a study of active screen nitriding

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176 Li et al. Active screen plasma nitriding

4 Nitrogen distribution in near surface of AS and dc plasmanitrided 722M 24 steel

and/or e-Fe2 3

N in the compound layer. The lowernitride c-Fe

4N has a theoretical nitrogen content of

59 wt-% and the higher nitride e-Fe2 3

N has a

nitrogen content of 77111 wt-%. In principle, thethickness of, and the phase constituents in thecompound layer can be estimated from the nitro-gen proles. In Fig. 4, the slightly deeper highnitrogen prole in the ASPN sample suggests thatAS nitriding produced a slightly thicker compoundlayer. This is in agreement with the microscopyobservations as shown in Fig. 2. The higher nitrogencontent in the AS nitrided compound layer suggestsit may contain more e phase, which has beenconrmed by XRD results as shown below.

2 Optical microstructures of dc and AS plasma nitridedX-ray diVraction analysis was performed on the722M24 steel: 500C for 5 h in 25%N

2+75%H

2

nitrided surface with a Philips X-ray diVractometerusing Cu Ka radiation (l=0154 nm). The scanningIndeed, very little microstructural variation was angle (2h ) was varied from 30 to 100 and theobserved between the AS and dc nitrided samples. scanning rate was 002 deg s 1 . The XRD patterns

Nitrogen concentrations in the nitrided surface shown in Fig. 5 indicate that both ASPN and DCPNwere measured with a glow discharge spectrometer treated sample surfaces consist of a mixed structure(Leco GDS 750). Figure 4 shows that AS nitriding of c-Fe

4N and e-Fe

2 3N. However, the relative peak

produced a similar nitrogen depth prole as dc intensities of the two phases are diVerent in twonitriding. Both are characterised by an initial high samples. The comparison becomes easier by justnitrogen region near the surface and graduallyreduced nitrogen regions underneath. However, asignicant diVerence exists in that the nitrogencontent in the AS nitrided surface is higher than that

in the dc nitrided surface up to a depth of about6 mm. It is known that the high nitrogen concentrationis related to the presence of iron nitrides of c-Fe

4N

5 X RD patterns ofa dc andb AS plasma nitrided 722M243 SEM microstructure of AS nitrided sample: 500C for5 h in 25%N

2+75%H

2sample surface

Surface Engineering 2002 Vol. 18 No. 3


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a study of active screen nitriding

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