Abstract:
An ultrasonic impact treatment method for remediating metal sensitization including introducing ultrasound compression wave energy through ultrasonic mechanical impulse impacts into an area of sensitized metal in a workpiece. The ultrasound compression wave energy and impulse impacts impart compressive residual stress to the workpiece thereby decreasing tensile stresses in the sensitized metal and modifying the grain structure of the workpiece. These changes to the structure of the workpiece combine to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, cause intergranular diffusion of alloying elements in the area of sensitized metal, return a portion of alloying elements in the area of sensitized metal to solution and reduce or eliminate substantially straight intergranular paths through the workpiece.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a method for remediating sensitization in metals, and more particularly, to remediating sensitization in metals by application of ultrasonic impact treatment (UIT). 
       BACKGROUND OF THE INVENTION 
       [0002]    Sensitized metals are those that when exposed to high temperatures for extended periods have alloying phases precipitate to the grain boundaries of the metal. Precipitation of the alloying phases makes the materials very susceptible to cracking and material failure. Stress corrosion cracking (SCC) refers to the growth of a crack in a susceptible material that is subjected to tensile stress above a threshold value and exposed to either a gaseous or liquid corrosive environment. A non-exhaustive list of examples of materials susceptible to SCC include carbon steels, low alloy steels, high strength steels, all 300-series stainless steels (including Types 304, 304L, 304H, 321, and 347), aluminum alloys from the 5XXX alloy family which may be sensitized, copper alloys and titanium alloys. Examples of corrosive environments include but are not limited to, hydroxides, nitrates, carbonates, bicarbonates, liquid ammonia, carbon monoxide/carbon dioxide/water, aerated water, chloride, sulfide, thiosulfate, polythionate, hydrogen sulfide, and methanol. When cracking or material failure occurs in sensitized metal, wholesale removal and replacement of the sensitized metal are required since there is no effective repair technique. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to the application of UIT to sensitized metals for effectively remediating the effects of metal sensitization or repairing the sensitized metals using conventional welding procedures. According to one aspect of the invention, there is provided a method for treating metal including providing a workpiece including an area of sensitized metal and decreasing tensile stresses in the area of sensitized metal by imparting compressive residual stress in the area of sensitized metal. Compressive residual stress is imparted to the workpiece by applying a multiplicity of shock pulses in the form of ultrasonic energy with an ultrasonic transducer to the area of sensitized metal thereby creating a treatment zone of plastic material in the metal structure. It is believed that the compressive residual stress imparted to the area of sensitized metal acts to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, causes intergranular diffusion of alloying elements in the area of sensitized metal, returns a portion of alloying elements in the area of sensitized metal to solution and reduces or eliminates substantially straight intergranular paths through the workpiece to a surface thereof. 
         [0004]    According to another aspect the invention, there is provided a method for treating metal including providing a workpiece including an area of sensitized metal, the area of sensitized metal including a grain structure including a plurality of crystal grains, and modifying the grain structure by arranging a major axis of each grain of a portion of grains of the plurality of grains to be essentially parallel to a surface of the area of sensitized metal. The grain structure is modified as described above by applying a multiplicity of shock pulses to the area of sensitized metal in the form of ultrasonic energy with an ultrasonic transducer in contact with a surface of the workpiece. Modification of the grain structure in this manner is believed to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, cause intergranular diffusion of alloying elements in the area of sensitized metal, return a portion of alloying elements in the area of sensitized metal to solution and reduce or eliminates substantially straight intergranular paths through the workpiece to a surface thereof. 
         [0005]    According to another aspect of the invention, there is provided a method for treating metal including providing a workpiece including an area of sensitized metal, replacing a portion of the area of sensitized metal with a replacement metal, and introducing ultrasound wave energy into the workpiece about a junction of the replacement metal with the workpiece. The ultrasound wave energy is introduced in the form of ultrasonic energy with an ultrasonic transducer in contact with a surface of the workpiece. The ultrasonic wave energy is believed to ultrasonically excite the base metal and relax stresses therein thereby making the base metal more susceptible to grain modification produced by the impact of a set of indenters coupled between the metal surface and the ultrasonic transducer. In this way, it is believed that the introduction of the ultrasound compression energy acts to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, causes intergranular diffusion of alloying elements in the area of sensitized metal, returns a portion of alloying elements in the area of sensitized metal to solution and reduces or eliminates substantially straight intergranular paths through the workpiece to a surface thereof. 
