JP2012021188A - Method for improving residual stress in pipe, and method for construction management - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for compressing a tensile residual stress working on an inner surface in the vicinity of a pipe welded part at a <350°C construction temperature.SOLUTION: A step is repeated at least twice, wherein, after heating the vicinity of the pipe welded part at the <350°C construction temperature by heater heating from the outer surface, cooling water is supplied to the pipe inside to rapidly cool the inner surface. In the construction management, a temperature difference between inner and outer surfaces is evaluated based on lowering speed of the outer surface temperature when the cooling water is supplied to rapidly cool the inner surface, and on a pipe plate thickness at a temperature measuring position, and it is confirmed that a thermal stress generated by the temperature difference between the inner and outer surfaces is equal to or higher than a yield stress of a pipe material.

Description

  The present invention relates to a method of improving a residual stress acting on an inner surface of a pipe in a compression direction and a construction management method thereof.

  Residual stress in the tensile direction may act on the inner surface near the welded portion of the pipe due to the thermal history during welding. This tensile residual stress contributes to the occurrence and development of stress corrosion cracking in high temperature water piping made of austenitic stainless steel. For this reason, if the tensile residual stress acting on the inner surface in the vicinity of the welded portion can be improved in the compression direction, preferably compressed, damage to the pipe due to stress corrosion cracking can be suppressed.

  Of the methods for improving the tensile residual stress acting on the inner surface near the welded part of the pipe in the compression direction, the method for improving the residual stress in the compression direction by rapidly cooling the inner surface after heating the pipe is possible by adjusting the heating temperature. Since the temperature difference between the inner and outer surfaces can be adjusted, the residual stress can be improved in the compression direction even in small-diameter pipes where the plate thickness is thin and it is difficult to apply a large temperature difference to the inner and outer surfaces.

  Typical methods for improving the tensile residual stress acting on the inner surface by rapid cooling of the inner surface after heating the pipe in the compression direction are listed in Patent Documents 1 to 3. In these patent documents, a pipe is heated from the outer surface to a predetermined temperature, and then a refrigerant is supplied to the inside, and the inner surface of the pipe is tensile-yield due to a thermal stress generated by a temperature difference between the inner and outer surfaces of the pipe. A method for improving the stress in the compression direction is described.

  In Patent Document 1, after the entire tube group is heated uniformly, a temperature difference is given to the inner and outer surfaces by flowing a coolant into the tube, and the tensile residual stress acting on the inner surface of the pipe is reduced by tensile yielding the inner surface. A method of relaxation or compression is described.

  Patent Document 2 describes changes in residual stress when heated to 200 ° C. to 900 ° C. after welding and soaked for 1 hour, then air-cooled and internally water-cooled to reduce residual stress, and the heating temperature is high It is described that the effect of reducing the residual stress in the inner surface axial direction is higher, and the effect of reducing the residual stress of inner surface water cooling is higher than that of air cooling. The reason why the residual stress in the axial direction of the inner surface becomes the compressive residual stress when the cooling method is the inner surface water cooling is after the heating temperature exceeds about 600 ° C.

  Patent Document 3 describes a method for improving the tensile residual stress acting on the inner surface of the pipe in the compressing direction by giving a temperature difference to the inner and outer surfaces by flowing a coolant into the pipe after the pipe is uniformly heated, and construction management. As a method, a method for defining the minimum value of the cooling water amount for each inner diameter of the pipe is described.

  As described above, in order to suppress damage due to stress corrosion cracking of austenitic stainless steel pipes, it is necessary to improve, preferably compress, the tensile residual stress generated by the thermal history during welding.

  In the method of improving the residual stress in the compression direction by rapidly cooling the inner surface after heating the piping shown in Patent Literatures 1 to 3, as shown in Patent Literature 2, the piping inner surface is increased as the heating temperature of the piping becomes higher. Residual stress reduction effect is improved. This is due to the fact that the temperature difference between the inside and outside of the pipe increases during the water cooling of the inner face by increasing the heating temperature of the pipe. Since the thermal stress generated when the temperature difference increases also increases, the amount of plastic deformation in the tensile direction generated on the inner surface of the pipe increases, and the effect of reducing the residual stress is improved.

