JPS5858596B2 - Aluminum plated steel pipe for heat exchanger - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an aluminum-plated steel tube for heat exchangers, particularly at the end of the tube that is fixed to the end plate of the heat exchanger, to prevent cracks from occurring during tube expansion processing for fixation. This relates to steel pipes that have undergone such measures.

Steel pipes for heat exchangers (carbon steel pipes) usually have an outer diameter of 1
6-140wnφ, wall thickness 1.2-12.5ff1m, and required length, for example, 5-10m, are provided.

The present invention is applied to such steel pipes.

On the other hand, steel pipes for heat exchangers are coated with aluminum to provide corrosion resistance on their inner and outer surfaces.

However, in order to fix this aluminum-plated steel pipe to the end plate, the end of the steel pipe was inserted into a hole drilled in the end plate.
When expanding a pipe by inserting a tool into the end of the pipe, cracks often occur at that end.

This crack may occur only in the plating layer at that location, but it may also start from a crack in the plating layer and propagate inside the steel pipe, reaching deep into the pipe wall.

This is an aluminum plating layer (aluminum layer + aluminum
This is due to the fact that the hardness of the alloy layer that forms the alloy layer (alloy layer of aluminum and steel) is extremely hard and difficult to deform, even though it has the property of being difficult to deform. The more there is, the stronger the tendency (・.

The thickness of the alloy layer is determined by the high temperature of the aluminum plating bath and the long time required for plating, but when it comes to aluminum plating steel pipes, the aluminum bath temperature and plating time are generally short. The degree of occurrence of the above-mentioned machining side deviation also depends.

Conventionally, measures taken to solve these problems include adding Si, Ti, and Be to the molten aluminum bath in order to suppress the formation of an alloy layer during plating.

Addition of Cr and other elements has been carried out.

However, in actual on-site work, the content of these elements in the bath constantly changes, making it difficult to maintain a constant level, and not only is it expensive, but it is difficult to completely prevent cracking. It is still difficult to do so.

Another method is to remove the plating layer on the part to be expanded when the tube is fixed to the end plate, and after expanding the tube, apply anti-corrosion paint to the surface of the area where the plating has been removed.

However, this requires a process of deletion and a process of painting, and the process of deletion is not particularly easy.

However, it is difficult to expect much from anticorrosion coatings, and their heat resistance and corrosion resistance are also poor.

The present invention aims to solve these problems, and provides a steel pipe for L/heat exchangers that does not require any special consideration during aluminum plating or when fixing to end plates. This is what I am trying to do.

That is, the present invention applies to aluminum-plated steel pipes for heat exchangers.
Therefore, the tube end that is fixed to the end plate is made of a material that suppresses the diffusion of aluminum into the steel during aluminum plating, and the short tube with the same diameter and wall thickness as the main steel tube portion is concentrically formed. This is an aluminized steel tube for a heat exchanger, characterized in that a welded and connected tube is dip-plated in a molten aluminum bath to form a thin aluminum plating layer on the tube end.

Next, the features of the aluminum-plated steel pipe for heat exchangers of the present invention will be explained in detail by illustrating the manufacturing process.

Ordinary carbon steel is used as the main part of the aluminum-plated steel pipe for heat exchangers of the present invention, and the JIS steel pipe symbols are 5TB30, 5TB33.5TB35.5TB42.
etc., and its dimensions are 16.0 to 140 in outer diameter
．． Use a steel pipe with a diameter of 0 and a wall thickness of 1.2 to 12.5 mm.

In addition, for the short pipe part that suppresses aluminum plating, a material that suppresses the diffusion of aluminum into the steel during plating and can be welded is used. (1) 13% Cr steel (JI
The S symbol is 5US405) or (2) 0.5 on the surface.
~2. Plain carbon steel with OfMt carburizing treatment is used.

Its dimensions are the same as the main steel pipe in terms of outer diameter and wall thickness.

The length is determined by the thickness of the end plate to be fixed and the outer diameter and wall thickness of the tube, but specifically, it is preferably about 2 to 3 times the outer diameter of the tube, and this is substantially the same as the end plate. When fixing, insert a tube expansion tool to cover the area affected by the tube expansion process.

When joining such a short pipe section to the main steel pipe section, the joint ends are first beveled and ordinary electric welding or gas welding is performed, or the short pipe is rotated at high speed. By friction welding.

After welding, remove any beads protruding from the surface as necessary to create a smooth surface.

The short tubes may be joined at both ends or at one end of the main steel pipe section, and when the short tubes are joined, they should have a predetermined length necessary for installation at a specific location in a specific heat exchanger.

Aluminum plating is pretreated by degreasing with a solution of caustic soda, soda carbonate, etc., pickling with a solution of L., sulfuric acid, hydrochloric acid, etc., followed by hot water washing and rapid heating drying at about 200°C.

