JP2008105050A - Residual stress reduction method and apparatus - Google Patents

Abstract

An inexpensive residual stress reduction method and apparatus while ensuring welding reliability is provided.
Residual stress of a welded structure is reduced by heating a welding construction surface with a heater after welding and then cooling with a cooler.
[Selection] Figure 1

Description

  The present invention relates to a method and apparatus for reducing residual stress in a welded structure.

  In general, it is known that a structure such as a reactor pressure vessel undergoes stress corrosion cracking (SCC) at or near a welding site in an environment under high temperature and high pressure. In this stress corrosion cracking, the residual stress resulting from welding is considered to cause the occurrence of stress corrosion cracking, its progress, and a decrease in fatigue strength. This residual stress is known to result from the occurrence of thermal expansion and plastic strain in the welded structure due to welding, and various measures have been conventionally taken to reduce this residual stress.

For example, the residual stress reducing means shown in Patent Documents 1 and 2 is a welding apparatus in which a welding electrode and a cooling nozzle are adjacently arranged and integrated on a rail provided along a welding line, and welding is performed. Later, the welding surface is rapidly cooled by spraying a coolant to relieve the tensile stress generated on the welding surface, thereby reducing the residual stress.
Japanese Patent No. 3732040 Japanese Patent Laid-Open No. 2006-7301

  The residual stress reducing means shown in the above-mentioned patent documents 1 and 2 rapidly cools the welding construction surface with a coolant immediately after the welding construction, thereby reducing the residual stress on the construction surface. At that time, it is necessary to perform cooling while the temperature of the welding surface is sufficiently high, and cooling must be performed immediately after welding. For this purpose, it is necessary to bring the welding torch and the cooling mechanism as close as possible. However, when the cooling mechanism is brought close to the welding machine, there is a problem that the arc discharge becomes unstable and the reliability of welding is lowered. In addition, it is conceivable to make the welding machine have a special structure in order to stabilize arc discharge, but there is a problem that the apparatus becomes complicated and the cost becomes high.

  In order to solve the above-mentioned problems, the present invention is characterized in that in the welding residual stress reduction method and apparatus therefor, the welding surface is heated by a heater after welding and then cooled by a cooler.

  The present invention efficiently reduces the residual stress of a welded structure while ensuring the reliability of welding by using a heater and a cooler together with an existing welder without using a special welder. Can be made.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings.
FIG. 1 (a) shows an example of a welded structure to which the residual reduction method according to the present invention is applied. It is shown in the figure. In this example, the cylindrical structure 1 is welded to each other by a weld 2 in the circumferential direction. Actually, in order to perform welding of the welded portion 2 of the cylindrical structure 1, it is necessary to perform, for example, 23-pass welding as shown by a weld bead path 4 illustrated in FIG. 1B is an enlarged cross-sectional view of the welded portion A, and 3 is a cross-section of the welded portion.

  Tensile residual stress usually occurs on the construction surface after welding due to expansion and contraction due to the heat of welding. FIG. 2 shows a configuration of a residual stress reducing device for reducing the residual stress. This residual stress reducing apparatus is composed of a welding machine 5, a heating machine 6 and a cooling machine 7, and this apparatus is used in the 23rd pass welding which is the final pass.

  In the present embodiment, the heater 6 and the cooler 7 are attached to the same rail 8 as the welder 5 so as to follow the welding torch of the welder 5 during the final pass welding. In this embodiment, the heater 6 and the cooler 7 are integrated and move at the same moving speed as the welder 5, but the welder 5, the heater 6 and the cooler 7 are integrated as one device. May be used. Of course, the distance between the heater 6 and the cooler 7 may be set and integrated in consideration of the heating / cooling time.

  In the present embodiment, a gas burner 10 is used as the heater 6 as shown in FIG. 3, but a high-frequency heating device 14 may be used instead of the gas burner 10 as shown in FIG. In the figure, reference numeral 14a denotes a high frequency generator.

