FI81609C - Method for improving residual stresses in a stainless steel tube or similar - Google Patents

Description

1 81609

A method for improving residual stresses in a stainless steel pipe or the like

The invention relates to a method for improving the residual stresses in a stainless steel pipe or the like, in which cooling water is brought into contact with the inner surface of a pipe with a wall thickness T mm and an average radius R mm and locally heated locally around the outer circumference of the pipe. , which point is radially outwards from the inner surface of the pipe, with the amount of heat supplied QJ / mm.

It is generally well known that if austenitic steel, which is used in large quantities in nuclear power plants and chemical plants, is subject to tensile stresses at the same time and is affected by corrosion-promoting factors, rapid corrosion cracking will occur.

When the metal materials are in the form of steel tubes, a method for improving residual stresses has been invented and shown, allowing cooling water to flow through the tube and heating the tube by the same induction heating method or the like so that the temperature difference between the outer and inner surfaces of the tube is sufficient to produce yield stresses. thereby forming residual compressive stresses in the weld seam of the pipe for which improvement of the residual stress is required. For example, the present inventors have disclosed the invention "Method for producing residual stresses by local heating" in JP Patent Application No. 154198/1977. This method uses a tungsten electrode to reheat the metal accumulated in the weld seam zone while cooling the inner surface of the tube with water, creating residual stresses in the welding zone. In addition, another method has been disclosed in which welding is performed with cooling water flowing in the same pipe and thus producing residual stresses in the weld seam zone. The same inventors have further proposed a method in which the outer cylinder-5 surface of the tube is welded on while cooling the inner surface of the tube.

However, the methods described above have the following problems depending on variations in stainless steel pipes such as diameter, wall thickness, 10, etc.: (1) The conventional induction injection method requires a high feed heat density for pipes with a relatively small diameter and a thin wall. The result is many technical limitations and increased costs due to the need to use a high frequency generator or the like.

(2) In the case of the method disclosed in the above-mentioned JP Patent Application No. 154198/1977, the heat treatment conditions must be determined in advance on the basis of tests performed on each of the 20 pipe diameters. For this reason, this method is laborious.

(3) In the case of the method of welding in the presence of cooling water, only heating conditions suitable for welding can be used, so that the range of use is limited.

(4) As in case (3), only the heating conditions suitable for welding can be used in the method of surface welding, and the metal welded on has an adverse effect on the ultrasonic examinations.

In the light of the foregoing, it is an object of the present invention to provide a method for improving residual stresses which can substantially overcome the problems encountered in conventional methods; which can avoid various restrictions imposed by the variation of the size of stainless steel pipes; with which residual stresses can be increased; which can improve the corrosion resistance of the inner surfaces of stainless steel pipes only by adjusting the heating conditions by simple calculations; and for which there are many different uses.

To achieve the above and other objects, the method according to the invention is characterized in that the amount of heat Q supplied satisfies the condition 2.6 RT £ Q £ 44 T2, where stresses exceeding the yield point are formed in the pipe wall between the heating point and the inner surface of the pipe. When the pipe is cooled so that a shrinkage corresponding to the plastic deformation takes place in the wall of the pipe, in the case of a stainless steel pipe, ice-15 compressive stresses of less than -10 kp / mm2 are formed on the inner surface of the pipe.

The foregoing and other objects, effects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment when considered in conjunction with the accompanying drawings.

Figure 1 shows a welding zone of a stainless steel tube or the like heated by a tungsten electrode discharge; Fig. 2 shows the temperature gradient prevailing in the wall of the tube under the conditions shown in Fig. 1; Fig. 3 shows the distribution of stresses generated in the wall of the pipe under the conditions of Fig. 2; Fig. 4 shows the residual stress distributions on the inner surface of the pipe after the welding zone has been heated according to Fig. 1 and after the residual stress enhancing treatment has been performed; And Figure 5 shows the relationship between the wall thickness of the pipe and the amount of heat supplied.

The method according to the present invention for improving the residual stresses in a stainless steel tube or the like will now be described in more detail with reference to Figs. In this embodiment, residual compressive stresses are generated on the inner surface of the stainless steel pipe P made of austenitic steel (in the vicinity of the welding-5 zone).

The cooling water is always forced to flow at a suitable flow rate, for example 0.3 m / s, and is brought into contact with the inner surface of the stainless steel pipe P. The heating point B is selected on the outer surface so that it is radially at the point A of the inner surface to be treated, which is in the middle of the zone in which the residual stresses are to be improved. A voltage is applied to the tungsten electrode S of the TIG welding machine so that the heating arc thus formed locally heats the heating point B in an argon gas atmosphere. The treatment is done in such a way that the heating station B is moved gradually periphery of the steel tube B in Figure 1, the arrow X as shown. The amount of heat supplied Q (J / mm) is adjusted to the range determined by Equation (i) so as to produce 20 residual compressive stresses below -10 kp / mm2 without causing the cooling water W to boil.

