JPH048986A - Method and device for preventing stress corrosion near welded joint - Google Patents

Abstract

PURPOSE: To prevent stress corrosion by providing a method comprising a step of applying radiation heat to a first surface of a welded joint and a workpiece area, a step of keeping a flow of a cooling fluid passing by a second surface of the welded joint and a working area, a step of regulating the quantity of radiation heat and a flow, and a step of cooling the first surface. CONSTITUTION: A welded part 10 of a stainless steel pipe 14 of a welded joint and an area indicated by a broken line are exposed to an external generated heat 16. A cooling fluid 30 is let flow to an inner wall 32 through the interior of the pipe 14, the external generated heat 16 is applied to the outer surface of the pipe 4 in the area 12 to generate local thermal stress exceeding the compressive yield stress and the quantity of external generated heat 16 and the quantity of the cooling fluid 30 are regulated to generated such a temperature difference as to generate a local thermal stress larger than a tensile yield stress in the inner wall 32. The welded part 10 and the area 12 are cooled to the atmosphere temperature. As a result, the particle-to-particle stress corrosion cracks near the welded joint can be decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the mitigation of stress corrosion cracking in steel products, and more particularly to an in-situ treatment method and apparatus for reducing intergranular stress corrosion cracking in the vicinity of welded joints in austenitic steel piping systems. .

Stress corrosion cracking, especially intergranular stress corrosion cracking in welded steel products in welded austenitic steel piping, is clearly due to the interaction of the corrosive environment, increased sensitivity of the steel to welding heat, alloying elemental composition, and other metallurgical factors. and due to the presence of residual stress near the weld area.

Intergranular stress corrosion cracking in the vicinity of welded joints in steel, particularly austenitic stainless steel piping used in cooling water lines of nuclear power plants, has long been recognized as a serious problem in the industry. Various solutions to this long-standing problem have been proposed, including early proposals for solution annealing, cooling fluid inside the pipe as taught by Hanneman et al., U.S. Pat. At the same time as welding, overlay welding that extends beyond the original weld and Eguchi
4.168.190, rapid liquid cooling followed by rapid heating of local sensitivity-increasing areas by generation of high-frequency alternating current in the pipe by induction or by internal IR heating. A recent proposal advanced in light of the knowledge that the probable cause of intergranular stress corrosion cracking in the vicinity of welded joints in nuclear power plant austenitic stainless steel piping is the presence of residual tensile stresses near the joint. ) Other U.S. Pat. No. 4,229,235
proposed heating the pipe while passing a cooling fluid through the pipe by passing an electric current between a pair of electrode elements spaced apart on the surface of the pipe. Mazda also pointed out the following: That is, with such heating, the residual tensile stresses normally present on the inner wall of the pipe are reduced and possibly converted into residual compressive stresses,
As a result, "corrosion fatigue" decreases. A recent proposal is Terasaki's U.S. Patent No. 4.354,
No. 883 and Sugihara et al., US Pat. No. 4,505,763, involve selecting the shape of the radio frequency heating element or coil in an attempt to control the temperature distribution over the area of application. The use of weld overlays has also been discouraged, at least in part, by the inherent inability to control temperature gradients within the metal and the local environmental difficulties of performing in-situ welds. The installation, in which current is passed through the pipe between the electrodes, has also not been found to be particularly effective. Radiofrequency heating of pipes is attractive in theory, but in practice it requires carefully placed shielding as well as special radiofrequency power supplies, impedance matching devices, and cooling media for induction coils and power cables. valves, which require expensive and bulky equipment such as , and associated pumping equipment, all requiring special design elements;
This constitutes a practical problem that is exacerbated by complex shaped installations such as T-fittings, elbows, U-tubes, etc.

In its broad aspects, the present invention maintains the flow of cooling fluid across the inner surface and at the same time provides a rapid temperature rise to the outer surface by exposing it to externally generated radiant heat, thereby generating an easily controllable temperature difference within the wall of the stainless steel. It can be briefly stated that an apparatus and method for achieving in-situ reduction of intergranular stress corrosion cracking in welded austenitic and other steel products, such as steel pipes. In a narrower aspect, the invention provides a plurality of high-temperature radiant heating systems that complementarily match the contours of the area to be treated, in connection with the easily possible selective control of the radiant heat generating coils and their distance from the workpiece. Moji S-Jkm heating furnace with built-in coil
Generator, &. . In a more narrow aspect, the invention includes a heat flow directing and insulation device to effectively maximize the transfer of generated heat to the workpiece.

