JPH111724A - Method for preventing peeling crack of pressure vessel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preventing the occurrence of hardening in the interface between a base material and a stainless steel layer due to stress relief annealing and also the occurrence of peeling crack as delayed fracture. SOLUTION: In this method, a member for pressure vessel, where a stainless steel plate is laminated on the surface of a base material, is welded and then stress relief annealing is applied to prevent the peeling crack of the resultant pressure vessel. In this case, prior to the welding, heat treatment is applied to the member for pressure vessel at 560 to 640 deg.C, or, a method where heat treatment is applied at 560 to 640 deg.C prior to stress relief annealing can be adopted. Moreover, the stress relief annealing means heat treatment consisting of holding at 650 to 720 deg.C for 5 to 30 hr.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing peeling cracks in a pressure vessel, and more particularly to a pressure vessel used for petroleum refining and the like. And a method for preventing peeling cracks occurring at the interface of the steel sheet.

[0002]

2. Description of the Related Art Generally, a pressure vessel requiring corrosion resistance is made of carbon steel as a base material and a stainless steel plate is laminated on the inner surface thereof. For example, in petroleum refining, useful light oils are extracted by utilizing heavy oil remaining after refining various petroleums from crude oil and reacting the heavy oil with high-temperature and high-pressure hydrogen in a pressure vessel. However, this reaction produces highly corrosive hydrogen sulfide as a by-product. Cr-Mo alloy steel or Cr-Mo-V alloy steel, which is usually used as a base material of a pressure vessel,
The corrosion resistance to hydrogen sulfide is low. Therefore, in order to protect the base material from the hydrogen sulfide, the inner wall surface of the container is made of SUS30.
9 and SUS347.

[0003] A pressure vessel for oil refining uses clad steel in which a stainless steel plate is closely adhered to the surface of the base material, or a material in which a stainless steel plate is overlay-welded to the surface of the base material by submerged arc welding or the like. The above material is processed into a ring-shaped or plate-shaped part, and these parts are connected to each other by joint welding to form a final product. Therefore, there are many joint welds, and heat treatment is performed after welding for the purpose of removing residual stress in the joint welds. This post-weld heat treatment is called stress relief annealing, and is also effective in releasing hydrogen dissolved during welding, adjusting the mechanical properties of the base material, and the like. This stress relief annealing is performed at a temperature of 650 ° C. or more and 720 ° C. or less for 5 to 3 times.
It is generally carried out for 0 hours, and in the pressure vessel manufacturing process, it is carried out by being carried into a large-scale annealing furnace in a substantially finished product state.

[0004] However, the stress relief annealing hardens the interface structure of the cladding weld portion and the cladding of the pressure vessel member, and furthermore, when the operation of the oil refining pressure vessel is stopped, hydrogen enters the hardened interface structure. As a result, the interface is embrittled, and there is a problem that peel cracking, which is a kind of delayed cracking, occurs at the interface between the base material and the stainless steel layer.

[0005] The reason why the stress-relieving annealing is performed after welding to cause peeling cracks at the interface between the base metal and the stainless steel material is that martensite structure is formed at the interface by the stress-relief annealing, and the hardness of the interface is greatly increased. Because it is easily broken. Therefore, a method of improving the peeling crack resistance by replacing the generated martensite structure with another structure such as troostite or sorbite by heat treatment can be considered. For example, there is known a method for preventing the stainless steel layer from peeling off by performing a heat treatment at about 600 ° C. after stress relief annealing in pressure vessel manufacturing (“DISBONDIN”).
G MECHANISMS, IMPROVEMENT OF OVERLAY BEHAVIOR '' CRE
USOT-LOIRE INDUSTRIE Seminar, 1992). However, in the process of manufacturing a pressure vessel for petroleum refining, it is not desirable from the viewpoint of manufacturing cost and construction period to perform heat treatment at around 600 ° C. again in a large annealing furnace in the state of the final product after stress relief annealing. Accordingly, there has been a demand for the development of a method capable of preventing peeling cracks without performing heat treatment on the pressure vessel after the stress relief annealing.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and prevents hardening of a base material-stainless steel layer interface by stress relief annealing, and prevents peeling cracks, which are a kind of delayed fracture. It seeks to provide a way in which it can be done.

