JPH0698387B2 - Reducing residual stress in stainless steel welded pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a stainless steel welded pipe having a relatively small diameter and a thin wall, and the residual stress of the stainless steel welded pipe is reduced by using an O press molding apparatus. The present invention relates to a stress reducing method.

[Conventional technology]

For example, small-diameter thin-wall 304 stainless steel welded pipes have come into use for water pipes.

This stainless steel welded pipe is formed by bending a long stainless steel strip having a predetermined width into an annular shape by passing it between a plurality of forming rolls, and welding both ends of the folded strip. It is a thing.

[Problems to be solved by the invention]

Since such a stainless steel welded pipe is one in which a strip is bent in an annular shape and both ends are welded, a tensile residual stress exists on the outer peripheral surface of the pipe. The tensile residual stress thus distributed in the pipe causes stress corrosion cracking, which is often a problem.

The compressive residual stress theoretically does not cause stress corrosion cracking. In the stress value, minus means a contracting force, but the compressive residual stress cannot exist alone and exists in a paired relationship with the tensile residual stress. That is, normally, the outer surface and the inner surface of the pipe are balanced, and the large negative value on the outer surface of the pipe means that the inner surface of the pipe also has a large tensile residual stress. The present invention has created a method for reducing residual stress in a stainless steel welded pipe from the viewpoint of this tensile residual stress.

In order to solve the problem of such stress corrosion cracking, it is known that the residual stress should be reduced. Conventionally, the diameter reduction rate of a welded pipe (that is, the diameter reduction rate, the same applies hereinafter).
A straightening method has been attempted in which the residual stress is reduced by imparting strain and strain in the longitudinal direction by applying the stress.

However, in the straightening process using, for example, a straightening roll, press, etc., even though the pipe is reduced in diameter and stretched in the longitudinal direction and strained in the straightening process as described above, in the pipe welding / cooling process. It is known that the resulting residual stress does not always decrease.

In view of the above-mentioned conventional problems, the present invention focused on the welding / cooling process of stainless steel welded pipe production, conducted temperature and stress analysis and measurement, and investigated the influence of production conditions on residual stress. As a result of various experiments conducted by using an O press forming apparatus to give a diameter reduction to the welded and cooled pipe and examine the effect of reducing residual stress, a satisfactory effect was obtained.

Therefore, an object of the present invention is to provide a method for reducing the residual stress of a stainless steel welded pipe, which can remarkably reduce the residual stress of the pipe.

[Means for solving problems]

In order to achieve the above-mentioned object, a method for reducing residual stress of a stainless steel welded pipe according to the present invention is to use an O press forming apparatus to apply a diameter reduction to a welded and cooled stainless steel welded pipe to give a residual stress of the pipe. In the method for reducing the above-mentioned problem, an O-press die having a die length of 1.3 times or more of the outer diameter of the pipe is used, and pressing is performed so that the diameter reduction rate is 1.8% or more.

[Action]

By setting the die length to 1.3 times the outer diameter of the pipe and the diameter reduction rate to 1.8% or more, the die length and diameter reduction rate can be suppressed to the necessary minimum, and the stress corrosion cracking of the pipe is possible. It is possible to suppress the residual tensile stress required to prevent the occurrence of 100 MPa or less in both the longitudinal and circumferential directions of the pipe.

〔Example〕

 Embodiments of the present invention will be described in detail below based on experiments.

That is, in the present invention, an experiment was conducted to examine the effect of reducing residual stress by giving a diameter reduction to the pipe after welding and cooling the stainless steel welded pipe.

The sections will be described separately below.

Experimental Conditions The pipe 1 after welding and cooling shown in FIG. 1 was molded using the O-press die 2 shown in FIG.

Table 1 shows the die hole die diameter Dd, the die length Ld, the plate thickness t of the welded pipe, and the pipe length Lp at this time. Also, pipe 1 after welding and cooling
The outer diameter Dp of the pipe obtained from the average outer peripheral length of is 22.5 to 22.6 mm. Furthermore, although the pipe length Lp was set to 500 mm only in the experiment for examining the change in strain in the longitudinal direction, it was 150 mm in other experiments.
And

Experimental Results [1] Relationship between Plate Thickness and Diameter Reduction Rate Fig. 3 shows the relationship between the nominal diameter reduction rate Rn and the true diameter reduction rate Rt when O-pressing was performed while changing the sheet thickness of the tube 1.
These were calculated by the following formula.

However, Dd: Die hole die diameter Dwc: Pipe average outer diameter after welding / cooling Dop: Pipe average outer diameter after O pressing From Fig. 3, it can be seen that Rt is slightly lower than Rn regardless of the change in plate thickness. You can see that Further, when Rn is set small, the diameter does not decrease.