Among the advantages associated with the practice of the present invention is the significantly improved design of the in-wall temperature gradient control system and the elimination of the specially designed transformers, cables and associated shielding and control equipment that are characteristic of high frequency heating systems. There are things to do. Other benefits include eliminating the need for cooling water and associated high-frequency generators, allowing the use of conventional industrial standard power supplies, cables and control equipment, and simplifying equipment and increasing mobility to connect adjacent equipment. This includes eliminating undesirable heat transfer or heat generation and significantly reducing power requirements. A further advantage is that it is applicable to various pipe and element shapes;
A high degree of selective control and placement of radiant heat generating modules is possible to control selective heating to various workpiece areas to create the required intra-wall temperature differences, resulting in This includes being able to process welded joints between pipes or elements of different alloys that require different heating rates.

It is an object of the present invention to provide an improved method and apparatus for the heat treatment of welded steel workpieces.

Another object of the present invention is to provide an improved method and apparatus for in-situ treatment of reducing intergranular stress corrosion cracking near weld areas in stainless steel piping, such as in nuclear power plants.

In accordance with the requirements of the patent laws, other objects and advantages of the present invention will emerge from the following description, made with reference to the accompanying drawings, which illustrate presently preferred embodiments of heat treatment apparatus incorporating the principles of the invention. It will become clear.

Referring first to FIG. 1, the method and apparatus of the present invention involves heating the weld (10) at the joint of two sections of stainless steel pipe (14) and the areas on either side thereof, indicated by dashed lines (12). Including on-site work that involves exposure to externally generated heat inside a furnace-like furnace. Such externally generated heat (16) is primarily
radiation generated by passing a controlled amount of electrical current through one or more resistance heating wires (18) of a selected size and/or shape placed in spaced relation to the pipe exterior surface and the weld surface (20); At the same time, a coolant is passed along the inner wall surface (32) of the vibrator. Furnace shaped, preferably made of ceramic, formed of a thermally insulating shield (22) having radiant heat reflective properties to trap and redirect the generated heat as indicated by the arrow (24) towards the pipe surface (20). A housing is disposed in surrounding relation to the wire (18).

a rigid shell (26) with thin walled side walls disposed in abutting relation to the pipe surface to complete the hearth-like enclosure;
Preferably, the thermally insulating and reflective shielding material (22) is backed and supported by.

A cooling fluid (
30) and the application of externally generated heat to the outer surface of the pipe in zone (12) creates a stress in the outer wall greater than the material's compressive yield stress and an inner wall greater than the material's tensile yield stress. It helps to preferably generate an intra-wall temperature differential gradient of a suitable nature so as to cause plastic deformation of the outer wall due to heating of sufficient magnitude to generate large stresses. An ideal situation for such a phenomenon is illustrated in Figure 2, where the tensile and compressive yield strengths are δyt and δy, respectively.
It is represented by c. A stress along the curve OA occurs on the outer surface of the pipe - a strain distribution occurs, and a stress along the curve OB occurs on the inner surface of the vibrator.
The outer surface of the vibrator is heated to create an intra-wall temperature difference of appropriate magnitude to create a strain distribution. As represented by points A and H, the outer wall temperature is at a height that produces a local thermal stress on the outer surface that exceeds the compressive yield stress of the pipe material and a local thermal stress on the inner surface that exceeds the tensile yield stress of the pipe material. The temperature difference has the property of giving When the externally generated radiant heat is stopped and the pipe is returned to a negative temperature, the stresses on the inner and outer surfaces become residual compressive stress on the inner surface of the pipe and residual tensile stress on the outer surface. Reducing the tensile stress state, preferably converting it to a residual compressive stress state, on the inner surface of the pipe near the weld joint increases the resistance of that area to stress corrosion and/or corrosion fatigue, and reduces intergranular stress corrosion cracking in that area. Work like you do.

