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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 Wasil
producer of electrical and high strength steel - Warren, Ohio, USA
2003

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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 on Amazonaffil 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
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.