Abstract:
A process for cladding a surface of a substrate that includes cleaning the surface just prior to applying the cladding. Oxides, such as base metal oxides, are removed from the surface during the step of cleaning. Cleaning methods include an ionized gas cleaning process that may include forming an arc between an electrode and the surface. Optionally, the step of cleaning can occur in a chamber that is substantially evacuated. The cladding can be applied to the cleaned surface immediately after it has been cleaned.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention relates generally to a method of cladding a surface of a substrate. More specifically, the present invention relates generally to a method of cladding a surface of a substrate using an electric arc, energy beam, or resistance weld cladding process immediately or soon after the surface has been cleaned using an arc-ion cleaning process. 
         [0003]    2. Description of Prior Art 
         [0004]    Structural members subjected to ambient conditions are typically treated to prolong their useful life. Treatment methods generally include forming the member from a corrosive resistant material, such as stainless steel, coating the member, or cladding the member. Coatings, such as paint or polymeric compounds, can protect the surface of a member from moisture and corrosive elements that can promote oxidation or galvanic action. However, most coatings wear over time, either from exposure or from rubbing contact with another object. Therefore, to protect such members, a cladding may be required that is bonded to the outer surface of the member and cannot be easily removed or eroded. 
         [0005]    Surfaces of an object to be protected are generally cleaned prior to application of any cladding. Cleaning a surface of the object prior to applying a cladding typically includes machining the surface and then applying a chemical cleaner, such as alcohol or acetone. The object is then usually secured and oriented so that cladding can be applied. During most cladding processes the object is often preheated to temperatures approaching 400° F. Temperatures at this level can cause oxides to form on the surface before cladding is applied. 
       SUMMARY OF THE INVENTION 
       [0006]    Disclosed herein is a method of and system for cladding a substrate. In an example, a method of forming a layer on a surface of a substrate is disclosed that involves ionizing a stream of gas and directing the ionized gas stream onto the surface. The ionized gas stream is applied to the surface for a period of time to remove oxides from the surface. A cladding material is applied onto the surface before oxides form on the surface. Alternatively, the step of ionizing a stream of gas includes flowing an inert gas adjacent an electrode and energizing the electrode to apply a negative charge to the gas. In one example, the electrode has a body with pointed members on one of its sides, passages are formed through the body that intersect with the side having the pointed members. When the gas stream flows through the passages it crosses the pointed members and becomes ionized. Electricity can be supplied to the electrode at about 80 volts, about 2500 Hz, and from about 1 milliamp to about 1 ampere. Optionally, the cladding can involve powering a welding electrode from about 8 volts to about 30 volts, from about 25 amperes to about 300 amperes. At these power levels the cladding can be welded onto the surface, this can result in an energy input to the surface from the welding electrode that ranges from about 3 KJ/in to about 180 KJ/in. In one example embodiment, the cladding material applied to the surface has a substantially uniform density and is substantially free of porous voids. Cladding the substrate can occur after anywhere from about 1 second to about 10 minutes after the surface has been cleaned with the cleaning electrode. Alternatively, the ionized gas stream can have an axis, wherein the ionized gas stream is repositioned so that the axis intersects the surface along a path on the surface, and wherein the cladding material is applied to the surface along the path and behind the ionized gas stream. The substrate may optionally be part of a tensioning mechanism used on an offshore platform or a valve on a tree. 
         [0007]    Also disclosed herein is a method of cladding a surface of a substrate; this example embodiment includes powering a cleaning electrode with electricity at about 2500 Hz, from about 1 milliamp to about 1 ampere, and about 80 volts and flowing a stream of gas adjacent a tip of the cleaning electrode to form a flow of ionized gas. Oxides are removed from the surface by directing the flow of ionized gas onto the surface; which defines an interface where the flow of ionized gas contacts the surface. The cleaning electrode is moved so that the interface moves along a path on the surface to define a clean path. A cladding is applied on the clean path. Applying a cladding may involve contacting the clean path with a welding electrode. Power to the welding electrode can be controlled so that energy input to the cladding ranges from about 3 KJ/in to about 180 KJ/in. In an optional embodiment, cladding takes place before oxides form on the clean path. The substrate may be part of a tensioning mechanism used on an offshore platform. Yet further optionally, the cladding can be applied by spraying the clean path with cladding material. 
