JPS62248574A - Manufacture of finned tube - Google Patents

Abstract

PURPOSE:To obtain the finned tube having excellent gap corrosion resistance and stresscorrosion cracking resistance by performing a continuous arc welding of the joining part of both side edge parts of the fin and heat transfer tube with winding the fin on the heat transfer tube and executing a shot peening. CONSTITUTION:The joining part of a heat transfer tube 11 and both side edge parts 12a, 12a of a fin 12 is continuously welded by a pair of MIG welding machines 13 with winding the strip shaped fin 12 made of a two phase stainless steel spirally on the outer periphery of the heat transfer tube made of the two phase stainless steel. A shot peening is then executed on the outer periphery of the heat transfer tube 11 depositing the fin 12 and the tensile stress remaining near the depositing parts 14, 14 of the heat transfer tube 11 is transferred to a compression stress. In this way the finned tube excellent in a gap corrosion resistance and stress corrosion cracking resistance can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing finned tubes used in, for example, dryers, heat exchangers, and the like.

(Prior art) Conventionally, when manufacturing this type of finned tube,
In order to ensure good heat conduction between the fins and the tube, the contact end of the fin is crimped to the snow surface, or the fin is pressed to the tube using high-frequency induction heating. Ta. A finned tube manufactured by the currently widely used high-frequency induction heating method is, for example, a fin with fins wound spirally around the outer circumference of a tube 1 and joined as shown in FIGS. 8 and 9. 2 and the tube 1, a non-uniform protrusion 3 is formed due to melting of the fin material, and a gap 4 is created. Then, such a gap 4
When a finned tube having a finned tube is used in a dryer or a heat exchanger that is often handled in a humid environment, there is a problem that the gap 4 is likely to become a starting point for crevice corrosion.

By the way, it is known to manufacture finned tubes by arc welding. However, this method is rarely used today for the production of finned duplexes for technical and economic reasons. This is due to the following reason.

When fins and heat exchanger tubes are heated by arc welding, these materials become hard due to their self-hardening properties.

Also, from a cost perspective, it is desirable to reduce the penetration of welding rods into the base metal. In particular, when stainless steel is used as a material, its coefficient of thermal expansion is large, so welding distortion and tensile residual stress are likely to occur.

For the above reasons, arc welding is carried out with the welding current kept as low as possible, and after welding, annealing is performed as a post-heat treatment to alleviate the above drawbacks and eliminate distortion.
In some cases, annealing was not possible.

Further, due to the special form of the finned duplex, the annealing carried out during post-heating may not be able to simultaneously perform the entire process by furnace annealing. Therefore, annealing relies on local annealing, but since sufficient quality control of the finned tube cannot be expected with this local annealing method, the above-mentioned high-frequency induction heating method, which does not apply electricity to the material, is currently in widespread use.

[Problem that the invention seeks to solve]

What the present invention seeks to solve is the problem of the gap 4 that tends to occur between the fin 2 and the tube 1 and that tends to become a starting point for corrosion, and the problem of a post-treatment that replaces annealing in the post-heat treatment.

The present invention has been made in view of the above circumstances, and its objectives are to prevent the formation of gaps at the joint between the fin and the tube, to have excellent crevice corrosion resistance, and to reduce thermal stress due to welding. An object of the present invention is to provide a method for manufacturing a finned tube that prevents stress corrosion cracking (SCC) that occurs in a material instead of an annealing treatment.

[Means for solving problems]

In order to achieve the above object, the present invention involves attaching fins to a heat transfer tube to increase heat transfer efficiency by continuous arc welding to both side edges of the fins, and then subjecting the tubes to shot peening. .

[Effect]

In the method for manufacturing a finned tube of the present invention, the fins are continuously arc welded to the heat exchanger tube in the form of fillet welding on both sides, so that a continuous built-up part with a substantially triangular cross section is formed at the joint. is formed, eliminating the occurrence of crevices and improving crevice corrosion resistance. In particular, the crevice corrosion resistance is improved in a medium to high temperature range and in an environment exposed to a humid atmosphere.

In general, stress corrosion cracking (S) of stainless steel, for example,
CC) is often caused by the crevice corrosion as the starting point, since the defective portion caused by the crevice corrosion becomes a stress concentration point. Therefore, the present invention suppresses this type of stress corrosion cracking in addition to improving the above-mentioned gap rrA corrosion resistance.

Further, by performing shot peening as a post-welding treatment, the tensile residual stress can be relaxed and actively transferred to compressive stress, thereby improving the anticorrosion effect.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on FIGS. 1 to 7.

In the figure, reference numeral 11 denotes a heat transfer tube made of duplex stainless steel for applying heat via a heat transfer surface using a heat medium.

Then, as shown in FIG. 4, the heat exchanger tube 11 is rotated and sent rightward at a predetermined speed, and a band-shaped fin 1 made of duplex stainless steel is attached to the outer peripheral surface of the heat exchanger tube 11.
2, connect the joints between the heat exchanger tube 11 and both side edges 12a, 12a of the fins 12 with the heat exchanger tube 11.
A pair of arc welds 11 fixed to the fixed part of the feeder of
Continuous welding is performed in step 3. As a result, the spiral fins 12 are welded to the outer circumferential surface of the heat exchanger tube 11, and there is no gap in the welded portion 14.14, which has a triangular cross section, as shown in FIG. Significantly improves crevice corrosion resistance.

