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Microstructure and corrosion behaviour of pulsed plasma-nitrided

AISI H13 tool steel

Rodrigo L.O. Basso a, Heloise O. Pastore b, Vanessa Schmidt a, Israel J.R. Baumvol a,c, Silvia A.C. Abarca d,Fernando S. de Souza d, Almir Spinelli d, Carlos A. Figueroa a, Cristiano Giacomelli a,*

a Centro de Cincias Exatas e Tecnologia, Universidade de Caxias do Sul, 95070-560 Caxias do Sul, RS, Brazilb Instituto de Qumica, Universidade Estadual de Campinas, 13084-862 Campinas, SP, Brazilc Instituto de Fsica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazild Departamento de Qumica, Universidade Federal de Santa Catarina, 88040-900 Florianpolis, SC, Brazil

a r t i c l e i n f o

Article history:

Received 8 March 2010

Accepted 25 May 2010

Available online 1 June 2010

Keywords:

A. Steel

B. Polarization

B. XRD

B. SEM

C. Pitting corrosion

a b s t r a c t

The effect of pulsed plasma nitridingtemperature and time on the pitting corrosion behaviour of AISI H13

tool steel in 0.9% NaCl solutions was investigated by cyclic polarization. The pitting potential (Epit) was

found to be dependent on the composition, microstructure and morphology of the surface layers, whose

properties were determined by X-ray diffraction and scanning electron microscopy techniques. The best

corrosion protection was observed for samples nitrided at 480 C and 520 C. Under such experimenta

conditions theEpit-values shifted up to 1.25 V in the positive direction.

2010 Elsevier Ltd. All rights reserved

1. Introduction

Pulsed plasma nitriding is a thermochemical process exten-

sively applied in materials science and surface engineering due

to its well-known potential for improving properties such as hard-

ness, wear, and corrosion resistance of metallic parts [1,2]. This

surface treatment technique consists of the implantation of nitro-

gen species at low energies (63 eV) into the steel substrate, and

their subsequent diffusion into the bulk at temperatures above

300 C. The interaction of nitrogen and steel constituents leads to

the formation of different types of metallic nitrides, which form

the so-called nitride layer. Starting from the solid surface, such

a modified layer usually comprises an oxide layer, a compound

zone, and a diffusion zone[3,4]. The resulting structure of these

domains depends on several processing parameters such as the

concentration of alloying elements, exposure time, substrate

temperature, and gaseous mixture[5,6].

The presence of a nitride layer obviously changes the mecha-

nisms of interaction betweenmetallicmaterialsand theirsurround-

ings, thus affecting their stability in aggressive environments [79].

In the case of austenitic steels, in particular, the incorporation of

nitrogen imparts better mechanical properties (friction and wear

resistance), but the dissolution kinetics (corrosion resistance) re-

mains closely related to the composition of the corrosive medium

[10,11].

In this context, the AISI H13 tool steel is largely employed in

industrial processes that take place in aggressive environments

For example, this steel is commonly used in the fabrication o

moulds, casting dies and screws for poly(vinyl chloride) (PVC

extrusion and the injection mould industry. One drawback associ-

ated with the high temperature processing of PVC-containing

materials is the non-negligible concentration of free chloride spe-

cies originating from the cleavage of CCl bonds, which may

strongly accelerate corrosion and wear rates.

However, this problem can be avoided by plasma nitriding the

AISI H13 tool steel[12,13]. Hard iron nitrides are originated during

the plasma treatment owing to nitrogen diffusion in the near

surface region at temperatures below the eutectic poin

(Te 593 C) [14]. Usually, two distinctive phases corresponding

to the e-Fe2-3N and c0-Fe4N nitrides are obtained, whose high hard

ness improves the strength, friction and wear resistance [15,16]

However, the highest wear resistance is normally achieved when

the close-packed hexagonal e-Fe2-3N phase is primarily at the sur-

face of the specimens. This is so because the composite nitride

layer of the e-Fe2-3N and c0-Fe4N phases is, in fact, stressed due

to a crystal lattice mismatch[1720].