         [0008]    A system for applying cladding to a surface is also provided herein that in one example embodiment is made up of a cleaning element with a tip selectively disposed proximate the surface. The tip is selectively energized with electrical potential to become an energized tip. When the energized tip is used to ionize a gas stream that is then directed at the surface, oxides are removed from the surface by the ionized gas to form a cleaned surface. The system also includes a source of cladding material that is selectively disposed proximate the cleaned surface. Alternatively, the cleaning element is made up of a body having a side on which the tip is disposed, and passages in the body that intersect the side having the tip. The system can optionally include a power source that provides electrical power to the cleaning element at about 2500 Hz, from about 1 milliamp to about 1 ampere, and about 80 volts. In an alternative, the source of cladding material is a welding electrode and a power source for the welding electrode is included, so that energy input from the welding electrode to the cladding material ranges from about 3 KJ/in to about 180 KJ/in. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a side partial sectional view of an example embodiment of a method of cladding a surface of a substrate in accordance with the present invention. 
           [0011]      FIG. 2  is an elevation view of a riser tensioner system having a surface clad in accordance with an example embodiment of the method of  FIG. 1 . 
           [0012]      FIG. 3  is an elevation view of an exemplary embodiment of a ram tensioner piston rod having a surface clad in accordance with an example embodiment of the method of  FIG. 1 . 
           [0013]      FIG. 4  is an overhead view of an example method of cleaning and cladding a surface of a substrate in accordance with an example embodiment of the method of  FIG. 1 . 
       
    
    
       [0014]    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 that embodiment. 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 INVENTION 
       [0015]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
         [0016]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the improvements herein described are therefore to be limited only by the scope of the appended claims. 
         [0017]      FIG. 1  illustrates in a side schematic view one example of applying a cladding  10  to a surface  12  of a substrate  14 . In an example embodiment, the substrate  14  includes iron, nickel, cobalt, copper, titanium, aluminum based alloy systems, combinations thereof, and the like. In the example of  FIG. 1 , a cleaning element  16  is provided for cleaning the surface  12  before the cladding  10  is deposited. The cleaning element  16  of  FIG. 1  includes a body  18  with a head  20  attached on one end of the body  18 . Electrodes  22  are shown provided on a side of the head  20  facing the surface  12 . In the example of  FIG. 1 , the electrodes  22  come to a point on their free end that faces the surface  12 . In one example embodiment, the electrodes  22  are formed from a material that includes tungsten. An ion flow  24  is shown existing the head  20  and striking the surface  12 . In the example of  FIG. 1 , the ion flow  24  is formed by discharging a gas from nozzles (not shown) in the head  20  while providing an electrical potential to the electrodes  22  that charges the gas molecules. The gas may be stored in a vessel  26  shown having a line  28  connected between it and the head  20  for transporting the gas to the head  20 . Flow passages  30  are formed at various locations in the head  20  that channel the gas to the nozzles. A flow control valve  32  in the line  28  may optionally be provided for regulating gas flow to the head  20 . Electrically charging the gas with the energized electrodes  22  generates ions that when directed towards the surface  12 , physically remove oxides  34  from the surface  12  to form a cleaned space  36 . Examples of the gas include argon, helium, any other inert gas, and combinations thereof; with or without active gases. 
         [0018]    Further in the example of  FIG. 1 , the cleaning element  16  is shown moving in the direction of arrow A and lateral to the surface  12 . In one example embodiment the cleaning element  16  continuously moves lateral to the surface  12  while the ion flow  24  is being generated. In one example, the cleaning element  16  moves at a substantially constant rate of travel. Optionally, the cleaning element  16  remains at a discrete location for a period of time while the ion flow  24  is occurring, then moves a designated distance to another discrete location where it remains for another period of time while the ion flow  24  continues. These steps of moving and remaining can repeat, and can continue until substantially all of the surface  12  is contacted by the ion flow  24 . Embodiments exist wherein the ion flow  24  operates only when the cleaning electrode  24  is not moving, operates only when the cleaning electrode  24  is moving, or combinations thereof. The designated distances between steps can vary depending on the particular substrate  14  being protected. Removing the oxides  34  from the surface  12  while moving the cleaning electrode  16  creates a cleaned space  36  on the surface  12  behind where the ion flow  24  contacts the surface  12 . 