[0007]

According to the present invention, which has solved the above-mentioned problems, the present invention is manufactured by welding a member for a pressure vessel in which a stainless steel plate is laminated on a surface of a base material, and then performing stress relief annealing. A method for preventing peeling cracks of a pressure vessel, wherein the pressure vessel member has 560
The gist of the invention is to perform a heat treatment at a temperature of not less than 640 ° C. Alternatively, 560 prior to the stress relief annealing.
A method of performing a heat treatment at a temperature of not less than 640 ° C and not more than 640 ° C may be employed. In the present invention, the stress relieving annealing means a heat treatment that is maintained at a temperature of 650 ° C. to 720 ° C. for 5 hours to 30 hours.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the first place, the mechanism of the formation of a martensite structure at the interface between a base material and a stainless steel layer is partially due to the continuous change in the concentration of alloying elements (Cr, Ni, etc.) at the interface. This is considered to be because the austenitizing temperature is very low, and the region partially austenitized by the stress relief annealing becomes martensite by cooling.

Therefore, if a region to be austenitized by stress relief annealing is eliminated in advance, a martensite structure is not generated. Therefore, in the present invention, a method is employed in which, prior to the stress relief annealing, a heat treatment for transforming all the crystal structure at the interface into ferrite + carbide by heating and holding at a temperature lower than the annealing temperature is adopted. The austenitizing temperature of the crystal structure once transformed into ferrite + carbide is sufficiently higher than the heating temperature of stress relief annealing, and it does not become austenite even if stress relief annealing is performed. It does not generate. Therefore, since the hardness of the interface does not increase even after the end of the stress relief annealing, the cause of delayed cracking is eliminated, and the peeling crack resistance is improved.

Further, the mechanism by which martensite is not generated even when stress relief annealing is performed by employing the method according to the present invention will be described in detail below. That is,
When stainless steel is welded (laminated) to a base material such as Cr-Mo steel, a martensite phase is generated at the interface. This martensite phase does not undergo austenite transformation when heated to about 600 ° C., and precipitates carbide (for example, chromium carbide) due to carburization from the base material, and the martensite phase is decomposed to change into a ferrite structure. . Even if such ferrite + carbide is heated to a high temperature (650 ° C. or more and 720 ° C. or less) during stress relief annealing, the already formed carbide does not form a solid solution in the ground phase.
In addition, since the composition of C, Cr, etc. existing in the ferritized geological phase is also reduced, the austenite transformation temperature is high, making it difficult for austenite transformation to occur. This is because martensitic transformation hardly occurs during the cooling process (hardening hardly occurs).

FIG. 1 shows the temperature of the heat treatment (retention time: 5 hours) applied before the stress relief annealing (690 ° C. × 21 hours).
4 is a graph showing a relationship between interface hardness after stress relief annealing.
The interface hardness is minimal at around 600 ° C., is 400 Hv or less at 660 ° C. or less, and 350 Hv at 640 ° C. or less.
v or less. If the interface hardness is 400 Hv or less, peeling crack resistance can be confirmed.
Since the resistance to peeling cracking is greatly improved by setting the temperature to the following, it is recommended to perform the heat treatment at 640 ° C. or lower. If the heat treatment temperature is too low, a sufficient heat treatment effect cannot be obtained, and a long holding time is required. If the holding time is too short, the ferrite and the carbide cannot be sufficiently separated, so that the holding time is preferably 1 hour or more, and more preferably 5 hours or more. On the other hand, if the holding time is too long, the strength of the base material may be reduced.
It is necessary to set the heat treatment time as short as possible, and from the viewpoint of production efficiency, the heat treatment time is preferably as short as possible, more preferably 20 hours or less.

As described above, according to the present invention, it is possible to prevent the formation of martensite structure and prevent the occurrence of peeling cracks without performing heat treatment after the stress relief annealing. Therefore, according to the present invention, heat treatment for preventing peeling cracks can be performed not on the entire pressure vessel but on individual pressure vessel members, and heat treatment can be performed only on members requiring improvement in peeling crack resistance. You can do it too. Since the heat treatment can be performed on the pressure vessel members instead of performing the heat treatment on the entire pressure vessel in this manner, the space efficiency and energy efficiency of the heat treatment furnace can be increased, and the pressure vessel members for a plurality of pressure vessels can be simultaneously formed. Since heat treatment can be performed, the construction period can be shortened. Since the method of the present invention is a method of preventing the formation of martensite by changing the crystal structure of the interface to ferrite + carbide, before the stress relief annealing, heat treatment is performed after assembling the pressure vessel member. May be.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and the present invention will be carried out with appropriate modifications within a range that can conform to the above and subsequent points. All of these are included in the technical scope of the present invention.

