JPS6023933B2 - Double pipe manufacturing method - Google Patents

Description

[Detailed Description of the Invention] (Technical Classification/Field) The disclosed technology belongs to the technical field of manufacturing corrosion-resistant and erosion-resistant double pipes for oil country tubular goods and the like.

<Explanation of the gist> Accordingly, in this invention, an inner pipe corresponding to a predetermined outer pipe is placed in a relative inner tube in advance, and then the inner pipe is ganged by hydraulic pressure or the like, and the inner pipe is made to have a port beyond the yield point. This invention relates to a manufacturing method in which the inner and outer tubes are closely integrated and amplified, and then tube expansion pressure is released to obtain a degree of tympanism and a self-tightening corrosion-resistant double tube. A material that produces martensite (martensitic phase generated from austenite phase with the help of machining below the defined Md point) is selected and iron-clad in the outer tube, and in that state, the Md point (martensitic phase of the steel material) is selected. When austenite transforms to martensitic during cooling, the highest transformation initiation temperature that can be raised by applying working strain is It is possible to cause the marnansite transformation at a temperature above the Md point (the temperature at which austenite to martensitic transformation starts) and below the Md point only by cooling without applying any heat. The outer tube is also enlarged through light enlargement, and the outer tube is processed, while martensitic transformation is prevented from occurring, and the tube expansion stress is released and the tube is shrunk at a predetermined diameter.
Therefore, the inner tube is cooled to a temperature below the starting temperature of the martensitic transformation point of the material to cause martensitic transformation in the inner tube and expand, resulting in a high degree of saddle density due to the difference in the inner and outer tubes. This invention relates to a method for manufacturing a double pipe by which a double pipe is obtained.

(Prior art) As is well known, corrosive fluid transport piping such as oil country tubular goods and oil transmission pipes must have pressure and heat resistance as well as corrosion resistance. tube is used.

However, the inner tubes of the clad steel tubes that have been used until now in the manufacture of double-walled double-walled tubes are often made of stainless steel with an austenitic structure, and are therefore soft and have the disadvantage that the cladding does not have sufficient binding force. In addition, there is a disadvantage that compressive residual stress is not formed even if compressive force is applied during manufacturing, and so-called stress corrosion cracking that occurs during operation cannot be prevented.

On the other hand, many of the applicant's earlier inventions have devised techniques to obtain a high iron content when manufacturing double-walled pipes by improving the thermal tube method, hydraulic pipe expansion method, etc. However, there are disadvantages in that the outer tube heating device and the like are large-scale and lead to high costs, and it is also difficult to form sufficient compressive residual stress uniformly in terms of design.

OBJECT OF THE INVENTION The purpose of the present invention is to solve the technical problem of manufacturing a corrosion-resistant double pipe based on the conventional technology described above, and to reduce the joining margin between the inner and outer pipes by utilizing the martensitic transformation of the pipe material. It is an object of the present invention to provide an excellent method for manufacturing double-layered pipes that can be obtained in a large amount and be strong enough to be manufactured without using any heating, thereby benefiting piping applications in various industries.

In order to solve the above-mentioned problems, the structure of the present invention, which is based on the above-mentioned object and has the gist of the above-mentioned claims, is to solve the above-mentioned problems. The material is cooled and kept at room temperature for so-called sub-zero treatment (when the austenite phase transforms into the martensitic phase, a residual austenite phase remains that is not transformed, but in order to promote the martensitic transformation of this phase, the material is generally kept at a temperature below room temperature. This is called Sapzero processing.

), the inner tube made of a material that produces deformation-induced martensite is relatively inserted into the outer tube, and the inner tube is heated to a temperature above the Md point using hydraulic pressure. The pipe is expanded by pipe expansion, etc., and the outer pipe is expanded as well, and the diameter is increased beyond the yield point, and the martensitic transformation is suppressed. It is shrunk and cooled to a temperature below the martensitic transformation start temperature to undergo martensitic transformation, and the expansion associated with the martensitic transformation increases strength and increases the iron content.
Technical measures have been taken to create compressive residual stress, maintain corrosion resistance, and prevent stress corrosion cracking. <Embodiments> Next, embodiments of the present invention will be described below with reference to the drawings.

As shown in Fig. 1, if the horizontal axis is the temperature involved in transformation T and the vertical axis is the diameter D of the inner and outer tubes, for example, the flea change of the carbon steel tube of the outer tube is linear, whereas that of austenite is linear. C2 of the stainless steel inner tube consists of the heating path (solid line) from before the tube expansion process to the tube expansion point P and the cooling path after the tube expansion plastic deformation (
It is known that expansion due to transformation occurs during cooling at the martensitic transformation (indicated by M) starting temperature (Ms point when no processing strain is applied).

