Abstract:
A method of metalworking a substrate ( 10 ) previously strengthened in a gas heat treatment to form precipitates throughout an entire volume of the substrate ( 10 ), where the precipitates have an active chemical element incorporated during the gas heat treatment. The method includes: melting a portion of the substrate ( 10 ) during a full penetration metalworking process to form a molten portion ( 12 ); generating a metalworking atmosphere ( 22 ) having a supply of an active chemical element in a gas state during the metalworking process; exposing the molten portion ( 12 ) to the metalworking atmosphere ( 22 ); and cooling the molten portion ( 12 ) while maintaining exposure to the metalworking atmosphere ( 22 ) to form a solidified portion ( 36 ) comprising precipitates comprising the active chemical element, where the precipitates are present throughout an entire volume of the solidified portion ( 36 ), and thereby re-strengthen the entire volume of the solidified portion ( 36 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to post heat-treatment metalworking of an alloy that has previously been strengthened in a heat treatment with reactive gasses. Specifically, the invention relates to exposing a subsequently metalworked portion of the previously strengthened substrate to reactive gasses during the subsequent metalworking process. 
       BACKGROUND OF THE INVENTION 
       [0002]    The recent introduction of advanced high temperature alloys that are strengthened by reactive gas heat treatment poses significant advantages as well as limitations. An example, but not meant to be limiting, of such a material is NS-163™ manufactured by Haynes International of Kokomo, Ind., USA. As manufactured, this material is very ductile, formable, and weldable. After forming and weld fabrication assemblies are then given a high temperature nitrogen atmosphere heat treatment to optimize mechanical properties. In this material the heat treatment causes nitridation throughout the part and results in considerable strengthening. The strengthening mechanism is largely attributed to the precipitation of titanium and columbium nitrides. This processing has limitations, however. For example, heat treatment strengthening is limited to relatively thin substrates, e.g. about 2 mm (0.08″) maximum. Also, after fabrication and heat treatment the assembly cannot be further processed by forming or welding because it is fully strengthened and not amendable to such metalworking. However, there are occasions where it would be advantageous to form and/or metalwork the material subsequent to the heat treatment. Consequently, there remains room in the art for improvement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The invention is explained in the following description in view of the drawings that show: 
           [0004]      FIG. 1  illustrates a full penetration metalworking process using reactive gas shielding. 
           [0005]      FIG. 2  is a cross-sectional view of a repair made using the process of  FIG. 1 . 
           [0006]      FIG. 3  is a cross-sectional view of an assembly made using the process of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    The inventor has discovered an innovative method for re-strengthening an alloy, for example a cobalt or nickel based alloy, that has previously been strengthened via a gas reactive heat treatment, where the alloy has been subjected to a post strengthening metalworking that otherwise would reduce or eliminate the strengthening effect. Some post heat treatment metalworking processes, such as welding, have an effect of reducing or eliminating the strengthening effect resulting from the original heat treatment. It is thought that this happens when heat from the subsequent metalworking process reduces or eliminates precipitates that formed during the original heat treatment process. As a result, these precipitates may not be present in the portion of the previously strengthened substrate that has subsequently been metalworked, and so the portion of the substrate that has subsequently been metalworked may be weaker than a remainder of the substrate. The inventor has discovered that exposing the subsequently metalworked portion of the previously strengthened substrate to a reactive element that was present in the original strengthening process helps reform precipitates in the subsequently metalworked portion of the substrate. This re-strengthens the portion of the previously strengthened substrate that has subsequently been metalworked, leaving a substrate of uniform or near uniform strength. 
         [0008]    In the case of Haynes NS-163™ alloy, the chemical composition of the substrate includes the elements titanium and columbium. When the NS-163™ alloy is heat treated in an atmosphere containing nitrogen the nitrogen combines with the titanium and/or columbium to form nitride precipitates throughout an entire volume of the NS-163™ alloy, and these precipitates strengthen the entire volume of the NS-163™ alloy. Conventional practice in metalworking alloys that are relatively reactive to the atmosphere requires that the metalworking be done in an inert environment. For example, argon and/or helium are typically used for shielding from the atmosphere during welding, cladding, or hardfacing stainless steels and alloys of nickel, cobalt, aluminum, and titanium. The objective of such shielding is to prevent excess oxidation and/or nitridation. For particular other reasons limited percentages of reactive (i.e. non-inert) shielding gasses are sometimes used or most especially combined with inert gasses. Certain processes, such as cladding or surface hardening, have used hydrogen as part of a metalworking atmosphere. However, the inventor is unaware of any full penetration metalworking process that utilizes a metalworking atmosphere that contains nitrogen in order to re-strengthen a substrate that has previously been strengthened through a heat treatment process utilizing a heat treatment atmosphere containing that same reactive element. 
         [0009]    The method described herein can be used with various metalworking processes where a full thickness of the substrate is traversed by the metalworking process, e.g. a full penetration metalworking process, including but not limited to laser beam welding (LBW), plasma arc welding (PAW), tungsten inert gas (TIG) welding, and metal inert gas (MIG) welding etc. Such a full penetration welding process includes keyhole welding processes when joining two substrates edge-to-edge. As shown in  FIG. 1 , in keyhole welding processes an entire thickness of a portion of the substrate  10  is melted via an energy source  11  into a molten pool  12  (a.k.a. weld pool), and a hole  14  is formed through the entire thickness of the molten pool  12  of the substrate  10 , and therefore an entire thickness of the substrate  10  itself. As the process traverses the substrate the molten pool  12  and associated hole  14  also traverse the substrate  10  in a direction of travel  15 . Molten substrate solidifies into a weld bead  16  behind the hole  14 . As the molten pool  12  heats and then melts previously hardened substrate material the strengthening effect that resulted from the heat treatment in the reactive gas atmosphere and indicated by strengthened region  18  diminishes or disappears altogether, as indicated by de-strengthened area  20 . 
