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for Metal Finishing 1989-2025

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Steel mill needs blue oxide removal
When annealing (1100 feet long continuous anneal furnace )high strength steel in Hydrogen / Nitrogen atmosphere During cooling we are encountering a surface oxidation that is sometimes objectionable. What Hydrogen to water ratio will reverse this process over a temperature range of 1200 to 400 °F during the cooling process. At this point it is more economically desirable to fix the result than to remove the cause.
Thomas R Wasilproducer of electrical and high strength steel - Warren, Ohio, USA
2003
2003
Does high strength indicate high carbon content or alloying with Cr, Ni, Mo, etc?
Neglecting alloying*, the applicable oxide reduction reaction is FeO + H2 = Fe + H2O. The Gibbs free energy is Delta G = 13,600 -- 7.75 T Joules/mole, with T in degrees Kelvin, from Introduction to Metallurgical Thermochemistry, 2nd Edn., p. 509.
From the equilibrium constant, the equilibrium p(H2)/p(H2O) ratio can be expressed as
p(H2)/p(H2O) = exp[ -1.268 + 2219/T(K)]
At 1200 F (922.0 K): p(H2)/p(H2O) = 3.12
At 1400 F (1033.15 K): p(H2)/p(H2O) = 2.41
To reduce iron oxide, use p(H2)/p(H2O) higher than the equilibrium value. However, the necessary p(H2)/p(H2O) ratio increases drastically at lower temperatures. Water vapor frequently causes blue oxide formation when cooling from 700 to 400 F. To prevent Fe3O4 formation at 400 F (477.6 K), p(H2)/p(H2O) > 50.8 is required. *Alloying elements which preferentially oxidize -- Cr, Mn and Si -- may further raise the necessary p(H2)/p(H2O). The The Making, Shaping and Treating of Steel [on Amazon affil links], 9th Edn., p. 1111, mentions that bright annealing commonly uses a 75% H2- 25% N2 gas produced by cracking anhydrous ammonia.
Potential problems: In a strongly reducing H2-N2 atmosphere, decarburizing of high C steel or even nitriding of alloy steel may occur. Hydrogen embrittlement may also be a problem. And of course, hydrogen is flammable and explosive. Is it burned off exiting the furnace? Perhaps Lee Gearhart or another heat treatment expert would care to comment.
Ken Vlach [deceased]- Goleta, California

Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
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