JPH05329557A - Flare working tube made of austenitic stainless steel - Google Patents

Abstract

PURPOSE:To prevent a crack generated on the inside surface of a frame part of a flare working tube by performing compression with cold working to the inside surface of the frame part of the tube end formed by a flare working. CONSTITUTION:Flare working is executed by pressing a die (cone) against the tube end of a stock tube while rotating it and spreading out the tube end. After forming a frame part by the flare working, cold working is performed by using a steel wire brush wheel and a nylon grindstone wheel 7. By this compression, the tensile residual stress of the surface is reduced, or the tensile residual stress is eliminated, and on the contrary, the compression residual stress is generated, and stress corrosion cracking and fatigue cracking come also to be scarcely generated. In such a manner, the flare working tube can be used safely for a long period with respect to a piping coming into contact with the environment containing oxygen and chlorine ions, whose concentration is comparatively high, a piping whose expansion and contraction by a wall are large, and a piping whose vibration is violent, etc., as well.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel pipe whose pipe end is formed into a brim by flare processing and which is joined by a loose flange.
The present invention relates to a pipe in which cracks are less likely to occur in the brim-shaped molding portion (flare processing portion).

[0002]

2. Description of the Related Art As one of piping methods in chemical plants and various structures, there is a method of joining pipes whose ends are flared with loose flanges. FIG. 1 shows the principle in a longitudinal sectional view of a tube. As shown in the figure, the two flare tubes 1 are formed in a brim shape at their ends, and a packing 2 is inserted therebetween, and the loose flange 3 is fastened with bolts 4 and nuts 5 to join them.

The flared tube is made of carbon steel,
There are various types such as low alloy steel and stainless steel, but for example, for water supply, hot water supply, drainage, or transportation of chemicals, austenitic stainless steel welded pipe such as JIS SUS 304 is used. Often. The pipe made of this material is extremely excellent in general corrosion resistance, but the bent part of the tsuba (the part circled with ○ in Fig. 1) during long-term use.
Fine cracks may occur on the inner surface (surface facing the inside of the pipe), especially near the weld beads. If such a crack progresses, internal fluid will leak from the joint, and in extreme cases, the joint will rupture and will have to be replaced even if the other parts of the pipe are not damaged. Not only is it troublesome, but many problems occur, such as waste of materials and interruption of plant operation.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to prevent cracks occurring in the bent portion of the flange of the flared pipe of austenitic stainless steel as described above.

[0005]

The gist of the present invention is that "the austenitic stainless steel is characterized in that the inner surface of the pipe end collar portion formed by flare processing is subjected to compression processing by cold working. Flare processing tube made of.

[0006]

The inventor of the present invention has investigated in detail the cause of the cracks generated in the bent portion of the flange of the flared pipe, and found that it is due to stress corrosion cracking and fatigue fracture. Then, it is considered that the cracks are likely to occur inside the bent portion of the brim in the processing history.

As shown in FIG. 2, the flaring of the pipe end is performed by pressing the molds (cones) 6 and 6'on the pipe end of the material pipe (welded steel pipe) while rotating the pipe end to spread the pipe end. Done by the method. Therefore, the bent portion A of the collar (see FIG.
Tensile stress remains on the surface of the (C) circled). The processed tube is used as it is (without being subjected to heat treatment or the like), so that the inner surface of the portion A is exposed to the medium flowing inside the tube while having a tensile residual stress.

Stress corrosion cracking occurs due to the synergistic effect of the corrosive environment and tensile stress. Therefore, the stress corrosion cracking susceptibility of the portion of the member having tensile residual stress is increased. In particular, the weld bead portion is often sensitized by the heat effect of welding, and is therefore susceptible to stress corrosion cracking.

Fatigue failure is mainly caused by repetitive stress generated by expansion / contraction of the pipe body, vibration, etc. However, this also easily occurs when the member itself has tensile residual stress. Further, if stress is repeatedly applied to a portion where a crack has occurred due to stress corrosion, the progress of the crack is further accelerated.

The pipe of the present invention is characterized in that, after forming the collar portion by flare processing, the inner surface of the bent portion is subjected to compression processing by cold working. By this compression processing, the above-mentioned tensile residual stress on the surface is reduced, or the tensile residual stress disappears and conversely compressive residual stress occurs, and stress corrosion cracking and fatigue cracking hardly occur.

