Mechanical Properties of
DC-Pulsed Plasma Nitridin
Steel (AISI 4140)
Luciano Dutrey,* Evangelina De Las Heras
Introduction
One of the greatest challenges imposed by the increasing
technological progress is to develop new materials or to
improve the existing ones in order to make them resistant
to severely demanding working conditions.[1] In this sense,
surface treatments can offer significant benefits, such us
producing specific surface properties maintaining the bulk
properties of the original material. Among them, plasma
nitriding treatments have had a great development due to
their advantages respect to conventional surface mod-
[2–4]
combination of metallurgical properties, namely, increases
of surface hardness, wear resistance, fatigue life, and, in
certain cases, corrosion resistance;[5,6] it is an environ-
mental friendly technology,[7] generates negligible volu-
metric changes, reduces the production costs, and
improves the productivity. Some of the main applications,
on a great diversity of substrates (steels, castings, sintered
materials, titanium, and other alloys), are: cutting tools,
matrices, valves, and other machine components. During
the plasma nitriding treatment two zones are produced in
low-alloy steels, namely, the so-called compound layer
-
The AISI 4140 is one of the most used low-alloy steels
in nitrided structural elements. The process influence on
Full Paper
of Industrial Technology, Av. Gral. Paz 5445, B1650WAB, San
Martı´n, Buenos Aires, Argentina
PN
m
e in
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pronounceductility loss, associated tothebrittle intergranular fracturesurfaceduetothenitride
S314

the mechanical properties in bulk and on the variation of
these from the surface toward the material core is a topic
of great technological interest. Although there exist
already a large amount of works devoted to diverse
aspects of plasma nitriding in low-alloy steels, there is
scarce information regarding systematic studies on the
mechanical properties and on the deformation and
Fax: (þ54) 11 4754 4072; E-mail: g-bio@inti.gob.ar
H. G. Svoboda
University of Buenos Aires, CONICET, Faculty of Engineering,
INTECIN, Materials and Structures Laboratory, Las Heras 2214, 1427
Buenos Aires, Argentina
P. A. Corengia
Fundacio´n INASMET-Tecnalia, Mikeletegi Pasealekua 2, E-20009
Donostia-San Sebastia´n, Guipu´zcoa, Spain

ification technologies: they produce an effective (adjacent to the surface), and the diffusion zone (under
neath the previous one). The depth and the hardness of the
diffusion zone depend on the alloy content and on the
nitriding time and temperature.
L. Dutrey, E. De Las Heras
Mechanics Research and Development Center, National Institute

precipitation in the grain boundaries.

Pablo A. Corengia
An industrial DC-pulsed plasma nitriding (DCP
AISI4140low-alloysteelwith fivedifferentconfig
diffusion zone, substrateþ diffusion zoneþ co
layer. The microstructures of the samples wer
and X-ray diffraction. Microhardness and te
mechanisms for the different materials wer
produced an increase in the elastic modulus a

Plasma Process. Polym. 2009, 6, S314–S320
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Layers Obtained by
g on a Low-Alloy
, Herna´n G. Svoboda,
) equipment was used to obtain samples of an
urations:substrate, substrateþ diffusionzone,
pound layer, and diffusion zoneþ compound
vestigated by scanning electron microscopy
ile tests were performed, and the fracture
finally analyzed and discussed. The DCPPN
ell as in the yield strengths, together with a

DOI: 10.1002/ppap.200932404

Mechanical Properties of Layers Obtained by . . .
.
Si Cr Mo Ni Cu Fe
0.25 0.81 0.15 0.18 0.21 Balance
e
%
%

Table 3. Different configurations studied.
Series Composition
1 100% substrate
2 Substrateþ diffusion zone
3 100% diffusion zone

