Inhibition of nitrogen uptake by SiO 2 surface films formed on stainless steel during annealing in H 2 /N 2 atmospheres
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I. INTRODUCTION DEFORMATION processes such as drawing, stamping, or bending create microstructural stresses in stainless steels that make further working difficult or impossible. Heating the metal to a high temperature in a protective atmosphere relieves these stresses. This process is called bright annealing because the steel surface of acceptable parts does not discolor by reacting with the gas atmosphere. Stainless steels were traditionally bright annealed in pure hydrogen or dissociated ammonia atmospheres, but these have been displaced in recent years by more economical H2/N 2 blends with low hydrogen contents. However, H2/N 2 blends suffer from two drawbacks: they can lead to nitride formation and discoloration. When these N2-rich blends are very dry, m stainless steels absorb unacceptably high levels of nitrogen during annealing. Nitrogen adsorbs on the metal surface and dissociates into nitrogen atoms, which diffuse into the bulk of the metal at annealing temperatures. I2j The dissolved nitrogen reacts to form nitrides with the components of the stainless steel, primarily chromium, either at annealing temperatures or during cool-down of the metal part. The nitrides precipitate along the grain boundaries, 131 sensitizing the steel to intergranular corrosion. Another problem encountered in these atmospheres is loss in brightness. Because of the low levels of H 2, small amounts of O2 and H20, ubiquitous impurities in typical refractory-lined industrial furnaces, react with metals in the stainless steel to form dull oxide films. These films are advantageous because they act as a barrier to nitrogen uptake, t4'51 However, costly cleaning procedures after heat treatment are then required to brighten the surface of the metal part. Appropriate control of the composition of the annealing atmosphere can solve the problems of discoloration and nitride formation. Discoloration is due primarily to formation of dulling Cr20 3 films, since chromium is the most easily J.F. KIRNER and E.J. KARWACKI, Principal Research Chemists, M. R. ANEWALT, Senior Research Chemist, and A. L. CABRERA, Principal Research Physicist, are with Air Products and Chemicals, Inc., Allentown, PA 18195. Manuscript submitted March 11, 1988,
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oxidized element of the major components of stainless steels. Therefore, one can prevent dulling by maintaining the H20/H2 molar ratio at a level thermodynamically reducing to chromium in the alloy. Ellison et al. calculated the maximum permissible H20/H2 ratios for an 18 pct Cr alloy at typical annealing temperatures. [61 On the other hand, low levels of H20 can still be beneficial for controlling nitride formation. At H20/H2 levels reducing to chromium, H20 can inhibit nitrogen absorption. Introducing other oxygen-containing compounds such as NzO or CO2 into the atmosphere also controls nitrogen absorption. I71 However, unlike the oxidant levels required to prevent excess oxidation, those required to inhibit nitrogen absorption have not been defined thermodynamically. Until now, the
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