0010-938X/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2010.05.036

* Corresponding author. Present address: Universidade Federal de Santa Maria

(UFSM), Centro de Cincias Naturais e Exatas (CCNE), Av. Roraima, 1000, Cid.

Universitria, Prdio 18, Bairro Camobi. 97105-900 Santa Maria, RS, Brazil. Tel.: +55

8291224932; fax: +55 8232141384.

E-mail address: [email protected](C. Giacomelli).

Corrosion Science 52 (2010) 31333139

Contents lists available at ScienceDirect

Corrosion Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o r s c i
http://dx.doi.org/10.1016/j.corsci.2010.05.036mailto:[email protected]://dx.doi.org/10.1016/j.corsci.2010.05.036http://www.sciencedirect.com/science/journal/0010938Xhttp://www.elsevier.com/locate/corscihttp://www.elsevier.com/locate/corscihttp://www.sciencedirect.com/science/journal/0010938Xhttp://dx.doi.org/10.1016/j.corsci.2010.05.036mailto:[email protected]://dx.doi.org/10.1016/j.corsci.2010.05.036

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Recent work has shown that the pitting corrosion resistance of

nitrocarburized AISI H13 tool steel can be significantly improved

by nitride layers consisting of e-Fe23(C,N) and c0-Fe4N phases

[21,22]. However, the effect of the nitride layer microstructure

on the pitting corrosion behaviour of AISI H13 tool steel is still

not fully understood. In this study, we address this question by

analysing the influence of plasma processing parameters (temper-

ature and time) on the corrosion behaviour and microstructure ofpulsed plasma-nitrided AISI H13 tool steel.

2. Experimental

2.1. Nitriding process

AISI H13 tool steel samples (20 10 2 mm) with nominal

composition of 90.6 Fe, 0.5 C, 0.4 Mn, 1.0 Si, 5.1 Cr, 1.4 Mo, and

0.9 V (wt.%) were used in this study. Before plasma nitriding, sam-

ples were polished with diamond powder (mesh size = 1.0lm) and

ultrasonically cleaned in ethanol. The nitriding process was carried

out in an automatic hot-wall Plasmatec 450 pulsed plasma system

with 60 A/1000 V of maximum capacity. The substrate tempera-

ture was the same as the reactor temperature, and will be referredto as the nitriding or processing temperature. In all cases, the plas-

ma was generated by a pulsed DC power supply operating at 7.1 A

and 380 V with a duty cycle of 0.25 (pulse on: 50 ls; pulse off:

150ls). Before nitriding and during the heating step to reach the

processing temperature, the specimens were ion-bombarded for

4 h in an Ar/H2 80/20 v/v plasma for cleaning purposes. A H2/N280/20 v/v gas mixture flow at 400 Pa was used throughout the

nitriding process. Samples were submitted to plasma nitriding

either at temperatures ranging from 360 to 520 C for 4 h, or at a

constant temperature of 400 C and nitriding time ranging from

1 to 36 h. After processing, the samples were left to cool down

slowly (6 h) inside a vacuum chamber.

2.2. XRD and SEM analyses

The samples for XRD and SEM analyses were mounted in con-

ductive Bakelite (Bakelite + copper powder) and mirror-polished

with colloidal silica (mesh size = 0.05 lm). The nitrided layers

were revealed at room temperature by chemical etching with Nital

(2% v/v nitric acid in absolute ethanol). Micrographs of cross-

sections of nitrided layers were recorded using a Shimadzu

SSX-550 scanning electron microscope. The phase composition of

the compound layers was determined by XRD analysis using a

Shimadzu XRD-6000 diffractometer. The diffractograms were re-

corded at room temperature using the BraggBrentano geometry

with monochromatic radiation (Cu Ka, k = 0.15418 nm).

