Abstract:
A method of eliminating dissimilar metal welds has been disclosed. The method includes the steps of providing a first part having a first alloy composition; providing a second part having a second alloy composition different from the first part; connecting a containment structure to the first part; pouring a powder into the containment structure such that the powder is in contact with the first part; positioning a portion of the second part in the containment structure such that the second part compresses the powder between the first and second parts; and performing hot isostatic pressing (HIP) to consolidate the powder and join the first and second parts together.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to a method of joining dissimilar metals, and more particularly to a method of eliminating dissimilar metal welds. 
         [0002]    For purposes of discussion, the invention will be described with reference to eliminating dissimilar metal welds (DMWs) for nozzle-to-safe end applications, such as those employed in nuclear and fossil plants; however, it should be appreciated that the invention is not limited to nozzle-to-safe end applications and is applicable to the elimination of DMWs in other applications. 
         [0003]    Joining nozzle-to-safe end dissimilar materials has always been accomplished using a welding process. However, nozzle-to-safe end welds are often difficult to produce since they require the use of multiple weld filler metals and an intermediate post weld heat treatment (PWHT) during their assembly. The weld joint is commonly comprised of an SA508 low alloy steel nozzle joined to a 304L or 316L stainless steel safe end. Due to the differences in the material compositions, the nozzle is first machined to the appropriate weld bevel and a nickel-based alloy such as Alloy 82 is applied to the face of the bevel. This is often referred to as “weld buttering”. The weld is then post weld heat treated (PWHT) to assure proper tempering of the low alloy steel just below the deposited nickel-based filler. Also, prior to PWHT, a weld clad using 308L/309L or Alloy 52 clad may be applied along the inside diameter surface of the nozzle. 
         [0004]    Once the nickel-based filler has been applied and the tempered condition has been generated for the nozzle (via the weld butter), a second weld is commonly required to join the buttered nozzle to the stainless steel safe end. A nickel-based alloy such as Alloy 182, 82, or 52 may be used to join these alloys. See  FIG. 1  for an example of a nozzle-to-safe end joint. 
         [0005]    Due to the number of different materials and the reflectors generated at the transition of the alloys, DMWs are also difficult to inspect. When damage is found, it is often isolated on the nozzle weld butter side (Alloy 182/82) of the joint which normally dictates that the entire weld is removed and then repaired. This can be both time consuming and costly to a utility company. 
         [0006]    Accordingly, there remains a need for a method that eliminates the DMWs entirely. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    This need is addressed by the present invention, which uses PM-HIP to consolidate a nickel-based powder between two substrate materials (a low alloy steel and a stainless steel) thereby eliminating the need for a weldment entirely. 
         [0008]    According to one aspect of the invention, a method of joining dissimilar metals includes the steps of providing a first part having a first alloy composition; providing a second part having a second alloy composition different from the first part; connecting a containment structure to the first part; pouring a powder into the containment structure such that the powder is in contact with the first part; positioning a portion of the second part in the containment structure such that the second part compresses the powder between the first and second parts; and performing hot isostatic pressing (HIP) to consolidate the powder and join the first and second parts together. 
         [0009]    According to another aspect of the invention, a method of joining dissimilar metals without welding includes the steps of providing a first part having a first alloy composition; providing a second part having a second alloy composition different from the first part; connecting and sealing a containment structure to the first part; pouring a powder into the containment structure such that the powder is in contact with the first part; positioning a portion of the second part in the containment structure such that the second part compresses the powder between the first and second parts and connecting and sealing the second part to the containment structure to form a complete assembly; and placing the complete assembly in a high pressure containment vessel and performing hot isostatic pressing (HIP) to consolidate the powder and join the first and second parts together. 
