Ceramic tubemill roll assembly

A roll assembly for use in the production of a thin-wall heat exchanger tube using a high-frequency induction welding tubemill, the roll assembly comprising:

• a roll having an arcuate profile formed on its periphery that defines one-half of a desired cross-sectional shape of the tube produced by the tubemill, the roll being formed entirely of a ceramic material chosen from the group consisting of silicon nitride and YSZ, the roll having an axial bore formed therein;
• and
• a shaft received in the axial bore of the roll such that the shaft bears against the surface of the axial bore, the shaft being formed entirely of an alumina material, such that the shaft has a composition that differs from that of the roll.

A high-frequency induction welding tubemill for the production of a thin-wall heat exchanger tube, the tubemill comprising a pair of roll assemblies, each of the pair of roll assemblies comprising:

• a roll portion having an axial bore formed therein, the roll portion being formed entirely of a first ceramic-based material;
• and
• a shaft member received in the axial bore of the roll portion such that a surface portion of the member is a bearing surface that bears against the surface of the axial bore, the shaft member being formed entirely of a second ceramic-based material having a composition that differs from the first ceramic-based material of the roll portion.