GB2044296A

GB2044296A - Method of Heat Treating an Object Having a Welded Joint

Info

Publication number: GB2044296A
Application number: GB8003065A
Authority: GB
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 1979-02-07
Filing date: 1980-01-30
Publication date: 1980-10-15
Also published as: DE3004233A1; FR2448574A1; JPS55122631A

Abstract

Stress corrosion cracking near weld joints in e.g. a piping system or vessel is minimized by heat treating the welded object whereby residual compression stresses are created at the surface exposed to a corrosive environment and residual tensile stresses are formed in the opposing surface. The heat treatment includes applying heat to one surface and applying a heat sink e.g. water to the other surface whereby a temperature gradient is established between the two surfaces while the one surface is heated and subsequently is allowed to cool. <IMAGE>

Description

SPECIFICATION Method of Heat Treating an Object Having a Welded Joint This invention relates generally to heat treating of materials, and more particularly the invention relates to a method of treating an object having a welded joint.

Temperature cycling of an object can create residual stresses which have deleterious effects in the subsequent use of the object. For example, stresses in weld joints in a piping system can result in cracking and early failure of the system.

In particular, stress corrosion cracking has been observed in sensitized zones created by welding of pipe. This sensitization of the material is the result of the pipe being heated to critical temperatures during the welding process.

Conventionally, attempts at minimizing the deleterious effects of these residual stresses have focused on eliminating the residual stresses through various processes of stress relief.

However, for a body to be effectively stress relieved the entire body must be heated to a uniform and high temperature, held there for an extended period of time so that stresses will relax, and then uniformly cooled so that new stresses are not created in the body. This is a very difficult task especially where large structures are involved.

A feature of the invention is the creation of beneficial residual stresses.

Briefly, in accordance with the invention, a welded object is treated to create residual compressive stresses on one surface and residual tensile stresses on another surface near a weld joint. This is accomplished by heating one surface in the vicinity of the weld to establish a temperature gradient between this first surface and the second surface. Maintaining this temperature gradient will in many cases require removing heat from the second surface. In a piping system, the residual stresses are established by circulating a coolant through the piping system while the exterior surface near a weld joint is heated.By establishing a temperature gradient between the exterior surface and the interior surface while at the same time minimizing the temperature increase of the interior surface through cooling, residual compressive stresses are formed at the interior surface near the weld joint while residual tensile stresses are created at the external surface near the weld joint. Importantly, stress corrosion cracking tends to occur in surfaces having residual tensile stresses, therefore the residual compressive stresses at the interior surface minimizes stress corrosion cracking.

This invention thus provides a method of treating a weld joint having first and second opposed surfaces to relieve tensile stress at the first surface, the method comprising heating the second surface whilst maintaining a temperature gradient between the second surface and the cooler first surface, and then causing or allowing the second surface to cool. Thus stress corrosion cracking near weld joints in a piping system or vessel can be reduced by heat treating the welded object so that residual compression stresses are created at the surface to be exposed to a corrosive environment and residual tensile stresses are formed in the opposing surface.The heat treatment usually includes applying heat to one surface and applying a heat sink to the other surface whereby a temperature gradient is established between the two surfaces while the one surface is heated and subsequently is allowed to cool.

The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.

The drawing illustrates one embodiment of the invention as applied to a piping system having a plurality of weld joints.

As described above, stress corrosion cracking often occurs in sensitized material having a surface which is left in a state of tension. To minimize such cracking, in accordance with the invention, the surface of the object which is exposed to the corrosive environment is treated so that residual compressive stress is created. As shown in the drawing, a portion of a pipe 10 of a closed piping system illustrated by dotted line 12 has a plurality of butt weld joints such as weld 14 in pipe section 1 0. After the weld joints have been completed, heat is applied to the exterior surface of the pipe section 10 in the vicinity of the butt weld 14, and a heat sink such as water is circulated through the piping system.By obtaining a temperature gradient between the exterior surface and the interior surface of the piping system in the vicinity of the weld joints, residual compressive stresses are created in the cooler interior surface while residual tensile stresses are created in the exterior surface.

The heating may be applied by a manual torch or by automatic tungsten inert gas (TIG) weld with no additional filler material being used. The axial width of the heated zone, the temperature of the external surface, and the rate of cooling of the object depends on the material and dimensions of the welded object.

While the invention has particular applicability to the treatment of welded pipes, the invention can be applied to any vessel or system in which one surface temperature can be controlled by means of a suitable heat sink such as liquid or chill blocks while the other surface is heated.

Moreover, for an object having relatively thick walls, a heat sink may not be needed for the interior surface of the exterior surface can be rapidly heated through a high rate of heat input.

The two major constraints in utilizing the invention are the rate of heat input and the rate at which heat must be removed in order to reach a desired temperature distribution. The invention also has applicability to weld repairs of pressure vessels.

Thus, while the invention has been described with reference to a specific environment, the description is illustrative of the invention and not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true principles and scope of the invention as defined by the appended claims.

Claims

1. A method of treating a welded object having a first surface and a second surface so that residual tensile stresses are formed in said first surface near the weld and residual compressive stresses are formed in said second surface comprising the steps of heating said first surface in the vicinity of the weld and cooling said second surface in the vicinity of the weld to establish a temperature gradient between said first surface and said second surface in the vicinity of the weld.

2. A method of treating a welded object as defined by Claim 1 wherein said object comprises a pipe and said temperature gradient is established by filling said pipe with a liquid while the exterior surface of said pipe is heated.

3. A method of treating a welded object as defined by Claim 2 wherein said liquid is circulated through said pipe.

4. A method of treating a welded object as defined by Claim 3 wherein said liquid is water.

5. A method of treating a welded object as defined by Claim 1 wherein said temperature gradient is established by applying a heat sink to said interior surface whereby heat removal from said second surface is facilitated.

6. A method of minimizing stress corrosion cracking within a pipe near a weld joint which comprises establishing residual compressive stresses in the interior surface of said pipe near the weld joint and-establishing residual tensile stresses in the exterior surface of said pipe near the weld joint.

7. A method as defined in Claim 6 wherein said residual stresses are established by heating said exterior surface in the vicinity of said weld joint and applying a heat sink to said interior surface in the vicinity of the weld joint.

8. A method as defined by Claim 7 wherein said heat sink comprises a liquid which is circulated through said pipe while said exterior surface is heated.

9. A method of treating a weld joint having first and second opposed surfaces to relieve tensile stress at the first surface, the method comprising heating the second surface whilst maintaining a temperature gradient between the second surface and the cooler first surface, and then causing or allowing the second surface to cool.

10. A method of treating a weld joint to relieve tensile stress at a surface thereof, the method being substantially as hereinbefore described with reference to the accompanying drawing.

11. A weld joint treated by a method according to any of claims 1 to 10.