Further, the outer periphery of the heat exchanger tube 11 to which the fins 12 are welded is subjected to so-called shot peening, in which steel particles are projected. As a result, the tensile stress remaining in the vicinity of the r8@ portion of the heat exchanger tube 11 is transferred to compressive stress, and stress corrosion cracking can be prevented.

In addition, the finned tube is made of duplex stainless steel (commercial name:
For example, by adopting stainless steel with a two-phase structure of austenite and ferrite, various aspects of corrosion can be realized, namely (1) general corrosion, (2) stress corrosion cracking, (3)
) Pitting corrosion and (4) crevice corrosion can be produced.

The residual stress characteristic diagrams shown in FIGS. 6 and 7 specifically show the above effect. In this figure, the sample material is
Steel pipe for heat transfer tube 11: 50AX4t (two-phase stainless steel! I), and steel strip for fin 12: PL4xl 2w
Welding method GMAW (MIG welding) welding wire Duplex stainless steel wire (1,2 m) Welding current 100-150 amperes Welding voltage 15-25 volts Welding speed 3
Welding was performed under the conditions of 0 to 100 cm/min shielding gas composition 02 containing flow rate 10 J/sin or more. The dimensions of each part of the finned tube are shown in FIG. 1 when the diameter D of the heat exchanger tube 11 is 6.
0.5m+, thickness t+=4m, width W of fin 12=12
alI, thickness t2=4 am, and pitch P=27 am. In addition, we made two types of finned tubes for testing.
One of them was kept in the welded state, and the other one was shot peened under the following conditions. and,
For each finned tube J-beam, the residual stress was measured at a distance of 1 to 7 m from the weld bead end using the splitting method as shown in Figure 5 (in Figures 6 and 7, the marks are shots). After peening construction, the ○ mark indicates the value after welding construction).

Shot peening conditions Shot material: Steel shot Particle size: Within 26 mmI Projection speed: 30 to 10001/SeC As is clear from Figures 6 and 7, in the case of welding only (see ○ mark)
, tensile stress (maximum 20.8
N#f/ms), while compressive stress remains in the circumferential direction of the tube, after shot peening (see the * mark), the residual stress shifts to the compressive side by approximately 10 to 2 ON#f/ms+. However, it was found that compressive stress existed in both the tube axis and tube circumferential directions.

Next, a stress corrosion cracking (SCC) test was conducted on the above two types of finned tubes, that is, the finned tubes that had not been subjected to shot peening and those that had been subjected to shot peening. This test was conducted under the condition of 35%-HoCfz (boiling), and the cracking life was measured by regular observation twice a day after the start of the test. As a result, the test time for all finned tubes subjected to shot peening exceeded 1000 hours, while some of the tubes that were not shot peened broke in about 350 hours. The effectiveness of shot peening against stress corrosion cracking has been demonstrated.

In addition, the above welding conditions (welding current 100-150 amperes, wJm voltage 15-25 volts, welding speed 30-10
Anything other than 0 cm/l1in ) is extremely disadvantageous from a technical or economic point of view. Further, in the above shot peening, if the shot particle size is larger than 2.0 mm, the shot peening surface becomes rough and non-uniform. Furthermore, the projection speed of shot peening is 10
If it exceeds 0 i/sec, a concave portion will appear on the second surface, and if it is slower than 30 n+/sec, the distortion cannot be removed.

〔Effect of the invention〕

As explained above, according to the present invention, the fins for increasing the heat transfer efficiency are attached to the heat transfer tube by continuous arc welding on both side edges of the fins, so that the fins and the tube are connected. It is possible to prevent gaps from forming at the joints of the pipes, improve crevice corrosion resistance, prevent stress corrosion cracking caused by crevice corrosion, and prevent shot peening of heat exchanger tubes with welded fins. By applying this, compressive stress can be left in the vicinity of the weld, and stress corrosion cracking that has conventionally occurred due to the presence of tensile stress can be reliably suppressed.

[Brief explanation of drawings]

1 to 7 show an embodiment of the present invention,
Figure 1 is a front view of the finned tube, Figure 2 is a cross-sectional view of the welded part, Figure 3 is a side view showing the welding method, Figure 4 is a front view showing the welding method, and Figure 5 is residual stress measurement. A cross-sectional view showing points, Figure 6 is a characteristic diagram of residual stress in the tube axis direction, Figure 7 is a characteristic diagram of residual stress in the circumferential direction of the tube, and Figures 8 and 9 show a conventional finned tube. FIG. 8 is a front view, and FIG. 9 is a sectional view. DESCRIPTION OF SYMBOLS 11... Heat exchanger tube, 12... Fin, 12a... Side edge part, 13... Arc welding machine.

Claims (1)

[Claims] 1) After attaching fins to increase heat transfer efficiency to a tube that uses a heat medium to impart heat through a heat transfer surface by performing continuous arc welding on both side edges of the fins. , a method for manufacturing a finned tube characterized by subjecting it to shot peening. 2) The above shot peening is performed using steel shot.
The particle size is within 20mm and the projection speed is 30-100m.
2. The method for manufacturing a finned tube according to claim 1, wherein the manufacturing method is carried out at a speed of 1/sec. 3) Use duplex stainless steel for the heat exchanger tubes and fins, and use continuous arc welding to attach them with the following welding specifications: Welding method MIG welding Welding current 100-150 amperes Welding voltage 15-25 volts Welding speed 30-100 cm/ 2. The method for manufacturing a finned tube according to claim 1, wherein the welding is performed under welding conditions having a min welding wire and a duplex stainless steel wire.