Abstract:
An improved centrifugally-cast tube is provided, along with a related method and apparatus of making the tube. The tube includes a plurality of grooves and bosses that are mechanically machined into an interior surface of the tube, such as by a broaching process The profile of grooves and bosses may be defined by a plurality of intersecting concave and convex radii. The tube is resistant to creep, carburization and metal dusting. The table also has an enhanced heat transfer rate, and a desirable surface roughness. A method of making the tube is also provided. The method essentially consists of mechanically deforming the interior surface of the tube by passing a series of cutting inserts on support rings having incrementally-differing dimensions over the interior surface. An apparatus is also provided which includes a telescoping shaft upon which is mounted at least one cutting tool that has a plurality of cutting inserts and is adapted to form a plurality of grooves and bosses in the interior surface of the tube. The grooves and bosses may be straight or spiraled.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field Of The Invention  
         [0002]     The present invention relates to furnace equipment for use in petrochemical plants, and more particularly, to improved centrifugally-cast tubes for use in such equipment and to a method and apparatus of making such tubes.  
         [0003]     2. Description Of The Related Art  
         [0004]     It is well known that there are two basic types of furnaces used in petrochemical plants, one being “steam cracker” furnaces, and the other being “steam reformer” furnaces. Steam cracker furnaces are mainly used to make ethylene, and steam reformer furnaces are mainly used to make hydrogen. Both types of furnaces include a number of tubes, generally arranged vertically, that form a continuous flow path, or coil, through the furnace. The flow path or coil includes an inlet and an outlet In both types of furnaces, a mixture of a hydrocarbon feedstock and steam are fed into the inlet and passed through the tubes. The tubes are exposed to extreme heat generated by burners within the furnace. As the feedstock/steam mixture is passed through the tubes at high temperatures the mixture is gradually broken down such that the resulting product exiting the outlet is ethylene in the case of a steam cracker furnace and hydrogen in the case of a steam reformer furnace.  
         [0005]     The petrochemical industry has in the past recognized at least three desirable features in a steam cracker or steam reformer furnace. First, it is important to maximize the heat transfer rate from the furnace burners through the walls of the tubes and into the mixture of hydrocarbons and steam in order to increase the efficiency of the faire. Second, it is important to make furnace tubes from materials that are resistant to what is known in the metallurgical are as “creep”. Third, it is important to make furnace tubes so as to be resistant to corrosion, carburization and metal dusting.  
         [0006]     With regard to the second important feature, “creep” is basically the gradual elongation of a metal when placed under stress and subjected to high temperatures. Various creepiresistant alloys are known to those of skill in the art. Two main methods have developed within the industry of making furnace tubes with creep-resistant alloys, one being to extrude the tube, and the other being to centrifugally cast the tube. A centrifugally-cast tube is one formed by pouring an alloy in liquid form into a tubular mold that is rotating at a high speed. The alloy is allowed to cool so as to form the centrifugally-cast tube. The internal bore of the tube is then mechanically-machined by boring to achieve the desired inner diameter, resulting in a cylindrical tube having a circular cross section with a generally constant inner and outer diameter. The industry has discovered, however, that centrifugally-cast tubes exhibit superior creep properties in comparison to extruded tubes. In particular, upon inspecting cross-sections of extruded and centrifugally-cast tubes, the industry has discovered that extruded tubes have a very fine grain metallurgical structure, whereas centrifugally-cast tubes have much larger, and columnar, grains. Further, extruded tubes have a lower carbon content when compared to the carbon content of centrifugally-cast tubes. The larger, columnar grains and higher carbon content are what give the centrifugally-cast tubes superior creep properties in comparison to the fine grain microstructure and lower carbon content of extruded tubes.  