  In the actual welding, the range of influence of the welding heat on the welded structure is within a range of a maximum of 15 mm outward from each of the melting lines 9 (two) that define the upper and lower ranges of the melted part based on preliminary tests and predictions. Therefore, as shown in FIG. 3, at least the range of the melt line 9 to 20 mm was heated by the flame 10a of the gas burner 10 of the heater 6 so that the heating range included the range.

  Furthermore, when the heating temperature is 500 ° C. or higher due to heating by the heater 6, material sensitization or the like occurs. When the heating temperature is 300 ° C. or lower, a sufficient residual stress improvement effect cannot be obtained. The amount of heat of the heater 6 is adjusted so that the temperature becomes 300 ° C to 500 ° C.

  Next, immediately after being heated by the heater 6, it is rapidly cooled by the cooler 7. In the present embodiment, dry ice is used as the coolant, and the dry ice 7b is sprayed onto the heating unit via the nozzle 7a. Of course, other cooling means may be used instead of the cooling by the dry ice 7b. The cooling range may be the same as the heating range.

  In the present embodiment, a cooling chamber 11 is provided as shown in FIG. 4 and the dry ice 7b is covered with the cooling chamber 11 of the cooler 7 so that the flame 10a of the gas burner 10 does not affect the cooling process. Only to be sprayed on. Thereby, the influence of the heat from the gas burner 10 is interrupted, and the cooling is efficiently performed until the temperature of the material surface 1a is sufficiently lowered.

  FIG. 5 is a graph showing the residual stress distribution on the construction surface after welding according to the present invention and the distance from the welding center as a parameter. In FIG. 5, 13 is a residual stress distribution by the welding method of the present invention, and 12 is a residual stress distribution by a normal welding method. From FIG. 5, when comparing the residual stress in the vicinity of welding, the tensile residual stress is not reduced in the distribution 12 of the normal welding, whereas the residual stress distribution 13 when using the welding method according to the present invention is It can be seen that the tensile residual stress is significantly reduced.

(A) is the schematic of a cylindrical structure, (b) is an expanded sectional view of the welding part A. FIG. 1 is a schematic diagram of a residual stress reduction device according to an embodiment of the present invention. Schematic of the heater which concerns on embodiment of this invention. Schematic of a heater and a cooler according to an embodiment of the present invention. The residual stress distribution map which concerns on embodiment of this invention. Schematic of the high frequency heating apparatus which concerns on embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Cylindrical structure, 1a ... Material surface, 2 ... Welded part, 3 ... Cross section of welded part, 4 ... Weld bead path, 5 ... Welding machine, 6 ... Heating machine, 7 ... Cooling machine, 7a ... Nozzle, 7b ... Dry ice , 8 ... rail, 9 ... melting line, 10 ... gas burner, 10a ... flame, 11 ... cooling chamber, 12, 13 ... residual stress distribution, 14 ... high frequency heating device, 14a ... high frequency generator.

Claims (6)

  A residual stress reduction method for reducing residual stress of a structure, wherein the welding surface is heated with a heater after welding and then cooled with a cooler.   The method for reducing residual stress according to claim 1, wherein the heating is performed by heating the welding surface in a range of 20 mm from the melt line at 300 ° C to 500 ° C.   3. The residual stress reduction method according to claim 1, wherein the heating is performed by a gas burner or high frequency heating.   4. The residual stress reduction method according to claim 1, wherein the cooling is performed by spraying dry ice onto the welding surface.   A residual stress reduction device for reducing residual stress of a structure, comprising: a welding machine; a heater for heating a welding construction surface after welding; and a cooling machine for heating and cooling the welding construction surface. Residual stress reduction device.   6. The residual stress reduction device according to claim 5, wherein at least the heating machine and the cooling machine are integrated among the welding machine, the heating machine, and the cooling machine.

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