2.3. Electrochemical measurements

Corrosion tests were carried out with an EG&G PAR model 263A

potentiostat/galvanostat interfaced to a personal computer using

the EG&G-PAR SoftCorr II Model 252/352 software for data

acquisition and analysis. The electrochemical cell contained five

openings: three of them were used for the electrodes, and two

served either for nitrogen bubbling prior to the experiments or

for keeping an inert atmosphere with a gentle nitrogen flow

through the cell during the experiments. The counter electrode

(CE) was a graphite rod, and the reference electrode (RE) was a sat-

urated calomel electrode (SCE) connected to the cell by a salt

bridge and a LugginHabber capillary. Unless otherwise indicated,

all potentials in the text are quoted with respect to this reference

electrode. The working electrode (WE) was a 0.55 cm2 (geometricalsurface area) AISI H13 tool steel disc mounted in a glass tube with

Araldite epoxy. Prior to the experiments, the electrode surface

was degreased with acetone, rinsed with deionised water, and

gently dried with nitrogen[23].

Cyclic potentiodynamic polarization curves were recorded

according to the ASTM G61 standard, which is recommended for

evaluating the pitting corrosion tendency of materials in chlo-

ride-containing media. In this pitting experiment, the open circuit

potential (OCP) was initially measured over 30 min. The potentialscan was then started from the OCP-value in the positive direction

until a threshold current density of 6.0 mA/cm2 was reached. At

this point, the potentiodynamic scan was reversed to the negative

direction, and continued down to 0.25 V/OCP. The pitting poten-

tial (Epit) was taken as the value at which the current sharply in-

creased during the positive scan. The values reported hereinafter

correspond to an average of three measurements, and exhibit stan-

dard deviations of 215 mV.

3. Results and discussion

The morphology and microstructure of nitrided layers produced

on near-surface regions of AISI H13 tool steel by plasma treatment

at different temperatures and exposure times were determined bySEM and XRD, respectively. SEM images can reveal up to two dis-

tinct types of surface layers, depending on the treatment parame-

ters. The SEM image in Fig. 1 shows two distinct layers, as obtained

for a sample plasma nitrided at 520 C for 4 h. One can see an out-

ermost layer well-known as compound layer or white layer, and

below it there is a modified region also known as diffusion layer

[3,4]. SEM micrographs of cross-sections of samples nitrided here

at different temperatures for 4 h are shown in Fig. 2. A non-

compact compound layer was generally obtained for samples trea-

ted at temperatures below 480 C (Fig. 2AC), whereas a compact

compound layer became evident when the nitriding process was

carried out at temperatures as high as 520 C(Fig. 2D). This tem-

perature-induced transition was corroborated by the XRD analysis,

as indicated by the results given in Fig. 3. One single diffractionpeak was observed at 2h= 44.0 for untreated AISI H13 substrates,

which was attributed to thea-ferrite phase. After plasma nitriding,

the e-Fe2-3N phase with characteristic diffraction peaks at 2h=

38.3, 41.0, 43.6 and 57.6 was identified for the whole tempera-

ture range investigated in this study. The peak at 2h= 43.6 was

slightly displaced towards higher angles for samples submitted

to pulsed plasma nitriding at high temperatures, as previously

Fig. 1. SEM micrograph of cross-section of a AISI H13 tool steel samples after

plasmanitriding for4 h at 520 C. Tree different well-defined regions can be seen inthis image.

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reported in the literature[13]. The diffractogram also revealed the

appearance of diffraction peaks at 2h= 41.0 and 47.6 forTP 440 C (see arrows inFig. 3), which are associated to the pres-

ence ofc0-Fe4N phase in the modified layer [24]. The diffraction

peak at 2h= 46could, in principle, be attributed to expanded mar-

tensite phase [25,26] or c0-Fe4N phase [27]. However, the set of

diffraction peaks identified in this study clearly indicates the for-

mation of the c 0-Fe4N phase.

The low diffusivity of nitrogen at low temperatures is very

likely at the origin of the formation of a non-compact compound

layer in which the e-Fe2-3N phase was predominant. For this rea-

son, the formation ofc 0-Fe4N is usually hindered at low tempera-

tures[28]. However, the synthesis of the latter is favoured above

440 C, as indicated by the progressive increase in the intensity

of its characteristic diffraction peaks as a function of the processing

temperature. This variation continues until a major transition oc-curs between 480 and 520 C (see diffractograms inFig. 3). These

results are consistent with the precipitation of e-Fe2-3N and

c0-Fe4N phases, thus producing the compact compound layer iden-

tified by SEM (Fig. 2D).