Table 1. Chemical composition of the AISI 4140 low-alloy steel studied
Element C Mn P S
wt.-% 0.38 0.79 0.032 0.007
Table 2. Nitriding parameters.
Time Temperature Pressure Atmospher
4 h 500 8C 6 hPa 75% H2þ 25
15 h 500 8C 6 hPa 75% H2þ 25

fracture mechanisms for the different zones observed on
nitrided materials.
The aim of the present work is to study the mechanical
behavior of the different zones obtained by DC-pulsed
plasma nitriding (DCPPN) on a Cr–Mo low-alloy steel, AISI
4140. Its objective is to provide a better understanding of
this behavior through the analysis of the tensile properties
and fracture modes in each region (for different fractions of
substrate, diffusion zone, and compound layer).
Experimental Details
The material used was a Cr–Mo low-alloy steel with the
chemical composition shown in Table 1.
A 12.7 mm thickness AISI 4140 steel plate of 120 mm
wide by 240 mm length was heat treated, applying an
austenitization at 840 8C (for 25 min.), followed by an oil
quenching and a tempering at 550 8C (for 2 h), in order to
obtain a tempered martensite structure. Later on, the piece
was machined and 1.5 mm thickness specimens were
extracted using wire cutting in a Charmilles Roboform 550
electrical discharge machining (EDM) equipment. After-
wards, the specimens were ground to 1 mm. The DCPPN
treatments were carried out under two different times,
4 Substrateþ diffusion zoneþcompound
5 Diffusion zoneþcompound layer
Plasma Process. Polym. 2009, 6, S314–S320
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Tension Current density Pulse on/off
N2 750 V 1.03 A cm2 20–700 ms
N2 750 V 1.03 A cm2 20–700 ms
Procedure
Quenched and tempered
4 h DCPPNþcompound layer removed
15 h DCPPNþcompound layer removed

namely, 4 and 15 ha; the process parameters are given in
Table 2. The compound layer was then removed from a
group of samples in order to obtain the five different
configurations, which are detailed in Table 3.
Once the five series of samples were obtained, a
microstructural characterization was performed using a
Philips SEM 505 scanning electronic microscope (SEM). The
specimens for metallographic analyses were Ni electro-
plated—at 40 8C with a 1.5mA current—to obtain a 10mm
thick protective layer, sectioned perpendicular to the
treated surface, embedded in a mounting resin, ground
with SiC paper, and polished using 1 mm alumina as the
last step. The microstructure was revealed using 2% Nital
(2mLHNO3 in 100mL ethanol) and Vilella’s (5mL HClþ 1 g
picric acid in 100 mL ethanol)—for the austenitic grain
boundaries—etchants.
layer 4 h DCPPN
15 h DCPPN
a The selection of these two treatment times was done in such a way
that for the short process N could diffuse up to approximately
a quarter of the sample thickness (250 mm),[8,9] and for the
long process up to approximately a half of the sample thickness
(500 mm).[8,9] Hence, samples were obtained therefore 50% of
substrate and 50% of diffusion zone for the first case, and 100% of
diffusion zone for the second case.
www.plasma-polymers.org S315

-
according to ASTM E8M Standard in an
s 1–5 cross-
e corre-
l etching can
e of the
r belonging to
zone N can be in
microstructure
e of nitrides
can be seen
s was
s the introduc-
certain point the
s expected, the
e grain bound-
e presented
the sample core.
are shown in
and 3, with the exception that the compound layer of the
outer surface was conserved). In both cases, a continuous
and uniform compound layer (unreactive to Nital etching)
was appreciated. The thickness was approximately 4 mm
for the short-treated sample and 6 mm for the long-treated
one. Once more, nitride precipitation can be observed at
the grain boundaries for Series 5.
Figure 2 shows the microhardness depth profiles,
acquired on the samples cross-sections, of Series 4 and
5. The diffusion zone thickness, as expected, was greater in
Series 3 and 5 than in Series 2 and 4. For the long treatment
(15 h), this zone covered the whole material volume,
reaching values of 40 HV above the substrate hardness
(371 HV0.25) in the sample center line region. In the short
treatment (4 h), the thickness of the diffusion zone was
around 230 mm. Therefore, it can be said that the two
expected structural configurations were achieved, i.e., one
consisting of a combination of diffusion zone plus
substrate (plus compound layer): short treatment—Series
L. Dutrey, E. De Las Heras, H. G. Svoboda, P. A. Corengia
Figure 1. SEM micrographs of samples: (a) Series 1, (b) Series 2, (c) Series 3, (d) Series 4,
and (e) Series 5.
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Instron Series IX 4467 testing machine.
The elastic modulus was determined by
the extensometry.
In the last characterization step, the
obtained fracture surfaces were analyzed
by SEM and the failure modes for the
different cases were determined.
Results and Discussion
Microestructural Characterization
Figure 1 shows SEM micrographs of Serie
sections. The tempered martensite microstructur
sponding to a sample of Series 1 after 2% Nita
be observed in Figure 1a. The microstructur
diffusion zone without compound laye
Series 2 is appreciated in Figure 1b. In this
solution or forming nitrides.[7,8] In the
corresponding to Series 3, the presenc
precipitated at the grain boundaries
(Figure 1c). The appearance of these precipitate
due to the longer treatment, which enable
tion of a greater N amount, exceeding at a
N solubility in the steel matrix.[11] A
preferential precipitation places were th
aries.[10,12] The adjacent zone to the free surfac
a higher N content that decreases toward
Microstructures of Series 4 and 5

