Effect of NH<SUB>3</SUB> Stepwise Increase in Gas Carbonitriding Atmosphere on Nitrogen Content of Low Carbon Steel

In conventional gascarbonitriding, voids appeared in a surface layer right after the nitrogen content reached about 0.4 mass%, and then the nitrogen content decreases rapidly. The authors have established the novel method named a retained ammonia rising method that gives higher nitrogen contents without void formation. The influence of retained ammonia rising rate in the new gascarbonitriding process on the void formation and nitrogen content was studied using pure iron and low carbon steel foil with a thickness of 0.25 mm. Wear-resistance and fatigue limits for bulk specimens that were treated by the retained ammonia rising method were studied using medium carbon steel.The critical nitrogen contents for void formation in pure iron [CNp] and in low carbon steel [CNl] were described by the logarithm equations with the variable treating times [t], respectively.(This article is not displayable. Please see full text pdf.) The rising rate of retained ammonia for void formation [R] was also described by the exponential equations with the variable CNp or CNl.(This article is not displayable. Please see full text pdf.) For example, at the treating time of 1.8 ks, by the ammonia rising method, the critical nitrogen content of pure iron foil for void formation was 0.78 mass%, that was about 1.5 times that in conventional gascarbonitriding. The critical nitrogen content of low carbon steel foil for void formation was 0.81 mass%, that was about three times that in conventional gascarbonitriding.The wear resistance of medium carbon steel treated by the ammonia rising methed was about twice that in conventional gascarbonitriding. This is due to the improvement in resistance to temper softening and also to the increase in surface hardness through martensite transformation of the retained austenite during the wear test.Fatigue limits of medium carbon steel treated by the ammonia rising method did not differ from those in conventional gascarbonitriding. The retained austenite which prevented cracks from initiation and propagation was transformed into martensite during the fatigue test.