Remediation of Sensitization in Metals

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.

It is claimed: 
     
         1 . A method for treating metal comprising:
 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.   
     
     
         2 . The method according to  claim 1  wherein imparting compressive residual stress in the area of sensitized metal causes one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof. 
     
     
         3 . The method according to  claim 1  further comprising applying a multiplicity of shock pulses to the area of sensitized metal thereby creating a treatment zone of plastic material in the metal structure. 
     
     
         4 . The method according to  claim 3  further comprising inhibiting, by reaction to the shock pulses in the treatment zone, the migration of alloying element precipitates of the workpiece to grain boundaries in the area of sensitized metal. 
     
     
         5 . The method according to  claim 3  further comprising applying the shock pulses in the form of ultrasonic energy with an ultrasonic transducer that is coupled with a surface of the workpiece. 
     
     
         6 . The method according to  claim 1  further comprising modifying a grain structure of the area of sensitization by arranging a major axis of each grain of a portion of grains of the grain structure to be essentially parallel to a surface of the area of sensitized metal. 
     
     
         7 . The method according to  claim 1  further comprising replacing a damaged portion of the area of sensitized metal with a replacement metal and introducing ultrasound compression wave energy into the workpiece about a junction of the replacement metal with the workpiece. 
     
     
         8 . The method according to  claim 1  further comprising contacting an external surface of the area of sensitized metal with a set of striking needles driven by an ultrasonic transducer emitting head. 
     
     
         9 . The method according to  claim 1  further comprising modifying the grain structure of the area of sensitized metal by re-orienting a grain structure of the workpiece into a pancake-like structure. 
     
     
         10 . A method a treating metal comprising:
 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   creating a treatment zone in the area of sensitized metal by modifying the grain structure by arranging a major axis of each grain of a portion of grains of the plurality of crystal grains to be essentially parallel to a surface of the area of sensitized metal.   
     
     
         11 . The method according to  claim 10  wherein arranging the major axis of each grain of the portion of grains of the plurality of crystal grains to be essentially parallel to the surface of the area of sensitized metal causes one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof. 
     
     
         12 . The method according to  claim 10  wherein each grain of the portion of grains of the plurality of crystal grains is arranged to be essentially parallel to a surface of the area of sensitized metal by applying a multiplicity of shock pulses to the area of sensitized metal. 
     
     
         13 . The method according to  claim 10  further comprising applying the shock pulses in the form of ultrasonic energy with an ultrasonic transducer that is coupled to a surface of the workpiece. 
     
     
         14 . The method according to  claim 10  further comprising removing a damaged portion of the treatment zone from the workpiece and replacing the damaged portion with a replacement metal. 
     
     
         15 . The method according to  claim 10  further comprising removing a damaged portion of the treatment zone from the workpiece, replacing the damaged portion with a replacement metal and introducing ultrasonic compression wave energy into the replacement metal. 
     
     
         16 . The method according to  claim 10  further comprising contacting an external surface of the area of sensitized metal with a set of striking needles driven by an ultrasonic transducer emitting head. 
     
     
         17 . A method for treating metal comprising:
 providing a workpiece including an area of sensitized metal, and   introducing ultrasound compression wave energy into the area of sensitized metal thereby creating a treatment zone in the area of sensitized metal.   
     
     
         18 . The method according to  claim 17  further comprising removing a first portion of the treatment zone from the workpiece. 
     
     
         19 . The method according to  claim 18  wherein the first portion of the treatment zone includes a crack. 
     
     
         20 . The method according to  claim 18  further comprising welding a replacement metal portion to the workpiece and within a hole formed by removal of the first portion. 
     
     
         21 . The method according to  claim 20  further comprising introducing an additional ultrasound compression wave energy into the workpiece about a junction of the replacement metal with the workpiece. 
     
     
         22 . The method according to  claim 21  wherein the additional ultrasound compression wave energy is introduced into the workpiece along a root pass weld. 
     
     
         23 . The method according to  claim 21  wherein the additional ultrasound compression wave energy is introduced into the workpiece along a cap pass weld. 
     
     
         24 . The method according to  claim 18  wherein introducing the ultrasound compression wave energy into the workpiece causes one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof. 
     
     
         25 . The method according to  claim 18  further comprising applying a multiplicity of shock pulses to the area of sensitized metal thereby creating the treatment zone of plastic material in the metal structure. 
     
     
         26 . The method according to  claim 25  further comprising applying the shock pulses in the form of ultrasonic energy with an ultrasonic transducer that is coupled with a surface of the workpiece. 
     
     
         27 . The method according to  claim 25  further comprising modifying a grain structure of the area of sensitization by arranging a major axis of each grain of a portion of grains of the grain structure to be essentially parallel to a surface of the area of sensitized metal. 
     
     
         28 . The method according to  claim 18  further comprising contacting an external surface of the area of sensitized metal with a set of striking needles driven by an ultrasonic transducer emitting head. 
     
     
         29 . A workpiece comprising:
 a sensitized metal portion including 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.   
     
     
         30 . The workpiece according to  claim 29  wherein the treatment zone exhibits one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof. 
     
     
         31 . The workpiece according to  claim 29  wherein 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. 
     
     
         32 . The workpiece according to  claim 29  wherein the sensitized metal portion includes a grain structure including a plurality of crystal grains, each grain of a portion of grains of the plurality of grains having a major axis arranged essentially parallel to a surface of the treatment zone. 
     
     
         33 . The workpiece according to  claim 29  further comprising an opening extending through the treatment zone created by removal of a damaged section of the sensitized metal portion. 
     
     
         34 . The workpiece according to  claim 29  further comprising a metallic replacement member welded to the workpiece within an opening through the treatment zone created by removal of a damaged section of the sensitized metal portion. 
     
     
         35 . The workpiece according to  claim 29  further comprising a stress corrosion crack in the treatment zone. 
     
     
         36 . The workpiece according to  claim 29  further comprising a depression formed within the treatment zone, the depression being created by removal of a crack, wherein the depression is filled with a replacement metal. 
     
     
         37 . A method for treating metal comprising:
 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.   
     
     
         38 . The method according to  claim 37  wherein the pulses of ultrasonic wave energy are introduced into a sensitized portion of the workpiece which contains the stress corrosion crack. 
     
     
         39 . The method according to  claim 37  further comprising removing a section of the workpiece that contains the stress corrosion crack and welding a replacement plate within an opening created by the removal of the section. 
     
     
         40 . The method according to  claim 39  further comprising introducing additional pulses of ultrasonic wave energy into the workpiece through additional periodic ultrasonic mechanical impulse impacts wherein the additional periodic ultrasonic mechanical impulse impacts are applied to a weld joint formed between the workpiece and the replacement plate. 
     
     
         41 . A method for treating metal comprising:
 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.   
     
     
         42 . The method according to  claim 41  wherein the pulses of ultrasonic wave energy are introduced into a sensitized portion of the workpiece which contains a stress corrosion crack.