The temperature of the molten aluminum bath is approximately 710 to 760°C, and the steel pipe is immersed in the bath.

When dipping, it is best to tilt the tube so that the air in the tube is expelled quickly, and to shake the tube after dipping (the dipping time will vary depending on the dimensions of the tube, bath temperature, and other conditions). Although it varies, it is about 30 seconds to 3 minutes.

When withdrawing from the bath, it is best to maintain the tilted state and raise the aluminum solution to 0 in order to improve drainage of the aluminum solution.
When pulling up, it is preferable to suck up hot air or steam to the inner and outer surfaces and scrub and wipe them.

The aluminum-plated steel pipe of the present invention coated with aluminum in this manner has a plating layer thickness of 0.1 to 0.2 mm on the main steel pipe portion, and the thickness of the alloy layer is 0.1 to 0.2 mm. 04~0.15mm (40~75φ of the whole layer
).

The higher the plating temperature and the longer the plating time, the greater the proportion of the thickness of the alloy layer.

The thickness of the plating layer on the short pipe part is (1) in the case of 13% Cr steel,
0.01 to 0.03rrtm, in which the thickness of the alloy layer can be suppressed to 0.005m or less, and (2)
In the case of ordinary steel with a carburized surface of 0.5 to 2.0 m, the above 1.
A plating thickness similar to that of 3% Cr steel was obtained, and the thickness of the alloy layer was extremely thin at 0.002 to 0.003 mm.

Through the above manufacturing process, a fixation test was conducted on a main steel pipe with a wall thickness of 2rrrm and a short pipe part length of 100rran to an end plate with a thickness of 20rrrm, and the clearance of the hole drilled in the end plate was set to 0.4 mm. The pipe was expanded at a pipe expansion rate of 5.0% to obtain the above (1) 13 q6Cr steel pipe, (2 bale surface carburized steel,
As a result of expanding and fixing 50 tubes each, not a single tube was cracked.

This is because the formation of the alloy layer is further suppressed as the aluminum plating layer is suppressed, and because it forms an extremely thin layer, cracks are less likely to occur during processing, and even if a crack occurs in the alloy layer, it will not occur. This is thought to be because the particles were so small that they did not spread to other places.

Since the expansion ratio of a steel pipe with a thickness of 2 to 7 mm in one direction in normal pipe expansion fixing is 1.6%, it is clear that the product of the present invention can sufficiently withstand this.

The attached drawing shows a cross section of an embodiment of the present invention in which an aluminum plated steel pipe for a heat exchanger is fixed to an end plate.
A heat exchanger steel tube, in which a main steel tube section 2 made of ordinary steel tube and a short tube section 3 made of 13% Cr steel tube or surface carburized steel tube are connected at a welded section 6, is aluminum immersion plated and coated with a thick plating layer. 4 and a thin C. plating layer 5 formed thereon, an aluminum-plated steel tube 1 for a heat exchanger is inserted into an end plate 7 and expanded and fixed using a tube expansion tool 8.

In the present invention, the plating layer is thick in the main pipe part and the ratio of the alloy layer is also small, but the plating layer is thin in the short pipe part, and the thickness ratio occupied by the alloy layer is extremely small.

In this way, the aluminum-plated steel pipe for heat exchangers of the present invention has almost no cracking even when expanded and fixed as described above, and also has corrosion resistance and heat resistance, and is carburized throughout its entire length. Compared to a steel pipe that is aluminum-plated at ℃°C or a full-length 13% Cr steel pipe that is aluminum-plated, the former has an expensive carburized quality, and the latter has a higher material cost. In this case, use a long piece of ordinary steel pipe or 13% Cr steel pipe that has been carburized in advance. However, it is much cheaper than the conventional method of removing the aluminum plating part, fixing it to the end plate, and then painting the removed part.

In addition, there is no need to take care to reduce the thickness of the alloy layer or manage the attachment, and there is no need to worry about fixing the end plates to the end plates on-site, so aluminum-plated steel pipes for heat exchangers can be used. It can be said that it is extremely suitable for fixing to end plates.

[Brief explanation of drawings]

The accompanying drawing is a cross-sectional view of an embodiment of the present invention in which an aluminum-plated steel tube for a heat exchanger is fixed to an end plate. In the figure 1...Aluminized steel pipe for heat exchanger, 2.
...Main steel pipe part, 3...Short tube sound doctor 4...Thickness L, plating layer, 5...Thin L, plating layer, 6...Welded part, 7
...End plate, 8...Pipe expansion tool.

Claims (1)

[Claims] 1. For aluminum-plated steel pipes for heat exchangers, the ends of the steel pipes for heat exchangers are made of a material that suppresses diffusion of aluminum during aluminum plating, and have the same diameter.
An aluminum-plated steel pipe for a heat exchanger, which is formed by dip-plating aluminum on a pipe in which short pipes of the same wall thickness are concentrically connected to form a thin aluminum plating layer on the short pipes.