JPS5817807B2 - Heat treatment method for piping - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of heat treating piping, and more particularly to a method of heat treating piping suitable for increasing the strength of piping against corrosion fatigue, stress corrosion cracking, etc.

Conventionally, the piping used in nuclear power plants, thermal power plants, chemical plants, etc. consists of many straight pipes, bent pipes, etc. joined by welding to form one long pipe. In pipe welding, welding heat input generates tensile residual stress on the inner and outer surfaces of the pipe in the vicinity of the weld.

It is generally known that in steel materials, when tensile stress and corrosion factors coexist, intergranular corrosion cracking rapidly progresses in a direction perpendicular to the tensile direction.

Therefore, if the fluid flowing through the pipe is a corrosive fluid, it goes without saying that the tensile residual stress existing on the inner surface of the pipe as described above will cause stress corrosion cracking and corrosion fatigue.

In particular, stress corrosion cracking has often been a practical problem in the case of austenitic stainless steel piping for atomic couplants, or because corrosive fluids flow through the piping in chemical plants, petroleum refining plants, and the like.

One way to solve the above-mentioned problems is to leave compressive stress on the surface of the pipe where corrosive factors exist, such as on the inner surface of the pipe if the fluid flowing through the pipe is a corrosive fluid, to prevent stress corrosion cracking and Corrosion fatigue can be significantly reduced.

Conventionally, as a means of generating such compressive residual stress, there is a processing method called shot peening, in which a small steel ball is blown onto the surface of an object with high-pressure air to cause plastic deformation on the surface. Although it is effective when used on pipes and short pipes, it has the disadvantage that it is extremely difficult to apply after the piping has been assembled, and it is impossible to uniformly treat the entire circumference of the pipe.

In addition, both sides of the welded part are heated with a gas flame, which is called the Linde method.
Although there is a method of alleviating the residual stress in the weld, this method only relieves the residual stress and cannot generate the opposite residual stress.

Furthermore, as is generally known, there is a method of generating thermal stress by heating and then rapidly cooling the treated material to obtain residual compressive stress.

However, in such conventional methods, the residual stress obtained is greatly affected by even slight differences in processing conditions, so reproducibility is poor and extremely uncertain.

This point was confirmed by the following experiment.

The experimental method was to heat a circular tube (shown in Figure 5) in which a straight 304 stainless steel tube and an elbow tube were welded together, then immerse it in water to cool it. The stress was measured at two positions: part) and B (straight pipe part).

Measurements were made at multiple locations on the inner circumferential surfaces of both tubes A and B.

As a result, there are locations where residual tensile stress (+ side) is generated in both A and B, as shown by broken lines in FIGS. 6 and 7, respectively.

As described above, with the conventional method of applying residual compressive stress using thermal stress, it is not always possible to obtain residual compressive stress, and there is a lack of reliability.

Moreover, it is practically impossible to heat and cool a long piping system at once after assembly.

As described above, at present, there is no suitable method for improving the residual stress caused by welding heat input in the piping of a plant or the like after it is assembled.

The present invention cools the piping by continuously flowing a coolant to the side where corrosion factors exist near the welded part of the piping after the piping system is assembled, and cools the piping system by keeping the side where no corrosion factors exist at a temperature lower than the transformation temperature. By heating for a certain period of time until the temperature gradient in the thickness direction becomes steady, a temperature difference is created between both sides of the heat-treated part that generates thermal stress in different directions that exceeds the yield point, and this is then brought to room temperature. By cooling, a homogeneous residual compressive stress is reliably generated on the side where the corrosion factor is present, and its controllability and reproducibility are improved.

An optimal embodiment of the present invention will be described below with reference to the drawings.

As shown in FIGS. 1 and 2, a high-output heating device 2 is arranged around the outer periphery of the pipe 1 so that its heat generating surface concentrically surrounds the outer periphery of the welded portion of the pipe 1.