From this result, it is understood that there is a relation of Rt = 0.846 Rn−0.306 between Rn and Rt.

[2] Effect of reduction rate of true diameter on residual stress In Fig. 4, the pipe thickness is 1.0 mm and the pipe 1 is when Rt is changed.
2 shows the residual stress distribution in the longitudinal and circumferential directions. An X-ray diffractometer was used for stress measurement.

It can be seen from FIG. 4 that the circumferential stress σθ does not depend on Rt and the absolute value of the stress is within 100 MPa.

On the other hand, regarding the longitudinal stress σz of the pipe, the residual stress is reduced by performing O-pressing, but when Rt is small, 100 to 2 near the pipe bottom 3 and the welding beat 4 are obtained.
There is a tensile residual stress of 00 MPa. Then, when Rt = 0, the same numerical value as Rt = 0.4% was confirmed, but when this is illustrated, it is omitted because the figure becomes complicated. That is, Rt =
If 0.4% is regarded as Rt = 0, the decrease in residual stress at Rt = 0.8% is clear.

However, it was found that the absolute value of the residual stress also greatly decreases with respect to θz by setting Rt to a large value.
Further, in this figure θz, the diameter reduction rate (reduction rate) and the residual stress do not have a linear proportional relationship, that the residual stress reduction effect is small in the range where the diameter reduction rate is small, and the dispersion of the experimental values is smaller. Because it's big, and statistically,
The increase in diameter reduction rate can be understood as the tendency of residual stress reduction.

By the way, the tensile stress of 304 stainless steel is about 100MPa.
It is generally known that the crack generation time becomes remarkably long under the following conditions.

In the case of small diameter stainless steel pipes used for general piping, it is considered that there is no risk of cracking even under a tensile residual stress of about 150 MPa. The residual stress to prevent it from occurring was set to 100 MPa. From the results shown in Fig. 4, if the true diameter reduction rate Rt = 1.8% is set, the residual stress is 10M.
It became Pa or less, and it was confirmed that there was no risk of stress corrosion cracking. Further, in FIG. 4, the peculiar behavior indicated by the behavior of the residual stress of “180” is the effect of welding, and the behavior of “45” is considered to be the symmetry of “135” and the measurement was omitted. There is a left-right difference in the machine from "180" to "360", but it is almost the same, and it is symmetrical from "0" to "180".

The behavior is significantly different between "0" and "90" because the welded pipe by roll forming is usually formed continuously from the strip plate to the pipe by using rolls of 10 to 20 stands. In the width direction, the length of the molding path is different between the center and the end portion, which is a downward sled in the longitudinal direction. On the other hand, at the position of “180”, thermal expansion due to welding and contraction due to cooling occur, which results in an upward warpage in the longitudinal direction. The bending stress of the upper and lower rolls is added to this, and residual stress is formed. The behavior of "90" is different because there are few left and right rolls.

In addition, in the case of the tube sheet thickness other than the tube sheet thickness shown in FIG. 4, the behavior is the same though the numerical values are different, and the influence is changed by changing the tube sheet thickness to four types in FIG. Shows.

Furthermore, in order to reduce the longitudinal residual stress σz,
As a reason for requiring a larger true diameter reduction rate than in the case of reducing the circumferential residual stress σθ, the longitudinal extension of the tube 1 with respect to the true diameter reduction rate was investigated. This is shown in FIG.

From the figure, the relationship between the true diameter reduction rate and the longitudinal spread is εz
= 0.178Rt + 8.3 × 10 ^-3 , which shows that about 20% of the given true diameter reduction rate is extended in the longitudinal direction. That is, if the diameter reduction rate is 1.8%, the elongation in the longitudinal direction is about 0.3%, which exceeds the proof stress, and plastic strain occurs in the longitudinal direction. Therefore, it was confirmed that the residual stress can be reduced.

[3] Effect of plate thickness on residual stress In Fig. 6, nominal diameter reduction rate Rn = 2.2% and plate thickness 4
The following shows the residual stress reduction status when changing to different types.

As shown in the figure, the circumferential residual stress σθ is
Although it is 100 MPa or less, it can be seen that with the residual stress σz in the longitudinal direction, when the plate thickness is 0.4 to 0.6 mm, a sufficient reducing effect is not recognized. This is because the diameter reduction was performed with the same die hole die diameter, so that the true diameter reduction rate Rt near the nominal diameter reduction rate Rn = 2.2% in FIG. 3 decreases as the plate thickness decreases.

