Improvement in ion nitriding elongated hollow steel bodies.

摘要

1. Method for ionic nitriding of an elongate hollow body of steel with variable internal diameter, using a hollow central anode conveying the gaseous nitriding mixture, perforated with orifices, and introduced into the said hollow body which is located in an enclosure subjected to a reduced and controlled pressure, this method being usable for hollow bobies subject to internal stresses, being intended to relieve, just before nitriding, the stresses in the hollow bodies to be treated, and being characterized by the following seven stages in succession : a) setting the enclosure containing the hollow body or bobies and its or their anodes under vacuum ; b) pickling under very reduced pressure, lower than 0.8 millibars, accompanied by a slight rise in temperature, less than 100 degrees C, with the use of a gaseous mixture of nitrogen and hydrogen containing between 1 % and 4 % of nitrogen by volume ; c) raising the temperature to 430 degrees C to 540 degrees C, accompanied by a progressive rise in pressure to the process pressure of 2.5 to 10 millibars and with the same gaseous mixture as during picking (b) ; d) maintenance at a constant temperature of between 430 degrees C and 540 degrees C for a duration of between one hour and six hours, with the gaseous mixture described hereinbefore at (b) always under a pressure of between 2.5 and 10 millibars ; e) further raising of the temperature, from 430 degrees C-540 degrees C to 460 degrees C-570 degrees C, with a gaseous mixture of nitrogen and hydrogen containing between 10 % and 15 % of nitrogen by volume ; f) maintenance at a maximum constant temperature of 460 degrees C to 570 degrees C for duration of 25 to 70 hours, and under the process pressure of 2.5 to 10 millibars, with the gaseous mixture described hereinbefore at (e) ; g) final cooling under vacuum.