Abstract:
A system and method inductively heats and stress relieves a weld joint area having a stress induced zone. A susceptor assembly is positioned over the stress induced zone. The susceptor assembly includes susceptor sheets manufactured to operate at different, preselected Curie temperatures. A housing is mounted over the susceptor assembly including an induction coil positioned adjacent to the susceptor assembly. An alternating electric current is applied to the induction coil. The alternating electric current causes the induction coil to generate a plurality of magnetic flux lines. The invention provides the advantage that the magnetic flux lines passing through the susceptor assembly heat the susceptor assembly providing localized and controlled temperature heat to the weld joint area to stress relieve the stress induced zone.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to induction heating and more specifically to systems and methods to use induction heating for post weld stress relief of welded parts. 
     BACKGROUND OF THE INVENTION 
     Following a welding procedure, many metals exhibit increased residual stresses in the area adjacent to the weld zone. A weld zone is defined herein as a weld joint and the adjacent area surrounding the weld joint in a metal, wherein the material properties of the metal are affected by residual stresses following the weld procedure. This residual stress can be modeled using an exemplary finite element model to identify the value of the stress and therefore identify the amount of post-weld heat required to relieve the stress in the area of the weld zone. Computational models therefore exist to identify a temperature gradient required adjacent to the weld zone to relieve the residual stress in the material. 
     If the residual stress remaining in the material following welding is not relieved by a post-weld stress relief procedure, the fatigue life of the material can be degraded. A post-weld stress relief procedure is therefore normally performed on many materials in order to regain a full or nearly full fatigue life cycle for the material. Post-weld stress relief procedures known in the art include providing resistive heating coils on or adjacent to the material where the weld joint or weld joints are formed. The resistive heating coils are placed in direct contact with the welded material, therefore the welded material is raised in temperature to permit the residual stress to relax in the material. 
     The disadvantage of known post-weld stress relief procedures using the resistive heating method is that the resistive heaters are bulky and the temperature gradient required to minimize the amount of heat input to the metal part is difficult to obtain. If the temperature gradient which is precalculated for the particular material and material size is not closely followed, overheating of the material can occur which can distort and damage the material. Under-heating of the material can also occur which will prevent effective reduction of the residual stress. It is also desirable to provide the highest post-weld stress relief temperature adjacent to the weld area, decrease the temperature as the distance from the weld area increases, and isolate surrounding structure from the elevated temperature. Resistive heating coils known in the art are inefficient at providing this gradual change of temperature away from the weld zone and at isolating surrounding structure. 
     It is therefore desirable to provide a post-weld stress relief method and system which avoids the drawbacks of the known resistive heating coil methods. It is further desirable to provide a system and method of accurately controlling the temperature gradient at and adjacent to a weld zone for post-weld stress relief. 
     SUMMARY OF THE INVENTION 
     According to a preferred embodiment of the present invention, an apparatus is provided to heat and stress relieve a metal plate area. A susceptor assembly is constructed using one or more sheets of material having preselected Curie temperatures. The Curie temperature is defined as the sheet temperature at which magnetic permeability equals unity. The individual susceptor sheets are assembled into a semi-flexible assembly by welding the individual sheets at their adjoining edges, forming pairs of sheets. The susceptor assembly is positioned over a metal plate in the area adjacent to a weld joint in the associated weld zone area. The susceptor assembly is sized to approximate the area where stress relief of the weld zone is required. A housing is then mounted over the susceptor assembly covering the susceptor assembly and a portion of the metal plate area. An induction coil is positioned within the housing adjacent to the susceptor assembly. An oscillating electric current is passed through the induction coil which induces an electromagnetic flux. This flux then couples with the susceptor assembly. The susceptor assembly has high magnetic permeability, which makes the susceptor assembly the lowest energy path for the electromagnetic flux to reside. Coupling the electromagnetic flux in the susceptor assembly causes an induced current flow with associated resistive losses (heating). Heat generated by this process conductively and convectively passes from the susceptor assembly to the weld zone of the metal plate area. The susceptor assembly, based on its multi-sheet design, allows a thermal gradient of temperatures to be applied to the metal plate area at the weld zone. 
