Heat transfer coefficients during quenching of inconel and AISI 304 stainless steel cylinders in NaNO2aqueous solutions

In order to reduce distortion and cracking when quenching in water, uniform cooling must be promoted while avoiding full-film boiling. The formation of a vapor blanket may be prevented either by imposing severe agitation or by quenching in aqueous salt solutions. Furthermore, with the addition of the optimum salt concentrations, the cooling rate may be favorably increased. In this work, the cooling characteristics of round-bars of Inconel (ø12.6 mm) and AISI 304 stainless steel (8, 12 and 20 mm in diameter) heated to 900°C and quenched by immersion into aqueous solutions of 2, 3, 4 and 9% sodium nitrite (NaNO ₂) were studied. Time-temperature cooling curves were obtained using embedded thermocouples within the probes and the heat transfer coefficients and cooling rates were calculated. It was found that an increase of NaNO ₂ concentration beyond 4% did not result in a corresponding increase in the cooling ability of the quenchant.