JPH076056B2 - Method for producing austenitic material coated with high aluminum content ferritic stainless steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a clad material mainly composed of an austenitic material used in a highly corrosive atmosphere at high temperature such as molten salt.

(Prior art) In a highly corrosive environment at high temperature such as molten salt, the surface of austenitic stainless steel or high nickel alloy is impregnated with aluminum (hereinafter abbreviated as Al)
A method of improving high temperature corrosion resistance by increasing the Al concentration is often adopted.

However, Al does not easily diffuse into austenitic stainless steel, and it is difficult to form a desired intermetallic compound with nickel to obtain a desired coating with high high temperature corrosion resistance. On the other hand, although Al easily diffuses in ferritic stainless steel and forms a film with high corrosion resistance, Al diffuses from the surface to the inside during use at high temperature, and
The disadvantages are that the concentration decreases quickly and corrosion resistance is lost prematurely. In addition, ferritic stainless steel has the drawback that its high temperature strength itself is weaker than that of austenitic materials.

In order to solve these drawbacks, the present inventors have developed that the surface of austenitic stainless steel is clad with ferritic stainless steel containing high Al, and Japanese Patent Application No. 1-9970.
Filed as an issue. Since this clad keeps Al in the ferrite layer, the high temperature corrosion resistance is less deteriorated, and the composite material is excellent in high temperature strength and workability.

(Problems to be solved by the invention) However, the ferritic stainless steel containing Al is
Since the workability is inferior to that of a material not containing Al or austenitic stainless steel, work such as plastic working and welding is difficult. That is, although it is clad with a good workability austenitic material, although it depends on the contained Al layer, the clad of a high Al content steel still has a workability such as plastic working, welding, etc. It is inevitable that it will be somewhat inferior to the case.

Therefore, the present invention aims to provide a composite material that overcomes this difficulty in workability, forms a surface layer containing a high concentration of Al, and has excellent properties such as workability, corrosion resistance, and heat resistance. And

(Means for Solving the Problems) In order to achieve the object, the present invention uses a heat resistant alloy such as austenitic stainless steel as a base material, and clad a ferrite stainless steel having a thickness of 2 μm or more in the heat resistant alloy. Form an aluminum layer on the ferritic stainless steel surface of the clad material, and keep it at 800 ° C in a vacuum or an appropriate atmosphere.
By subjecting aluminum to a diffusion treatment at a temperature of up to 1200 ° C, the surface is made of a high aluminum content ferritic stainless steel having good high temperature oxidation resistance.

Here, the formation of the high aluminum layer is performed by pasting an aluminum foil on the surface of the ferritic stainless steel, spraying aluminum, immersing in an aluminum bath, and then heating in a vacuum furnace or a heating furnace in an appropriate atmosphere. , To diffuse aluminum. In addition, when forming a high aluminum layer using an aluminum foil coil, after the aluminum foil coil is overlaid and fixed on the surface of the ferritic stainless steel, an oxide film is formed in advance as a release material. Laminating thin plate coils of Al-containing chrome heat resistant steel, heating these laminated coils,
It may be peeled off after cooling.

(Function) In the present invention, a material containing high Al having poor workability is used from the beginning and is not used as a clad, and a clad of a ferritic stainless steel containing no Al and an austenitic material is first manufactured. This is processed into a predetermined shape as needed. After that, the method of diffusing Al in ferritic stainless steel is adopted. When forming a layer containing a high amount of Al in this way, although the number of steps is increased, advantages are obtained as a whole. That is, in the case of an austenitic material alone, Al does not easily diffuse and permeate, but Al diffuses quickly into the ferrite layer and easily permeates to significantly improve the oxidation resistance. This is because Al is difficult to form a solid solution in austenitic stainless steel, and the diffusion rate is slow, but it is easy to form a solid solution in ferritic stainless steel and the permeation is rapid.

In addition, a clad of a ferritic stainless steel containing no Al and an austenitic material is first manufactured, and in order to process this, the clad is subjected to plastic working such as rolling, cutting,
Welding is applied. At this time, for example, the clad undergoes thermal deformation due to the bimetallic effect, and we want to reduce the thickness of the ferrite layer in order to reduce this, but as shown in Japanese Patent Application No. 1-9970, the ferrite system of several tens of μm or less from the beginning is used. Manufacturing the stainless steel thin plate increases the rolling cost, and there is a limit to the manufacturing plate thickness. Prepare a clad material that is thicker than the target thickness,
By rolling the thick clad with good workability to the target thickness, you can easily reduce the thickness of the ferrite layer,
Then, by diffusing and permeating Al, a thin layer of high Al ferritic stainless steel having good oxidation resistance can be obtained.