As the high-output heating device at this time, it is preferable to use, for example, an induction heating device.

Next, the heating device 2 heats the outer surface of the piping 1 at a temperature lower than the transformation temperature for a certain period of time until the temperature distribution in the thickness direction of the piping system becomes steady, and coolant 3 (for example, water) is supplied inside. It flows continuously to create a temperature difference between the outer and inner surfaces of the pipe 1.

Now, when the heating temperature is To and the temperature of the coolant 3 flowing inside the pipe is Tw, the inner surface of the pipe 1 is cooled by the temperature of the coolant 3 and becomes Ti.

Therefore, assuming that the temperature distribution is linear, the thermal stress generated on the inner and outer surfaces of the pipe 1 is expressed by the following equation.

σα=σψ−=−E−α・(To −T i )/2
(1-!' )+ However, σα: axial stress, σψ: circumferential stress, E: Young's coefficient, α: coefficient of linear expansion, C: Poisson's ratio, and the inner surface of pipe 1 is positive (tension). Stress, the outer surface corresponds to negative (compressive) stress.

From this equation, it can be seen that the greater the temperature difference between the inner and outer surfaces of the pipe 1, the greater the thermal stresses σα and σψ.

The stress distribution in the axial direction in the pipe 1 at this time is as shown in FIG. 3, where σy is the yield stress.

By the way, taking atomic couplant piping as an example, the piping is made of austenitic stainless steel, and the Young's modulus E
is around 1.9 x 10'kg/m4, and Poisson's ratio ν is around 0.3 to 0.5 kg/m4.

Similarly, the yield point varies depending on the temperature, and the higher the temperature, the lower the yield point, but on average it is around 9 in 20.

By substituting these values into the above equation, it is derived that the temperature difference (To-Ti) between the inner and outer surfaces of the pipe 1 is approximately 200°C.

Therefore, in order to generate a thermal stress higher than the yield point on the inner and outer surfaces of the pipe 1, a temperature difference of at least 200°C is required.

In order to obtain this temperature difference (200 degrees Celsius or more), if the coolant 3 that cools the inside is allowed to stagnate in a valve or the like in the piping system, the coolant will quickly become hot. Along with this temperature change, the heating temperature must also be increased, and such control is practically extremely difficult.

If the inner surface temperature of the pipe 1 is kept almost constant by continuous flow as in the present invention, it is sufficient to keep the outer surface heating temperature almost constant, and this control is extremely easy.

In this way, a temperature difference is generated between the inner and outer surfaces of the pipe 1,
When the pipe 1 is returned to normal temperature after being subjected to a thermal stress equal to or higher than the yield point, compressive stress remains on the inner surface and tensile stress remains on the outer surface, as shown in FIG.

The residual stress generated in this way can be controlled by making the temperature distribution in the thickness direction of the pipe constant.

In the present invention, heating is performed for a certain period of time until the temperature distribution in the thickness direction of the pipe becomes steady, and this heating time can be determined from the temperature diffusion coefficient and the plate thickness using the following relational expression.

2 τ〉−X O, 7 t: Plate thickness, a: Temperature diffusion coefficient, τ: Heating time Therefore, the heating time can be easily set according to the part of the piping system to be heat treated,
The generated residual stress can be controlled.

In this case as well, if the cooling member that cools the inside of the piping system is allowed to stay, the heating temperature will not be stabilized due to temperature changes in the cooling member, and therefore the control of the heating time to reach the steady state described above will be unstable. Practically impossible.

As another example, in the case of a plant where corrosion factors exist on the outside surface of the tube, by heating the inside surface of the tube and cooling the outside surface, compressive stress is applied to the outside surface of the tube, contrary to the first embodiment. If corrosion factors exist both inside and outside the tube, the tube itself may be energized to cool the inside and outside of the tube.

In short, according to the present invention, compressive stress can be left on the side where the corrosion factor is present.