The diffraction line at 2h= 37.2 is typically associated with the

CrN phase, which is well-known to precipitate during plasma

nitriding at high temperatures (above 440 C in the present case

[29,30]. No evidence of the formation of the Cr2N phase was found

in the diffractograms recorded for treated samples.

Considering the results reported above, different nitride layers

canbe synthesized on the surface of AISI H13 tool steel by adjustingthe plasma processing temperature. Briefly, samples nitrided a

T6 400 C exhibit a modified region near the surface whose

structure consists mainly of the e-Fe2-3N phase. The formation o

c0-Fe4N and CrNphases isfavouredabove 440 C. A clear microstruc

tural rearrangement that leads to the precipitation ofe-Fe2-3N and

c0-Fe4N phasesto form a compact compoundlayer, takesplacewith-

in the range of 480520 C.

The corrosion behaviour of pulsed plasma-nitrided AISI H13

tool steel in 0.9% NaCl solutions reflected clearly the variations in

terms of the microstructure and phase composition of the near-

surface layers described above. Cyclic polarization experiments

were carried out in order to evaluate the pitting corrosion resis-

tance, which was the main purpose of this study. Before cyclic

polarization, the open circuit potential (OCP) was measured over30 min. This is an electrochemical parameter that can provide

valuable information on the resistance of passive surface films

against degradation (corrosion) via non-destructive analysis. The

OCP-value is generally dictated by the structure, chemical compo-

sition and thickness of the protective film, as well as by the mor-

phology and nature of the film/solution interface[31].

The results are shown inFig. 4A for representative samples. In

general, the OCP decreased very soon after the electrode immer-

sion in the aggressive medium, and remained virtually constant

thereafter. Interestingly, the values measured after 30 min of expo

sure are strongly dependent on the plasma processing protoco

(i.e., on the structure and composition of the near-surface layer

as evidenced by data given in Table 1, entries 16. The average

OCP increased as a function of nitriding temperature from0.797 V for untreated H13 specimens up to 0.265 V/SCE fo

Fig. 2. SEM micrographs of cross-sections of AISI H13 tool steel samples after plasma nitriding for 4 h at 360 C (A), 440 C (B), 480 C (C), and 520 C (D).

Fig. 3. XRD patterns of AISI H13 tool steel surface after plasma nitriding for 4 h at

different temperatures, as indicated.

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samples nitrided at T= 440 C. Above this temperature, a sudden

potential decrease to 0.555 V/SCE was observed, followed by

another increase to 0.288 V/SCE for specimens nitrided at T=

520 C. The increases in the OCP-values (up to 500 mV in

selected cases) described above for plasma-nitrided AISI H13 sam-

ples immersed in de-aerated 0.9% NaCl solutions are considerablyhigher than those previously reported in the literature for the same

material in aerated 3.5% NaCl after surface treatment via plasma

immersion ion implantation (60 mV on average) [21,32]. It

should be noted, however, that this study was carried out in a less

aggressive environment, and therefore such differences must be

cautiously interpreted. To the best of our knowledge, the OCP-

value of AISI H13 in de-aerated 0.9% NaCl solutions has not been

previously reported.

The cyclic polarization curves shown inFig. 4B were typical of

materials in the passive state with critical pitting corrosion poten-

tials (Epit), whose values revealed essentially the same behaviour as

that described above. According to the data summarized in Table 1,

theEpit-values tend to increase with the nitriding temperature, in

spite of a remarkable decrease for T= 480 C. Most importantly,these experiments proved that the Epit can be displaced by

approximately 1.25 V in the positive direction (DEpit= [+0.787

V/SCE (0.472 V/SCE)] = +1.25 V), therefore implying that the

pitting corrosion tendency is considerably reduced. Furthermore,

the dissolution processes after pit formation were considerably

less active for nitrided samples, as evidenced by the increase inthe slope ofEvs. log(i) curves inFig. 4B.

