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Choosing the right material
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The material of which the sphere would be made had to meet the following conditions :
l. Great constitutional strength and high tenacity against external stress
2. Resistance to rust and corrosion
3. Stable structural and chemical characteristics over a wide range of ambient temperatures
4. Resistance to stress during welding
5. Ideally the material should have been developed in Japan.
After exhaustive studies of the various materials available, a special stainless steel designated NTK-22AT was chosen for the capsule. NTK-22AT is an austenitic stainless steel containing 22% nickel, 20% chromium and very little carbon. A titanium additive inhibits progressive crystalline phase transformation caused by the diffusion of carbon. In other words, NTK-22AT is a high-grade alloy which is both extremely strong and highly resistant to corrosion. However, NTK-22AT was developed originally as a material suitable for forming into rods and plates. Its practical applications include artificial heart valvcs, pins and plates for use in orthopaedic surgery and components for high-quality watches.
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In the case of the capsule, the metal would need to be formed into a large, hollow sphere. |
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Although NTK-22AT is known to be stable during mechanical processing- that is, forming and bending-it was decided to keep processing to a minimum and manufacture the capsule by casting. This decision presented many difficult technical problems in view of the size and shape of the container and the need to preserve the characteristics of the alloy. Ultimately, the technical committee and their advisers developed a special method of reverse-casting which proved to be entirely successful. A test capsule was cast in April, 1969, and the chemical composition of the cast body was as follows:
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Sealing and welding
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Many of the objects destined to go into the capsule were made of materials that deteriorate progressively in the presence of oxygen and carbon dioxide. For this reason, air would be extracted from the interior of the sealed capsule and replaced with argon gas. The retention of this inert replacement atmosphere was one factor in determining the best method of sealing the capsule; the other factor was the prevention of corrosion.
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NTK-22AT stainless steel is known to be highly resistant to the dispersion of carbon. Clearly it was necessary to find a method of sealing the capsule which would have equally good characteristics in this respect. Many conventional methods were unsuitable: plastic packing, for example, is highly suspect as a source of carbon. Metal packing may produce intra-crystalline corrosion. Both types of packing may set up stress and metal fatigue. Bolts are also unreliable due to the possibility of corrosion and metal fatigue. |
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After careful consideration of the long and short-term efficiency of several different materials and methods, it was decided to seal both inner and outer lids by electric welding. To facilitate welding and eventual remove of the lids, both lids were formed with a flange and groove-welded. The chemical composition of the electrode was similar to that of NTK-22AT stainless steel. |