     The susceptor assembly is constructed using sheets of material containing a combination of cobalt, nickel, and iron material. Higher concentrations of cobalt produce a higher Curie point for the susceptor material. The magnetic flux coupled through the susceptor material causes a rapid heat-up of the susceptor material up to the Curie point of each sheet at which point the susceptor material becomes an inefficient conduit for the magnetic flux since the material above its Curie point becomes non-magnetic. By varying materials in the susceptor sheet and using a plurality of individual sheets having different Curie temperatures, a susceptor assembly is formed which allows a temperature gradient to be induced into the plate material surrounding a weld joint. The development of susceptor sheet material is disclosed in U.S. Pat. No. 5,728,309 issued to Matsen, et al. which is incorporated herein by reference. 
     The housing which surrounds the susceptor assembly contains the induction coil which is held in place using an insulation material which in a preferred embodiment is formed from a castable ceramic material. The insulation (e.g., the ceramic material) spaces the induction coil away from both the susceptor assembly and from the housing walls. A coolant such as water is induced to flow through the tubular body sections of the induction coil to remove residual heat generated by the current flow through the induction coil. An alternating electric current flows through the induction coil to generate the lines of magnetic flux. In a preferred embodiment, the housing is held in place adjacent to the weld zone by a vacuum sheet which is a flexible material applied over the perimeter of the housing and about a small surface area of the metal plate. A vacuum seal is formed at the contact points between the vacuum sheet material and the metal plate and a partial vacuum is drawn to hold the housing and the susceptor sheet against the metal plate. 
     A ferritic material in the form of a ferritic plate is also cut and formed to fit on a perimeter of the susceptor assembly. The ferritic material is selected from a non-electrically conductive material also having high magnetic permeability. The ferritic material captures and induces the lines of magnetic flux from a perimeter of the susceptor sheets and directs the lines of magnetic flux toward the weld joint located approximately at the center of the susceptor assembly. The ferritic plates act as both a concentrator and as an insulator to prevent the lines of magnetic flux from heating the surrounding area outside of the susceptor assembly therefore containing heat output from the susceptor assembly in the immediate area of the weld zone. 
     The alternating current is induced to flow in the induction coil and the susceptor assembly heats up to the Curie temperature of each associated sheet of the susceptor assembly. Although the lines of magnetic flux are continuously generated during the procedure, once the Curie temperature is reached for each of the sheet materials of the susceptor assembly, the temperature of the individual sheets is maintained at a constant level. This acts as an automatic temperature control which follows a predetermined temperature gradient calculated for the particular material the post-weld stress relief is being performed on. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a plan diagram showing a preferred embodiment of an induction heating tool of the present invention located adjacent to a repair weld; 
         FIG. 2  is a cross-sectional view taken along section line  2 — 2  of  FIG. 1  showing an assembly of components for a preferred embodiment of the present invention; 
         FIG. 3  is a plan view of an exemplary induction coil of a preferred embodiment of the present invention; 
         FIG. 4  is a plan view of an exemplary susceptor assembly of the present invention having a circular form having individual sheets joined by welding; 
         FIG. 5  is a cross-sectional view similar to that of  FIG. 2  further showing the lines of magnetic flux generated by the induction coil; and 
         FIG. 6  is a graph showing an exemplary distribution of temperatures about a centerline of a weld joint using the circular susceptor assembly of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , an induction heating system  10  according to a preferred embodiment of the present invention is shown. The induction heating system  10  includes a tool body  12  which is temporarily affixed to a weld surface  14 . The tool body  12  is held in place on the weld surface  14  at a vacuum seal  16 . A vacuum device  18  draws a partial vacuum within the vacuum seal  16  through a vacuum tube  20 . A geometrically arranged induction coil  22  is disposed within the tool body  12 . A supply of coolant  24  is provided to cool the induction coil  22 . The coolant  24  flows within the generally tubular shaped induction coil  22  as will be further described in reference to FIG.  3 . The coolant  24  is provided via a coolant supply pipe  26  and returns after cooling the induction coil  22  via a coolant return pipe  28 . 
     A susceptor assembly  30  is disposed between the induction coil  22  and the weld surface  14 . The susceptor assembly  30  is generally centered over a weld joint  32  which is formed in the weld surface  14 . The susceptor assembly  30  is shown having a generally circular shape, however, a variety of shapes (e.g., oval, rectangular, square, etc.) can be used which provide the necessary dimensions to cover the weld joint  32 . An alternating current (A/C) power source  34  is shown connected to the induction coil  22  via a set of power lines  36 . The vacuum seal  16 , the induction coil  22 , and the susceptor assembly  30  are shown in a partial sectioned view within  FIG. 1  for clarity. 
