JPS5848016B2 - Solution treatment method for austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solution treatment method for austenitic stainless steel, and particularly to a solution treatment method capable of locally treating sensitized regions.

Products using austenitic stainless steel are widely used in nuclear power and chemical plant equipment, etc. However, since these products are made by welding, welding creates sensitized areas, and therefore, if used while welded, they may become sensitive. Since cracks occur in the oxidized region, solution annealing is usually performed.

In the conventional solution treatment method, the product is put into a heating furnace either as a whole or in parts, and the whole product is heated and maintained at the solution temperature, and then rapidly cooled.

However, such solution treatment methods cannot be applied to large single products that cannot be divided, products that deform when heated as a whole, or plant products after on-site assembly. In some cases, it has been necessary to use the product without solution treatment.

In order to overcome the above-mentioned situation, the present invention aims to provide a solution treatment method that allows local treatment without the need to heat the entire product, including the sensitized area or its vicinity. A solution treatment process in which a region is rapidly heated to a solution temperature, held at this solution temperature for a time shorter than the time the sensitized region was exposed to the sensitization temperature, and then rapidly cooled is performed over a constant heating range. It is characterized by being carried out in a state.

The present invention was achieved by the inventors as a result of the following ideas regarding the sensitization of austenitic stainless steel by heating and the solution phenomenon of the sensitized region.

Regarding the solid-liquid and precipitation of carbon contained in austenitic stainless steel, the results shown in FIGS. 1, 2, and 3 have been reported.

FIG. 1 shows the relationship between sensitization temperature ('G) and retention time (hr) using carbon content as a parameter, where A indicates the sensitized region and B indicates the non-sensitized region.

As is clear from this characteristic curve diagram, for example, austenitic stainless steel with a carbon content of 0.07 to 0.05% becomes sensitized in 5 to 30 minutes when held at 650 to 700°C, but carbon It is found that it takes several hours or more to become sensitized when the amount is less than 0.03%.

Figure 2 shows the relationship between carbon content (%) and solution temperature ('C). Region C is a region without carbide, region D is a region where a small amount of carbide exists, and region E is a region in the base. It shows a region with carbides, indicating that the solution temperature increases as the carbon content increases; for example, austenitic stainless steel with a carbon content of 0.02 to 0.08% has a temperature of 900 to 1100%. This shows that it becomes a solution when kept above ℃.

Figure 3 shows the solution treatment time (minutes) and the amount of dissolved carbides (%) using temperature as a parameter.
It can be seen that if the temperature is maintained at 00°C, almost equilibrium is reached in 1 to 30 minutes.

Therefore, as in the past, for example, when the entire product including the sensitized area is held at the solution temperature by welding, the entire product will naturally be exposed to the liquefaction temperature, but the sensitized area will also be included. If you are attempting to solutionize only areas that include or are close to sensitized areas of the product, it is important to avoid creating sensitized areas within the heating boundaries, since heating boundaries will occur if these areas are held at the solution temperature. become.

That is, FIG. 4 shows welded steel plates, where 1 is a steel plate;
2 is the weld metal, 3 is the weld boundary, and 4 is the sensitized area caused by welding.

Incidentally, when austenitic stainless steel plates are welded, the sensitized region 4 occurs within about LOmyn from the weld boundary 3, and its width is about several mm.

When only the sensitized region 4 of the steel plate 1 is locally maintained at the solution temperature, the temperature distribution becomes, for example, a distribution similar to that shown in FIG. 5.

In this figure, the horizontal axis shows the distance from the heating center in the axial direction of the steel pipe, and the vertical axis shows the temperature (°C) of the steel pipe.

In the figure, if L represents the part of the weld metal 2 and the sensitized region 4 that is to be solutionized and is locally heated, that is, the heating width, then the temperature is There will be a heating boundary L2 due to the distribution, thus creating a portion L2 within the heating boundary that is exposed to the sensitization temperature.

That is, to solutionize the sensitized area caused by welding,
Sensitization of non-sensitized areas becomes inevitable.

The present inventors solved the problem of how to reduce the sensitization caused by this inevitable solutionization by considering the above-mentioned figures 1 to 3, and as is clear from figures 2 and 3,
The time to be maintained at the solution temperature was regulated based on the fact that the solution phenomenon occurs relatively faster than the sensitization phenomenon.

That is, if the time to be held at the solution temperature is shorter than the time the sensitized region was exposed to the sensitization temperature, if an equal or longer time is used, This is because, although the sensitized region undergoes solution treatment, a sensitized region equal to or greater than the non-sensitized region of the heating end result is generated.

Note that the time during which the sensitized region is exposed to the sensitization temperature means, for example, the time during which each layer is exposed to the sensitization temperature during welding in multiple welding, respectively, 1, 12, . . .
....tn means Σtn.

Furthermore, in the present invention, it goes without saying that the reason why the product is rapidly heated to the solution temperature and rapidly cooled from the solution temperature is to avoid sensitization.

Furthermore, when performing local heating, it is effective to increase the temperature gradient in order to reduce the effect on the heating boundary, and furthermore, it is effective to increase the temperature gradient in order to reduce the effect on the heating boundary, and furthermore, it is effective to apply local solution treatment to areas where sensitized regions occur due to unavoidable solution treatment. It is also effective to select a heating width so that it is applied to the strong parts of the product to be heated.

Examples will be described below with reference to the drawings.

FIG. 6 shows a heating and cooling device for performing the solution treatment method of the present invention on austenitic stainless steel pipes.
Heating is performed by supplying power from a heating power source 7 to a heating coil 6 wound around the outer periphery of a portion of the steel pipe 5 that includes the sensitized region 4 .