[4] Effect of Die Length on Residual Stress By the way, when a residual stress reducing method using an O press molding apparatus is introduced into an actual stainless steel welded pipe manufacturing line, the O press mold 2 is a pipe 1 The compression, unloading, and movement are repeated in synchronization with the manufacturing speed of. If the die length is long, the number of times of compression is small, but the moving distance is long and the molding load is large. Also, if the die length is shortened, the forming load is small and the moving distance is short, but it is doubtful to what extent the residual stress reduction effect when the die length of 80 mm adopted in the experiments so far can be applied.

Therefore, in the experiment of the present invention, in addition to the case where the die length is 80 mm, the die lengths of 10, 20, and 30 mm are prepared.
The residual stress distribution after pressing was examined.

The results are shown in FIG. In this case, the plate thickness was 1.0 mm and the true diameter reduction rate was Rt = 1.8%.

When the die length is 10 and 20 mm, both σz and σθ have large variations in the circumferential position and deviate from the allowable values. When the die length is 30 mm, the residual stress value at each position is within 100 MPa, which is close to the result when the die length is 80 mm.

The reason for this is that when the die length is short, the O-press-molded portion is subject to front and rear restraints, and is therefore locally bent and deformed. Therefore, a good flat portion corresponding to the die length was not obtained, and it is considered that this also affected the residual stress distribution.

From the above results, the outer diameter of the pipe is 22.5 to 22.6 mm, and the residual stress value at each position in the circumferential direction of the pipe is 100 MPa due to the die length of 30 mm. It was found that a die length of 1.3 times the outer diameter of the pipe or more is necessary to make the die sufficiently flat.

[5] Shape accuracy after O-press molding The residual stress reduction effect by O-press molding and improvement in shape accuracy were measured and compared with the one-part roll forming method.

Plate thickness change Fig. 8 shows the plate thickness strain distribution in the circumferential direction. In O press molding, the true diameter reduction rate increases and the plate thickness increases, and this tendency is particularly remarkable at the side portion of the tube. In FIG. 8, the difference between the left and right data is the sum of the machining accuracy of the machine, the variation of the material, and the variation of the measurement, and the obtained data is based on a single experiment. is there.

In addition, when comparing it with the result of FIG. 5, it can be seen that the majority of the true diameter reduction rate is converted into the plate thickness increase. It can be seen that when the true diameter reduction rate is the same, the plate thickness increase by the roll forming method is not so large. This is because the longitudinal extension of the tube is promoted as the roll rotates.

Circumferential distribution curvature Fig. 9 shows the circumferential curvature distribution by O-press forming method and roll forming method when the plate thickness is 0.4 mm. Fig. 10 shows the plate thickness 1.0 m.
The curvature distribution in the circumferential direction at each true diameter reduction rate at m is shown.
The horizontal lines in FIGS. 9 and 10 are calculated curves,
The numerical value means the outer diameter of the pipe at that time. These figures are the data obtained by comparing the circumferential curvature distributions of the roll forming and the O press forming depending on the diameter reduction. If the diameter reduction ratio is within the scope of the claims of the present invention, the dimensional accuracy of the pipe is Is improving.

When the true diameter reduction rates are almost the same, no difference due to the molding method is observed. Also, at the same reduction rate, when the plate thickness is 1.0 mm, the O press molding method has a slightly poor curvature distribution, but within the true diameter reduction rate range where residual stress is sufficiently reduced, the O press molding method is more accurate. You can see that it is improving.

Pipe outer diameter and cut deformation Figure 11 shows the changes in the pipe outer diameter aspect ratio and the aspect ratio after pipe cutting. When the pipe outer diameter is measured in the pipe circumferential direction, the outer diameter between the weld bead and the pipe bottom is the largest, and the outer diameter between the side parts of the pipe is the smallest. This was made dimensionless and expressed as an aspect ratio.
The cutting in this figure may be cut to the required length when using the tube for practical purposes, and when residual stress is uneven and large, it is assumed that the accuracy of the tube will deteriorate due to the phenomenon of cut deformation. That is, it demonstrates the improvement of the accuracy of the pipe in the O press molding of the present invention.

Before O-press molding, the shape was 4-5% oval due to the influence of welding and cooling, but the accuracy is improved to 1% or less after molding. The aspect ratio is the best when the true diameter reduction rate Rt is about 0.7%, and the aspect ratio is not always improved even if the diameter reduction is applied thereafter.

Examining the measurement results of the pipe outer diameter at each position in the circumferential direction, the elastic recovery amount from the set hole-shaped radius is uniform at each position in the circumferential direction near the reduction rate Rt = 0.7%.