     Referring to  FIG. 2 , the susceptor assembly  30  is flexible allowing it to be formed along the contoured outer surface of the weld surface  14 . The induction coil  22  is spaced adjacent to and separated from the susceptor assembly  30 . The vacuum seal  16  is shown as a ring. The vacuum seal  16  is formed preferably from a tacky, flexible material such as a clay known in the art, or any other suitable sealing material or compound. The advantage of using the flexible vacuum seal  16  is that the material will adhere temporarily to the weld surface  14  but when removed after the stress relief process, will not leave a residue on the contacted surface of the weld surface  14 . The tool body  12  is preferably formed of a semi-rigid material capable of being modified to suit the geometry of the weld surface  14 . The material of the tool body  12  is dielectric, such that the material is non-magnetic and will not alter or absorb the lines of magnetic flux generated by the induction coil  22 . In a preferred embodiment, the material of the tool body  12  is a polymeric material, including polyvinyl chloride. In the embodiment shown in  FIG. 2 , the tool body  12  is represented as a partial section of a pipe or tube having a circular body and an end cap or cover. 
     To hold the tool body  12  in physical contact with the weld surface  14 , a vacuum sheet  38  is disposed about the perimeter of the tool body  12  and about the vacuum seal  16 . A partial vacuum drawn using the vacuum device  18  (shown in  FIG. 1 ) collapses the vacuum sheet  38  about both the tool body  12  and the vacuum seal  16  thus pressing the tool body  12  against the weld surface  14  during the time that the susceptor assembly  30  is in use. An insulation material  40  is disposed within the tool body  12  and surrounds each segment of the induction coil  22  to retain a spacing between the induction coil  22  and the susceptor assembly  30 . In a preferred embodiment, the spacing between the induction coil  22  and the susceptor assembly  30  is maintained between approximately 0.4 to 0.5 inches (1.01 to 1.27 cm). This spacing prevents physical contact between the induction coil  22  and the susceptor assembly  30 . The spacing can be varied from the value given depending upon a variety of conditions including the spacing and size of the induction coil  22 , the geometry of the weld surface  14 , the amperage of the current flowing through the induction coil  22 , and the desired temperature profile generated by the susceptor assembly  30 . In a preferred embodiment, the composition of the insulation material  40  is a castable fused silica ceramic, but other suitable insulating materials may also be used. 
     Ferrite plates  42  are disposed about a perimeter of the susceptor assembly  30 . The ferrite plates  42  are formed of a cintered magnetic material which is essentially electrically non-conductive. The material for the ferrite plates  42  is selected such that a high magnetic permeability allows it to absorb magnetic energy generated as lines of electromagnetic flux from the induction coil  22 . The ferrite plates  42  also form an additional thermal barrier between the weld surface  14 , the tool body  12  and the induction coil  22 . This thermal insulation property helps to limit the heat input into the weld surface  14 . The ferrite plates  42  are shown surrounding a perimeter of the susceptor assembly  30  and generally abut the perimeter of the susceptor assembly  30 . 
     Referring now to  FIG. 3 , an exemplary induction coil assembly  44  is shown. The induction coil assembly  44  includes a plurality of segments of the induction coil  22  preferably formed as shown in a helical arrangement. The induction coil  22  preferably comprises a copper tube internally carrying the coolant  24  (shown in FIG.  2 ). A current bus  46  connects the induction coil assembly  44  to the power lines  36  (shown in FIG.  1 ). The current bus  46  feeds a current supply tubing section  48  and completes the circuit via a current return tubing section  50 . Alternating electric current enters the induction coil assembly  44  via the current supply tubing section (i.e., conductor)  48  in a current supply direction B as shown. Current flow through the induction coil  22  is generally along the perimeter of the induction coil  22  tubular body in a coil current flow path direction C as shown. The current flow exits the induction coil assembly  44  via the current return tubing section (i.e., conductor)  50  in a current return direction D. The coolant  24  flowing within the induction coil  22  is supplied from a coolant source  51  via the coolant supply pipe  26  and returns from cooling the induction coil  22  via the coolant return pipe  28 . The direction of coolant flow can be in parallel with the coil current flow path direction C or in a counter-current flow direction (not shown). 
