Magnet assembly for dental magnetic attachment

1. A magnet assembly for a dental magnetic attachment comprising: a cup-shaped yoke of substantially Ni-free ferritic stainless steel, a permanent magnet received in a recess of said cup-shaped yoke, a seal member sealing an opening of said cup-shaped yoke, and an abutment-welded portion of said seal member and said cup-shaped yoke; said seal member integrally comprising:

a center portion of substantially Ni-free ferritic stainless steel;

an intermediate portion outside said center portion, which is made of substantially Ni-free austenitic stainless steel having a nitrogen content of 0.5-2.0% by mass; and

a peripheral portion outside said intermediate portion, which is made of substantially Ni-free ferritic stainless steel having a nitrogen content of 1.3% or less by mass, wherein the nitrogen content in said peripheral portion is smaller than that in said intermediate portion,

wherein said cup-shaped yoke and said seal member have a Ni content of 0.2% or less by mass, and

wherein the abutment-welded portion welds said peripheral portion of said seal member to said cup-shaped yoke.

2. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein a center of said welded portion is deviated from an abutment boundary of said seal member and said cup-shaped yoke toward said cup-shaped yoke.

3. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein said cup-shaped yoke and said seal member have a Cr content of 17-32% by mass.

4. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the average radial length of said peripheral portion is 20-80% of the radial length of said welded portion.

5. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the average radial length of said peripheral portion is 50-400 μm.

6. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the average radial length of said intermediate portion is 50-400 μm.

7. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the average radial length of said intermediate portion is larger than the average radial length of said peripheral portion.

8. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein part of the austenitic stainless steel of said intermediate portion is transformed to a ferrite phase or a mixed phase of ferrite and Cr nitride by heating, in a boundary of the austenitic stainless steel of said intermediate portion and the ferritic stainless steel of said center portion.

9. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein said seal member is obtained by cutting a round rod to a disc, the round rod integrally having a core portion of ferritic stainless steel, an intermediate portion of austenitic stainless steel, and a surface layer portion of ferritic stainless steel.

10. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the nitrogen content in said intermediate portion is 1-2.0% by mass.

11. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the nitrogen content in said intermediate portion is 1.5-2.0% by mass.

12. The magnet assembly for a dental magnetic attachment according to claim 1 , wherein the nitrogen content in said intermediate portion is greater than 1% by mass and less than or equal to 2.0% by mass.

13. The magnet assembly for a dental magnetic attachment according to claim 2 , wherein the center of the welded portion is deviated by 20 μm from the abutment boundary of said seal member and said cup-shaped yoke toward said cup-shaped yoke.

14. A method for producing a magnet assembly for a dental magnetic attachment comprising: a cup-shaped yoke of substantially Ni-free ferritic stainless steel, a permanent magnet received in a recess of said cup-shaped yoke, a seal member sealing an opening of said cup-shaped yoke, and an abutment-welded portion of said seal member and said cup-shaped yoke; said seal member integrally comprising:

a center portion of substantially Ni-free ferritic stainless steel;

an intermediate portion outside said center portion, which is made of substantially Ni-free austenitic stainless steel having a nitrogen content of 0.5-2.0% by mass; and

a peripheral portion outside said intermediate portion, which is made of substantially Ni-free ferritic stainless steel having a nitrogen content of 1.3% or less by mass, wherein the nitrogen content in said peripheral portion is smaller than that in said intermediate portion,

wherein said cup-shaped yoke and said seal member have a Ni content of 0.2% or less by mass, and

wherein the abutment-welded portion welds said peripheral portion of said seal member to said cup-shaped yoke, comprising the steps of placing the permanent magnet in the recess of the cup-shaped yoke of substantially Ni-free ferritic stainless steel, inserting the disc-shaped seal member into an opening of the recess, and welding an abutment portion of the cup-shaped yoke and the seal member,

wherein the seal member is obtained by subjecting a round rod of substantially Ni-free ferritic stainless steel to a nitrogen-dissolving treatment to austenize a surface layer of the round rod to a predetermined depth, denitriding a surface portion of the austenized portion for ferritization, and then transversely cutting the round rod to a disc shape.