         [0007]     One approach to achieving two of the above-identified objectives is disclosed in U.S. Pat. No. 6,250,340 (“the &#39;340 patent”). In particular, the &#39;340 patent discloses a method of modifying a centrifugally-cast tube by adding a series of longitudinally-disposed fins and valleys in the typically-circular internal bore of the tube. In this manner, the internal surface area of the tube is increased, thereby increasing the heat-transfer rate therethrough. As such, the &#39;340 patent results in a tube that is resistant to creep (since it is centrifugally cast from a creep-resistant alloy) and has an increased heat transfer rate (y virtue of its modified internal profile). A key drawback to the tube disclosed in the &#39;340 patent however, is that it is not resistant to corrosion, carburization or metal dusting. This is because the tube in the &#39; 340 patent is made using an electrochemical machining (CECM method, as opposed to a mechanical machining process (e.g., the boring process traditionally used to provide the desired diameter in a centrifugally-cast tube). As is known in the art, use of the ECM method results in an electropolished surface and does not provide adequate deformation and/or orientation of the subsurface or material lattice of the inner surface of the tube. In this regard, it is well known that an electropolished surface is not resistant to corrosion, carburization or metal dusting. See, e.g., MATERIALS AND COROSION,  Carburization, Metal Dusting and Carbon Deposition , ISSN 0947-5117, Vol. 49, No. 4/5, April/May 1998, pp. 221-225 and 328-335. These articles compare the effect of machining or any other surface deformation (e.g., grinding, blasting, peening, honing, etc.) to electropolishing and clearly show the advantage of conventional machining over electropolishing on resistance to carburization and metal dusting. An additional drawback to the ECM process is that it results in a tube having an interior surface with an inferior surface roughness and dimensional accuracy when compared to the interior surface that has been prepared by mechanical machining. A still further drawback to the ECM process is that it is more expensive relative to the cost of mechanical machining.  
         [0008]     As such, there remains a need in the art for a centrifugally-cast tube, and method and apparatus of ranking same, that (1) has an increased heat-transfer rate, (2) is resistant to creep, (3) is resistant to corrosion, carburization and metal dusting, (4) has a desirable surface roughness and dimensional accuracy, and (5) is cost-efficient. The present invention has been developed to overcome the foregoing deficiencies and meet the above-described needs.  
       SUMMARY OF THE INVENTION  
       [0009]     In one aspect the present invention may be a centrifugally-cast tube comprising: a tubular body made from a creep-resistant alloy, the body having an exterior surface and an interior surface, the interior surface including a plurality of bosses and a plurality of grooves, and the plurality of bosses and grooves being mechanically machined into the interior surface. Another feature of this aspect of the invention may be that the plurality of bosses and grooves are defined by a plurality of intersecting concave and convex radii. Another feature of this aspect of the invention may be that the plurality of bosses and grooves are formed by a broaching process. Another feature of this aspect of the invention may be that the interior surface is resistant to carburization and metal dusting. Another feature of this aspect of the invention may be that the surface roughness and dimensional accuracy of the interior surface is superior to that of a centrifugally-cast table wherein the plurality of bosses and grooves are formed by a electrochemical machining process. Another feature of this aspect of the invention may be that the plurality of bosses and grooves form a profile that is at least 5% longer than a circumference of a smallest circle that encompasses the entire resulting profile. Another feature of this aspect of the invention may be that the interior surface has a surface roughness of less than 125 RMS (CLA).  
         [0010]     In another aspect, the present invention may include a method of improving a centrifugally-cast tube comprising: passing a first cutting tool having a plurality of first cutting inserts through a bore of the tube so as to mechanically remove a first quantity of material from the bore and to commence the formation of a plurality of grooves and bosses; passing a second cutting tool having at plurality of second cutting inserts through the bore so as to-mechanically remove a second quantity of material from the bore and to continue the formation of the plurality of grooves and bosses, the plurality of second cutting inserts having dimensions different than corresponding dimensions of the plurality of first cutting inserts; and continuing to pass additional cutting tools having a plurality of additional cutting inserts through the bore so as to continue to mechanically remove additional quantities of material from the bore until a desired profile of grooves and bosses is achieved, each set of additional cutting tools having dimensions different than corresponding dimensions of the cutting inserts employed in the immediately preceding pass. Another feature of this aspect of the invention may be that the dimensions of each subsequent set of cutting tools are larger than the corresponding dimensions of the cutting tools employed in the immediately preceding pass. Another feature of this aspect of the invention may be that the dimensions incrementally increase at a rate of between 0.05 mm and 0.1 mm per pass.  
         [0011]     In another aspect, the present invention may include a method of improving a centrifugally-cast tube comprising forming a plurality of grooves and bosses in an interior surface of the tube by mechanically deforming the interior surface. Another feature of this aspect of the invention may be that the plurality of grooves and bosses are gradually formed by passing a series of cutting tools having incrementally-differing dimensions over the interior surface.  
