JPS61186461A - Manufacture of heat transfer tube made of ti - Google Patents

Abstract

PURPOSE:To obtain the titled tube without being damaged even if sea water, etc. in tube are frozen, by drawing seam welded Ti tube so that sectional area reduction ratio becomes the limited value or more, next heat treating said tube in a specified temp. range. CONSTITUTION:A seam welded Ti tube is drawn worked by >=30% sectional area reduction ratio, then, said tube is heat-treated at 600-800 deg.C and recrystallized to obtain metallic structure superior in ductility at a welded zone. At drawing said tube having larger diameter than that of product upto the product diameter, it is desirable to draw a large tube together with a core metal inserted into the middle part, to finish the outer diameter and wall thickness of the tube to the product size. By such a way, the titled tube without causing crack, etc. even if sea water, etc. in tube is frozen is obtd., and the safety of heat exchanger incorporated with this tube can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a Ti heat exchanger tube that has excellent ductility and does not burst even when seawater or purified water flowing inside the tube freezes. It is.

[Conventional technology]

Inside the heat exchanger, seawater or purified water is poured into the pipes.
There are heat exchangers that bring a medium at a temperature lower than 0°C (such as LNG) into contact with the outside of the tube, and evaporate the low-temperature medium using the heat retained in seawater. A specific example of such a heat exchanger is a shell-and-tube type heat exchanger. Examples include utensils. The materials for heat transfer tubes in such heat exchangers are corrosion resistant (seawater resistant, etc.)
Ti is used because of its excellent properties, and seam-welded pipes made of Ti are widely used, which are manufactured by bending a Ti band in the width direction and welding the butted ends.

However, the above Ti1ll! Although seam-welded pipes fully meet the requirements in terms of corrosion resistance, the ductility of the welded parts is poor, so when the seawater flowing inside the pipe freezes, the pipes tend to crack, and in severe cases, they may burst. It has the disadvantage of leading to. In other words, foreign matter adheres to the inside of the Ti seam welded pipe due to the reasons described below, and when the flow of seawater, etc. becomes poor due to the presence of the deposits, the seawater, etc. in the area where the flow is impaired is excessively cooled by the low-temperature medium, and as it freezes, the seawater, etc. The circulation may deteriorate further, the amount of frozen water may increase, and eventually blockage may occur. As a result, the TL seam welded pipe is subjected to large internal pressure due to the expansion of the pipe's internal volume due to freezing of seawater, etc., and cracks occur in and around the welded part, leading to rupture.

[Problem that the invention seeks to solve]

Therefore, the present inventors investigated the metallographic structure of the seam welded part of a Ti heat exchanger tube in order to clarify the cause of the occurrence of the cracks, etc., and found that the metallographic structure of the welded part and its vicinity exhibited an acicular structure. It was confirmed that the ductility of this part was lower than that of the base material, making it easier for cracks to occur. Therefore, it was considered necessary to improve the weld metal structure in order to improve ductility.

The present invention was completed as a result of further research based on these findings, and is an attempt to manufacture a Ti heat exchanger tube that will not be damaged even if seawater, etc. in the tube freezes. .

[Means for solving problems]

The gist of the method of the present invention that achieves these objectives is that a TiJll pipe manufactured by seam welding is drawn so that the reduction in area is 30 inches or more, and then heat treated at 600 to 800°C. It is something.

[Function] In the present invention, in order to improve the ductility of the metal structure in the welded part and its vicinity, a Ti seam welded pipe having a larger diameter than the product diameter is manufactured. Next, this is subjected to drawing processing to obtain a Ti seam welded pipe of the product diameter, but the cross-sectional reduction rate in the drawing processing must be set to - or more. This is a transition that exists within the Ti tube due to drawing processing (cold working).

This is to increase the number of lattice defects such as point defects to facilitate recrystallization], thereby lowering the recrystallization temperature (heat treatment temperature). In other words, if the area reduction rate is 30% or less, uniform recrystallization cannot be achieved even at the above heat treatment temperature, and the welded part becomes a mixed grain state).
T with uniform quality with large variations in material strength and ductility values
I can't get an i-tube.

Next, the Ti1li seam welded pipe drawn as described above is heat treated to recrystallize it, and the heat treatment temperature at this time is 6.
By the above-mentioned heat treatment, which needs to be set at a temperature of 00 to 800°C, the entire metal structure including the weld zone and its vicinity is improved to a uniform, equiaxed crystal structure with excellent ductility and large grain size. That is, according to the research conducted by the present inventors, in order to obtain satisfactory ductility, it is necessary to set the average grain size of Ti metal to 20 to 100 μm, but when the heat treatment temperature is less than 600°C, the average grain size does not reach 20 μm, and a sufficient improvement in ductility cannot be expected. On the other hand, if the heat treatment temperature exceeds 800° C., the average crystal grain size exceeds 100 μm, and the ductility etc. decrease too much, making it easy to break.

The basic configuration of the present invention is as described above, but when drawing a Ti seam welded pipe with a diameter larger than the product diameter to the product diameter, insert the core metal 1 into the center as shown in Figure 1. Then, pull out the large diameter Ti tube 2 together with the core metal 1 in the direction of the arrow and insert it into the die 3.
It is also desirable to compress the tube 2 from both sides of the core metal 1 to finish the tube to the outside diameter and wall thickness t - product dimensions. This not only improves the dimensional accuracy of the tube, but also smoothes the protrusion caused by the weld bead on the inner surface of the tube.

