These results suggest the optimum surface design as a
combination of carburizing and nitriding. The process
thickens the hard layer by simultaneous nitriding and
carburizing as the first process. The surface concentration
peak of carbon must be pushed-ahead by the nitriding as a
second process for corrosion resistance. The process details
are listed in Table 2 as 4(C+N)–4N. This process thickens
the nitrogen-S phase by nitriding throughout the process.
The microstructure and the GDOES profile of 4(C+N)–4N
specimen are shown respectively in Fig. 8 and Fig. 9. The
duplex layer thickness is greater than 11 mm. Carbon
concentration at the surface is reduced by the second
nitriding process. The hardness profile of this specimen is
displayed in Fig. 10. The thickness of the maximum hardness
is identical to that of the 8.N specimen. Bulging of the
intermediate hardness range by carbon S phase is apparent.
Corrosion resistance, already listed in Table 3, is as good as
the 4C–4N process.