         [0006]    According to another aspect of the invention, there is provided a workpiece including a sensitized metal portion having a treatment zone, the treatment zone being constructed and arranged by introducing pulses of ultrasonic wave energy into the sensitized metal portion through periodic ultrasonic mechanical impulse impacts. As a result of the introduction of the ultrasonic wave energy through ultrasonic mechanical impulse impacts, a grain structure of the sensitized metal portion, which includes a plurality of crystal grains, is modified so that each grain of a portion of grains of the plurality of grains has a major axis arranged essentially parallel to a surface of the treatment zone. It is believed this modification of the grain structure causes a reduced rate of enrichment of alloying elements at grain boundaries within the workpiece, an improved intergranular diffusion of alloying elements in the workpiece, intergranular diffusion of alloying elements in the area of sensitized metal, and a reduction of substantially straight intergranular paths through the workpiece. Preferably, the workpiece is constructed of a material selected from a group consisting of a carbon steel, a low alloy steel, a high strength steel, a 300-series stainless steel, an aluminum alloy with a magnesium content greater than three weight percent, a copper alloy and a titanium alloy. 
         [0007]    According to another aspect of the invention, there is provided a method for treating metal including providing a metal workpiece including a stress corrosion crack, and introducing pulses of ultrasonic wave energy into the workpiece through periodic ultrasonic mechanical impulse impacts. The pulses of ultrasonic wave energy are introduced into a sensitized portion of the workpiece which contains the stress corrosion crack in order to stabilize the metal surrounding the crack for cutting or grinding. After metal stabilization, the section of the workpiece that contains the stress corrosion crack is removed, and a replacement plate is welded within an opening created by the removal of the section. During welding of the plate to the workpiece, additional pulses of ultrasonic wave energy are introduced into the workpiece through additional periodic ultrasonic mechanical impulse impacts that are applied to the root weld and cap weld passes. Instead of removing a section of the workpiece and replacing it with a metal sheet, the crack may be ground out thus leaving a depression in the workpiece. Thereafter, the depression can be filled in with weld metal and treated with additional UIT. 
         [0008]    According to yet another aspect of the invention, there is provided a method for treating metal including exposing a metal workpiece to a corrosive environment, wherein the workpiece is susceptible to stress corrosion cracking, and introducing pulses of ultrasonic wave energy into the workpiece through periodic ultrasonic mechanical impulse impacts. The ultrasonic wave energy and periodic ultrasonic mechanical impulse impacts are applied to the workpiece in order to stabilize the workpiece metal thereby making it less susceptible to stress corrosion cracking. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a plan view of a metallic workpiece surface susceptible to sensitization. 
           [0010]      FIG. 2  is a plan view of an metallic workpiece surface that is susceptible to sensitization and exhibiting indeterminate sensitization. 
           [0011]      FIG. 3  is a plan view of a sensitized metallic workpiece surface exhibit sensitization. 
           [0012]      FIG. 4  is a sectional view of a metallic workpiece that is susceptible to sensitization. 
           [0013]      FIG. 5  is a sectional view of a sensitized metallic workpiece that has undergone UIT sensitization remediation in accordance with a preferred embodiment of the present invention. 
           [0014]      FIG. 6  is a plan view of a sensitized metallic workpiece including a stress corrosion crack. 
           [0015]      FIG. 7  is plan view of the sensitized metallic workpiece of  FIG. 6  illustrating a UIT treatment zone about a section of the workpiece containing the stress corrosion crack. 
           [0016]      FIG. 8  is a plan view of the sensitized metallic workpiece of  FIG. 7  illustrating removal of the section of the workpiece containing the stress corrosion crack. 
           [0017]      FIG. 9  is a perspective view of the sensitized metallic workpiece of  FIG. 8  illustrating a replacement metal plate welded thereto. 
           [0018]      FIG. 10  is a plan view of a sensitized metallic workpiece including a stress corrosion crack. 
           [0019]      FIG. 11  is plan view of the sensitized metallic workpiece of  FIG. 10  illustrating a UIT treatment zone about the stress corrosion crack. 
           [0020]      FIG. 12  is a plan view of the sensitized metallic workpiece of  FIG. 11  illustrating removal of the stress corrosion crack. 
           [0021]      FIG. 13  is a perspective view of the sensitized metallic workpiece of  FIG. 8  illustrating a weld pass along a depression formed by removal of the stress corrosion crack. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The present invention is directed to the application of UIT to sensitized metals for effectively remediating the effects of metal sensitization and/or repairing the sensitized metals using conventional and emergent welding methods. As used herein, sensitized metal refers to metal having an alloying element precipitate out of solution and congregate at the metal grain boundaries thereby forming a continuous or solid film of the alloying element along the metal grain boundaries. The film may extend to the surface of the metal. By forming a continuous or solid film, interconnected intergranular pathways are formed along the grain boundaries of the metal. 