Finally, for the different configura
tions, standardized tensile samples were
machined by EDM and tensile tested

Microhardness profiles were carried
out in a Shimadzu HMV-2000 equipment
using loads of 0.25 N and a Vickers
indenter. The compound layer and diffu-
sion zone thicknesses were determined
for each case applying the Metals Hand-
book method.[10] (i.e., where the surface
hardness differ in more than 20 HV
respect to the substrate).
The phases formed in the treated AISI
4140 were then characterized by X-ray
diffraction (XRD) using a Cu-Ka
(l¼ 1.5406 A˚) radiation in a Philips-
Rigaku X-ray diffractometer, with an
average penetration depth in the mate-
rial of 3 mm. A conventional u/2u Bragg–
Brentano symmetric geometry was used,
from 30 to 908, with a step size of 0.058
and a velocity of 18 min1.

Figure 1d and e (these samples are similar to Series 2
Plasma Process. Polym. 2009, 6, S314–S320
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 and 4, and the other consisting completely of diffusion
DOI: 10.1002/ppap.200932404

as the reason for this broadening.[15] Nakata et al.[16]
indicate that the peak broadening, associated to the
presence of microstrains in the matrix, could be caused by
the appearance of a coherent pre-precipitation phase.
Nevertheless, it cannot be ignored that the compound
layer elimination, performed by mechanical removal,
could have introduced residual stresses in the material
surface as well.[17] For Series 3 samples—diffusion zone in
the long treatment—, peaks of a-Fe were detected, with a
more significant broadening with respect those of Series 1.
In addition, small indications of nitride presence were
observed. This fact was consistent with the micrographs,
where the presence of precipitates was appreciated. For
Series 4 samples—short treatment—, it was observed that
the compound layer was constituted by the phases g 0-
Fe4N and, to a lesser extent, e-Fe2–3N; in accordance with
the reported data for this material compound layer.[10,13,14]
Similarly, in Series 5 samples—long treatment—, the
with a loss of ductility.
Mechanical Properties of Layers Obtained by . . .
Figure 2. Microhardness depth profiles of the DCPPN samples for
Series 4 and 5.

zone (plus compound layer): long treatment—Series 3 and
5. The surface hardness measured in Series 4 and 5 was
practically the same for the 4 h treatment (851 HV0.25) as
for the 15 h treatment (845 HV0.25), consistently with the
values reported in the literature.[13,14]
In order to determine the present phases in the samples
after the DCPPN process, an XRD analysis was performed
on the surface. As reference, analyses of Series 1 samples—
the substrate—were performed and peaks of only one
component were identified, namely, a-Fe.[10] For Series 2
samples—diffusion zone in the short treatment—, mainly
peaks of a-Fe were observed, which presented a broad-
ening with respect to those observed for Series 1. In the