     The helical design of the induction coil assembly  44  shown is an exemplary preferred embodiment of the present invention. The induction coil  22  can be formed in any geometric pattern which is suited to provide coverage above a susceptor assembly  30  (shown in FIG.  1 ). It is also noted that the approximately even spacing shown in  FIG. 3  between segments of the induction coil  22  is also an exemplary preferred arrangement. The spacing between the individual segments of the induction coil  22  can vary from that shown in  FIG. 3  depending upon the geometry of the tool body  12  and the susceptor assembly  30 . 
     Referring to  FIG. 4 , an exemplary susceptor assembly  30  of a preferred embodiment of the present invention is shown. The susceptor assembly  30  includes a first susceptor sheet  52  enclosed within a second susceptor sheet  54  which in turn is enclosed within a third susceptor sheet  56 . The first susceptor sheet  52 , the second susceptor sheet  54 , and the third susceptor sheet  56  form adjacent pairs of susceptor sheets. Each of the adjacent pairs of susceptor sheets are joined by a weld joint. Other processes can be used to join susceptor sheets which meet or exceed the operating temperature range of the susceptor assembly. In the exemplary embodiment shown in  FIG. 4 , a weld joint  58  joins the first susceptor sheet  52  to the second susceptor sheet  54  and a weld joint  60  joins the second susceptor sheet  54  to the third susceptor sheet  56 . By using multiple sheets to form the susceptor assembly  30 , a variety of Curie temperatures can be used to develop a susceptor assembly. In the exemplary embodiment shown in  FIG. 4 , the first susceptor sheet  52  can be selected to have the highest Curie temperature of the three sheets shown. The highest Curie temperature sheet is then positioned adjacent to the weld joint (e.g., weld joint  32  shown in FIG.  1 ). Each adjacent sheet in the susceptor assembly can then be formed using a successively lower Curie temperature such that a desired temperature gradient is formed in the weld surface  14  (shown in FIG.  1 ). It will be appreciated that any combination of Curie temperatures can be used for a susceptor assembly. In a preferred embodiment, however, each sheet of a multi-sheet susceptor assembly has a progressively reducing Curie temperature starting at the susceptor sheet adjacent to a weld joint. The susceptor assembly  30  shown in  FIG. 4  has an exemplary circular shape. As noted above for the induction coil assembly  44  (shown in FIG.  3 ), any geometric shape can be used for the susceptor assembly  30  including having sheet segments which have different widths or cross sections from sheet to sheet or within each sheet such that a temperature gradient can be varied dependent upon the geometry of the weld surface. 
     Referring to  FIG. 5 , the sectioned elevation view of  FIG. 2  further shows an operating condition for the induction heating system  10  of the present invention. Magnetic flux lines are shown which are generated by the induction coil  22  with an alternating electric current passing through them. For clarity, a clockwise magnetic flux line group  62  is shown and a counter-clockwise magnetic flux line group  64  is similarly shown. Each of the clockwise magnetic flux line group  62  and the counter-clockwise magnetic flux line group  64  emanate away from the induction coil  22 . Both the ferrite plates  42  and the susceptor assembly  30  are high magnetic permeability materials, therefore the lines of magnetic flux are concentrated by the ferrite plates  42  and enter at the outside perimeter areas of the susceptor assembly  30  where they converge adjacent to the weld joint  32 . The clockwise magnetic flux line group  62  and the counter-clockwise magnetic flux line group  64  excite electrons in the susceptor assembly  30  which generates heat within the susceptor assembly  30 . This heat is conveyed primarily conductively from the susceptor assembly  30  to the weld surface  14 . The amount of heat generated by the susceptor assembly  30  varies between each of the susceptor sheets of the susceptor assembly  30 . 
     As shown in  FIG. 5 , the induction coil  22  extends beyond a perimeter of the susceptor assembly  30 . It is desirable to extend the induction coil  22  to prevent a drop off of flux density on the perimeter of the susceptor assembly  30 . The ferrite plates  42  both collect the lines of magnetic flux and act as a thermal barrier between the weld surface  14  and the induction coil  22  such that the outwardly extending induction coil  22  transmits minimum quantities of heat into the weld surface  14 . 