  However, it takes a long time to heat the piping to a high temperature at a uniform temperature. Furthermore, when kept at a high temperature for a long time, depending on the temperature zone, material deterioration such as material embrittlement or carbide precipitation may occur. For example, in the case of austenitic stainless steel, it is known that σ phase embrittlement occurs in a temperature range of 600 to 900 ° C. Further, even in the case of austenitic stainless steel, the welded portion contains a ferrite phase in the weld metal, so that 475 ° C embrittlement may occur at a temperature around 475 ° C. For this reason, it is desirable that the heating temperature of the pipe is low from the viewpoint of shortening the construction time and reducing material deterioration, and compressing the residual stress acting on the inner surface in the vicinity of the welded portion even at a low heating temperature can reduce the residual stress. It is an issue for improvement.

JP 54-94415 A Japanese Patent No. 4196755 Japanese Patent Laying-Open No. 2005-320626

  An object of the present invention is to provide a method for compressing a tensile residual stress acting on an inner surface in the vicinity of a welded portion of a pipe at a construction temperature of less than 350 ° C.

  The method for improving residual stress according to the present invention is a method for improving the residual stress on the inner surface of the pipe in the compression direction by rapidly cooling the inner surface of the pipe after heating the pipe. After raising the temperature, the process of supplying cooling water to the inside of the pipe and rapidly cooling the inner surface near the pipe welded portion is repeated twice or more.

  According to the present invention, since the tensile residual stress acting on the inner surface in the vicinity of the welded portion of the pipe can be improved to the compressive residual stress, by applying it to a high temperature water pipe (for example, made of austenitic stainless steel), stress corrosion cracking can be achieved. It is possible to suppress the occurrence. Furthermore, since the construction temperature is as low as less than 350 ° C., 475 ° C. embrittlement and σ phase embrittlement do not occur, and the construction time can be shortened by shortening the heating time.

The figure explaining the specific example of a construction procedure about the residual stress improvement method of this invention. The figure explaining the specific example in the case of applying the residual-stress improvement method of this invention to piping butt-welding part vicinity. The figure explaining the specific example which evaluates a temperature decreasing rate from the time-dependent change of piping outer surface temperature at the time of water-cooling the inner surface after piping heating in the residual stress improvement method of this invention. The figure explaining the specific example of the residual stress improvement effect obtained by applying the residual stress improvement method of this invention.

  The present invention has the following features.

  After heating the vicinity of the welded portion of the pipe to a construction temperature of less than 350 ° C. by heating from the outer surface, the process of supplying cooling water to the inside of the pipe and rapidly cooling the inner surface is repeated at least twice. In addition, regarding the management at the time of construction, the temperature difference between the inner and outer surfaces is evaluated based on the rate of decrease in the outer surface temperature when cooling water is supplied and the inner surface is rapidly cooled and the pipe thickness at the temperature measurement position. Confirm that the thermal stress caused by the above is greater than the yield stress of the piping material. The outer surface temperature is measured by attaching a temperature measuring device such as a thermocouple to the outer surface of the pipe near the weld.

  Hereinafter, the present invention will be described in detail.

  In the method for improving the residual stress of small-diameter pipes according to the present invention, the range in which the residual stress in the vicinity of the pipe weld or the pipe inner surface is desired to be improved in the compression direction is heated to a construction temperature of less than 350 ° C. by heating from the outer surface, and then the pipe The process of supplying cooling water to the inside and quenching the inner surface (hereinafter referred to as “quenching after heating”) is repeated at least twice. By setting the construction temperature to less than 350 ° C., for example, the residual stress reduction effect is reduced as compared with the case where rapid cooling after heating is performed at a construction temperature of 600 ° C. or higher. Thereby, in the rapid cooling after the first heating, there may be a case where tensile residual stress remains in a portion where the initial residual stress is locally high. Since the high tensile residual stress is locally reduced by the rapid cooling after the first heating, the tensile residual stress remaining on the inner surface of the pipe can be compressed by the rapid cooling after the second heating.