         [0012]     In yet another aspect, the present invention may include an apparatus for improving a centrifugally-cast tube comprising: a first and a second guide adapted to hold the tube; a telescoping shaft adapted to pass longitudinally through a bore of the tube; and at least one cutting tool attached to the shaft and including a plurality of cutting inserts, the cutting tool being adapted to mechanically form a profile of grooves and bosses within the bore of the tube. Another feature of this aspect of the invention may be that the cutting inserts are indexable. Another feature of this aspect of the invention may be that the cutting inserts are provided with concave cutting surfaces. Another feature of this aspect of the invention may be that the cutting inserts are provided with convex cutting surfaces. Another feature of this aspect of the invention may be that the at least one cutting tool includes a plurality of cutting tools attached to the shaft, and further including at least one spacer collar disposed about the sit and adapted to maintain the cutting tools in spaced relationship. Another feature of this aspect of the invention may be that the apparatus may filter include means for lubricating the apparatus. Another feature of this aspect of the invention may be that the apparatus may further include means for attaching the at least one cutting tool to the shaft. Another feature of this aspect of the invention may be that the apparatus may further include means for attaching the at least one cutting tool to the shaft. Another feature of this aspect of the invention may be that the apparatus may farther include at least one centering guide. Other features and aspects of the present invention will be explained below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  illustrates a cross-sectional view of one embodiment of a centrally-cast tube of the present invention.  
         [0014]      FIG. 2  is a cross-sectional view similar to  FIG. 1 , but illustrating certain dimensions of a specific embodiment of a tube of the present invention.  
         [0015]      FIG. 3  is a schematic illustration of an apparatus of the present invention.  
         [0016]      FIG. 4  is a longitudinal view in partial cross-section that illustrates a specific embodiment of a cutting assembly for use in making a tube of the present invention.  
         [0017]      FIG. 5  is a top view of a support ring of the present invention.  
         [0018]      FIG. 6  is a side cross-sectional view of the support ring shown in  FIG. 5 .  
         [0019]      FIG. 7  is a top view of a cutting tool of the present invention which includes a support ring with a plurality of concave cutting inserts attached thereto.  
         [0020]      FIG. 8  is a side cross-sectional view of the cutting tool illustrated in  FIG. 7 .  
         [0021]      FIG. 9  is a top view of a cutting tool of the present invention which includes a support ring with a plurality of convex cutting inserts attached thereto.  
         [0022]      FIG. 10  is a side cross-sectional view of the cutting tool illustrated in  FIG. 9 .  
         [0023]      FIG. 11  is side view, partially in cross-section, of another specific embodiment of a cutting assembly of the present invention.  
         [0024]      FIG. 12  is an end view of a collar of the present invention.  
         [0025]      FIG. 13  is a side view of a specific embodiment of a collar of the present invention.  
         [0026]      FIG. 14  is a side view of a specific embodiment of a collar of the present invention,  
         [0027]      FIG. 15  is a side view of a specific embodiment of a collar of the present invention.  
         [0028]      FIG. 16  is a side view of a specific embodiment of a collar of the present invention.  
         [0029]      FIG. 17  is a cross-sectional view taken along line  17 - 17  of  FIG. 11C , and illustrates a cross-section of a slide ring of the present invention.  
         [0030]      FIG. 18  illustrates a specific embodiment of a cutting tool of the present invention, and, in particular, illustrates a maximum diameter cutting tool that is used for cutting-grooves in a tube of the present invention.  
         [0031]      FIG. 19  illustrates a specific embodiment of a cutting tool of the present invention, and, in particular, illustrates a minimum diameter cutting tool that is used for cutting grooves in a tube of the present invention.  
         [0032]      FIG. 20  illustrates a specific embodiment of a cutting tool of the present invention, and, in particular, illustrates a minimum diameter cutting tool that is used for cutting bosses in a tube of the present invention,  
         [0033]      FIG. 21  illustrates a specific embodiment of a cutting tool of the present invention, and, in particular, illustrates a maximum diameter cutting tool that is used for cutting bosses in a tube of the present invention.  
         [0034]      FIG. 22  is a top view of a cutting insert of the present invention and illustrates that the cutting insert can be indexable to maximize the useful life of the cutting insert.  
