Tempering method using a two phase flow of cryogen

Abstract

A method of tempering an article is disclosed in which an article is heated so that the material making up the article is in an essentially stress-free state and the article thereafter is quenched and cooled. At the conclusion of the quenching, a stress pattern in initiated through the thickness of the article in which the midplane layer is in tension and the surface layers are in compression. This stress pattern is further developed during the cooling stage. The article is quenched by spraying the surfaces with a coolant, preferably comprising either liquid nitrogen or liquid air in the form of a two phase flow of liquid and vapor. The liquid is sprayed so that the surface tensile stress of the surface layers are not exceeded and the liquid does not accumulate on the surface layers as a film.

Dependencies are inferred from the claim text.