  In the method for improving the residual stress of small-diameter pipes according to the present invention, since the construction temperature is low, it is important to manage that the residual stress on the inner surface of the pipe has been improved by rapid cooling after heating. Whether or not the residual stress on the inner surface of the pipe is improved depends on whether or not the thermal stress generated on the inner surface of the pipe due to rapid cooling after heating exceeds the yield stress of the pipe material. Although the thermal stress generated on the inner surface of the pipe cannot be directly measured, the thermal stress generated on the inner surface of the pipe due to the temperature difference between the inner and outer surfaces of the pipe is generated on the inner surface of the hollow cylindrical tube shown in the following equations (1) and (2). It can be evaluated by the thermal stress formula.

ここで、σ_θは周方向の熱応力、σ_aは軸方向の熱応力、αは線膨張率、Ｅは縦弾性係数、νはポアソン比、ΔＴは配管内外面の温度差、ａは配管の内半径、ｂは配管の外半径を示す。

Where σ _θ is the thermal stress in the circumferential direction, σ _a is the thermal stress in the axial direction, α is the linear expansion coefficient, E is the longitudinal elastic modulus, ν is the Poisson's ratio, ΔT is the temperature difference between the inner and outer surfaces of the pipe, and a is the pipe The inner radius of b indicates the outer radius of the pipe.

For example, when a temperature difference of 150 ° C. is applied to the inner and outer surfaces of an austenitic stainless steel pipe having a pipe outer diameter of 60.5 mm and a pipe plate thickness of 5.5 mm, α is 15.14 × 10. ^-6 K ^-1 , E is 195 GPa and ν is 0.3, it can be evaluated that 337 MPa thermal stress is generated on the inner surface of the pipe. Since the yield stress of austenitic stainless steel used as piping material, such as SUS304 steel and SUS316 steel, is smaller than 337 MPa, plastic deformation in the tensile direction is applied to the inner surface of the piping by applying a temperature difference of 150 ° C to the inner and outer surfaces of the piping. The residual stress after construction can be improved in the compression direction.

  Regarding the temperature difference between the pipe inner and outer surfaces, it may be difficult to measure the temperature of the pipe inner surface during construction. For this reason, in the present invention, focusing on the fact that the temperature decrease rate measured on the pipe outer surface has a strong correlation with the temperature difference between the pipe inner and outer surfaces and the pipe plate thickness, the pipe plate thickness and the pipe at the outer surface temperature measurement position The temperature difference between the inner and outer surfaces of the pipe is evaluated based on the temperature decrease rate of the outer surface. Specifically, the temperature decrease rate of the pipe outer surface increases with an increase in the temperature difference between the pipe inner and outer surfaces. In addition, the temperature decrease rate of the outer surface of the pipe decreases as the pipe plate thickness increases.

  By utilizing the physical properties described above, in the present invention, in the method for improving the residual stress near the inner surface of the pipe weld or the inner surface of the pipe, the rate of decrease and the temperature of the outer surface of the pipe when cooling water is supplied to quench the inner surface of the pipe. Based on the pipe thickness at the measurement position, it is determined that the residual stress on the inner surface of the pipe has been improved, and construction management is performed. In addition, since the decrease in the outer surface temperature that occurs when the inner surface of the pipe is cooled with water is an event that is completed in a short time of about several seconds, in this patent, the temperature of the outer surface of the pipe is measured at intervals of 0.1 seconds or less. From the temperature of the pipe outer surface, the temperature decrease rate of the pipe outer surface is evaluated.

  Hereinafter, examples of the residual stress improvement method and construction management method for small-diameter pipes according to the present invention will be described. In the following examples, a case where the inner surface of the pipe near the butt weld portion of the pipe made of austenitic stainless steel (SUS304 steel or SUS316 steel) is applied will be described as an example.

  An embodiment of a method for improving residual stress and a construction management method for small-diameter pipes according to the present invention will be described with reference to FIGS. FIG. 1 is a diagram illustrating a specific example of a construction procedure for the residual stress improving method of the present invention. FIG. 2 is a diagram for explaining a specific example when the residual stress improving method of the present invention is applied in the vicinity of a pipe butt weld.

  In this embodiment, first, the plate thickness of the pipe 1 at each outer surface temperature measurement position is measured. In addition, about the outer surface temperature measurement position, since the rate of temperature decrease measured on the outer surface of the pipe has a strong correlation with the pipe plate thickness, the inner face groove processing portion 3 in which the pipe plate thickness continuously changes due to the curved surface. It is desirable to measure the temperature of the outer surface of the pipe outside this range.