By using a radiant heat source located in a furnace-like housing in spaced relation to the workpiece surface, the external heating elements can be configured in modular form with different shapes to accommodate welding joints at various locations. becomes possible. One of the most commonly found locations for welded joints is generally between two pipes, as shown in FIG. Referring now to Figures 3 through 5, two straight stainless steel vibrators are shown.
An assembled circular gIJ shell consisting of a plurality of segments (40, 42), at least two, of sufficient length to extend on either side of the weld (44) of two sections (46, 48). A mold heating element assembly (36) is illustrated. Figures 4 and 5
As best seen in the figure, the partially cylindrical section consists of a plurality of elongated non-conductive ceramics wrapped around a resistance heating wire (52) which is terminally connected to a busbar (54) carrying, for example, a three-phase 480μ alternating current. It includes a support member (50).

Ceramic support members (50) are terminally supported by shell insulation (56) and maintained in a predetermined spacing relationship with each other, also with a radiant heat reflector 1, suitably made of a heat-resistant ceramic material.
! This is confirmed by (58). The entire assembly of busbar (54), shell insulation (56) and reflective wall (58) is 3.
Surrounded on one side by a stainless steel housing (60). As best seen in FIG. 5, the lateral dimensions of the shell insulation (56) are such that the resistance heating wires (52)
) and also serve as the end wall of a hearth-like enclosure. As schematically illustrated in FIG. 3, a plurality of thermocouples (70) are mounted on the outer surface of the pipe section (46, 48) to provide a continuous flow of temperature information regarding the actual temperature at the pipe surface. It is therefore desirable to be able to quickly control the heating rate. Electric cable (
72) serves to provide power to the busbar (54), and a suitable rheostat (not shown) regulates the amount of power supplied.

Figures 6-8 schematically illustrate the weld heating configuration of the vibrator section and the convenient application of a modular radiant heating assembly to the weld.

FIG. 6, for example, schematically shows a three-piece cylindrical heating assembly of 120' section (80). Figure 7 shows the installation of an assembly of the type shown in Figure 6 over one of the welds (82) connecting a straight pipe section (84) to a valve (86) in the general form of a T-joint. This is a schematic diagram. Figure 8 shows the reducer type displacement pipe section (94) and the reduced diameter pipe section (94).
6) shows an inclined heating assembly (90) mounted over the weld (92) between FIG. In this type of assembly, one set of radiant heating elements is disposed in parallel spaced relation to the reducer section (94) and a second set of heating elements is disposed parallel to the surface of the pipe section (96). be done.

FIG. 9 is a schematic diagram of a system for controlling the rate of heating to the outer surface of a workpiece (110). As shown, a thermocouple (70) sends a continuous stream of temperature data indicative of the surface temperature of the workpiece to a comparator device (100), which also communicates via a sensor (102) the temperature data on the inner surface of the workpiece. Continuously accepts cooling water temperature data. This input data is compared to a programmed data value indicating the desired temperature on a scheduled basis and the difference is used to provide a series of control signals (104) to the power controller (106) to control the external power supply ( 108) to the radiant heating element (52).

Clearly, the modular construction described above not only accommodates different workpiece contours, but also provides easily controlled heating of the workpiece and its parts. Thus, the disclosed arrangement is easily adaptable to metals with different coefficients of thermal expansion, with suitable and different intra-wall temperature differences in each alloy and/or suitable axial direction of the pipe near the weld zone. Gives a temperature difference. It will now be appreciated that radiant heating elements other than the resistance wires described above could be used in certain types of equipment and processing areas. For example, high-energy lamps using quartz filaments or other heat-resistant ceramic or metal-ceramic mixture heating elements.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of an embodiment of the present invention in the process of welding joints of stainless steel piping, such as those used in nuclear power plants; FIG. FIG. 3 is a stress-strain ideal diagram showing the progressive stress changes that occur in a welded pipe workpiece in response to the presence and subsequent cooling of a stainless steel pipe. 4 is an internal view of a portion of a radiant heating module incorporating the principles of the present invention; FIG. 5 is a vertical sectional view of the module portion of FIG. 4; FIG. FIG. 8 is a schematic perspective view of a radiant heat module of selected configurations adapted to accommodate different workpiece surface contours; FIG. 9 is a schematic diagram of a power control system for a heating assembly of the type described; 18... Heating wire 22... Reflection/shielding device 26.
... Shell body 30 ... Cooling fluid 36 ... External heat source (module) 50 ... Support member 52 ... Resistance wire 5
8... Reflector device 60... Housing FIG, 3 Patent application agent FIG, 4

