Structural characterization of fluidized bed nitrided steels

The nitriding behaviour of 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels was investigated nitrided in the fluidized bed processes. The nitriding processes were carried out at a temperature of 575 °C for treatment times of 6, 12 and 18 h. The nitrided samples were fully characterized using metallographic, microhardness and XRD techniques. Test results indicated that thickness of the compound layer on the steel surface changed in the range from 10 to 18 μm depending on steel type and treatment time, and γ′-Fe4N and ε-Fe2−3N formed in the compound layer. The hardness of the diffusion layer was over 1000 HV. Depending on the chemical composition of steels, the case depth ranged from 155 to 525 μm. Kinetics studies showed that the effective diffusion coefficients are 298×10−14, 525×10−14 and 68.8×10−14 m2 s−1, for 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels, respectively. The fluidized bed process realizes the highest hardness of the case layer, 1095 HV, with fairly high growth rates, 27 μm/h.