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1 Zeszyty Naukowe. Mechanika / Politechnika Opolska

1 2001

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Degradation processes in ammonia synthesis conditions

Číhal V., Stránský K.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

2001

z. 72

67-72

Nitriding by ammonia synthesis gas causes a modification of the carbides, a decarburization and sensitization to hydrogen corrosion within a relatively thin metal layer near to the inner surface of the converters. Subsequently a sensitization to hydrogen attack occurs due to the reaction of diffusing hydrogen with carbon released by the carbides and a decar-burization by carbon diffusion to the surface. High local internal stresses perpendicular to the surface caused by surface nitriding lead to small inter-granular separations parallel to the surface without any significant effect on the integrity of the pressure vessel. As the nitrided layer hinders further nitriding of the steel, this phenomen is without any long-term consequence. On the other hand, if sufficiently high load and/or residual stresses apply in the axial or tangential direction to the vessel wall, cracks have to be expected propagating slowly across the wall, as crack growth is controlled by diffusion of nitrogen and carbide transformation. Consequently, the Nelson diagram for NH3-containing synthesis gas should be applied with only great care. Efforts are necessary to quantify as well the critical stress level for crack initiation, as the propagation rate of such cracks is controlled by nitrogen diffusion and the complex carbide reactions finish by chemically induced hydrogen cracking - hydrogen corrosion.