JOURNAL OF MATERIALS SCIENCE 35 (2000)71–77
DC extended arc plasma nitriding of stainless
and high carbon steel
A. SAHU
∗
‡
, B. B. NAYAK
∗
, N. PANIGRAHI
∗
, B. S. ACHARYA
∗
, B. C. MOHANTY
∗
∗
Regional Research Laboratory, Bhubaneswar 751013, India
‡
Centre of Plasma Physics, Dispur, Guwahati 781006, India
E-mail: bijan
nayak@yahoo.com
Stainless steel (SS 302) and high carbon steel (HCS) substrates were nitrided in a pot type
arc plasma furnace in the temperature range 1100–1200
◦
C under different gas (Ar, N
2
,H
2
)
mixture configurations for twenty minutes each. The nitrided surfaces were characterized
by XRD, SEM, metallography and microhardness. Depth of the nitride layer grown was
found between 40 to 50 µm. Microhardness for SS 302 was observed to increase by about
three to four times but for HCS the increase was not more than two times. The major
compound phases to grow by this method were indentified to be Fe
2−3
N(ε), (Cr, Fe)N
1−
x
and CrN in case of SS 302 whereas for HCS the phases were recognised as Fe
2−3
N(ε),
(Cr,Fe)N
1−
x
,Fe
2
N(ξ) and WN. Further details about the experiment and characterization of
arc plasma nitrided steel are reported and discussed in this paper.
C
2000
Kluwer
Academic Publishers
1. Introduction
Nitriding, basically a surface hardening process, pro-
duces improvements in wear and corrosion resistance,
hardness and fatigue lifetimes of metals. Thus, iron
and steel components, particularly engineering com-
ponents, are often nitrided to prevent their premature
failure due to progressive decay of surface materials
by erosion, abrasion, corrosion etc. under extreme duty
conditions.
The conventional nitriding processes adopted in in-
dustry use either cyanide-cyanate fused salt bath or
gases like N
2
or NH
3
for obtaining case depth of var-
ious thicknesses (usually less than 0.5 mm). Due to
the toxicity involved in cyanide-cyanate salt, its han-
dling in industrial scale poses a major problem. Gas
nitriding, on the other hand, requires relatively more
time (usually in the range of several hours) for devel-
oping desired case depth. With a view to overcoming
the drawbacks highlighted above, plasma nitriding has
now come to centre-stage in industrial nitriding pro-
cess. Besides being a clean and non-toxic process that
involves considerably less nitriding time, a close con-
trol over the nature and structure of the nitrided layer
is possible in plasma process [1–6]. The white brittle
layergrown inallconventionalnitridingprocesses does
not appear in the plasma process. This is a significant
advantage as the need to machine steel components in
order to remove the white layer is avoided.
Though most commercial plasma nitriding reactors
presently in operation use low pressure dc glow dis-
charge plasma [7], interests have also grown in recent
times in arc plasma nitriding [8] due to the following
advantages: a) faster diffusion and reaction kinetics of
theactivatedspeciesonsubstratesurface,b)higherden-
sity of reactive species and c) reduced processing time.
In the present work we have made an attempt to nitride
two widely used engineering materials, namely, stain-
less steel (SS 302) and high carbon steel (HCS) in a pot
type arc plasma furnace under different gas mixtures
consisting of Ar, N
2
and H
2
. The nitrided samples have
been characterized by X-ray diffraction (XRD), met-
allography, scanning electron microscopy (SEM) and
microhardness and the results are discussed to throw
some light on high temperature nitriding phenomenon.
2. Experimental
The nitriding substrates were taken in the form of cir-
cular discs (25 mm dia, 3 mm thickness) for SS 302
and square plates (12 ×12 ×3 mm) for high carbon
steel. The surfaces of the substrates were mechanically
ground and polished first by silicon carbide paper (120,
320, 400 & 600 grit) and then by alumina slurry (1.0,
0.05 & 0.03 µm grade) to yield a mirror finish. The
compositions of SS 302 and HCS are given in Table I.
The plasma nitriding furnace (Fig.1) is a vertical pot
furnace indigenously designed and developed by the
SpecialMaterialsDivisionof RegionalResearch Labo-
ratory,Bhubaneswar.Thefurnacehasprovisiontowork
both in transferred and non-transferred arc mode. Ni-
triding of substrate surfaces was carried out in the non-
transferred mode in order to avoid melting of substrate.
The hearth of the furnace constitutes of a salamander
(clay bonded SiC) crucible insulated by bubble alu-
mina. Another crucible made of graphite is fixed at the
centre of the hearth and rests on a magnesia block at
the bottom. The graphite crucible is laterally supported
by a plurality of graphite rods (four numbers) which
also act as electrical feed throughs. Graphite wool is
filled into the interveningspace ofthe hearth to provide
0022–2461
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2000 Kluwer Academic Publishers
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