These results corroborate the variations in the nature of nitride

layers formed at different plasma processing temperatures. Within

the 360440 C temperature range the corrosion resistance in-

creases because of the formation of a e-Fe2-3N surface layer that

retains chromium atoms in its crystalline structure. Indeed, this

phase has been reported to be the most corrosion resistant among

those formed during the plasma nitriding of steels [17,19,20]. The

formation of CrN as the substrate temperature exceeds 440 C (see

discussion related toFig. 3) is apparently at the origin of the corro-

sion resistance decrease, in good agreement with results reported

elsewhere[29,30]. On raising the nitriding temperature to 520 C,

the pitting corrosion resistance was markedly recovered (i.e.,

Epit-values increased again) due to the precipitation of both

e-Fe2-3N and c0-Fe4N phases. Such a compound layer imparts bet-

ter corrosion protection because it is more compact than those

generated below 520 C.

In an attempt to further improve the pitting corrosion resis-

tance of plasma-nitrided AISI H13 tool steel, the processing tem-

perature was fixed at 400 C and the effect of processing time

was evaluated.Fig. 4shows the SEM micrographs of cross-sections

of the samples nitrided for different periods of time. The images

suggest the presence of a diffusion layer in the near-surface region,

and the absence of a well-defined compound layer as a conse-

quence of the low substrate temperature during the surface mod-

ification process, as discussed above. The thickness of the layer on

the steel surface was determined from SEM images. The data pre-

sented inTable 2confirm that the layer thickness increases with

the increase in the nitriding time, as expected. Needle-shapedstructures typical of c0-Fe4N nitrides [3] were evidenced on the

surface layer of specimens nitrided for relatively long times

(36 h) as indicated inFig. 6. In fact, the c 0-Fe4N phase can be pro-

duced not only at high temperatures (see above) but also at low

temperatures when longer nitriding times are selected [3,33,34].

Fig. 4. Representative open circuit potential (A) and cyclic potentiodynamic

polarization curves (B) recorded in 0.9% NaCl solutions for AISI H13 steel specimens

after plasma nitriding for 4 h at different temperatures, as indicated.

Table 1

Corrosion parameters obtained from cyclic potentiodynamic polarization curves

recorded in 0.9% NaCl solutions for AISI H13 tool steel specimens before and after

plasma nitriding under different conditions.

Entry Nitriding

time (h)

Substrate

temperature (C)

OCP (V/SCE) Epit(V/SCE)

1-H13 0.797 0.472

2 4 360 0.529 0.292

3 4 400 0.490 +0.0624 4 440 0.265 +0.445

5 4 480 0.555 0.288

6 4 520 0.288 +0.787

7 1 400 0.520 0.241

8 4 400 0.490 +0.062

9 9 400 0.357 +0.665

10 16 400 0.395 +0.725

11 36 400 0.424 +0.580

Table 2

Layer thickness obtained from SEM images for AISI H13 tool steel samples nitrided at

400 C for different periods of time.

Nitriding time (h) Layer thickness (lm)

01 25.2

04 36.1

09 44.0

16 50.0

36 66.8


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The findings described above were confirmed by the XRD re-

sults (Fig. 7). In general, the interpretation of diffraction peaks de-

scribed in detail above also applies to the present case. Depending

on the nitriding time, three different phases corresponding to a-

ferrite (steel matrix), e-Fe2-3N and c0-Fe4N were identified in the

modified layer. The c0-Fe4N phase, however, was only detected

unambiguously for samples nitrided for long treatment periods

(36 h) at 400 C. Furthermore, no compact compound layer of e-

Fe2-3N and c0-Fe4N was detected under these experimental condi-

tions, as observed in the SEM imaging analysis (Fig. 5). This is inpart due to the fact that nitrogen diffusion is not sufficient to allow

such a structural rearrangement to occur within the nitriding time

scale investigated in this study. Therefore, the main phase in the

nitride layer should be e-Fe2-3N. According to XRD results, the e-

Fe2-3N phase is present in the diffusion layer and if the SEM images

do not show a compact compound layer, then this phase precipi-

tated as dispersed nitrides in the diffusion layer near the surface

region. Importantly, the precipitation of CrN could be effectively

avoided by selecting a reasonably low nitriding temperature and

preferentially for periods of time up to 16 h, since no diffraction

peaks characteristic of such a nitride are seen inFig. 7under such

conditions.