literature, the introduction of compressive residual
stresses related to the plasma nitriding process is reported
Table 4. Tensile test results for the five tested series.
Series Yield strength 0.2% Tensile strength
MPa MPa
Average Deviation Average Deviation Averag
1 1 126 23 1 269 18
2 1 288 24 1 432 26
3 1 693 32 1 693 32
4 1 284 24 1 314 25
5 1 533 30 1 533 31
E.E.: Electric extensometer.
Plasma Process. Polym. 2009, 6, S314–S320
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Series 2 and 4—4 h process—presented slight plastic
deformation, more significant in the sample without
compound layer. Series 3 and 5—15 h process—showed
Elongation %
ef% ¼ lfl0l0 100
Reduction of area
to fracture %
qf% ¼ A0AfA0 100
Young
modulus
GPa
e Deviation Average Deviation E.E.
17 1.5 47 1.9 201
1 <1 2 <1 214
<1 <1 1 <1 220
<1 <1 1 <1 217
<1 <1 1 <1 224

occurrence of g 0-Fe4N phase in the compound layer was
confirmed, whereas e-Fe2–3N has practically disap-
peared.[10,13,14]
Mechanical Properties
The results obtained from the tensile tests on the five
analyzed series are shown in Table 4. The numerical values
correspond to an average of three tests per series.
The values obtained for the substrate are consistent
with the data reported in the literature for this material.
Regarding the nitrided series, 2–5, an increase of the
resistance and of the elastic modulus was observed, along

www.plasma-polymers.org S317

%) were nearly zero, especially for the
, and due to
e and,
t in the entire
increase was
d to the
treatment than
to a crystalline
d solution. In
g processes
in the samples
w phases in the
e struc-
modulus—may
e surfaces in
substrate, presented a typical cup and cone ductile fracture
for rectangular sections,[19] in accordance with the tensile
test results. A significant reduction of the cross-sectional
fracture area is observed. Series 2 and 4 (Figure 3b and d),
4-h DCPPN, presented a mixed fracture, with the central
part of the sample at 458 and flat outer regions. Although
in this central fraction of the sample the material is
relatively ductile—substrate—, the presence of the
diffusion zone prevents the necking formation; therefore,
inside the sample triaxiality could not take place. Hence,
the central part fractured following the maximum shear
stress plane.[12] Series 3 and 5 (Figure 3c and e), 15-h
DCPPN, presented a classic brittle fracture with flat
surface,[19] in accordance as well with the tensile test
results. No reduction was appreciated in the cross-
sectional fracture area.
The flat fracture zone thicknesses from Series 2 and 4
was measured on the micrographs, and corresponded well
with the diffusion zone thicknesses determined by Vickers
L. Dutrey, E. De Las Heras, H. G. Svoboda, P. A. Corengia
Figure 3. SEM images of the fracture surfaces: (a) Series 1, (b) Series 2, (c) Series 3, (d)
Series 4, and (e) Series 5.
S318

strength were obtained in Series 3 and
the minimum in Series 1. In the short as
well as in the long treatment, the
samples with compound layer (4 and
5) showed lower tensile strength than
their homologous without compound
layer (Series 2 and 3, respectively). This
was associated to the microcracks gen-
eration in the compound layer during
the deformation process, as result of the
different elastic modulus of each mate-
rial fraction; i.e., the sample can be
modeled as a composite working in an
isodeformation condition.[12] During the
tensile test, the compound layer with
higher elastic modulus holds greater stresses
its brittleness originates the cracked appearanc
finally, the propagation of the fracture fron
sample.
Concerning the elastic modulus, an
observed in the nitrided samples compare
substrate, with higher values for the 15 h
those for the 4 h one. This was associated
structure modification due to the N soli
addition, it was observed that, in both nitridin
the modulus reaches higher magnitudes
with compound layer. The presence of ne
compound layer, which present other crystallin
tures and bond types—with higher elastic
explain the mentioned increase.
Fracture Surfaces
Figure 3 shows SEM images of the fractur