         [0035]     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     Referring to the drawings in detail, wherein like numerals denote identical elements throughout the several views, there is shown in  FIG. 1 a  specific embodiment of a centrifugally-cast tube  10  constructed in accordance with the present invention. In a specific embodiment, the tube  10  may include a body  12  made from a creep-resistant alloy, such as, for example, Manaurite 36X. The body  12  may include an exterior surface  14  and an interior surface  16 . The interior surface  16  may include a plurality of bosses  18  and a plurality of grooves  20  disposed therebetween. Awle the specific embodiment shown in  FIG. 1  illustrates a tube  10  having eight bosses  18  and eight grooves  20 , those numbers should not be taken as a limitation, but, instead, the present invention covers any number of bosses  18  and grooves  20 . In a specific embodiment, as shown in  FIG. 2 , the peaks of the bosses  18  may intersect a circle C 1  having a radius R 1 . In a specific embodiment, the radius R 1  may be 19 millimeters (mm). Similarly, the lowermost points of the grooves  20  may intersect a circle C 2  having a radius R 2 . In a specific embodiment, the radius R 2  may be 22.5 mm. In a specific embodiment, the radius R 3  of each groove  20  may be 5 mm and the radius R 4  of each boss  18  may be 5 mm. In a specific embodiment, the resulting profile results from the intersection of a plurality of concave and convex radii at the bottom and top of the grooves  20  and comprises an integer number of grooves  20  and bosses  18 . In a specific embodiment, the length of the resulting profile of the internal surface  16  is at least 5% longer than the circumference of the smallest circle that encompasses  20  the entire resulting profile. In a specific embodiment, the depth of the grooves  20  may be in the range of from 3.5 mm to 6.35 mm, but larger and smaller depths are also encompassed by the present invention.  
         [0037]     Each of the plurality of bosses  18  and grooves  20  is mechanically machined into the interior surface  16  so as to deform and orient the subsurface or material lattice of the interior surface  16  of the tube  10 . As such, the result is a centrifugally-cast tube  10  that is resistant to corrosion, carburization and metal dusting. In addition, since the tube  10  is centrifugally cast, it is resistant to creep, and since the addition of the bosses  18  and grooves  20  result in an increased surface area of the interior surface  16  when compared to the interior surface of a tube of circular cross-section, the tube  10  also has an improved heat-transfer rate.  
         [0038]     The present invention also includes a new method and apparatus of manufacturing the tube  10 . A specific embodiment of an apparatus  22  of the present invention is shown schematically in  FIG. 3 . A plurality of tubes  24  that have been centrifugally cast are positioned on a platform  26  proximate the apparatus  22 . The tubes  24  are of the centrifugally-cast type that have been traditionally formed by pouring an alloy in liquid form into a tubular mold that is rotating at a high speed. The alloy is allowed to cool so as to form the centrifugally-cast tube. The internal bore of the tube is then mechanically-machined by boring to achieve the desired inner diameter, resulting in a cylindrical tube having a circular cross section with a generally constant inner and outer diameter. In a specific embodiment, the inner diameter may be advantageously chosen equal to the diameter of the smallest circle that intersects the bosses  18 . A crane  28  is used to lift and rotate a tube  30  from the platform  26  into position on the apparatus  22 . The tube  30  is then aligned and secured between a first guide  32  and a second guide  34 . The apparatus  22  also includes a broaching machine  36  that has a telescoping shaft  38  extending therefrom. In a specific embodiment the broaching machine  36  may be a Berthier model having a travel of 10 meters and a 50 KW power rating. The apparatus  22  may also include oil tanks  40  and  42  for providing lubrication to the broaching machine  36 . In a specific embodiment, the lubricating oil may be of the type sold under the name “PERFOLUB 40” by Wynns, 92 Courbevoie, France. As will be described in more detail below, the present invention provides for a series, of cutting tools to be mounted to the shaft  38  and then passed lengthwise through the tube  30 . Numerous passes are contemplated with cutting tools of gradually increasing size so as to gradually shear away metal shavings from the interior surface  16  of the tube  30  until the profile illustrated, for example, in  FIGS. 1 and 2  is achieved. The details of the cutting tools will now be explained.  