Cooling is performed from the inner and outer surfaces of the tube. Blind lids 8a and 8b are set at both ends of the steel pipe 5, and one blind lid 8a has a tube inner cooling water nozzle 9, and the other blind lid 8b has a tube inner cooling water nozzle. A water discharge nozzle 10 is provided, and the outer surface is cooled by being supplied from a tube outer surface cooling water supply nozzle 11 and discharged from a tube outer surface cooling water discharge nozzle 12, and the cooling water is supplied and drained by a cooling water supply and discharge equipment 13. .

The blind lids 8a and 8b are provided with an inert gas supply nozzle 14 and an inert gas discharge nozzle 15, respectively, and inert gas is supplied from the inert gas supply equipment 16 and purged into the steel pipe 5. It is then released into the atmosphere.

When solution treatment is performed using such a device, the steel tube 5 is rapidly heated to the solution temperature by the heating coil 6 while inert gas is purged into the tube to prevent oxidation inside the tube. Hold for a predetermined time.

The heating at this time is performed in a constant heating range, and at the same time as the heating by the heating coil 6 is stopped, cooling water is supplied to the tube outer surface cooling device and the tube inner surface cooling device to perform rapid cooling.

Such local solution treatment is applied to SUS. 304.2B
．． Sce. TIG (Tungus) 80 steel pipes
An example of butt-welded piping will be described.

In this welding, the sensitization temperature (500-8000C)
It was held for 3 minutes.

This piping was heated to 1100°C at a heating rate of 40°C/second, held at 1100°C for 30 seconds, and then cooled at a cooling rate of 500°C/second.
It cooled down in seconds.

As a result of this treatment, the weld sensitization in the weld zone disappeared, and at the same time some sensitization occurred at the heated boundary, but it was minor compared to the weld sensitization before solution treatment.

Figures 7a and b show metallurgical micrographs of the structure before and after treatment in the weld sensitized region. This indicates that thick carbides are precipitated.

On the other hand, FIG. 7b shows a structure that has undergone local solution treatment, and the grain boundaries are straight and thin, indicating that solution treatment has occurred.

Also, SUS. 304. 3B Sch. Almost the same results were obtained when the same type of treatment was performed on No. 80.

FIG. 8 shows a heating and cooling device for carrying out a method for making the solution treatment more effective. As shown in FIG. 5, in this method, when the weld sensitized region is maintained at the solution temperature Since there will always be a heating boundary exposed to the sensitizing temperature in the non-sensitized area of the welded product, the temperature gradient of the heating boundary (17 in Figure 8 indicates the heating boundary) will be sharpened to remove the sensitized area. The aim is to reduce the

Therefore, in this case, in addition to the external cooling device shown in FIG. 6, an external cooling device is provided at both ends of the heating coil, and the cooling water supply nozzle 18 during heating and the cooling water discharge nozzle 19 during heating are configured. has been done.

When solution treatment is performed using this apparatus, cooling water is supplied from the heating cooling water supply nozzle 18 while the steel pipe 5 is being subjected to the solution treatment.

When such an operation is performed, the width of the heating boundary portion of the temperature distribution becomes L4, as shown in M in FIG. 9, and the width exposed to the sensitization temperature becomes L5, and the width of the heating boundary portion of the temperature distribution becomes L5, as shown by the dotted line in the device of FIG. 6. If the width is N, the width can be made narrower than N.

Furthermore, in the solution annealing method of the present invention, it is not completely avoidable that non-sensitized areas become sensitized by solution annealing, but it is possible to sensitize non-sensitized areas by solution annealing. If there is a strong part, such as a part, if the heating width for solutionization is selected so that the heating boundary is located at that part, the effect on the product can be minimized.

As is clear from the above description, the solution treatment method for austenitic stainless steel of the present invention enables localized solution treatment, thereby causing problems such as deformation due to heating of the entire large single product made of austenitic stainless steel that cannot be divided. This enables solution treatment of products that will become products and plant products after on-site installation, and the industrial effects obtained are significant.

[Brief explanation of the drawing]

Figure 1 is a characteristic curve diagram showing the relationship between solution temperature and sensitization time of austenitic stainless steel, Figure 2 is a characteristic curve diagram showing the relationship between carbon content and solution temperature, and Figure 3 is the same. A characteristic curve diagram showing the relationship between the solution treatment time and the amount of dissolved carbide, FIG. 4 is a cross-sectional view showing the sensitized region generated by welding, and FIG. 5 is an axial temperature distribution diagram in the solution treatment of the present invention. FIG. 6 is a partial cross-sectional front view of an apparatus for carrying out an embodiment of the solution treatment method of the present invention, FIG. 7 is a metallurgical microscope photograph before and after the solution treatment of the present invention, and FIG. FIG. 9 is a partially sectional front view of the apparatus for implementing the embodiment, and is an axial temperature distribution diagram when the apparatus of FIG. 8 is used.

Claims (1)

[Scope of Claims] 1. A region including the sensitized region or its vicinity is rapidly heated to a solution temperature, and the sensitized region is heated to the solution temperature for a period longer than the time during which the sensitized region was exposed to the sensitization temperature. A solution treatment method for austenitic stainless steel, which is characterized by holding the stainless steel for a short period of time and then rapidly cooling it in a constant heating range. 2. When a region including a sensitized region or its vicinity is maintained at a solution temperature, the temperature distribution in the sensitization temperature range at the heating boundary is a dog. Claim 1
Solution treatment method for austenitic stainless steel as described in .