     The insulation material  40  is selected from a group of materials which exhibit a very low coefficient of thermal expansion. A low thermal expansion coefficient is necessary for the insulation material  40  due to the high thermal gradient generated between the weld surface  14  adjacent to the susceptor assembly  30  and the tool body  12 . The insulation material  40  prevents excessively high temperatures from reaching the tool body  12 , the vacuum sheet  38 , or the vacuum seal  16  which could cause these materials to reach or exceed their melting points. Temperatures of approximately 1400° F. (760° C.) and higher are common in a stress relief procedure. The Curie temperature of cobalt-iron-nickel alloys used to produce the susceptor assembly herein can range from approximately 675° F. to approximately 2050° F. (355° C. to 1120° C.). These temperatures are sufficient to exceed the melting points of the tool body  12 , the vacuum sheet  38 , or the vacuum seal  16 . It is desirable to maintain these materials at or near ambient temperature. 
     Referring back to  FIG. 1 , the A/C power source  34  provides alternating current to the induction coil  22 . A frequency of preferably approximately 3 KHz is used as a base frequency to operate the induction heating system  10 . The current and voltage used will vary depending upon variables of each application of the present invention. The voltage and current are also subject to the size of the induction coil assembly  44  (shown in FIG.  3 ). In a preferred embodiment of the present invention, water is used as the coolant  24 . The type of coolant  24  that is used can vary depending upon the availability of coolant and the ability to both pump and remove heat from the coolant. In a preferred embodiment, the vacuum device  18  is a vacuum pump. Other means to hold the tool body  12  in contact with the weld surface  14  can be used, including mechanical means or weights attached to the tool body  12 . 
     Referring to  FIG. 6 , a thermal gradient for the 3-sheet design susceptor assembly  30  shown in  FIG. 3  is graphed relative to both the type of alloy material used in the susceptor sheets and the temperature versus distance from a weld centerline. In the preferred embodiment shown, the first susceptor sheet  52  has the highest Curie temperature, followed by the second susceptor sheet  54  and finally by the third susceptor sheet  56 . By varying the alloys used in each of the susceptor sheets, a thermal profile  68  can be generated as shown. The thermal profile  68  is one of a plurality of exemplary thermal profiles that are available depending upon the geometry of the susceptor assembly designed for the application. 
     Referring back to  FIG. 5 , an outer ring  66  of the vacuum sheet  38  is formed between the tool body  12  and the vacuum seal  16  when a partial vacuum is drawn within the vacuum sheet  38 . The vacuum sheet  38  is a flexible polymeric material known in the art. Connection between the vacuum tube  20  and the vacuum sheet  38  is formed by a standard fitting (not shown) also known in the art. Exemplary materials used for the vacuum sheet  38  include flexible polymeric materials capable of reaching temperatures up to approximately 250° F. (121° C.). 
     The induction heating system  10  of the present invention can be used for an extended period of time to provide post-weld stress relief or annealing required following a welding operation on a metal surface. The advantage of using the susceptor assembly of the present invention is that the temperature reached by each sheet of a multi-sheet susceptor assembly is maintained at a constant temperature due to the change in magnetic property upon reaching the Curie temperature. An extended “soak period” can be used to reduce the resultant thermal stresses in the welded material. Upon reaching the end of the stress relief period, the partial vacuum is released and the tool body  12  is removed from the weld surface  14 . It is desirable that none of the materials used in the induction heating system  10  provide any residue which is left on the weld surface  14 . This minimizes later clean-up of the weld surface following the use of the induction heating system  10  of the present invention. 
     The induction heating system  10  of the present invention offers several advantages. The use of a susceptor assembly permits a calculated thermal gradient to be accurately met. The temperature generated by the susceptor assembly can be carefully locally controlled in the immediate area of the weld surface. By providing cooling to the induction coil of the present invention, heat generated by the current flow through the induction coil does not damage the tool body  12  or the surrounding materials which hold the tool body  12  in position during the operation. The use of a coolant also reduces the overall operating temperature of the induction heating system  10  of the present invention. The induction heating system  10  of the present invention can be used for post-weld stress relief of metals including titanium which are particularly susceptible to post-weld stress retention. The induction heating system  10  of the present invention can be used on any metal following a welding process. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, the induction coil is described herein as a copper tube. Other forms for the induction coil can be used including cables and solid wire, providing cooling is provided adjacent to the induction coil. Other electrically conductive materials can also be used for the induction coil. The induction coil can comprise multiple assemblies in a tool body of the present invention if desired, providing proper spacing for the magnetic flux lines is provided. The susceptor assembly is described having 3 rings of sheet material. Arrangements having any number of sheet portions can be used. Such variations are not to be regarded as a departure from the spirit and scope of the invention.