EFFECT OF NH3 STEPWISE INCREASE IN GAS CA RBONITRIDING ATMOSPHERE ON NITROGEN-CONTENT OF LOW-CARBON STEEL

In conventional gascarbonitriding, voids appeared in a surface layer r ight after the nitrogen content reached about 0.4 mass%, and then the nitrogen content decreases rapidly. The authors have established the n ovel method named a retained ammonia rising method that gives higher n itrogen contents without void formation. The influence of retained amm onia rising rate in the new gascarbonitriding process on the void form ation and nitrogen content was studied using pure iron and low carbon steel foil with a thickness of 0.25 mm. Wear-resistance and fatigue li mits for bulk specimens that were treated by the retained ammonia risi ng method were studied using medium carbon steel. The critical nitroge n contents for void formation in pure iron [C(Np)] and in low carbon s teel [C(Nl)] were described by the logarithm equations with the variab le treating times [t], respectively. Pure iron: C(Np) = 1.46 - 0.16 lo g t Low carbon steel: C(N)] = 1.49 - 0.16 log t The rising rate of ret ained ammonia for void formation [R] was also described by the exponen tial equations with the variable C(Np) or C(Nl). Pure iron: R(p) = 2.2 2 x 10(-8) x exp (9.95C(Np)) Low carbon steel: R(l) = 3.78 x 10(-10) x exp (14.4C(Nl)) For example, at the treating time of 1.8 ks, by the a mmonia 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 co nventional gascarbonitriding. The critical nitrogen content of low car bon 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 twi ce that in conventional gascarbonitriding. This is due to the improvem ent in resistance to temper softening and also to the increase in surf ace hardness through martensite transformation of the retained austeni te during the wear test. Fatigue limits of medium carbon steel treated by the ammonia rising method did not differ from those in conventiona l gascarbonitriding. The retained austenite which prevented cracks fro m initiation and propagation was transformed into martensite during th e fatigue test.