         [0023]    An exemplary metal that is susceptible to sensitization is 5456-H116 aluminum. 5XXX aluminum alloys are commonly used in naval ship structures. These alloys provide high strength-to-weight ratios while maintaining good as-welded strength and excellent corrosion resistance. However, alloys like 5XXX aluminum alloys with above 3 wt % magnesium (Mg) are susceptible to thermal instability. At relatively low temperatures (−70° C.) over varying periods of time from a few years to 10-20 years, the Mg in the aluminum diffuses to grain boundary regions. When the local concentration of Mg is high enough, beta phase (Al 3 Mg 2 ) forms in order to lower the stored energy in the material. The beta phase is anodic to the matrix of alloy in seawater and sea air and this potential difference provides the driving force for dissolution of the beta from the grain boundaries causing intergranular corrosion. 
         [0024]    Depicted at  FIGS. 1 through 3  are micrographs of 5XXX aluminum workpiece surfaces.  FIG. 1  depicts a workpiece surface exhibiting little to no precipitation of beta phase Al 3  Mg 2  along the grain boundaries. In this micrograph, the grain boundaries of the aluminum are visible as disjointed dots across the surface of the workpiece.  FIG. 1  is representative of a metal that is not sensitized.  FIG. 2  depicts a workpiece surface exhibiting indeterminate aggregating of beta phase Al 3 Mg 2  along the metal grain boundaries. The grain boundaries are more defined than in  FIG. 1  and are visible as disjointed dots and disjointed, short lines along the metal grain boundaries. This is the result of migration of the beta phase Al 3 Mg 2  to the grain boundaries making the boundaries more visible than the boundaries in the 5XXX aluminum workpiece of  FIG. 1 . However, since the grain boundaries are not visible as continuous or solid lines, indicating only nominal beta phase migration, the 5XXX aluminum workpiece depicted in  FIG. 2  is not a sensitized metal.  FIG. 3  depicts a workpiece surface exhibiting substantial aggregation of beta phase Al 3 Mg 2  along the 5XXX aluminum workpiece grain boundaries. In this instance, the grain boundaries are visible as series of solid, interconnected lines along the metal grain boundaries. These lines represent a film of beta phase Al 3 Mg 2  at the boundaries and is indicative of a sensitized metal. 
         [0025]    Sensitization of metals is problematic since sensitized metals are susceptible to stress corrosion cracking. Stress corrosion cracking occurs when a material susceptible to stress corrosion, such as a sensitized metal, is exposed to a corrosive environment and tensile stresses are experienced in the material above a threshold value. In a sensitized metal, stress corrosion cracking results from the penetration of corrosive elements of the corrosive environment into the metal along pathways created by intergranular corrosion of the metal along the grain boundaries by the continuous film of precipitated alloying elements. By exposing the internal grain boundary surfaces of the metal to the corrosive elements, the metal is further degraded along the grain boundaries causing further intergranular corrosion and the formation of cracks which are exacerbated by the presence of tensile stresses. 
         [0026]    It has been discovered that by treating sensitized metal with UIT, the susceptibility of the metal to stress corrosion cracking can be reduced or eliminated. It has further been discovered that metals exhibiting a stress corrosion crack can be repaired more efficiently than utilizing present methods if the metal undergoes UIT before, during and after the stress corrosion crack is removed. UIT, as used herein and described in detail in U.S. Pat. Nos. 7,431,779; 7,344,609; 7,301,123; 7,276,824; 6,932,876; 6,843,957; 6,289,736, and 6,171,415, all of which are incorporated herein by reference in their entireties, refers to a process of introducing pulse wave energy in combination with ultrasonic mechanical impulse impacts into a load bearing work body&#39;s interior structure in such magnitude as to affect or improve the grain structure and the residual stress patterns therein. In particular, the pulse wave energy and impulse impacts cause compressing of the top layer of the metal body and expanding of the top metallic layer in all directions parallel to the metal&#39;s surface. The surface layer expands beyond its elastic limit and experiences plasticity, which means that it experiences a permanent tensile strain. The surrounding elastically deformed material opposes this tensile strain thereby imparting compressive residual stresses in the surface of the metal in directions parallel to the surface. By expanding the top layer of the metal, the corrosive elements of the corrosive environment are prevented access to the internal grain boundaries of the metal. Thus, the corrosive elements cannot penetrate the metal. Further, by imparting compressive residual stresses in the metal, the effects of the tensile stresses can be ameliorated. 
