ANNEALING OF CARBON STEELS IN NONCRYOGENICALLY GENERATED NITROGEN

摘要

An improved process for producing high-moisture containing nitrogen-basedatmospheres suitable for oxide and decarburize annealing of carbon steels fromnon-cryogenically generated nitrogen is presented. These nitrogen-based atmospheres areproduced by 1) mixing non-cryogenically generated nitrogen containing less than 5.0vol.% residual oxygen with a specified amount of hydrogen, 2) humidifying the gaseousfeed mixture, 3) feeding the gaseous mixture into the heating zone of a furnacethrough a diffuser, and 4) converting in-situ the residual oxygen present in it tomoisture. According to the present invention, the total amount of hydrogen requiredfor producing suitable atmospheres can be minimized by simultaneously humidifyingthe feed gas and controlling the residual oxygen level in it. The key features of thepresent invention include a) humidifying the feed gas prior to introducing it into theheating zone of a furnace operated above about 600°C, b) selecting the level ofresidual oxygen in the feed gas in such a way that it minimizes hydrogen consumption,and c) using enough amount of hydrogen to convert completely the residual oxygenpresent in the feed gas to moisture and to maintain pH2pH2O ratio in the heatingzone of the furnace below about 2 for oxide annealing and at least 2 for decarburizeannealing carbon steels.