Claims (1)

[Claims] 1. A method for preventing stress corrosion in the vicinity of a welded joint of a steel workpiece, comprising: a. one surface of the welded joint and an area of the workpiece in its vicinity that normally experiences localized compressive stress; b. applying radiant heat emitted from an external heat source disposed in close and distant proximity thereto; b. a second surface of said weld joint and a workpiece area in its vicinity normally subject to localized tensile stress; c) maintaining a flow of cooling fluid across the first surface and adjacent areas of the workpiece, and applying a localized thermal stress exceeding the compressive yield stress of the workpiece to the second surface and adjacent areas; adjusting the amount of radiant heat applied and the amount of flow of the cooling fluid so as to generate a temperature difference across the workpiece of such a nature as to cause a localized thermal stress in excess of the tensile yield stress of the workpiece; d cooling the first surface and an area adjacent thereto to ambient temperature. 2. The method of claim 1, wherein the steel workpiece is austenitic steel. 3. The method of claim 1, wherein the steel workpiece is stainless steel. 4. The method of claim 1, wherein the radiant heat source is disposed within a furnace-like enclosure surrounding the first surface of the weld joint and an area adjacent thereto. 5. A method for preventing stress corrosion near a welded joint in a steel piping system, comprising: a. maintaining a flow of cooling fluid across the welded joint within the pipe; b. an outer surface of the welded joint and its (c) subjecting the immediate area to radiant heat emitted from an external heat source located in close and distant proximity thereto; adjusting the amount of radiant heat applied and the amount of flow of said cooling fluid to create a temperature difference across the pipe wall of such a nature as to create a localized thermal stress on the inner surface that exceeds the tensile yield stress of the pipe; d cooling the area of the pipe exposed to the radiant heat to ambient temperature. 6. The method of claim 5, wherein the steel is stainless steel. 7. The method of claim 5, wherein the radiant heat source is disposed within a furnace-like enclosure. 8. The method of claim 5, wherein the heat source comprises a resistance wire that can be heated to incandescence by passing an electric current through it. 9. The method of claim 5, wherein the radiant heat includes both direct and reflected radiant heat emitted from the heat source. 10. A device for preventing stress corrosion in the vicinity of welded joints of steel workpieces, comprising: a. one surface of said welded joint and a workpiece area in its vicinity that is normally subjected to compressive stress, which is closely spaced; a device for applying radiant heat from an external heat source located nearby; b. a device for maintaining a flow of cooling fluid across the second surface of said welded joint and a workpiece area in its vicinity that is normally subject to local tensile stresses; an apparatus for enclosing a workpiece area at and near a surface of the weld joint and the radiant heat source to concentrate heating; 11. The radiant heat source includes a configurable module having a contour that complements the contour of the workpiece for disposing the heat source in close and distant proximity to the workpiece surface. The device described. 12. The apparatus of claim 10, wherein the radiant heat source includes a resistance wire supported by an insulating member. 13. The apparatus of claim 10, wherein the apparatus for enclosing the radiant heat source includes a radiant heat reflector and an enclosure housing apparatus forming a furnace enclosure around a workpiece area exposed to radiant heat. . 14. Applying a local thermal stress exceeding the compressive yield stress of the workpiece to the first surface and an area adjacent thereto, and applying a local thermal stress exceeding the tensile yield stress of the workpiece to the second surface and an area adjacent thereto. 10. The method of claim 10, further comprising an apparatus for adjusting the amount of heat applied and the amount of cooling fluid flow so as to generate a desired temperature difference across the workpiece of a nature such that . The device described. 15. The apparatus of claim 10, wherein the workpiece is a stainless steel pipe, the first surface is an outer surface of the pipe, and the second surface is an inner surface of the pipe.