[0014]

EXAMPLE 1 A pressure vessel for petroleum refining using 2.25Cr-1 Mo steel as a base material, and two layers of SUS309 and SUS347 stainless steel were formed on the inner surface of the base material by band electrode submerged arc welding. Steel material (60m
mφ) was used as a test material. After performing heat treatment at various temperatures shown in Table 1 for 5 hours, stress relief annealing was performed at 690 ° C. for 21 hours, and the hardness of the interface between the stainless steel layer and the base material was measured. The hardness is measured by measuring 20 places for each sample, obtaining the maximum hardness of each sample, and averaging them. Table 1 also shows the results of the test.

[0015]

[Table 1]

If the heat treatment temperature before the stress relief annealing is 640 ° C. or less, the interface hardness after the stress relief annealing is about 350 Hv.
It turns out that it becomes as follows.

Example 2 The test material No. 1 obtained in the same manner as in Example 1 was used. 1 was used to evaluate the susceptibility to peeling cracking. The test conditions for the peeling crack test were a test temperature of 480 ° C. and a hydrogen pressure of 200 kg / cm.
^{2. The} holding time was 48 hours, and after the holding was completed, the sample was cooled to room temperature at a cooling rate of 300 ° C./hour. After leaving for 336 hours (2 weeks) after reaching room temperature, an ultrasonic flaw detection test was performed to measure the peeled area. Test material No. Table 2 shows the results obtained when heat treatment (600 ° C. × 5 hours) for preventing peeling cracks was performed before and after stress relief annealing and when heat treatment was not performed.
Shown in

In place of the submerged arc welding (SAW) of the first embodiment, gas tungsten arc welding (G
TAW). 2 and further quenching before heat treatment of stress relief annealing (960 ° C × 2.5Hr; water cooling)
Specimen material No. 2 which had been subjected to tempering and tempering (650 ° C. × 2.5 hr; air cooling). Using No. 3, the above-mentioned peel cracking test was performed. The results are shown in Table 2.

[0019]

[Table 2]

When no heat treatment for preventing peeling cracks is performed, peeling cracks occur. However, according to the method of the present invention, peeling cracks can be prevented similarly to the conventional method. I understand.

[0021]

As described above, the present invention is configured as described above, so that peeling cracks generated at the interface between the base material of the pressure vessel and the stainless steel layer can be prevented without performing heat treatment after stress relief annealing. Provision of a method became possible. Therefore, heat treatment for preventing peeling cracks can be performed not on the entire pressure vessel but on individual pressure vessel members, and heat treatment only needs to be performed on members that require improvement in peeling crack resistance. Was.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a heat treatment temperature before stress relief annealing and an interface hardness after stress relief annealing.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C21D 9/50 101 C21D 9/50 101Z

Claims (3)

[Claims] 1. A method for preventing a pressure vessel manufactured by welding a pressure vessel member having a stainless steel plate laminated on a surface of a base material and then subjecting the pressure vessel to stress relief annealing, wherein the welding includes: A method for preventing peeling cracks of a pressure vessel, comprising subjecting the pressure vessel member to a heat treatment at 560 ° C. or more and 640 ° C. or less beforehand. 2. A method for preventing a crack of a pressure vessel manufactured by welding a member for a pressure vessel in which a stainless steel plate is laminated on a surface of a base material and then performing stress relief annealing, the method comprising: A method for preventing separation cracking of a pressure vessel, comprising performing a heat treatment at 560 ° C. or more and 640 ° C. or less prior to annealing. 3. At a temperature of 650 ° C. or more and 720 ° C. or less,
The method for preventing separation cracking of a pressure vessel according to claim 1 or 2, wherein the stress relief annealing is performed by holding the pressure for 5 hours or more and 30 hours or less.