GB975202A - Improvements in process for chromizing and product - Google Patents

Abstract

In a process of chromizing ferrous material in a closed retort at 1500 - 2300 DEG F. a gas containing at least 2% by volume of a halogen, preferably in a hydrogen carrier, is continuously circulated through a source of chromium and the ferrous material, which is out of contact with the chromium source. The chromizing gas <PICT:0975202/C6-C7/1> is preferably hydrogen bromide, chromous bromide or bromine or a mixture of halides and the chromium source may be chromium or ferrochromium in the form of minus 1/4 inch plus 60 mesh pieces. The dew point of the exit gases from the retort should not be more than +5 DEG F. and preferably should be lower than -40 DEG F. The ferrous material treated should have a maximum carbon content of 0.03% and may be silicon steel, chromium and nickel stainless steel or steels stabilized with titanium, niobium, vanadium or tantalum. Prior to chromizing the steel may be decarburized in the retort by treating at 1300 - 1400 DEG F. with wet hydrogen. In one embodiment, Fig. 1, of a batch process, open coil steel strip 38 is supported by a grid 35 of plenum chamber 31 in a bell type furnace retort 54. The chromium source is supported in stacked trays 40 and the halogen gas is supplied via inlet 61 and circulated by fan 25 through the chromium and the coil, a series of baffles directing the gas through the coil in the direction of the arrows. The source of chromium may alternatively be in the form of a porous bell, Fig. 8 (not shown), or as a filling in the hollow core of the ferrous coil, Fig. 9 (not shown). In a continuous process shown in Fig. 4 steel strip or wire is continuously passed through perforated tubes 75 enclosed in a sealed retort 74 the space between the tubes being packed with the source of chromium 85. The circulating gas is supplied via inlet 96 and the treated wires are removed from plugs 92. In another embodiment, Fig. 6 <PICT:0975202/C6-C7/2> (not shown), a shaker hearth extends into the retort and conveys the workpieces into the retort for continuous treatment with the chromizing atmosphere and the treated pieces are then quenched directly in a quenching tank. In the batch type process the chromized work may be given a heat treatment in the same retort, the halogen gas being first purged and the heat treatment effected in hydrogen or other carrier gas. In the continuous process the heat treatment is effected in a separate furnace free of halogen gas or a non-chromizing flushing gas may be introduced into the discharge end of the continuous retort.