GB1246498A - Heat treatment of alloy coated articles - Google Patents

Abstract

1,246,498. Heat treatment of alloy coated metal articles. E.I. DU PONT DE NEMOURS & CO. Dec. 24, 1968 [Dec. 27, 1967], No.61489/68. Headings C7F and C7N An alloy coated metal substrate is heat treated in a melt comprising a molten metal transfer agent such as Ca, Ba, Sr, at a temperature between 900‹ C. and the substrate melting point for a time to allow carbon to migrate into the melt without allowing further alloying, and then quenching. Heat treating may be effected by dipping in the fused bath containing 10-99% Ca, Ba or Sr preferably at least 50% or by coating with a solidified melt such as drag out from the coating bath and then heating in a non oxidizing or inert atmosphere to fuse the solidified melt. The melt in addition to Ca, Ba or Sr may contain diluents such as Cu, Pb, Sn and the nitride of Ca, Ba or Sr and also diffusion elements such as Cr, Ni, Mn, Co, Zn, Mo, Nb, Si, Al. The melt containing the diffusion elements may have been used to form the initial alloy coating but retreatment time in this melt is insufficient to allow further alloying e. g. 60-120 secs. at 925- 1000‹ C. or 10-30 secs. 1000-1100‹ C. The quenching may be effected in liquid Na optionally containing K, Cs or Rb, or in a liquid comprising a hydrocarbon, silicone oil, water or brine, or in a gas or fluidized bed. The examples relate to the coating of steel substrates in a molten bath containing Ca, Ni, Al and Cr to form an alloy coating containing 26% Cr, 3% Ni, 0À3% Al. The coated articles are then heat treated in the same bath, both coating and heat treating being effected in an inert atmosphere. Alternatively the coating may be by a gas diffusion process. The heat treatment removes stains, increases the corrosion resistance and reduces the "orange peel" effect on deformation.