti0 ₂ and mixtures and combinations thereof. Application of the coating material can be carried out using such methods as plasma spray coating, sputtering, and chemical vapor deposition.

[0066] Joints, including those formed mechanically and chemically, such as weld joints, riveted joints, bolted joints, threaded joints, and the like, tend to be the weak link of a metal structure. Failures tend to occur in these areas. Therefore, strengthening of the joints in a multi-segment metal structure is particularly useful. The method of the present invention can be advantageously used to achieve this effect. By subjecting the joint area of the metal structure to steps (i) and (ii) in a desired direction, strengthening of the joint can be realized.

[0067] In an experiment, samples of 0.040 inch (1000 μπ thick by ¾ inch (1.90 cm) wide Pt-Rh alloy stripes comprising 20 wt% rhodium (Pt-20Rh) were tested at 1265°C for their sag resistance by placing them in a refractory fixture that had a 7 inch (17.8 cm) unsupported span. Additional weight was added to the center of the unsupported span to increase the maximum stress on the span to 1305 psi (8.998xl0 ³ kpa).

[0068] Periodically during the test, the samples were removed from the furnace and the amount of sag from the original horizontal plane was measured. The more the sample sagged, the worst the creep or sag resistance. During the testing, it was noted that there was a range in sag level from 0.5 inches (12.7 mm) to 1.4 inches (3.56 cm). Further analysis indicated that samples had various degrees of out-of-plane bow along their length due to the grit blasting. FIG. 4 below shows this bow along the length of the samples. In FIG. 4, the horizontal axis shows the out-of-plane curvature of the sample, i.e., the thickness increase, and the vertical axis shows the total sag, i.e., the distance from the lowest point of the sample to the horizontal line it began from in the experiment. As can be seen, the larger the out-of-plane bow, the smaller the sag. A three-times reduction in sag is observed when the out-of-plane bow is increased from 0.02 inch (500 zm) to about 0.042 inch (about 1067 zm).

[0069] This discovery is significant because a three-times reduction in sag is seen with curvatures on the order of the thickness of the metal being tested.

[0070] It will be apparent to those skilled in the art that various modifications and alterations can be made to the present invention without departing from the scope and spirit of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.