JPH022516B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for preventing stress corrosion cracking at a joint when connecting stainless steel pipes with a joint. BACKGROUND ART Conventionally, piping such as steel pipes and copper pipes has often been connected in a pipe-to-fitting-to-pipe arrangement. Typical examples of conventional connections between such pipes and joints are shown in FIGS. 1 and 2. In other words, the tightening member D has a female thread on the inner surface.
By screwing into the fitting B which has a male thread on the outer surface of the end, the packing C is pressed through the washer E to press the outer circumferential surface of the pipe A and the inner surface of the end of the fitting B, and the fluid is pumped. It is sealed. Also,
In many cases, the pipe is machined in advance to form a recess Aa, the snap spring F is fitted, and the tightening member D is tightened to lock the pipe A and the fitting B together, thereby increasing the pressure resistance in the axial direction. ing. As the seal C, there are O-rings (Fig. 1) and triangular rings (Fig. 2), which are more preferable because of their large crimping area (those slightly deformed from the triangular shape as shown in Fig. 2 are also referred to as triangular rings). has been used. On the other hand, stainless steel pipes such as SUS304 and SUS316 have good corrosion resistance and have recently come to be used for indoor piping, especially hot water supply pipes. However, if stainless steel pipes are connected using the conventional method and used as hot water pipes, no matter which type of gasket is used, the stainless steel pipe that comes in contact with the gasket will crack, causing water leakage in a short period of time.
The drawback was that it became unusable. As a result of studying a method for connecting stainless steel pipes that can be used for a long period of time without the above drawbacks, the present inventor found that the cracks are stress corrosion cracks originating from crevice corrosion that occurs on the outer periphery of the stainless steel pipe in contact with the packing. In addition, the propagation speed of stress corrosion cracking is significantly higher than that of crevice corrosion, and the residual stress value at the contact area with the packing on the outer circumferential surface of the pipe is 20.
Stress corrosion cracking can be prevented by keeping the distance from ^the tube end side contact point with the packing on the tube outer circumferential surface to the tube end side starting point G of the convex part on the inner circumferential surface of the concave part of the tube so that it is less than Kg/mm 2. The present invention was achieved by finding that the lifespan of the connecting portion can be significantly extended. In other words, the inner diameter is larger than the outer diameter of the stainless steel pipe to be connected, and the inner circumferential surface is continuous with the parallel part parallel to the axial direction, and the enlarged part where the inner wall slopes and expands toward the end. Insert the fitting into the pipe, and insert the packing between the pipe and the enlarged part of the fitting.
A tightening member whose position in the axial direction of the pipe is regulated by a snap spring fitted into a recess provided in the circumferential direction on the outer periphery of the pipe is pressed, and the seal is pressed between the inner circumferential surface of the enlarged part of the joint and the pipe. When crimping and sealing the tube to the outer circumferential surface, the concave portion of the tube is moved from the tube end side contact portion with the packing on the outer circumferential surface of the tube so that the residual stress value at the contact portion with the packing on the outer circumferential surface of the tube is 20 kg/mm ² or less. This is a method for preventing stress corrosion cracking of a stainless steel pipe joint, which is characterized in that the distance between the protrusion on the inner circumferential surface of the pipe and the starting point on the pipe end side is set. The present invention will be explained in detail below based on Examples. Example In the connection method shown in Figs. 1 and 2, the distance from the tube end side contact part with the packing on the outer peripheral surface of the stainless steel pipe to the tube end side starting point of the convex part on the inner peripheral surface of the recessed part of the pipe (first Nine sets of connecting pipes were created by connecting stainless steel pipes by varying the distance (abbreviated as "distance" in the table). The diameter of the stainless steel pipe is 40A (outer diameter 48.6mm).
The size is 80A (outer diameter 89.1mm), and the plate thickness is 1.2mm. Also, 40A is a non-annealed tube, and 80A is an annealed tube. Chlorine concentration of 200ppm was applied to each of these nine sets of connecting pipes.
Water with a pH of 6 was maintained at a temperature of 80°C and flowed through it at a flow rate of 0.3 m/sec for 12 months, and the presence or absence of crevice corrosion, stress corrosion cracking, and water leakage was investigated. The results are shown in Table 1. In Table 1, Nos. 3, 4, 6, 8, and 9 are examples of the present invention, and Nos. 1, 2, 5, and 7 are comparative examples.
Further, Table 1 shows the results of measuring the residual stress value at the contact portion of the outer circumferential surface of the connecting tube with the packing using an X-ray stress measuring device. As can be seen from Table 1, crevice corrosion occurred in all of the comparative examples, and water leakage occurred within 30 days after water was passed. However, in all of the examples of the present invention, although crevice corrosion occurred, stress corrosion cracking did not occur, and as a result, the water flow period until water leakage started was significantly longer than in the comparative examples. In other words, by reducing the residual stress value at the contact portion of the outer peripheral surface of the pipe with the packing to 20 kg/mm ² or less, the life of the stainless steel pipe joint can be significantly extended. Furthermore, favorable results were obtained when the distance from the tube end side contact portion with the packing on the tube outer circumferential surface to the tube end side starting point of the convex portion on the inner circumferential surface of the recessed portion of the tube was 13 mm or more. In addition, for the stainless steel pipe connections with the structure shown in Figures 1 and 2, the tightening amount is equal to the washer E.
Thickness and diameter of recess Aa and snap spring F
The tightening is uniquely determined by the size of the joint B, and the tightening is completed when the washer E comes into contact with the joint B. In addition, although the tightening is normally done by an operator using a tightening jig, fitting B, tightening member D, and washer E are all made of hard materials such as stainless steel, and when washer E comes into contact with fitting B, Even if the tightening force is changed, there is no difference in the force with which the snap spring F presses the recess Aa. In other words, in order to reduce the residual stress value at the contact area with the packing on the outer peripheral surface of the pipe to 20 kg/mm ² or less, the convex part on the inner peripheral surface of the recessed part of the pipe should be The distance to the starting point on the pipe end side is the pipe outer diameter,
Depending on the presence or absence of annealing, for example, the examples shown in Table 1 may be used.

【table】

【table】 [Brief explanation of drawings]

FIGS. 1 and 2 are upper half cross-sectional views showing how the pipes are connected. A...pipe, Aa...recess, B...fitting, C...
Packing, D...Tightening member, E...Washer,
F...Snat spring, G...Starting point on the tube end side of the convex portion on the inner peripheral surface of the concave portion of the tube.

Claims (1)

[Claims] 1 The inner diameter is larger than the outer diameter of the stainless steel pipe to be connected, and the inner peripheral surface has an enlarged part at the end where the inner circumferential surface is continuous with the parallel part parallel to the axial direction and the inner wall slopes and expands toward the end. A fitting is inserted into the pipe, and a gasket is inserted between the pipe and the enlarged part of the joint, and a fastening whose position in the pipe axial direction is regulated by a snap spring fitted into a recess provided in the circumferential direction on the outer periphery of the pipe is used. When pressing the packing with the attached member to press the packing against the inner circumferential surface of the enlarged part of the joint and the outer circumferential surface of the pipe and sealing, the residual stress value at the contact part of the outer circumferential surface of the pipe with the packing is 20 kg/ A stainless steel pipe connection part characterized in that the distance from the tube end side contact part with the packing on the tube outer circumferential surface to the tube end side starting point of the convex part on the inner circumferential surface of the ^recessed part of the tube is set so as to be less than mm 2 How to prevent stress corrosion cracking.