GB748396A - Improvements in and relating to the siliconizing of alloys containing iron - Google Patents

Abstract

Silicon-containing protective layers are formed on alloys containing Fe and at least 3 per cent Mo or W or both, by treatment with a volatile silicon-halogen compound in a non-oxidising atmosphere at a temperature above 1000 DEG C. and below the melting point of the alloy. The Mo or W may be present only in a thin surface layer produced e.g. by spraying, cladding, electrolysis, diffusion, vapour deposition or in a fused bath as described in Specification 635,595. Siliconizing gas may be formed by passing argon, helium, hydrogen, nitrogen, cracked ammonia or mixtures thereof freed from oxygen and water vapour, through liquid Si Cl4 at 0-30 DEG C. or by passing HCl vapour over ferro-silicon. Such gas may be passed over a large surface area of metallic silicon or silicides at a temperature higher than the siliconizing temperature to convert part of the Si Cl4 vapour into sub-chlorides. The treatment may be applied to steels the free carbon content of which does not exceed 0-4 per cent, the remaining carbon being bound by Ti, Nb, Ta, V, Zr, Cr, Mn or to alloys of iron with Cr, Ni, Co, Zr, Mn, Ti, Nb, Ta, V, even when the iron content is less than that of e.g. Ni or Co. Examples refer to the treatment of an Fe-Mo alloy containing 7 per cent Mo; of a steel molybdenised in a fused bath of Na2 MoO4; of a steel coated by spraying with an Fe-W alloy containing 8 per cent W; and of a steel coated with Mo by treatment with H Cl gas passed over Mo powder.