         [0039]     With reference to  FIG. 4 , a specific embodiment of a cutting assembly  44  is shown attached via an attachment mechanism  46  to the shaft  38  of the broaching machine  36 . The cutting assembly  44  may include a shaft extension  48  having a central lubricating channel  50  disposed therein with a plurality of radial lubricating channels  52  leading therefrom to an outer surface  54  of the shaft extension  48 . The lubricating channels  50  and  52  ate in fluid communication with one or more of the oil as  40 . The cutting assembly  44  may include a first centering guide  56  and a second centering guide  58  disposed at opposite ends of the shaft extension  48 . The first centering guide  56  may be disposed adjacent the attachment mechanism  46 . The cutting assembly  44  includes a plurality of cutting tools  60  disposed about the shaft extension  48 . The specific embodiment of the cutting assembly  44  shown in  FIG. 4  includes four cutting tools  60 , but the present invention is not limited to any particular number of cutting tools  60 . Each cutting tool  60  includes a support ring  62  and a plurality of cutting inserts  64 . In a specific embodiment, the cutting inserts  64  may be made of carbide and have a cutting radius of 5 mm, and may, for example, be of the type known as KX 15 sold by Safety, 92 Boulogne-Billancourt, France. The cutting inserts  64  may be connected to the support ring  62  in any known manner (e.g., by screws, etc.). In the specific embodiment shown in  FIG. 4 , the cutting tools  60  are disposed between the first and second centering guides  56  and  58 , and are separated by spacer collars  66 . As further discussed below, the spacer collars  66  may be provided in varying lengths to insure that metal shavings cut by the cutting inserts  64  are not allowed to damage the interior surface  16  of the tube  10 . A locking nut  68  is threadably attached to the end of the shaft extension  48  to hold the centering guides  56 ,  58 , cutting tools  60  and spacer collars  66  in place.  
         [0040]     The cutting tools  60  are reillustrated in  FIGS. 5-10 .  FIGS. 5 and 6  illustrate a support ring  62  without any cutting inserts  64  attached thereto.  FIGS. 7 and 8  illustrate a support ring  62  with a plurality of concave cutting inserts  64   a  releasably connected thereto. The concave inserts  64   a  are used to form the bosses  18  in the tube  10 . In a specific embodiment, the cutting radius of the concave inserts  64   a  may be 5 mm. A number of different support rings  62  are provided, each having a slightly different size. For example, the size of the support rings  62  may increase initially in 0.1 mm increments, and then, as the profile nears its final size, the size may increase in smaller increments, such as, for example 0.05 mm.  FIGS. 9 and 10  illustrate a support ring  62  with a plurality of convex cutting inserts  64   b  releasably connected thereto. The convex inserts  64   b  are used to form the grooves  20  in the tube  10 . In a specific embodiment, the cutting radius of the convex inserts  64   b  may be 5 mm. The support rings  62  on which the convex inserts  64   b  are mounted ( FIG. 7 ) are provided in gradually increasing sizes in the same manner as explained above for the concave inserts  64   a . The support rings  62  are configured to hold eight cutting inserts  64 , and may be used to create a tube  10  having the profile of eight bosses  18  and eight grooves  20  illustrated in  FIG. 1 . Again, however, that specific number is not a limitation of the present invention.  
         [0041]     Another specific embodiment of a cutting assembly  44 ′ is shown in  FIGS. 11A-11D . The cutting assembly  44 ′ includes a first centering guide  56 ′ and a second centering guide  58 ′ disposed about a shaft extension  48 ′. In a specific embodiment, the centering ′des  56 ′ and  58 ′ may include Teflon pads  57  and  59  to more precisely guide the cutting assembly  44 ′ along the center of the tube  10  to which the desired profile is being applied. The shaft extension  48 ′ may also include a central lubricating channel  50 ′ and a plurality of radial lubricating channels  52 ′ (see  FIG. 17 ) that are in fluid communication with a source of oil (e.g., oil tanks  40  shown in  FIG. 3 ). As shown in  FIG. 11A , the cutting assembly  44 ′ may include a looking nut  68 ′ and a thrust washer  69 . The cutting assembly  44 ′ also includes a plurality of cutting tools  60 ′. Each cutting tool  60 ′ may include a support ring  62 ′ and a plurality of cutting inserts  64 ′. The cutting tools  60 ′ may be separated by spacer collars  66 ′.  FIG. 12  is an end view of a collar  66 ′ and shows that the collars  66 ′ are provided with a keyway  67  adapted to cooperate with a  1   5  corresponding keyway (discussed below) on the shaft extension  48 ′.  FIGS. 13-16  are side views of various sizes of collars  66 ′. In a specific embodiment, the collar  66 ′ in  FIG. 13  may have a length of 20 mm, the collar  66 ′ in  FIG. 14  may have a length of 30 mm, the collar  66 ′ in  FIG. 15  may have a length of 40 mm, and the collar  66 ′ in  FIG. 16  may have a length of 55 mm. With reference to  FIG. 11A , the distance D between leading edges of the cutting inserts  64 ′ may be varied depending on the length of the spacer collars  66 ′ that are employed. In a specific embodiment, the distance D may be 75 mm. The distance D should be sized so as to allow sufficient space in which metal shavings cut from the interior surface  16  of the tube  10  (see  FIG. 1 ) may be temporarily housed without completely filling an annular space defined by the interior surface  16  of the tube  10 , the collar  66 ′ and the adjacent cutting tools  60 ′ between which the collar  60 ′ is disposed. This is important because the metal shavings or chips must be stored during the complete length (which may be greater than 3 meters) of the machining pass through the tube  10  by the cutting assembly  44 / 44 ′ so as to avoid destroying or damaging the surface roughness of the profile being cut into the internal surface  16  of the tube  10 .  
