Premature Failure of a Type 309 Stainless Steel Pan for a Lead Bath

Severely reduced wall thickness was encountered at the liquid line of a lead-bath pan that was used in a continuous strip or wire oil-tempering unit. Replacement of the pan was necessary after six months of service. The pan, 6.9 m (22.5 ft) long, 0.6 m (2 ft) wide, and 38 cm (15 in.) deep with a 2.5-cm (1-in.) wall thickness, was a type 309 stainless steel weldment. Operating temperatures of the lead bath in the pan ranged from 805 deg C (1480 deg F) at the entry end to 845 deg C (1550 deg F) at the exit end. Analysis (visual inspection. metallographic analysis, moisture testing, and etched micrographs using Murakami's reagent) supported the conclusions that thinning of the pan walls at the surface of the molten lead resulted from using coke of high moisture content and from the low fluctuating coke level. Recommendations included reducing the supply of oxygen attacking the grain boundaries and the hydrogen that readily promoted decarburization with the use of dry (2 to 3% moisture content) coke. Maintaining a thick layer of coke over the entire surface of molten lead in the pan would exclude atmospheric oxygen from the grain boundaries.