Abstract: A method for increasing the resistance to thermal shocks of the oxide layer of metallic heat conductive materials which contain 3% to 10% aluminum, 10% to 26% chromium, up to 3% zirconium and/or titanium and/or hafnium and/or niobium and/or silicon and/or 0.002% to 0.3% total of rare earths and/or yttrium in metallic form or as finely dispersed oxides, the remainder being iron and/or nickel and/or cobalt as well as the trace elements normally present in steels. The materials develop primarily aluminum oxide and/or chromium oxide on the surface when heated in a temperature range of 700.degree. C. to 1350.degree. C. in an oxygen-containing atmosphere. The materials are first heated in an oxygen-free atmosphere under conditions which cause recrystallization in their surface zone. They then are oxidized in an atmosphere which contains oxygen in chemically bound form.

Abstract: The invention relates to a metallic semi-finished product based on iron and/or nickel and/or cobalt and containing 2 to 16% of aluminum, 12 to 30% of chromium and at least one highly reactive element X, in particular from the group consisting of the rare earth metals and/or their dispersed oxides. The characteristic feature of the invention is that metallic columnar crystals are formed at least in the surface region of the semi-finished product and columnar crystals consisting predominantly of aluminum oxide and/or chromium oxide grow out of the said metallic columnar crystals, the axes of the two types of columnar crystals being predominantly at right angles to the geometric surface of the semi-finished product.