When the stainless steel tube is heated as shown above with reference to Fig. 1, the heating effect of the tungsten electrode S is limited only to the point to which the arc 25 is directed. The result is the formation of a point-like melting point and the formation of a temperature gradient as shown in Fig. 2 between the treated point A of the stainless steel pipe and the heating point B. As the temperature of the area near the heating point B rises, thermal expansion occurs in various parts of the stainless steel pipe P in both the axial and circumferential directions. These thermal expansions are limited by those parts of the stainless steel pipe P which do not heat, so that, as shown in Fig. 3, stresses are created in the wall of the pipe between the point A to be treated and the heating point B. When the stresses (half 5 816Q9 cross-stresses, -σ) in the vicinity of the heating point B exceed the yield point, a corresponding local plastic deformation results.

When the tungsten electrode S is moved and the high-temperature tube walls are cooled, residual compressive stresses corresponding to the stretching forces caused by the plastic deformation (shrinkage) in the vicinity of the heating point B are generated.

10 Immediately after the welding step, the stresses near the point A to be treated are thus tensile stresses + σ, as indicated by the solid line in Fig. 4, but the tensile stresses after treatment change to compressive stresses -o, as indicated by the broken line. Because 15 tungsten electrodes are transferred to a stainless steel tube

Ph in the circumferential direction, results in a corresponding circumferential shrinkage, the so-called ring tension phenomenon, so that the residual compressive stresses are also formed in the circumferential direction.

20 The relationship between the amount of heat supplied Q and the residual compressive stresses will now be described. The above condition, i.e. Equation (i), was obtained by analytical calculations (e.g., applying the method described in JP Patent Application No. 154198/1977 described above) and by analyzing experiments performed to determine the relationship between the amount of heat input Q and the resulting residual stresses. An additional condition is that the residual compression stresses should be less than -10 kp / mm2 and that film boiling of the cooling water should be avoided. Figure 5 is a graph obtained by applying experimental analysis to stainless steel pipes (type 80) connected by welding to a pipeline. In Fig. 5, the curves φ, ®, @ ... represent stainless steel pipes SUS304 (AISI TYPE 304) (type 80), the wall thickness T and the central radius H 35 of which are shown in Table 1.

6 81 609

table 1

Pipe dimensions _Wall thickness T (mm) Center radius R (mm) 5 φ 4.5 14.75 (2) 5.0 27.75 © 6.8 41.15 © 7.7 53.3 © 9.8 77, 7 10 ® 11.7 102.3 @ 13.8 126.8

The invention will now be described in more detail with reference to Fig. 5. It is assumed that residual stresses below -10 kp / mm2 are generated at 15A (absolute values are higher because the residual compression stresses are negative) and no cooling water film boiling occurs on the inner surface of the stainless steel pipe. In this case, a shaded area delimited by two dashed lines is obtained, which is shown in Fig. 5. As a result, when the supplied amount of heat Q is adjusted to this shaded area, residual compressive stresses -σ as shown in Fig. 4 are formed at and near the point A to be treated.

In the extreme case, in the case of a thin-walled tube with a central radius R very much larger than the wall thickness T, Equation (i) may not be applicable. In practice, however, Equation (i) can be applied to almost all stainless steel pipes used to build 30 pipelines.

In this embodiment, a TIG welding machine is used for local heating, so that the treatment to improve the residual stresses can be performed immediately after the welding step. In addition, the amount of heat Q to be supplied can be easily determined and adjusted by monitoring the voltage and current of the arc.

II

7 81609 tail. It is to be understood, however, that suitable heating means, such as a laser welding machine, a plasma arc welding machine, or a local high-frequency reheating device capable of heating a spot or a circumferential portion, may also be used. In the embodiment described above, a forced flow of cooling water W is used, but it is to be understood that the heating point can be selected to be on the bottom or side outer surface of the stainless steel pipe P so that the cooling water W 10 can circulate by a natural convection mechanism.

As described above, according to the present invention, while cooling water is brought into contact with a stainless steel tube or similar inner surface, the outer surface corresponding to the inner surface of the stainless steel tube is heated by a locally supplied amount of heat obtained from a simple equation depending on the stainless steel tube wall thickness. As a result, stresses exceeding the yield point are formed in the wall of the pipe between the heating point and the inner surface of the pipe, and then the surface of the pipe is cooled. Therefore, after cooling, residual compressive stresses greater than the specified value are formed on the inner surface of the tube. Although the tube is heated, in this case the cooling water membrane is prevented from boiling, and the heating efficiency can be varied over a wide range so that the formation of residual stresses is easily feasible. Thus, the corrosion resistance of a stainless steel pipe or the like can be improved. Furthermore, the method according to the present invention can be carried out simply and has a wide range of applications.

Claims (1)

20 Förfarande för förbättring av restspänningar i ett av rostfritt stäl framställt rör ell motsvarande, i vilket förfarande kylvatten leds i kontakt med inre ytan av röret, vilket har en väggtjocklek av T mm och en me-delradie av R y, Medan vilken ligger 25 radellt utät frän rörets inre yta, uppvärms lokalt runt rörets hela peripheral med en tillförd värmemängd QJ / mm, kännetecknat därav, att den tillförda Energin uppfyller villkoret 2.6 RT SQS 44 T! 30 varpenom uppkommer sträckgränsen överstigande spänningar i rörväggen me11an uppvärmningspunkten och rörets inre yta, varefter röret avkyles. li