GB787958A - Improvements in methods of forming superficial diffusion alloys on metal pieces and especially refractory metal pieces - Google Patents

Abstract

Chromizing of parts made from an alloy based on Fe and/or Ni and further containing a metal imparting refractory properties viz Cr, Mo, Co, Al, Ti, or Mn, is effected by a halide diffusion process followed by a reheating in a non-chromizing atmosphere to cause Cr introduced into piece to diffuse towards the inner layers. After the chromizing step the halide vapours in the container can be condensed away from the parts so that the rediffusion step can be performed in the same container. The parts preferably are heated before the condensation step to draw off any remnants of liquid or solid halide. The part may further be treated by quenching in an oxidizing bath and briefly heated in a fluorine containing reducing atmosphere at a temperature higher than that necessary to reduce the oxide layer but lower than that at which substantial metal diffusion will take place. The parts may be mechanically shaped before chromizing and then the chromium introduced to produce an alloy which could not have been worked to the shape of the part initially. Before chromizing, any of the metals which form precipitates in the mass of the part can be oxidized out of the part by electro-thermal sodium scouring or sodium scouring or sodium hydride scouring. Silicon may be introduced into the part during the treatment to prevent carbide precipitation.