Patent ID: 6179936
Filing Date: 2001-01-30
Classification: C21D,Y02P

Abstract:
A method of quench-hardening a hollow cylindrical workpiece, said workpiece having an outside surface and an inside surface defining a wall therebetween, said wall having an outside portion defined between said outside surface and a position spaced from said outside surface by a distance greater than one fourth of a thickness of said wall and less than one half of a thickness of said wall, an inside portion defined between said inside surface and a position spaced from said inside surface by a distance less than one half of a thickness of said wall, and a core portion defined between said outside portion and said inside portion, said method comprising:first:conveying a workpiece one after another in a substantially horizontal direction without a space between adjacent workpieces,induction-heating at a heating portion an entire cross-section of said wall of said workpiece simultaneously with said conveying to a temperature equal to or higher than an Ac.sub.3 transformation temperature and equal to or lower than a temperature 200.degree. C. higher than said Ac.sub.3 transformation temperature from said outside surface of said workpiece only,making a temperature of said workpiece substantially uniform in a longitudinal direction and in a thickness direction of said workpiece by utilizing a time period required for moving said workpiece to a cooling portion spaced from said heating portion, andcooling said workpiece from said outside surface only, so that the entire cross-section of said wall of said workpiece is quench-hardened, said cooling being initiated before the temperature of said workpiece falls to an Ar.sub.3 temperature; andsecond:conveying said workpiece one after another, the entire cross-section of said wall of which has been quench-hardened, without a space between adjacent workpieces in a horizontal direction,induction-heating said outside portion of said wall of said workpiece only simultaneously with said conveying to a temperature equal to or higher than said Ac.sub.3 transformation temperature and equal to or lower than a temperature 200.degree. C. higher than said Ac.sub.3, transformation temperature from said outside surface of said workpiece only, andcooling said workpiece immediately after said heating from said outside surface of said workpiece only, so that said workpiece has an effective hardness at a first position located within said outside portion, a portion located between said first position having the effective hardness and said outside surface has a hardness greater than the effective hardness, and a portion located closer to said core portion than said first position having the effective hardness has a hardness less than the effective hardness, while said workpiece has the effective hardness at a second position located within said inside portion, a portion between said second position having the effective hardness and said inside surface has a hardness greater than the effective hardness, and a portion located closer to said core portion than said second position having the effective hardness has a hardness less than the effective hardness.