In the present invention, the lower limit of the thickness of the ferritic stainless steel is 2 μm because it is difficult to manufacture a clad having a ferrite layer thinner than this, and sufficient oxidation resistance is maintained when the thickness is thinner than this. Because it is difficult to do. Also, the upper limit of the thickness of ferritic stainless steel is not particularly limited, but as the thickness of ferrite increases, the diffusion process of Al into the entire ferrite layer will take longer, and the required amount of Al In addition, it becomes difficult to make uniform penetration and diffusion. In this respect,
It is not a good idea that the ferritic stainless steel becomes too thick, and generally 200 μm or less is preferable. However, if technically possible, the thickness of the ferritic stainless steel may be more than this.

Regarding the diffusion temperature of Al, if the diffusion temperature is too low, the diffusion heat treatment time is too long even if the thickness of the ferrite layer is the lower limit of 2 μm, which is not practical, so the lower limit of the diffusion temperature was set to 800 ° C. In addition, at a high temperature exceeding 1200 ° C, heat treatment cost is required and the structure is severely deformed. Therefore, the upper limit is set to 1200 ° C.

Furthermore, although the amount of Al varies depending on the target oxidation resistance, Al is diffused and permeated by adhering only an appropriate amount of Al in a range of 3 to 30% by weight in the faylite.

Example 1 Example 1 1.5 mm thick SUS310S stainless steel and 0.08 mm (80 μm) thick
A clad with 22Cr ferritic stainless steel was prepared in advance. The clad was used to assemble the components of the published molten carbonate fuel cell. That is, the electrolyte layer 4 sandwiched between the flange 2 and the wet seal portion 3 is disposed between the bipolar sheet 1 (Ni / SUS), and the fuel gas channel 5 is formed between the bipolar sheet 1 and the wet seal portion 3. Insert the ribbed anode 6. Further, the current collector plate 7 and the cathode (NiO) 8 are incorporated into the flange 2 provided between the bipolar sheet 1 and the wet seal portion 3. In this way, the gap between the current collector 7 and the cathode 8 serves as a fuel gas channel.

Of these parts, the flange 2 that contacts the molten carbonate
The wet seal part 3 was made to have a ferritic stainless steel surface. A thickness of 35 is applied to the wet seal portion 3.
After plasma spraying by μm, vacuum heat treatment was performed at 1100 ° C. for 1 hour. As a result, the thickness of the ferritic stainless steel was about 130 μm, and the Al concentration on the surface (measured by EPMA) was about 12%.

Example 2 0.06 mm (60 μm) on both sides of 4 mm thick SUS310S stainless steel
When 22 Cr ferritic stainless steel having a large thickness was clad and further rolled to 0.3 mm, the ferritic stainless steel had a thickness of about 4.5 μm. And after processing into oil heating combustion parts by punching and pressing, Al fine powder is thinly plasma sprayed,
Furthermore, diffusion treatment was performed for 950 × 0.5 hours in a high vacuum. As a result, some unevenness and Al oxide were seen on the surface,
Most of Al was in solid solution in ferrite, and the thickness of ferrite had grown to about 10 μm. The obtained product also had very good oxidation resistance, and although it was red-hot as a burning part, the oxidation was very little even after 100 hours of use.

(Effect of the invention) As described above, in the present invention, after the ferritic stainless steel which does not contain Al and has good workability is clad with the austenitic material, Al is contained in the ferritic stainless steel on the surface. Is diffused to form a ferrite layer containing high Al. Therefore, the target shape can be easily processed, and the Al concentration in the surface layer can be maintained at a target high concentration. as a result,
The obtained product has excellent properties in all aspects such as workability, heat resistance, oxidation resistance, and corrosion resistance.

[Brief description of drawings]

The attached drawings show essential parts of a molten carbonate fuel cell as an example to which the present invention is applied. 1: Bipolar sheet 2: Flange 3: Wet seal part 4: Electrolyte layer 5: Fuel gas channel 6: Ribbed anode 7: Current collector 8: Cathode (NiO)

Claims (2)

[Claims] 1. A base material is a heat-resistant alloy such as austenitic stainless steel, and the heat-resistant alloy is clad with a ferritic stainless steel having a thickness of 2 μm or more. Or, in a suitable atmosphere, by subjecting aluminum to a diffusion treatment at a temperature of 800 ° C to 1200 ° C, the surface is made of a high aluminum content ferritic stainless steel with good high temperature oxidation resistance, and a high aluminum content. A method for producing an austenitic material coated with ferritic stainless steel. 2. The clad material according to claim 1 is a coil-shaped plate, and the manufacturing method thereof is an aluminum foil coil on a ferritic stainless steel surface, and an oxide film is formed on the outside thereof as a release material in advance. % Al-containing chromium-based heat-resistant steel sheets are stacked and wound, and the combined coil is subjected to aluminum diffusion treatment at a temperature of 800 to 1200 ° C in a vacuum or in an appropriate atmosphere to make the surface resistant to high-temperature oxidation. A method for producing an austenitic material coated with a high aluminum-containing ferritic stainless steel, which comprises forming a good high aluminum-containing ferritic stainless steel and removing the release material after cooling.