         [0010]    According to an aspect of the invention, a method of joining dissimilar metals includes the steps of providing a first part having a first alloy composition; providing a second part having a second alloy composition different from the first part; connecting a containment structure to the first part; pouring a first powder capable of joining with the first alloy into the containment structure such that the first powder is in contact with the first part; pouring a second powder capable of joining with the first powder and the second alloy such that the second powder is in contact with the first powder; positioning a portion of the second part in the containment structure and in contact with the second powder such that the second part compresses the first and second powders between the first and second parts; and performing hot isostatic pressing (HIP) to consolidate the first and second powders and join the first and second parts together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures, in which: 
           [0012]      FIG. 1  illustrates a nozzle-to-safe end weld joint; 
           [0013]      FIG. 2  shows Charpy impact testing results for a first configuration using PM-HIP nozzle-to-safe end joining; 
           [0014]      FIG. 3  shows Charpy impact testing results for a second configuration using PM-HIP nozzle-to-safe end joining; 
           [0015]      FIG. 4  shows Charpy impact testing results for a third configuration using PM-HIP nozzle-to-safe end joining; 
           [0016]      FIG. 5  shows Charpy impact testing results for a fourth configuration using PM-HIP nozzle-to-safe end joining; 
           [0017]      FIG. 6  shows Charpy impact testing results for a fifth configuration using PM-HIP nozzle-to-safe end joining; 
           [0018]      FIG. 7  shows Charpy impact testing results for a sixth configuration using PM-HIP nozzle-to-safe end joining; 
           [0019]      FIG. 8  shows Charpy impact testing results for a seventh configuration using PM-HIP nozzle-to-safe end joining; 
           [0020]      FIG. 9  shows a microstructure of an Alloy 690 interlayer and 690-508 interface associated with the seventh configuration of  FIG. 8 ; 
           [0021]      FIG. 10  shows a microstructure of an Alloy 82 interlayer and 82-508 interface associated with the first configuration of  FIG. 2 ; 
           [0022]      FIG. 11  shows a microstructure of an Alloy 600M interlayer and 600M-508 interface associated with the fifth configuration of  FIG. 6 ; and 
           [0023]      FIG. 12  is an exploded view of a nozzle-to-safe end joining by PM-HIP. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Recent advances in powder metallurgy-hot isostatic pressing (PM-HIP) have demonstrated PM-HIP is capable of providing high quality low alloy steel, nickel-based alloys, and stainless steels for pressure retaining applications that are readily inspectable, couple two different alloys such as bi-metallics for corrosion resistance or wear applications, and that eliminates the need for DMWs. 
         [0025]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 2-11  illustrate test results for various configurations using the method of the current invention. The current invention uses powder metallurgy and hot isostatic pressing (PM-HIP) to consolidate a nickel-based powder between two substrate materials (a low alloy steel and a stainless steel) thereby eliminating the need for a weldment entirely. 
         [0026]    Several tests have been conducted to determine which PM-HIP process provides the best properties (tensile, toughness, U-bends, hardness, and microstructure)—seven different joint configurations are discussed herein. These configurations have utilized one or more consolidated powders that bridge the gap between the two dissimilar materials and are shown in  FIGS. 2-8 . 
         [0027]    The primary advantage of the PM-HIP joint versus a welded joint is that multiple alloys and weld joint angles are eliminated. Straight side walls are employed between the nickel-based powders and the two substrate materials (SA508 and austenitic stainless steel) resulting in an easily inspectable interface. The interface regions found in dissimilar metal welds today exhibit multiple weld bevel angles and geometric reflectors associated with welding. 
         [0028]    The second advantage of the proposed PM-HIP application described herein is that the use of 600M or other niobium bearing powders minimizes the formation of detrimental chromium carbides at the 508 interface region, thereby producing good properties, see  FIG. 11 . 
         [0029]    Low Alloy Steel Side of Joint. The best configuration was determined to be one that included an Alloy 600M consolidated powder between the two alloys,  FIG. 6 . Detailed investigation of several of the joint configurations suggested that during HIP consolidation and processing, and subsequent heat treatment, carbon diffusion from the SA508 low alloy steel readily occurs at the interface boundary between the 508 material and a nickel-based powder ( FIGS. 9 and 10 ). The result is a distinct layer of M23C6 carbides at the interface region which leads to low ductility and toughness in this area. 
         [0030]    Utilization of the Alloy 600M powder as the joining media minimizes the development of the M23C6 carbides (M is usually Chromium). Stable niobium carbides are formed near the interface region, as shown in  FIG. 11 . Nb-carbides do not appear to detrimentally affect tensile, hardness, bend, or toughness properties at the interface region. The premise behind this effort is that niobium ties up the carbon before it is able to join up with the chromium (which as noted earlier produces detrimental properties). 