Therefore, in this invention, in order to prevent martensitic transformation, the diameter is increased above the Md point and then cooled.
Increasing the diameter by causing martensitic transformation at a temperature below the starting temperature of the martensitic transformation point of the material was used to form the diameter of the material.

As an example, as shown in FIG. 2, the austenitic stainless steel of the outer pot Di is appropriately inserted and layered into the carbon steel of the inner pot Do.

It goes without saying that the austenitic stainless steel of the inner tube is one type of steel that produces deformation-induced martensite.

Therefore, the austenitic stainless steel (SUS32
The highest temperature Md at which martensite does not occur even if the processing is applied in 1) is 12,500, and the martensite transformation point starting temperature Ms when no processing is applied is -700.

In this way, the inner tube of the layered inner and outer tubes is filled with a warm body such as oil heated to 125°C or higher, that is, the inner tube is heated to a temperature higher than the Md point by, for example, a jacking action of a hydraulic machine, etc. When a tube expansion force is applied to the inside of the tube, the inner tube curve C rises and expands based on the stress F-strain curve as shown in Fig. 2 above, and the outer tube comes into contact with the outer tube at the Do diameter. , furthermore, the inner and outer tubes are enlarged to form a concomitant integral body, and the stress F-strain curves C, , C of both tubes are obtained.
o increases friction beyond the yield point, undergoes plastic deformation processing, releases pressurized water at the set flea R, and contracts the exchange pipe, reducing the inner pipe diameter to Di' and the outer pipe diameter to Do'. .

Therefore, when the inner tube is immediately filled with soot, such as liquid nitrogen, at a temperature below the starting temperature of the martensitic transformation point of the material, and the inner tube is rapidly cooled, deformation-induced maransite is formed in the inner tube. Due to martensitic transformation, the inner tube expands from the above Pi' and forms a tightness △R at Di'', and as a result, the reduced outer tube diameter Do' is equal to Dj'' - Do'). An effective thickening degree △D is obtained,
A strong interference is obtained, a self-imposed double pipe is obtained, and a compressive residual stress is formed.
Therefore, the self-accumulating double tube obtained by subsequently discharging the coolant is not only resistant to insects but also strong and free from slippage, as well as free from stress corrosion cracking, erosion, etc. becomes. In the above-mentioned embodiment, the inner pipe has a higher yield stress, but there is no substantial difference in the reverse case. For example, as shown in FIG. 3, the inner pipe diameter Di after pipe expansion is In addition to forming a considerable interference between the outer tubes Do', it is possible to obtain an even larger interference ΔD due to martensitic transformation.

It goes without saying that the embodiments of the present invention are not limited to the above-mentioned embodiments; for example, other deformation-induced martensitic materials may be used, and the outer tube may be heated before relative insertion of the inner tube. Various methods are used, such as expanding the pipe, cooling it after shrinking the pipe, and the target double pipe can be applied not only to oil country tubular goods and oil transmission pipes but also to plant piping, etc.

<Effects of the Invention> As described above, according to the present invention, the inner tube made of a material that produces process-induced martensite is inserted relative to the outer tube, and the inner tube is heated at a temperature higher than the Md point with respect to the inner tube. The tube is expanded without causing martensitic precipitation, plastically deformed to release the expansion force, and then cooled to a temperature below the martensitic transformation point start temperature of the material to achieve martensitic transformation and increase the diameter of the inner tube. By expanding the tubes, it is basically possible to connect the two tubes once by expanding the tubes, and to accumulate them through martensitic transformation without using any means such as heating, and to apply compressive stress. This strengthens the structure and imparts compressive residual stress, resulting in improved corrosion resistance as well as
The excellent effect of preventing stress corrosion cracking and erosion can be achieved.

In addition, since it takes advantage of the fact that martensitic transformation occurs when cooled, it does not require mechanical means or special equipment, and has the advantage of being simple and low cost.

[Brief explanation of drawings]

Figure 1 is an illustration of temperature expansion during martensitic transformation;
FIG. 3 is a process explanatory diagram of an embodiment of this invention. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. In a method for manufacturing a double pipe having a degree of fit by relatively fitting an inner tube into an outer tube and then expanding the inner tube, the inner tube made of a material that produces deformation-induced martensite is relatively fitted into the outer tube, Then, the inner tube is expanded at a temperature higher than the Md point, brought into contact with the outer tube and expanded integrally, and both tubes are plastically deformed, and then both tubes are shrunk.
Next, the inner tube is cooled to a temperature below the starting temperature of the martensitic transformation point of the material, and a high degree of fitting is obtained due to the expansion diameter accompanying the martensitic transformation, so that the inner and outer tubes are tightly connected. Production method.