         [0042]     As shown in  FIG. 11C , the cutting assembly  44 ′ may also include a compensation washer  70  that is adapted to compensate wear in the various components of the broaching machine  36 . In a specific embodiment, the washer  70  may have a minimum rating of 4,000 daN. The attachment mechanism  46 ′ of this embodiment may include a fast-clamp slide ring  72 , a push spring  74 , and a return spring  76 . In operation, the slide ring  72  is shifted so as to compress the return spring  76 . The push spring  74  is then compressed so as to separate the two conical bearing surfaces. The cutting assembly  44 ′ may then be disengaged from the shaft  38 . Another cutting assembly  44 ′ having slightly larger cutting tools  60 ′ may then be engaged by the opposite way. In service the push spring  74  maintains the two conical bearing surfaces in contact. The slide ring  72  is further illustrated in  FIG. 17 , which is a cross-sectional view taken along line  17 - 17  of  FIG. 11C . As shown in  FIG. 11D , this embodiment of the cutting assembly  44 ′ may also include a nut  78  and locknut  80  for securing the cutting assembly  44 ′ to the shaft  38  of the broaching machine  36  (see  FIG. 3 ) and which are adapted to enable angular adjustment of the cutting assembly  44 ′.  
         [0043]      FIGS. 18-21  illustrate specific embodiments of cutting tools  60 ′ of the present invention.  FIG. 18  illustrates a maximum diameter cutting tool  60 ′ that is used for cutting the grooves  20  in the tube  10  (see  FIG. 1 ).  FIG. 19  illustrates a minimum diameter cutting tool  60 ′ that is used for cutting the grooves  20  in the tube  10  (see  FIG. 1 ).  FIG. 20  illustrates a minimum diameter cutting tool  60 ′ that is used for cutting the bosses  18  in the tube  10  (see  FIG. 1 ).  FIG. 21  illustrates a maximum diameter cutting tool  60 ′ that is used for cutting the bosses  18  in the tube  10  (see  FIG. 1 ). As explained above, the present invention contemplates numerous different sizes of support rings  62  with only minor incremental size differences between the various support rings  62 .  FIGS. 18-21  further show that this embodiment may include indexing keys  82  adapted to cooperatively engage corresponding keyways  84  and  86  in the cutting tools  60 ′ and shaft extension  48 ′, respectively. As shown in  FIG. 22 , in a specific embodiment the cutting inserts  64 ′ may be indexable so as to maximize the useful life of each insert. Each insert  64 ′ can be releasably attached to the cutting tool  60 ′ by a screw  84 . When a section of the insert  64 ′ becomes worn, instead of discarding the insert  64 ′, the screw  84  can be loosened, the insert can be rotated 120 degrees so that a unused section of the insert  64 ′ is positioned for cutting, and the screw  84  can then be retightened to lock the insert  64 ′ in place. In a specific embodiment of the invention, it has been learned that a single 120-degree section of an insert  64 ′ becomes worn and no longer useful after about 30 passes. As such, if it is rotated three times, a single insert  64 ′ may be used for up to  90  cutting passes through the tube  10 .  
