Sub-zero treatment for altering microstructure of materials - esp for metals polyurethane and nylon involves controlled cooling to avoid cracking, maintaining at the low temp, then warming to r.t

摘要

The microstructure of a non-textile matl. is altered by a cold-treatment, the matl. being cooled from normal temp. to a very low temp. using a speed which is not so drastic that fissures or cracks occur, held for a prescribed time at the low temp. and then restored to room temp. The matl. is pref. held 18-30 hrs. at the low temp., esp. at-196 degrees C, and it may be cooled in 2 or 3 stages to a temp. above or below the threshold temp. or the 2 threshold temps. at which cracks form. A pref. cooling velocity is 1.56-2.22 degrees C/minute to 5.94-8.33 degrees C/minute. The matl. is pref. metal, nylon, or polyurethane. Esp. for metals, for the conversion of iron alloys from an austenitic structure to a martensitic structure, e.g. to improve wear-resistance. A pref. method of cooling consists of the gradual immersion of the matl. in a cold liq., esp. liq nitrogen.