On the other hand, when the true diameter reduction rate Rt is 1% or more, the tube side portion has a substantially set hole die diameter, whereas the elastic recovery amount between the tube bottom portion and the bead portion is large and deviates from the set hole die diameter.

From this, it can be seen that when the diameter reduction rate becomes large, the compression state of the pipe differs depending on the circumferential position, and in order to perform O-press molding of the pipe with higher accuracy, the die hole shape should be slightly round rather than round. It indicates that the shape must be elliptical.

[6] Stress Corrosion Cracking Test As described above, when a stainless steel welded pipe is subjected to tensile stress and exposed to a chloride environment, an accident due to a metal fracture phenomenon called stress corrosion cracking occurs in units of several months to several years. Has occurred and has become a social problem. The present invention is based on this point of view. In order to prove that the residual stress is reduced and the stress corrosion cracking susceptibility is reduced, an accelerated stress corrosion cracking test is performed. In other words, a tube whose residual stress has been reduced by O press molding is used in a 42% magnesium chloride boiling solution.
A stress corrosion cracking test of dipping in ℃ was performed, and the time until the occurrence of cracking was compared with that of a commercial product. The crack was confirmed by a microscope and a color check. The result is shown in FIG.

In the commercially available product, circumferential cracking due to σz occurred in the entire pipe 30 minutes after the start of the test. Further, before the O-pressing and sizing steps, that is, after the welding / cooling, longitudinal cracks in the longitudinal direction due to σθ occurred at the bottom of the tube. This is consistent with the analysis result that tensile residual stress is distributed on the outer surface of the pipe bottom.

On the other hand, the pipe after the sizing process by the roll forming method is 1
Θ = 135 ° to 160 ° and 45 near the weld bead after time
A crack due to σz occurred around °. On the other hand, the O-press-molded tube showed no cracking after 3 hours,
It was a good result. From this result, it can be confirmed that the O-press-formed product according to the present invention has the longest time until the occurrence of cracks and is favorable from the relationship between the diameter reduction ratio and the crack occurrence time.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY As described above, the present invention provides the following residual stress reducing effect and accuracy improving effect in the method of reducing the residual tensile stress caused by welding and cooling of a stainless steel welded pipe by O press molding. Is.

Tensile residual stress is 10 in both longitudinal and circumferential direction
It was found that the diameter reduction rate must be 1.8% or more and the die length must be 1.3 times or more the outer diameter of the pipe in order to reach 0 MPa or less.

Therefore, the method of the present invention is carried out so as to satisfy the above-mentioned numerical values, and with the minimum necessary die length and diameter reduction rate, the tensile residual stress in both the longitudinal direction and the circumferential direction is determined.
It is possible to obtain a pipe reduced to 100 MPa or less, and it has an excellent effect of eliminating stress corrosion cracking due to residual tensile stress distributed in the pipe.

Moreover, the shape accuracy of the welded pipe subjected to O press molding is superior to that of the roll forming method, and the pipe straightening method by O press molding is straightened by the roll forming method even in terms of stress corrosion cracking resistance. It has advantages such as being superior to.

[Brief description of drawings]

FIG. 1 is a sectional view of a stainless steel welded pipe for carrying out the method of the present invention, FIG. 2A is a front view of an O press die, FIG. 2B is a side view, and FIG. 3 is a plate thickness and diameter of the pipe. A graph showing the rate of decrease,
FIG. 4 is a graph showing the residual stress distribution, FIG. 5 is a graph showing the elongation in the longitudinal direction due to the diameter reduction of the pipe, FIG. 6 is a graph showing the residual stress distribution due to the change in the plate thickness of the pipe, and FIG. 7 is the die. Graph showing residual stress distribution due to change in length, No. 8
Fig. 9 is a graph showing the plate thickness distribution, Fig. 9 is a graph showing the circumferential curvature distribution by the press forming method and roll forming method, and Fig. 10 is a graph showing the circumferential curvature distribution at each true diameter reduction rate.
FIG. 11 is a graph showing the aspect ratio of the welded pipe, and FIG. 12 is a graph showing the results of stress corrosion cracking test of the pipe. 1 ... Stainless steel welded pipe, 2 ... O press mold, 3 ...
… Tube bottom, 4… Weld beads.

Claims (1)

[Claims] 1. A method of reducing the residual stress of a pipe by applying a diameter reduction to a welded and cooled stainless steel welded pipe by using an O press forming apparatus, wherein the die length is 1.
Uses an O-press die that is 3 times or more the diameter reduction rate.
A method for reducing residual stress in a stainless steel welded pipe, characterized by pressing to 1.8% or more.