         [0031]    While this discussion has focused on 600M powder, any Nb-bearing nickel-based alloy powder, such as alloys 600M, 713C, 718, 725, 52M and any other suitable powder, would serve the same purpose. 
         [0032]    Stainless Steel Side of Joint. The above discussion has focused solely on the SA508 side of the joint. For the stainless steel side of the joint, it was shown that 690 powder produces an excellent joint. Unfortunately, 690 powder produces inferior properties when joined to the SA508 material as it does not contain Nb-carbides. 
         [0033]    In terms of a single powder, it has been found that the best combination is a 600M powder used with SA508 low alloy steel and 316L stainless steel. Another solution is to have a joint that includes SA508-to-600M-690-austenitic stainless steel. In other words, utilization of two powders would be employed: Alloy 600M and 690. Alloy 600M provides a good transition to the 508 low alloy steel, while the 690 provides a good transition to the austenitic stainless steel side of the joint. 
         [0034]    Another example of where dissimilar welds are used is in components where low alloy steel components are joined to nickel-based components. The method described above, may also be used to join these components. In this example, the nickel-based powders described above would still be used in a PM-HIP process as described above to eliminate dissimilar metal welds. 
         [0035]    In use,  FIG. 12 , a first part of a first alloy is joined to a second part of a second alloy during the manufacturing process. For clarity, the following discussion is related to a nozzle (first part) and safe end (second part) of dissimilar metals being joined; however, it should be appreciated that the method is generally related to the joining of dissimilar metal parts without the use of DMWs. 
         [0036]    The method joins a nozzle  10  and a safe end  30  of dissimilar metals. The method uses a nozzle having a wall  12  with an inner wall surface  14 , an outer wall surface  16 , and a bore  18  defined by the inner wall surface  14  and attaches a containment structure such as a shroud and/or can  20  thereto via welding or other suitable attachment means. The can  20  includes an inner sleeve  22  and an outer sleeve  24 . The inner sleeve  22  has an outside diameter “d1” similar to and/or equal to a diameter “d2” of the bore  18  to allow the inner sleeve  22  to be inserted into the bore  18  and connected to the inner wall surface  14  of the nozzle  10 . The outer sleeve  24  has an inner diameter “d3” similar to and/or equal to an outer diameter “d4” of the wall  12  to allow the outer sleeve  24  to slide over a portion of outer wall surface  14  and be connected thereto and form a channel  26  between the inner and outer sleeves  22 ,  24 , substantially equal to a thickness of the wall  12 . 
         [0037]    Once the channel  26  is formed, the appropriate alloy powder and/or powders (i.e. two different alloy powders are used) may be poured into the channel  26 , such that the powder rests in the channel  26  and against an end  28  of the nozzle  10 . With the powder contained in the channel  26 , a safe end  30  having a wall  32  with an inner wall surface  34 , an outer wall surface  36 , and a bore  38  defined by the inner wall surface  34  is positioned in the channel  26  such that the wall  32  is positioned in the channel  26  with the inner sleeve  22  positioned in the bore  38  and the outer sleeve  24  extends over a portion of the outer wall surface  36 . The inner sleeve  22  and outer sleeve  24  are connected to the safe end  30  in the same manner as they are connected to the nozzle  10  to seal the can  20  around the nozzle  10  and safe end  30 . It should be understood that the powder contained in the channel is compressed between the end  28  of the nozzle  10  and an end  40  of the safe end  30 . 
         [0038]    With the nozzle  10 , safe end  30 , can  20 , and powder all properly assembled into assembly  100 , a vacuum is pulled on the entire assembly  100  through one or more access ports  44  (fill stems). The access ports  44  are then crimped and welded shut. The assembly  100  is then placed into a high pressure containment vessel and subjected to hot isostatic pressing (HIP) where elevated temperatures and gas pressures are used to consolidate the powder and join the nozzle  10  to the safe end  30 . Once the HIP process has finished, the can  12  is removed from the assembly and any needed finish machining can be performed. 
         [0039]    The foregoing has described a method of eliminating dissimilar metal welds. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0040]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0041]    The invention is not restricted to the details of the foregoing embodiment(s). The invention extends any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.