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.

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 ...

AI Technical Summary

Benefits of technology

[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.

[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.

Problems solved by technology

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.

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