Next, an example of an experiment conducted to confirm the above-mentioned example will be shown.

The experimental method was to use an L-shaped circular tube (3
A heating coil of a high-frequency heating device is wrapped around the outer circumferential surface of the tube at the position to be measured, and water is continuously passed through the tube to cool and heat the inner surface of the tube. The coil was energized for a predetermined period of time.

Measurements were made at two locations A and B in FIG. 5, each along the inner circumferential direction of the tube.

In this experiment, position 8 is a welded part where two straight pipes are welded together.

The heating conditions using the high frequency heating device are as shown in Table 1.

The experimental results are shown in solid lines in FIGS. 6 and 7 for comparison with the conventional results described above.

As is clear from the figure, residual compressive stress is generated at all measurement points.

As described above, according to the heat treatment method of the present invention, it is possible to reliably obtain residual compressive stress at a desired location, and its reliability is much better than that of the conventional method.

Note that the present invention can be implemented even in cases where stress remaining due to welding heat input during piping assembly is distributed throughout the piping system, and it is necessary to improve stress throughout the piping system.

As described above, the present invention heats one side of the piping where no corrosive factors exist to a temperature lower than the transformation temperature, and continuously flows a coolant to the other side of the piping, so that the inner and outer surfaces of the piping are heated to a high temperature. (temperature difference required to generate thermal stress exceeding the yield point) and thereby generate residual compressive stress on the side of the piping where the corrosion factor exists. Residual tensile stress generated by welding heat input during assembly can be reduced as necessary, so pipe damage such as stress corrosion cracking can be avoided even in the presence of corrosive fluids, and pipe damage can also be improved. It can also be applied to small diameter or long objects.

In addition, in order to reliably generate the residual compressive stress, it is necessary to apply a temperature difference of the predetermined size between the inner and outer surfaces of the pipe, and until the temperature distribution in the thickness direction of the pipe becomes steady so as to obtain a preferable residual compressive stress. It is essential to continue heating for a certain period of time, but since the coolant that cools the tube surface is placed in a flow state, the temperature distribution on the cooling surface is uniform and constant, and the temperature cannot be controlled by the coolant supply side. This makes it extremely easy to ensure the temperature difference and control the heating time, and it is also possible to control the stress generated.

Therefore, compared to the conventional method of imparting residual compressive stress using thermal stress (method of cooling after heating), it is much superior in terms of reproducibility and reliability.

Furthermore, it has an excellent effect that has not been seen before in that it can be applied even after repairing a portion where the piping has deteriorated after operation of a plant or the like.

Furthermore, it has extremely great practical effects, as it can be applied to pipe materials that are useful under corrosive conditions, such as austenitic stainless steel pipes, which do not undergo phase transformation upon heating.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an embodiment of the present invention, Fig. 2 is a view taken from arrow A=A in Fig. 1, and Figs.
A stress distribution diagram on the outer surface, FIG. 5 is a diagram showing the tube material used in the experiment, and FIGS. 6 and 1 are stress distribution diagrams in the circumferential direction of the tube material at the measurement part. 1...Piping, 2...Heating device, meter...
...coolant.

Claims (1)

[Claims] 1. A method for heat treating a piping system in a plant, etc. that is used under conditions where corrosive factors exist, in which, after the piping system is assembled, a coolant is continuously applied to one side of the piping system near the welds where the corrosive factors exist. At the same time, one side of the side where no corrosion factors exist is heated at a temperature lower than the transformation temperature for a certain period of time until the temperature distribution in the wall thickness direction of the piping system becomes steady, and the difference between the two sides near the welded part is heated. After generating a large temperature difference that causes a thermal stress higher than the yield point in the direction, by returning the area near the weld to room temperature, residual compression is created on the side of the piping system where the corrosion factor exists near the weld. A piping heat treatment method characterized by generating stress and residual tensile stress on the side where no corrosion factors are present.