Although the present invention does not particularly limit the material of the Tii tube, it is desirable to limit the content of impurity elements Fe and O to 0.08% by weight or less and 0.10% by weight or less, respectively. . In other words, the increase in these impurity elements is caused by precipitation or solid solution in the Ti material. The content must also be below the above.

〔Example〕

Experiment 1 Outer diameter 25.4fnfl wall thickness 1.24 by seam welding method
We manufactured a large-diameter Ti1l seam welded pipe. This (m
Area reduction rate: 50%). Next, the product diameter tube was annealed at a heat treatment temperature of 700° C. for 0.25 hours, and the obtained sample was subjected to a repeated freezing test, and the results shown in Table 1 were obtained. A similar test was conducted using a Ti seam welded pipe obtained by the conventional method, and this was used as a comparative example.

After filling the sample tube with water and sealing it completely, heat it to -50°C.
It was immersed in a test tank to freeze. After the ice was thawed, the tube was further filled with water, completely sealed, and immersed in the test tank.The operation was repeated until the tube cracked, t, to determine the number of times.

Note) Amount of impurities in Ti: Fe: 0.041! ffi%Q
: 0.066% by weight As shown in Table 1, in No. 2, cracks already appeared in the pipe after 1 or 2 freezing cycles, but in No. 1, the pipes had already been frozen 5 times. For the first time, a crack appeared.

In addition, in the above example h, when the metallographic structure of the Ti seam welded pipe before drawing and the metallographic structure of the Ti seam welded pipe after heat treatment are examined, it is found that The results shown in FIG. 4 (photograph substituted for a drawing showing the structure after heat treatment) and FIG. 4 (photograph substituted for a drawing showing the structure after heat treatment in the portion corresponding to FIG. 3) were obtained.

As shown in Figure 2.3, an acicular structure was observed in the welded area and its vicinity before drawing, but after heat treatment it improved to a uniform, large-diameter, equiaxed crystal structure as shown in Figure 4. We were able to.

Experiment 2 Large-diameter seam-welded pipes made of Ti were manufactured so that the cross-sectional reduction rates during drawing were 25%, 30%, and 50 inches, and after drawing to the product diameter, they were heat-treated at 700°C. When the metallographic structure of the welded part was investigated, the results shown in FIGS. 5 and 6 (both photographs substituted for drawings) were obtained.

As shown in Figure 5, when the cross-sectional reduction rate is 25 inches, it is approximately 1
It was a state in which crystal grains with a size of 0 to 110 μm were mixed. In such a mixed state, there were large variations in ductility and strength near the weld, and the number of icing cycles was less than three times in some cases. The cross-sectional reduction rate for the fin is 30 inches and 50 inches.
In the case of cII, the cross-sectional reduction rate is 25% as shown in Figure 6.
Compared to that, it has a more uniform and constant size of 48 μm.
) grain structure was obtained, and sufficient permeability was obtained. From the above results, it was found that the area reduction rate needs to be set to 30% or more.

Experiment 3 Ti seam welded pipes (drawn material) with a cross-section reduction rate of 50% were annealed at various heat treatment temperatures. The relationship between the crystal grain size of the welded part of the obtained welded pipe and the number of repeated freezing cycles was investigated, and the results shown in Table 2 were obtained.

*The number of cycles of freezing at the time of rupture or the center of the welded part of the welded pipe and 25.5 in the length direction from the center.
When the elongation in the circumferential direction at each point 0.100 mm apart was measured and graphed in correspondence with the weld zone grain size, the results shown in FIG. 7 were obtained.

As shown in Table 2 and Figure 7, the average crystal grain size is 16 μm.
In the case of 110 μm, the elongation in the circumferential direction was low and the number of cycles of freezing was also low. On the other hand, the average grain size is 48
In the case of μm, excellent elongation in the circumferential direction was obtained and the number of cycles of freezing was also satisfactory.

From the above results, it was found that in order to obtain sufficient ductility, it is necessary to set the average grain size to 20 to 100 μm, and for this purpose, the heat treatment temperature must be set to 600 to 800°C.

Appearance of welded pipe before freezing test, average grain size force L4Bμm
The appearance of the 110 μm welded pipe after rupture was as shown in Reference Photo 1).

〔Effect of the invention〕

The present invention is constructed as described above. 9. A Ti seam welded pipe is drawn so that the cross-section reduction rate is 30 inches or more.
Since it is then heat treated at 600 to 800°C, the metal structure of the welded part can be made to have excellent ductility, and to provide a Ti heat exchanger tube that does not cause cracks even if seawater or the like inside the tube freezes. be able to. In this way, the safety of the heat exchanger incorporating the Ti heat exchanger tube was dramatically improved.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional explanatory diagram showing an example of a drawing method;
Figure 3 is a photograph substituted for a drawing showing the metallographic structure of the weld before drawing, Figure 4 is a photograph substituted for a drawing showing the metallographic structure of the weld after heat treatment, and Figures 5 and 6 are photographs used as a substitute for a drawing showing the metallographic structure of the weld after heat treatment. FIG. 7, a photograph substituted for a drawing of the structure, is a graph in which the elongation in the circumferential direction at the longitudinal displacement point from the center of the weld was investigated for each crystal grain size.

Claims (1)

[Claims] Ti pipes manufactured by seam welding have a cross-section reduction rate of 30
% or more, then 600-800℃
1. A method for producing a Ti heat exchanger tube, the method comprising: heat-treating the tube.