samples with compound layer. The
ductility loss along with the tensile
strength and the Young modulus
increase in the nitrided samples are
associated to the N diffusion into the
a-Fe crystalline matrix and to the
nitrides precipitation.[18] In other words,
the strength increase is produced by
solid solution hardening and by coherent
precipitates presence.[18] In addition, the
strong ductility loss could be associated
to the embrittlement induced by inco-
herent precipitation at grain bound-
aries.[18]
The maximum yield and tensile

practically a linear elastic behavior. For
all nitrided samples, the elongation (ef %)
and the reduction of area to fracture (qf

perspective of the five studied series. Series 1 (Figure 3a),
Plasma Process. Polym. 2009, 6, S314–S320
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

microhardness test.
DOI: 10.1002/ppap.200932404

Mechanical Properties of Layers Obtained by . . .

In Series 4 and 5—samples with compound layer—
secondary cracks on the surface were observed, which
extended into the sample core perpendicular to the load
direction. These cracks, were later seen, propagated through
the grain boundaries; and its formation was associated to
the embrittlement caused by nitride precipitation.
Figure 4 shows SEM images from the fractured
surfaces observed on the different samples series. In
Figure 4a, corresponding to Series 1, characteristic dimples
from a ductile fracture can be appreciated,[19] and also
microvoids related to the inclusions. In Figure 4b, Series 1,
secondary microcracks and decohesion of the metallic
matrix is observed. For Series 2, an intergranular fracture
associated to nitride precipitation at grain boundaries is
clearly distinguished (Figure 4c). This type of fracture was
observed on the diffusion zones of the nitrided series. For
Series 3, Figure 4d, a mixed fracture mode (with inter-
granular fracture) was observed in the sample edge zone—
without compound layer. In Figure 4e, corresponding to
Series 4, the fracture surface in the compound layer region
the central part
maximum shear
Samples with diffusio
fracture with flat
The DCPPN sample
mode in the diffusio
at grain boundaries
nular fracture wer
close to the compoun
Acknowledgements
technical and financial assistance granted by the Argentinean
National Institute
would also like
Amorphous Solids Laborator
tion, IONAR SA, G. Meye
Rosa from FIUBA.
Received: January
10.1002/ppap.2009324
Keywords: mechanica
ing; steel
Figure 4. Micrographs of the fracture surface for: (a,b) Series 1, (c) Series 2, (d) Series 3,
(e) Series 4, and (f) Series 5.
Plasma Process. Polym. 2009, 6, S314–S320
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

of Industrial Technology (INTI). The authors
to thank the collaboration received from
y—FIUBA, INASMET-Tecnalia Founda-
r from Balseiro Institute—CNEA, andH. De
10, 2009; Accepted: February 11, 2009; DOI:
04
l properties; nitrides; pulsed plasma nitrid-

were obtained for samples with only
diffusion zone. Samples with a com-
pound layer showed lower yield and
tensile strengths, associated to the
microcracks generation during the defor-
mation process.
The elastic modulus increased for the
DCPPN samples. This increase was more
noticeable for samples with compound
layers.
Fracture surfaces for untreated sam-
ples presented a typical cup and cone
ductile fracture for rectangular sections.
Samples with substrate plus diffusion
zone presented a mixed fracture, with
of the sample at 458—following the
stress planes—and flat outer regions.
n zone presented a classic brittle
surfaces.
s showed intergranular fracture
n zone, related to nitride precipitation
. Few cleavage facets from transgra-
e also found, especially in the region
d layer.
: The authors are very grateful for the

canbe appreciated, identifyinga different
morphology of brittle fracture for this
zone. In Figure 4f, corresponding to Series
5, the presence of a few cleavage facets
from a transgranular fracture can be seen
(characteristic also of brittle fractures)
along with intergranular fracture.
Conclusion
The treated samples presented a com-
pound layer and a diffusion zone,
characteristic of the DCPPN.
An increase of the yield and tensile
strength in the DCPPN samples com-
pared to the untreated samples were
observed, together with a pronounced
ductility loss. The maximum strengths

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DOI: 10.1002/ppap.200932404