The corrosion properties were found to depend on the micro-

structure changes that were induced by the variation of the nitrid-

ing time. The OCP-values showed almost no variation afte

immersion in 0.9% NaCl (Fig. 8A), implying that reasonably stable

film/solution interfaces were produced during the surface treat-

ment. However, an exception was found for samples nitrided for1 h (the shortest time). In this case, a slight increase in the OCP oc-

curred during immersion, but the small amplitude of this variation

(DOCP 50 mV) means that the results are not conclusive. The

average OCP-values measured after 30 min of exposure (Table 1

entries 711) increased with nitriding time from 0.797 V/SCE

for untreated H13 specimens up to 0.357 V/SCE for samples ni

trided for 9 h. A small decrease to0.424 V/SCE was then observed

Fig. 5. SEM micrographs of cross-sections of AISI H13 tool steel samples after plasma nitriding at 400 C for 1 h (A), 9 h (B), 16 h (C), and 36 h (D).

Fig. 6. SEMmicrograph taken at high magnificationfor a cross-section of a AISI H13tool steel sample after plasma nitriding at 400 C for 36 h.

Fig. 7. XRD patterns of AISI H13 tool steel surface after plasma nitriding at 400 Cfor different exposure times, as indicated.


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for longer treatment times due to the presence of the c0-Fe4N phase

whose formation is favoured during long nitriding periods (Fig. 7).

Interestingly, however, the Epit-values did not immediately reflect

this drawback in terms of OCP-values (see Table 1, entries 711).

TheEpitincreased from0.472 V/SCE for untreated H13 specimens

up to +0.725 V/SCE for samples nitrided for 16 h, with a small

reduction to +0.580 being observed for samples nitrided for 36 h.In fact, the reduction in the Epitvalue in the latter case can be asso-

ciated with the CrN precipitation.

The cyclic polarization curves (Fig. 8B) showed an interesting

feature for this series of samples. The current density continued

to increase by almost one order of magnitude once the threshold

current was achieved and the potential scan direction was re-

verted, as indicated by the dotted line. This behaviour suggests a

very active and localized electrode dissolution (pitting corrosion).

Nevertheless, the onset of such a process can be significantly re-

tarded by plasma nitriding.

4. Conclusions

The microstructure and composition of nitride layers producedby plasma surface modification of AISI H13 tool steel can be con-

trolled by suitably selecting the substrate temperature and nitrid-

ing time during pulsed plasma processing. These parameters have

a direct effect on the pitting corrosion behaviour of the resulting

materials in 0.9% NaCl solutions.

The results of the present investigation indicate that e-Fe2-3N is

the main phase obtained at nitriding temperatures below 400 C.

The precipitation of this phase on the surface as well as in the dif-

fusion layer near the surface region can significantly improve thepitting corrosion resistance of AISI H13 tool steel. The formation

of a non-compact compound layer ofe-Fe2-3N and c0-Fe4N phases

above 400 C, in addition to the precipitation of CrN at tempera-

tures higher than 440 C, leads to a decrease in the pitting corro-

sion resistance. However, this drawback can be successfully

avoided by raising the substrate temperature to 520 C. Under such

conditions, a compact compound layer ofe-Fe2-3N and c0-Fe4N can

be produced, which offers the best pitting corrosion protection, as

confirmed by a substantial 1.25-V increase in the Epit-values.

The pitting corrosion resistance can also be improved by plasma

treatment at 400 C during periods of time varying between 1 and

36 h. At such a low substrate temperature the precipitation of CrN

is suppressed, and the c0-Fe4N phase does not appreciably affect

the corrosion performance, which remains practically unchanged

for nitriding times longer than 9 h.

Acknowledgments

The authors are grateful to UCS, CNPq, CAPES and FAPESP for

financial support. We are also indebted to the Laboratrio de

Implantao Inica e Tratamento de Superfcies (LIITS). I.J.R.B.,

A.S., C.G., C.A.F., and H.O.P. are CNPq fellows.

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Fig. 8. Representative open circuit potential (A) and cyclic potentiodynamic

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specimens after plasma nitriding at 400 C for different exposure times, as

indicated.


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