         [0027]    More particularly, depicted at  FIGS. 4 and 5  are sectional views of the grain boundaries of two metal workpieces. In  FIG. 4 , the represented workpiece has not undergone UIT treatment. In this instance, the crystal grains of the metal have a cuboidal shape or cross-section. The grain boundaries arranged between the cuboidal-shaped crystal grain extend generally vertically and laterally. In a sensitized metal, the vertically-extending grain boundaries present pathways  10  along which stress corrosion cracks can form and exit to the surface of the metal thereby increasing the likelihood of failure of the metal workpiece. In  FIG. 5 , the represented workpiece has undergone UIT treatment. In this instance, crystal grains near the surface of the workpiece have been expanded in all directions parallel to the workpiece metal surface. The individual metal grains are transformed from a cuboidal shape to a flattened or pancake shape having major axes that extend parallel to the surface of the workpiece surface. By flattening of the metal crystal grains near the surface of the workpiece in a sensitized metal, the intergranular pathways  12  along which stress corrosion cracking can occur become more convoluted and longer than the intergranular pathways  10  in untreated sensitized metals. The result of this grain structure modification is that subsurface defects in the material lack a clear intergranular path to the surface, thus delaying cracks from propagating to a workpiece surface. Grain modification thereby forces a crack to try and propagate across a grain itself, which is a more difficult path requiring more energy to propagate. Further, when the workpiece is located in a corrosive environment, the rate at which the corrosive elements penetrate through the surface of the metal and into the workpiece is reduced. 
         [0028]    The penetration of the corrosive elements into the workpiece along pathways  12  may further reduced or altogether eliminated utilizing UIT. Microstructural investigation has shown that UIT treatment of metals produces ultrafine grain structure in the nanocrystalline regimen of the metal down to a depth from the surface of about 6-10 μm. This grain refinement process has been suggested to follow formation of high dislocation density and twining structure following further straining, formation of microbands structure, subdivision of microbands structure into submicron grains, and further breakdown of the subgrains to be equiaxed. Thus, UIT treatment of metals can achieve nanocrystallization of the surface layer for the metal, which is believed to improve in the corrosion and fatigue properties of the materials. In the context of UIT treated sensitized metal, it is believed the nanocrystallization of the surface layer for the metal can likely prevent essentially all penetration by corrosive elements into the workpiece. 
         [0029]    In addition to reducing or preventing corrosion element penetration of the metal workpiece and increasing the energy required to propagate a crack within the workpiece, it is believed that UIT treatment of a sensitized metal slows further enrichment of alloying elements at grain boundaries. As explained above, sensitization is a result of enrichment of one or more alloying elements at the grain boundaries. An example are aluminum alloys with magnesium content greater than three weight percent, such as the 5XXX alloy family. The beta phase Al 3 Mg 2  rich in magnesium tends to migrate to the grain boundaries. This results in intergranular corrosion and/or greater susceptibility to external, environmental corrosion factors. Accelerated corrosion may occur along a path of higher than normal corrosion susceptibility, which is the along the grain boundaries of a sensitized material where precipitates have migrated to, with the bulk of the material typically being passive. By imparting compressive residual stresses and stress relaxation with UIT, coupled with engineered repairs, the migration of precipitates to the grain boundaries may be slowed to such an extent that stress corrosion cracking no longer effectively influences the service life of the structures. This may be due in part to stabilization of the metal by the introduction of ultrasonic energy, ultrasonic or impulse relaxation, compressive residual stresses or a combination thereof. UIT is also believed to reverse metal sensitization by causing intergranular diffusion of alloying elements thereby eliminating enrichment of alloying elements at grain boundaries and regenerating the metal. To do so, the energy imparted to the base metal by UIT must be of sufficient magnitude to cause the precipitates to return to solution. 