         [0019]    In the example method of  FIG. 1 , the cladding  10  is applied by a welding electrode  38  shown depositing cladding material  40  onto the surface  12 , where the welding electrode  38  can be an electric art, an energy beam, a resistance application, or the like. In an example embodiment, the cladding material  40  can be tungsten, nickel, cobalt, iron, chromium, aluminum, yttrium, combinations thereof, and the like. Optionally, the welding electrode  38  can be part of a welding circuit so that by contacting the surface  12  with the welding electrode  38 , a circuit is closed causing material of the welding electrode  38  to arc into contact with the surface and form the cladding material  40 . The cladding material  40  can be applied onto the cleaned space  36  before oxides  34  can reform on the surface  12 . In an example embodiment, closely following the cleaning element  16  with the welding electrode  38  allows application of cladding material  40  onto the cleaned space  36  without oxides  34  being on the surface  12 . Optionally, the cladding material  40  is applied to the cleaned space  36  within a designated time frame after the cleaning element  16  treats the surface, and thus the interface  28  have been moved forward of the cleaned space  36 . Optionally, the designated time frame can range from one or more seconds to multiple minutes and any time within this range. Moreover, the upper and lower limits of the time frame can be any value within the range. In one example, the designated time frame is less than a time in which oxides  34  could reform on the cleaned space  36 . 
         [0020]    The cleaning element  16  is shown separate from the welding electrode  38  in the example of  FIG. 1 . Though alternate embodiments exist where the cleaning element  16  and welding electrode  38  are connected to one another. Similarly, the gas feed line  28  could have a dedicated nozzle (not shown) and be separate from the cleaning electrode  16 . In one example embodiment, cleaning and cladding the substrate  14  of the present method can be done at atmospheric conditions (i.e. standard temperature and pressure) and outside of an enclosure. Further optionally included are control lines  46 ,  48  connected respectfully to the cleaning element  16  and welding electrode  38 . In an example embodiment, control lines  46 ,  48  provide control signals and power respectively to the cleaning element  16  and welding electrode  38 . An optional controller  52  is shown connected to the control lines  46 ,  48  and in communication with the cleaning element  16  and welding electrode  38 . In one example embodiment, the controller  52  provides control and power for operating the cleaning element  16  and welding electrode  38 . 
         [0021]    In an example of operation the cleaning element  16  is operated at a frequency of at least 2500 Hz with amperage from 1 milliamp to about 1 ampere, including all values of amperage between 1 milliamp and 1 ampere. The operating voltage of the cleaning element  16  can vary depending on distance from the surface  12 . In an example, a voltage of about 80 volts is provided to the cleaning element  16  when the lowermost tips of the electrodes  22  are about 1.0 inch from the surface  12 . Known welding methods typically generate more heat than needed to form a weld, where the extra heating is for removing oxides from the weld. Whereas a typical prior art tungsten inert gas (TIG) welding process imparts about 10 KJ/in onto the cladding  10  while operating at an amperage of about 50 to 100 amperes and a voltage of about 10 to 15 volts. A typical prior art metal inert gas (MIG) welding process imparts about 70 KJ/in while operating at an amperage of about 100 to 300 amperes and a voltage of about 20 to 30 volts. These prior art heating values can form voids in the clad deposit that can range up to 0.060″ in diameter; which generally requires repair welding. Not all oxides respond to the higher heat input and may still remain within the weld, which can cause disbonding between the cladding  10  and surface  12 . 
