Nitriding-Quenching of Low-Alloy Steel
Using Atmospheric-Pressure Plasma Jet
Takashi Inoue1, Ryuta Ichiki1, Hirokazu Nagamatsu1
Masashi Yoshida2, Shuichi Akamine1, and Seiji Kanazawa1
1
Oita Univ, Japan
2
Shizuoka Institute of Science and Technology, Japan
Up to now, we have developed a new plasma nitriding using the pulsed-arc (PA) plasma jet and
realized the steel surface hardening with plasma nitriding under atmospheric pressure [1,2]. By taking
advance of the experience of plasma-jet nitriding, we have newly started to develop nitridingquenching (NQ) treatment using atmospheric-pressure plasma jet. NQ treatment is composed of the
following three phases: 1.[Heating phase] Heat steel up to the austenite range. 2.[Nitriding phase]
Supply nitrogen atoms into the austenite phase. 3.[Quenching phase] Cool the steel down rapidly
enough to invoke martensite transformation. In this treatment, low alloy steels can be hardened in
contrast to nitriding treatment [3].
N2/H2 mixture gas is introduced from the upper part of the nozzle (the coaxial cylinder electrode)
shown in Fig.1. Pulsed arc discharge is generated with a high-frequency power source (4.5 kV in
voltage, 1 A in discharge current, and 20 kHz) between the internal and the external electrode. The
afterglow is injected through the orifice onto sample surface. The distance between nozzle tip and
sample is kept at 4 mm for rising the temperature to 900○C. After the nitriding phase, the sample was
dropped into distilled water for water quenching. The tip of plasma jet is fitted with a cover made of
quartz to purge oxygen from treatment atmosphere as shown in Fig.1. As a sample, we used SPCC
which is low alloy steel containing almost no alloy elements. The hardness of SPCC base material is
approximately 150 Hv.
Fig.2 shows the hardness profile of sample cross-section. We can see that, SPCC was partially
hardened up to 800 Hv by NQ treatment. However, the hardness profile is considerably non-uniform.
The hardest part was formed into ring shape with a center of jet splaying point. Fig.3 shows
metallographic structure of cross-section of the hardest part. The metallographic structure indicates
that the hardest part is likely the iron-nitrogen martensite. The iron-nitrogen martensite formation
requires optimum nitrogen density and treatment temperature. We consider that these conditions were
optimum at the radius of hardest ring.
[1]H. Nagamatu et al., Surf. Coat. Technol., in press. (DOI: 10.1016/j.surfcoat.2013.03.012)
[3]R. Ichiki et al., Mater. Lett. 71, 134 (2012)
[3]M. Chiba et al., J. Jpn. Inst. Met. Metar. 76, p256-264 (2012) [in Japnese ].
N2/H2
Mixture Gas
Pulsed Power
Supply
18mm
Quartz Cover
Sample
Fig.1 Schematic of
experimental setup.
Depth from Surface [m]
Radial Position [mm]
-30
0
-20
-10
0
10
20
30Hv0.01
200
400
600
800
1000
Fig.2 Hardness profile of treated
sample cross-section.
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100
Fig.3 Metallographic structure
of cross-section in hardest part.