         [0010]    In the inventive process a full volume of the molten pool is exposed to the reactive element in the metalworking atmosphere  22  via a shielding gas delivery path  24 . The metalworking atmosphere  22  covers the top of the molten pool  12  and is also forced through the hole  14 . This entrains the reactive element within the molten pool  12 . This entrained reactive element then reacts with the elements in the substrate to strengthen the portion of the substrate that has been melted during the metalworking process. As the melted portion of the substrate cools the strengthening effect brought about by the reactive element begins to return, as indicated by first restrengthening region  26 , and once cooled the re-strengthened region  28  is fully re-strengthened to the level of the substrate  10  prior to the metalworking process. As a result, the entire volume of the substrate  10 , including that which was metalworked subsequent to the strengthening heat treatment, is strengthened to a uniform or near-uniform level. 
         [0011]    The reactive element may also be delivered directly to a back side  30  of the weld as a backing gas  32  via a backing gas delivery path  34  during the metalworking process in order to entrain nitrogen in the molten material at the bottom of the weld pool, or molten material that has worked around to the back side  30  of the substrate. The reactive element may also be delivered to a solidified portion  36  of the subsequently metalworked portion as trailing gas  38  delivered via a trailing gas delivery path  40  so nitrogen may still be incorporated into a surface  42  of the solidified weld. 
         [0012]    The nitrogen may likewise be delivered in various ways. For example, the nitrogen may be in gas form and independently delivered discretely or mixed with another gas to a molten pool. It may be a gas delivered discretely or mixed with another gas via a gas delivery path that is already incorporated into a welding process. Alternately it may be incorporated into a solid material such as a flux and released as a gas during the metalworking process. 
         [0013]    When delivered independently to the molten pool, an existing metalworking process would simply need to be supplemented with a supply of nitrogen and a shielding gas delivery path  24  to deliver the nitrogen to the molten pool. When delivered by using an existing shielding gas delivery path  24 , the nitrogen may take the place of gasses that formerly occupied that shielding gas delivery path, or it may be mixed with those gasses. For example, in LBW the gaseous nitrogen may be delivered via the incorporated shielding gas path or the incorporated optical assist/protective gas path. In PAW the gaseous nitrogen may be delivered via the incorporated shielding gas path or the incorporated orifice gas path. In TIG and MIG welding the gaseous nitrogen may be delivered via the incorporated shielding gas path. The nitrogen may also be delivered via a backing gas delivery path  34  or a trailing gas delivery path  40 . When nitrogen is incorporated into a solid, for example a flux, it may be delivered in any manner acceptable for delivering a flux. For example, the nitrogen containing flux could be a coating on an electrode, or a delivered separately flux, such as a powder applied to the substrate prior to the metalworking process, or a powder mixed and delivered with or in place of powder flux conventionally used in the metalworking process. 
         [0014]    Metalworking using the method disclosed herein may be used for a variety of purposes. For example, the method may be used in a welding process where two substrates are joined edge to edge. The method may also be used as a way to repair substrates, or as a way to build smaller substrates into larger assemblies. 
         [0015]    When used to repair a component, as shown in  FIG. 2 , a substrate  50  may have a crack or other unwanted imperfection that is removed by excavation etc, leaving a hole into which a repair piece  52  may be inserted. Both the substrate  50  and repair piece  52  would have already been subjected to a strengthening heat treatment. The repair piece  52  may be joined to the substrate  52  using the method described herein, and the resulting repaired component  54  would then have all the strength of an original component. 
         [0016]      FIG. 3  shows an assembly  60  with a plurality of substrate sheets  62 . The assembly  62  indicates several ways the technique described herein may be applied to a repair or to creating a new, built-up new component. In terms of repair and as opposed to placing a repair piece  52  in a hole as disclosed in  FIG. 2 , a repair could be made where the repair piece  64  is positioned in an excavation of an assembly  60  and welded into place. This technique requires that the weld fully penetrate the repair piece  62 , but it is not necessary that the weld fully penetrate the component  60 . 
         [0017]    The method disclosed herein may also be used to build-up several smaller substrate sheets  62  into a larger assembly  60 . This can happen in any or all of a number of ways, including welding sheets  62  in layers to form a thicker assembly  60 , and welding sheets  62  edge-to-edge to an assembly  60 , or assemblies  60  edge-to-edge etc. To form a layered assembly  60 , a second substrate  66  may be placed on top of a first substrate  68  and welded thereto. The welding may occur at the second substrate edges  70 , or may occur through a substrate sheet, as indicated by weld  72  of third substrate sheet  74 , which was welded to the second substrate at third substrate edges  76  and at a region  78  of the third substrate sheet  74  not at the edges. A fourth substrate sheet  80  has been shown as welded edge-to-edge to the second substrate sheet  64  and layered to the first substrate sheet  68 . A fifth substrate sheet  82  has been to a sixth substrate sheet  84  to form a second mini assembly  86  that has been edge-to-edge welded to a first mini assembly  88  to form the assembly  60 . Any or all of these techniques, or any technique that applies the teachings herein, may be used to form a complex assembly  60  of substrate sheets  62 . 
         [0018]    The present inventor has developed a technique for re-strengthening a previously strengthened substrate that has subsequently been metalworked. The technique utilizes existing technology in a way not yet practiced, and thus it will be easy to implement. Further, the method is inexpensive yet permits assembly and repair of materials in ways not previously possible, and thus it represents an improvement in the art. 
         [0019]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.