The cold working method may be any method as long as it produces a compressive residual stress on the surface. Mechanical methods include roller application, shot peening, and the like, and heat treatment methods include carburizing, nitriding, and surface hardening. However, it is only necessary to apply compressive stress to the inner surface of the bent portion of the tsuba, so for example, a steel wire brush wheel or a nylon grinding wheel (wheel-shaped nylon fiber non-woven fabric is used for abrasive grinding). It is practical to use a method in which grains are adhered).

FIG. 3 is a diagram showing an outline of the method.
Shows the processing method when the pipe has a large diameter, and (b) shows the processing method when the pipe has a small diameter. 7 is the steel wire brush wheel or the nylon grindstone wheel described above.

The compressive stress to be applied is effective even if the tensile residual stress in the surface layer portion is reduced, but it is desirable to apply the compressive stress to the extent that the compressive residual stress is generated. The specific method will be described in the following examples.

[0014]

[Examples] Under the following conditions, the occurrence of stress corrosion cracking in the flange portion of a flared pipe and the occurrence of cracks due to fatigue fracture were examined.

Sample material Austenitic stainless steel (SUS 304) welded pipe was flared, and the dimensions shown in FIG. 4 (outer diameter D = 76.3 mm, wall thickness T = 3.0 mm, flange height H = 10 mm, A flared tube having a bent portion (R = 5 mm) was prepared and used as a test material.

Inner surface treatment of the collar portion A steel wire brush wheel or a nylon grindstone wheel is used, and the pushing amount (t in FIG. 3) and the treatment time are changed as shown in Tables 1 and 2 as shown in FIG. 3 (a). It was carried out.

Stress Corrosion Cracking Test A pipe having a collar was cut into a length of 100 mm, immersed in a boiling 42% magnesium chloride solution for 24 hours, and cracks on the inner surface of the bent portion were detected by a penetrant flaw detection method.

Fatigue Fracture Test As shown in FIG. 5, the test was conducted in a state where the flared pipe 1 of the sample material was joined by the flange 3. The distance between the fulcrums is 500mm centered on the flange, and 2000kgf on the flange.
A compressive stress and a tensile stress of (the inner surface of the collar portion is 18.9 kgf / mm ² ) were repeatedly applied. The number of repetitions was 1 million times (2 Hz).

Table 1 shows the results of the stress corrosion cracking test, and Table 2 shows the results of the fatigue fracture test. The cracks are generated radially on the inner surface of the collar as shown in Fig. 4 (b). Fatigue cracks were defined as "present" if at least one penetration crack had occurred.

For Test Nos. 1, 4, 7, 11, 13, and 15 in Table 1, the residual stress on the inner surface of the flange portion was measured by the X-ray diffraction method. The values are also shown in Table 1.

[0021]

[Table 1]

[0022]

[Table 2]

As shown in Table 1, a large number of stress corrosion cracks were generated in the flare-processed sample which was not subjected to the compressive stress application (test No. 1). However, treatment with a steel wire brush wheel or a nylon grinding wheel reduces this cracking. As the wheel push-in amount and the processing time increase, the number of cracks that occur will decrease. This is because the residual stress changes from the tensile residual stress to the compressive residual stress, and its value increases. It is clear from Table 2 that cracks due to fatigue fracture are not generated by the above treatment.

[0024]

EFFECTS OF THE INVENTION The flared pipe of the present invention has excellent corrosion resistance inherent to austenitic stainless steel and is less likely to cause cracks on the inner surface of the bent portion of the collar. This tube is relatively simple to manufacture, and not only for the common applications of flared austenitic stainless steel tubes,
It can be safely used for a long period of time in pipes that come in contact with environments containing relatively high concentrations of oxygen and chloride ions, pipes that expand and contract greatly due to heat, and pipes that violently vibrate.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a usage state of a flared tube.

FIG. 2 is a diagram illustrating a method for manufacturing a flared tube.

FIG. 3 is a diagram showing an example of cold working of the inner surface of the flange portion when manufacturing the flared pipe of the present invention.

FIG. 4 (a) is a diagram showing the dimensions of the test material used in the test of the example, and (b) is a crack occurrence situation after the test.
FIG.

FIG. 5 is a diagram showing a fatigue fracture test method for flared pipes.

[Explanation of symbols]

1: Flared tube, 2: Packing, 3: Loose flange 4: Bolt, 5: Nut, 6 and 6 ': Rotating mold (cone) 7: Steel wire brush wheel or nylon grinding wheel

Claims (1)

[Claims] 1. A flared pipe made of austenitic stainless steel, characterized in that the inner surface of a flange portion of the pipe formed by flaring is compression-worked by cold working.