Surface hardening of steel by nitriding and then chromising - esp. to obtain tough surface in bore of rifled for machine guns or aircraft cannon

摘要

Stell is nitrided (I) at 500-570 degrees C., and is then chromised (II) at a suitable temp., cooled, washed, and next reheated to more than 25-30 degrees C above the Ac3 point and cooled in air. Coating (II) is pref. obtd. at 850-1050 degrees C when the steel contains 0.8-2% C. Pref. steel has compsn. 0.5-1.5% Cr, 0.15-0.25% Mo, 0-3.5% Ni, and max. 1.2% Al. Coatings (I, II) are pref. applied to the bore of a tube suspended in a furnace with a protective atmos.; and after final heating and air cooling, the steel pref. has a sorbitic or troostitic structure. Steel components pref. include sharp edges and are used esp. in cannons. Combined coating is much tougher than the individual coatings, so the sharp edges in rifled gun barrels are not easily damaged.