         [0044]     Referring back to  FIG. 3 , the manner in which a desired profile is formed in the internal surface  16  of the tube  10  will now be described. First, a centrifugally-cast tube  30  is made in the traditional manner as discussed above and placed into position between the first and second guides  32  and  34 . A cutting assembly (such as cutting assembly  44  or  44 ′) is connected (such as by attachment mechanism  46  or  46 ′) to the shaft  38  of the broaching machine  36 . For the first pass of the cutting assembly  44 / 44 ′ through the tube  30 , the cutting tools  60 / 60 ′ are each equipped with a convex cutting insert  64 / 64 ′ on a support ring  62  having a minimum radius. During this first pass, an initial cut is made at a depth of, for example, 0.1 mm. In this manner, the formation of the grooves  18  has commenced. After this first pass, the broaching shaft  38  is then retracted and the first cutting assembly is replaced with another cutting assembly that is equipped with convex inserts on a support ring  62  having a slightly larger radius. For example, the radius may be increased by 0.05 mm or 0.1 mm. This process is repeated until the desired groove depth is achieved. Next, the same process is repeated with concave cutting inserts and numerous passes are made, each pass being made with a support ring  62  having a slightly larger radius, until the desired profile for the bosses  18  is achieved. In a specific embodiment, each cutting assembly  44 / 44 ′ may be provided with four cutting tools  60 / 60 ′ of slightly different sizes, each successive one having, for example, a difference in machined depth of 0.1 mm. In this manner, a′total cut of 0.4 mm would be made in a single pass.  
         [0045]     In a specific embodiment, each “roughing” pass made at a depth of 0.1 mm may be made at a speed of 12 meters/minute, and each “finishing” pass made at a depth of 0.05 mm may be made at a speed of 18 meters/minute. As explained above, as the final size of the profile is approached, the size of the incremental increases in the sizes of the support rings  62  may be lowered, for example, to less than 0.05 mm, in order to achieve a more desirable surface roughness and dimensional accuracy, and thereby result in a tube having a greater resistance to carburization and metal dusting. In actual testing, a tube of the present invention has been achieved wherein the internal surface  16  has a surface roughness of 0.8 Ra μm, which equates to 32 RMS (CLA). “RMS” means Root Mean Square and in micro inch is a United States unit for surface roughness. “CLA” means Center Line Average and in micro inch is a United Kingdom unit for surface roughness. “Ra” in micrometer is a European unit for surface roughness. The surface roughness of 32 RMS (CLA) achieved by the present invention is far superior to the surface finish of 130 RMS (CLA) reported in the &#39;340 patent, and also to a surface roughness of 125 RMS (CLA), which is the usual surface roughness targeted in connection with fined tubes.  
         [0046]     From the above description it should now be apparent that the present invention has a number of advantages. Use of the method and apparatus of the present invention results in a centrifugally-cast tube having a macrostructure with a large grain size and high creep properties. In addition, subsurface deformation and orientation is achieved by the cutting passes of the hard metal (carbide) cutting inserts over the interior surface. Deformation of the primary carbides in the lattice at the interior tube surface can even be observed at high magnification.  
         [0047]     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials or embodiments shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. For example, while the term “tube” has been used to describe the present invention, it should be understood that the present invention applies equally to any conduit of any cross-sectional geometry (e.g, square, rectangular, etc.), and is not limited to a tube of circular cross-section. In addition, while the tube  10  and related method and apparatus  22  has been illustrated and discussed in the context of a particular geometric profile (e.g, bosses  18  and grooves  20  having semi-circular profiles), the present invention is intended to cover bosses and grooves of any geometric or other profile, and is not intended to be limited to semi-circular profiles. In addition, while the present invention has been illustrated in the context of grooves and bosses that are aligned with the longitudinal axis of the tube, the present invention is also intended to cover tubes and the related method and apparatus in which the grooves and bosses are formed in a spiral or helical pattern within the tube. This may be achieved by the present invention by rotating the cutting assemblies  44  at a constant rate as they are being passed through the tube. Further, while the present invention has been explained in the context of steam reformer furnaces and steam cracker furnaces, the present invention may also be useful in other applications. For example, the present invention may be applied to heaters used in DRI Direct Reduction of Iron) plants, which is a main area where metal dusting is observed Accordingly, the invention is therefore to be limited only by the scope of the appended claims.