  Next, the target value for decreasing the outer surface temperature at each outer surface temperature measurement position is evaluated from the measured pipe plate thickness. The rate of temperature decrease measured on the pipe outer surface has a strong correlation with the temperature difference between the pipe inner and outer surfaces and the pipe plate thickness, so the pipe plate thickness determines the temperature difference between the pipe inner and outer surfaces from the rate of decrease in the outer surface temperature. Can be evaluated. As a result, the temperature difference at which thermal stress sufficient to cause the inner surface to yield yield can be set as the target value for the lowering rate of the outer surface temperature.

  Next, the outer surface temperature measuring thermocouple 8 is attached to the outer surface of the pipe at the outer surface temperature measurement position. The outer surface temperature measuring thermocouples 8 are attached at least at one or more locations, and preferably at four locations at 90 ° intervals downstream of the butt weld 2 with the cooling water 10 supply side as the upstream side. The heating temperature control thermocouple 6 is preferably attached to the outer surface of the pipe near the center of the heating range 11 where the heating temperature is considered to be the highest because it is desirable to control the maximum heating temperature.

  Thereafter, the heater 4 is attached to the outer surface of the pipe in the heating range 11, and the heat insulating material 5 is attached to the heater 4 in the range including the heating range 11 and the outer surface of the pipe. The heat insulating material is attached to improve the heating efficiency of the pipe 1 by the heater 4 and to improve the evaluation accuracy when evaluating the temperature difference between the inner and outer surfaces of the pipe from the rate of decrease in the outer surface temperature.

  Next, power supply to the heater 4 is started from the heater power supply 7 with a heating temperature control function, and the pipe 1 in the vicinity of the butt weld 2 is heated to the target construction temperature. In the present invention, the construction temperature (upper limit of the heating temperature of the pipe) is set to a low temperature of less than 350 ° C. from the viewpoint of shortening the construction time and preventing material deterioration. After the temperature of the pipe 1 is raised to the construction temperature, the temperature measurement unit 9 starts measuring the outer surface temperature of the pipe, and when the temperature difference between the inner and outer surfaces of the pipe is evaluated based on the reduction in measurement noise and the rate of decrease in the outer surface temperature. In order to improve the evaluation accuracy, the power supply from the heater power supply 7 with the heating temperature control function is stopped, and the cooling water 10 is supplied to the heating range 11. In addition, about the supply amount of the cooling water 10, let it be the flow volume which the cooling water 10 can reach to the heating range 11 in a full state.

  For construction management, the maximum value of the rate of decrease in the external surface temperature is evaluated from the time-dependent change in the external surface temperature of the pipe measured by the temperature measurement unit 9. In the residual stress improvement method of the present invention, a specific example in which the temperature decrease rate is evaluated from the time-dependent change of the pipe outer surface temperature when the inner surface is water-cooled after the pipe is heated will be described with reference to FIG. The piping 1 in the heating range 11 is heated with the construction temperature (less than 350 ° C.) as the upper limit temperature. When the cooling water 10 is supplied to the inside of the pipe 1 in this state, the pipe 1 is rapidly cooled from the inner surface. On the outer surface of the pipe, the surface temperature starts to decrease with a time difference from the start of the inner surface water cooling. In the small-diameter pipe, since the plate thickness is thin, the rapid decrease in the outer surface temperature is completed in a short time of about several seconds. For this reason, the temperature of the pipe outer surface is measured at intervals of 0.1 seconds or less, and the time change (that is, the temperature decrease rate) of the pipe outer surface is evaluated from the measured temperature data of the pipe outer surface. In addition, since the measurement interval is short, it is desirable to perform a moving average process (about five-point average) on the temperature measurement data of the pipe outer surface used for the temperature decrease rate.

  Whether or not the construction is appropriate is evaluated based on whether or not the maximum value of the outer surface temperature decrease rate evaluated from the time-dependent change of the pipe outer surface temperature satisfies the outer surface temperature decrease rate target. In addition, about the fall rate target of the outer surface temperature in each temperature measurement position, a construction target changes with piping board thickness measured at the said position. Specifically, when the plate thickness is thin, the outer surface temperature decrease rate target tends to increase, and when the plate thickness is thick, the outer surface temperature decrease rate target tends to decrease. This is because even when the temperature difference between the inner and outer surfaces is the same, the rate at which the outer surface temperature is decreased increases as the plate thickness is thinner.