         [0022]    Removing oxides  34  from the surface  12  reduces the power input required to the electrode  38 . In an example, the cladding  10  is deposited at a temperature less than that required if oxides  34  were on the surface  12  when the cladding  10  is applied. For example, heating values from the welding electrode  38  range from about 3 KJ/in to about 180 KJ/in. Thus advantages of the lower power/heating input include eliminating porosity in the cladding  10  and creating a stronger higher quality bond between the cladding  10  and surface  12  to form a cladding  10  of higher strength without the need for repair. Also, heat input when applying the cladding  10  on a surface  12  substantially free of oxides  34  can be less that the heat input required when oxides  34  are present. Lowering the heat input reduces how much of the cladding  10  penetrates into the substrate  14 , which in turn reduces how much material from the substrate  14  flows into the cladding  10 . Less material from the substrate  14  in the cladding  10  means a lower amount of cladding  10  is required to protect the surface  12 . The amount of cladding applied using the present method can be as low as 50% of that of prior art methods. 
         [0023]    The article being treated and/or protected may be a part of a system used for producing hydrocarbons from a subsea wellhead. In one example, the article is included in a riser tensioning device used in a subsea well. The riser tensioning device can be what is referred to in the art as a “pull-up” type of a “push-up” type. With reference now to  FIG. 2 , an example of a tensioning mechanism  54  is shown in a side view. A riser  56  extends downwardly from a platform  58  to a subsea wellhead (not shown). Riser  56  has a longitudinal axis  60  and is surrounded by a plurality of hydraulic cylinders  62 . Each hydraulic cylinder  62  has a cylinder housing  64  having a chamber (not shown). A piston rod  66  has a rod end  68  that extends downward from each cylinder housing  64  and hydraulic cylinder  62 . The piston ends of rods  66  opposite rod ends  68  are disposed within the respective chambers (not shown) of cylinder housings  64 . Hydraulic fluid (not shown) is contained within the housing  64  for pulling piston rods  66  upward. Each hydraulic cylinder  62  also has accumulator  70  for accumulating hydraulic fluid from hydraulic cylinder  62  and for maintaining high pressure on the hydraulic fluid. A riser collar  72  rigidly connects to riser  56 . The piston rods  66  attach to riser collar  72  at the rod ends  68 . Cylinder shackles  74  rigidly connect cylinder housings  64  to platform  58 . In a specific example of use, the treating method described herein is used to protect a piston rod, such as the piston rod  66  of  FIG. 2 . 
         [0024]    In another embodiment, a cladding method disclosed herein can be applied to a ram tensioner piston rod. An example of a hydro-pneumatic tensioner unit  76  is provided in a side view in  FIG. 3 . On the tensioner unit  76  upper end is a rod end cap  78  used for connection to a top plate (not shown) to provide tension to a riser system. The rod end cap  78  is shown as threadingly attached to a shoulder or flange  80  formed of or attached to the main body of a tensioner piston rod  84 ; bolts  82  are shown coupling the end cap  78  and piston rod  84 . In an embodiment, the lower end of the tension unit  76  is connected to the operational marine platform (not shown). The tensioner piston rod  84  reciprocates in a housing  86  in response to movement of the operational platform  58  ( FIG. 2 ). 
         [0025]    Shown in  FIG. 4  is an overhead view of an example method of cleaning and cladding a surface  12  of the substrate  14  and taken along lines  4 - 4  of  FIG. 1 . As shown in the example of  FIG. 4 , the cleaning element  16  ( FIG. 1 ) moves laterally above the surface  12  so that the interface  28  moves along a path  88  on the surface  12 . As discussed above, creating the interface  28  on the surface  12  forms a cleaned space  36  that remains behind after the cleaning element  16  ( FIG. 1 ) and interface  28  have moved along the surface  12 . By following the path  88  and within a time frame so that oxides  34  do not reform in the cleaned space  36 , the welding electrode  38  ( FIG. 1 ) deposits cladding material  40  ( FIG. 1 ) to form a cladding  10  on the surface  12 . By moving along the path  88  as set out on  FIG. 4 , substantially all of the surface  12  is cleaned and clad without oxides  34  being present between the cladding  10  and surface  12 . 
         [0026]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, the cladding process can processes where the material is deposited via chemical vapor deposition, a plasma spray, a high velocity air fuel, a high velocity oxygen fuel, and the like. 
         [0027]    These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.