GB727567A - Improvements in or relating to the surface treatment of metals - Google Patents

Abstract

Ferrous articles are chromized by packing in a mixture, preferably granular, of ferro-chromium and an inert, porous, refractory material impregnated with anhydrous chromous chloride in an airtight retort, and heating whilst passing therethrough gaseous hydrogen chloride, at least at the commencement of the chromizing, and hydrogen. Impregnation of the refractory material may be effected by passing a stream of hydrogen chloride for 2-4 hours through the mixture at 1000-1100 DEG C. and then cooling under reducing conditions; during the heating up to reaction temperature a hydrogen atmosphere should be maintained. The refractory may be sillimanite or calcined bauxite. Air may be flushed from the retort by passing hydrogen until the temperature reaches 450 DEG C. when the supply is reduced to just sufficient to maintain reducing conditions; hydrogen chloride is passed through the retort whilst the temperature rises from 1000 DEG to 1050 DEG C. and thereafter cut off or greatly reduced; chromizing continues at 1050 DEG to 1150 DEG C. for 8-2 hours; cooling takes place in the presence of hydrogen, but without hydrogen flow through the retort. The ferrous articles may be formed of alloys containing C up to 0.5 per cent; titanium is also preferably present, the Ti:C ration being 4:1 to 5:1, alloys containing C 0.1 per cent and Ti O.5 per cent, and C 0.2 per cent and Ti 1.0 per cent being referred to; Mn up to 3.5 per cent, Mo up to 1.0 per cent, Ni up to 5 per cent and V up to 0.5 per cent may also be present. Chromized titanium-containing alloys may be nitrided at 450-550 DEG C. in anhydrous ammonia, or coated with aluminium by painting, heating at 600-800 DEG C. in aluminium powder or by spraying. Other alloys referred to, which may be chromized, contain carbon, silicon, sulphur, phosphorus and (a) Mn 0.31 per cent, Ni 4.62 per cent, Cr 0.08 per cent, Mo 0.09 per cent; (b) Mn 3.70 per cent, Ni 0.43 per cent, Cr 0.08 per cent; (c) Mn 0.42 per cent, Cu 0.98 per cent; (d) Cr 0.86 per cent, V 0.12 per cent; (e) Mn 0.95 per cent, Ni 8.2 per cent, Cr 18.8 per cent; (f) Mn 0.45 per cent, Ni 0.4 per cent, Cr 1.3 per cent, Mo 0.25 per cent, V 0.15 per cent, W 2.1 per cent. Articles referred to comprise hydraulic shafts, rams and spindles. Specification 617,849 also is referred to.