As shown in FIG. 3, the rate of decrease in outer surface temperature evaluated at each temperature measurement position becomes maximum with a time difference from the start of inner surface water cooling, and then decreases with the passage of time.
In the method for improving the residual stress of a small-diameter pipe according to the present invention, the thermal stress generated by the transient temperature distribution exceeds the yield stress of the pipe material, thereby applying a plastic deformation in the tensile direction to the inner surface of the pipe and compressing the residual stress in the compression direction. It has been improved. For this reason, the maximum value of the outer surface temperature decrease rate evaluated at each temperature measurement position satisfies the outer surface temperature decrease rate target, thereby improving the residual stress at each temperature measurement position. In this example, since the goal is to compress the residual stress around the entire inner surface of the pipe, the case where the construction target is satisfied at all temperature measurement positions is evaluated as proper construction, but only the residual stress at a specific angle is evaluated. In the case of compressing, the case where the construction target is satisfied at the temperature measurement position of the angle is evaluated as proper construction.

  In the method for improving residual stress of small-diameter pipes according to the present invention, the process of supplying cooling water to the pipes and rapidly cooling the inner surface is repeated at least twice. If it is done, 1 is added to the number of times of construction, and if it is not appropriate, the number of times of construction is left as 0. Thereafter, the water in the pipe 1 is drained, and the rapid cooling after the heating of the pipe 1 is repeated until the number of executions becomes two.

A specific example of the residual stress improvement effect obtained by applying the residual stress improvement method of the present invention will be described with reference to FIG. In the present invention, since the construction temperature is set to a low temperature of less than 350 ° C., for example, the residual stress reduction effect is lowered as compared with the case where rapid cooling after heating is performed at a construction temperature of 600 ° C. or higher.
Thereby, in the first construction (rapid cooling after heating), even if the compressive residual stress is on average, the tensile residual stress may remain in a portion where the initial residual stress is locally high.
Since the high tensile residual stress is locally reduced by the first construction, the tensile residual stress remaining on the inner surface of the pipe can be compressed by the second construction. In addition, when the piping material is austenitic stainless steel, the absolute value of the stress that initiates tensile and compressive yield increases due to work hardening by repeated application, so the maximum value of residual stress increases and the residual stress reduction effect also increases. I think that. For these reasons, the residual stress acting on the inner surface of the pipe after welding, which has large variations at each position and the average value is tensile, decreases as the number of constructions increases, and the average value also decreases in the compression direction. To be improved.

DESCRIPTION OF SYMBOLS 1 Piping 2 Butt welding part 3 Inner groove processing part 4 Heater 5 Thermal insulation material 6 Heating temperature control thermocouple 7 Heater power supply with heating temperature control function 8 External surface temperature measurement thermocouple 9 Temperature measurement unit 10 Cooling water 11 Heating range

Claims (6)

In the method of improving the residual stress on the inner surface of the pipe in the compression direction by rapidly cooling the inner surface of the pipe after heating the pipe,
After the temperature in the vicinity of the pipe weld is raised to the construction temperature by heating from the outer surface of the pipe, the process of supplying cooling water to the inside of the pipe and rapidly cooling the inner surface in the vicinity of the pipe weld is repeated twice or more. A method for improving residual stress in pipe welds.   2. The method for improving residual stress in a pipe weld according to claim 1, wherein the pipe is made of austenitic stainless steel.   In Claim 1, the said construction temperature is less than 350 degreeC, The residual stress improvement method of the pipe welding part characterized by the above-mentioned. A construction management method for use in the method for improving residual stress in a pipe weld according to claim 1,
Construction that determines whether or not construction has been performed properly based on the maximum rate of decrease in pipe outer surface temperature when cooling water is supplied and the pipe inner surface is rapidly cooled and the pipe thickness at the temperature measurement position Management method.   5. The construction management method according to claim 4, wherein the pipe outer surface temperature when the cooling water is supplied to quench the pipe inner surface is measured at intervals of 0.1 seconds or less.   6. The construction management method according to claim 5, wherein the temperature of the outer surface of the pipe is measured outside the groove processing range of the inner surface of the pipe.

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