         [0030]    To impart the requisite pulse wave energy and ultrasonic mechanical impulse impacts to a metal body to obtain the metal grain and metal grain boundary modifications discussed above, an ultrasonic impact operating system as described in U.S. Pat. No. 6,932,876 can be used. That system employs a set of ultrasonically movable impacting elements, presented typically as sets of three or four spaced members, for impacting a metallic work surface under control of an ultrasonic transducer head. A periodic pulse energy source, typically operable at ultrasonic frequencies up to 100 kHz, induces oscillations into the transducer head, preferably subject to feedback frequency and phase control processing feedback from the working transducer head to aid in matching resonance characteristics of the head when working on the work surface in the manner more particularly set forth in the parent applications of U.S. Pat. No. 6,932,876. The impacting element set creates at the work surface and extending into the sub-surface region of a metallic work body, plasticized metal permitting the surface texture to be machined and sub-surface structural modifications in the work body material to be retained, UIT imparts both ultrasonic relaxation and impulse relaxation within the material. These two components of UIT reduce the magnitude of the tensile residual stresses in the material at greater depths than the plasticity induced compressive stresses which are a surface phenomenon. These methods of relaxation or combinations thereof may result in the resultant tensile stress to be below the threshold value that is a pre-requisite for stress corrosion cracking. 
         [0031]      FIGS. 6  though  13  depict two engineering repair methods utilizing UIT on a sensitized metal workpiece including stress corrosion-induced damage. In  FIGS. 6 through 9 , the damaged workpiece includes a crack and sufficient metal sensitization around the crack that the crack and a portion of the surrounding workpiece metal must be removed and replaced. In  FIGS. 10 through 13 , the damaged workpiece includes a crack with a level of sensitization of the metal around the crack that only the surface of the metal defining the crack is removed and replaced. 
         [0032]    More particularly, referring to  FIGS. 6 and 7 , there is depicted a sensitized metal workpiece  16 , such as a 5XXX aluminum alloy workpiece, including a crack  18  created by stress corrosion. According to this method, a treatment zone  20  is produced in workpiece  16  by introducing pulse wave energy and ultrasonic mechanical impulse impacts to workpiece  16  by utilizing the ultrasonic impact operating system discussed above and thereby modifying the metal as described above. Treatment zone  20  is formed around a section  22  of workpiece that includes crack  18  and other metal that is sufficiently sensitized or otherwise damaged metal to require that the metal be removed from the workpiece. By forming treatment zone  20 , the metal therein is stabilized allowing for a cut to be made within the treatment zone and removal of damaged zone  22  from the workpiece without causing additional potentially damaging stresses to the metal. 
         [0033]    Referring to  FIGS. 8 and 9 , following pre-treatment of workpiece  16  with UIT, section  22  and, optionally a portion of treatment zone  20  situated adjacent to section  22 , are cut from workpiece  16  thereby forming an opening  24  within workpiece  16  and treatment zone  20 . Opening  20  is covered by placing a replacement metal sheet  26  sheet and butt welding sheet  26  within opening  20  along joint  28 . Following deposition of the root pass weld along joint  28 , UIT is applied along the root pass body and toes to strengthen the weld metal against sensitization and to relax any stress within the metal caused by cutting, fit up and welding. Where multi-pass welds are required, UIT can be applied to the fill passes. A final cap pass UIT application is applied along the cap pass weld of joint  28 . Following UIT application along the weld passes, additional UIT is applied to the weld heat affected areas of replacement metal sheet  26  and workpiece  16  adjacent joint  28 . 
         [0034]    Referring to  FIGS. 10 and 11 , there is depicted a sensitized metal workpiece  30 , such as a 5XXX aluminum alloy workpiece, including a crack  32  created by stress corrosion. According to this method, a treatment zone  34  is produced in workpiece  30  by introducing pulse wave energy and ultrasonic mechanical impulse impacts to workpiece  30  by utilizing the ultrasonic impact operating system discussed above and thereby modifying the metal as described above. Treatment zone  32  is formed around crack  32 , including crack  32 , and follows the general shape of crack  32 . By forming treatment zone  32 , the metal therein is stabilized allowing for crack  32  to be removed from the workpiece without causing additional potentially damaging stresses to the metal. 
         [0035]    Referring to  FIGS. 12 and 13 , following pre-treatment of workpiece  30  with UIT, crack  32  is removed from workpiece  16  by grinding thereby forming a depression  36  within workpiece  30  and treatment zone  34 . Depression  36  may or may not extend through workpiece  30 . Following removal of crack  32 , a weld material  38  is deposited within depression  36  thereby filling the depression. Following deposition of the root pass weld within depression  36 , UIT is applied along the root pass to strengthen the weld metal against sensitization and to relax any stress within the metal caused by cutting, fit up and welding. A final cap pass UIT application is applied along the cap pass weld of depression  36 . As described above, for  FIGS. 6 through 9 , UIT can be applied to the fill passes when multi-pass welds are required. Further, following UIT application along the weld passes, additional UIT is applied to the weld heat affected areas of replacement metal sheet  26  and workpiece  16  adjacent joint  28 . 
         [0036]    As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the claims below.