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Surface & Coatings Technology 187 (2004) 63–69

Microstructure and corrosion behaviour of DC-pulsed plasma nitrided

AISI 410 martensitic stainless steel

P. Corengiaa,*, G. Ybarraa, C. Moinaa, A. Cabob, E. Broitmanc

a Instituto Nacional de Tecnologıa Industrial, Av. Gral. Paz 5445, C.C. 157, (B1650 WAB) San Martın, Argentinab Ionar S.A., Arias 3422, C1430CRB Ciudad de Buenos Aires, Argentina

cDepartment of Chemical Engineering, Carnegie Mellon University, Pittsburg, PA 15213, USA

Received 5 August 2003; accepted in revised form 20 January 2004

Available online 27 March 2004

Abstract

AISI 410 martensitic stainless steel samples with different metallurgical structures were DC-pulsed plasma nitrided at 623, 723 and

773 K. The samples were ion nitrided in an industrial equipment using a gas mixture consisting of 25% N2+75% H2 under a pulsed

DC glow discharge. Optical and scanning electron microscopy, as well as glancing angle X-ray diffraction and microhardness

measurements have been used to study the ion nitrided surfaces. All plasma nitrided samples showed surface hardness values higher

than 1000 HV; the highest value was obtained at 673 K. The case depth was approximately 30 Am for samples nitrided at 673 and

773 K, while the sample nitrided at 623 K showed an ‘expanded ferrite’ phase (aN) and an incipient precipitation of Fe4N. Depending

on the treatment temperature and time, two fronts were formed, termed diffusion front and transformation front; the latter produces a

softening of the nitrided case. Electrochemical measurements showed a decrease of corrosion resistance in the samples nitrided during

20 h at 673 and 773 K. Conversely, the sample nitrided at 623 K presented a low corrosion current and more noble corrosion

potential.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Pulsed plasma nitriding; Martensitic stainless steel; Corrosion

1. Introduction

Stainless steels are usually employed as engineering

materials due to high corrosion resistance. However, low

wear resistance and poor tribological behaviour limit their

use in some applications. Therefore, there is an increasing

interest in improving surface properties through plasma

assisted thermochemical treatments, notably ion nitriding

[1–3].

Although stainless steels can be nitrided with the

consequential increase in surface hardness, which improves

their tribological performance, this is accompanied by a

loss of corrosion resistance of the nitrided case [1,4]. In

the last years, several investigations have been carried out

to solve this problem. Plasma nitriding has been success-

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.01.031

* Corresponding author. Tel.: +54-11-4724-6297; fax: +54-11-4752-

0818.

E-mail address: corengia@inti.gov.ar (P. Corengia).

fully employed at relatively low temperature (below 723 K)

[5,6], rather than at conventional nitriding temperatures of

approximately 873 K [7]. Further, it has been observed that

plasma nitriding at low temperature in austenitic stainless

steels can produce an extremely hard layer with good

corrosion resistance [8,9]. This layer contains a supersatu-

rated interstitial solid solution of N in austenite; this phase is

also termed expanded austenite (gN).

Although several studies were carried out on plasma

nitriding of austenitic stainless steels, few have been done

on martensitic stainless steels [10–12]. Nitrided martensitic

stainless steels remain a good choice for mildly corrosive

environments and low temperature applications [13,14].

However, the studies of Refs. [10–12] did not include any

corrosion study. In the present work, an AISI 410 martens-

itic stainless steel with different metallurgical structures

(annealed and quenched-tempered) was DC-pulsed plasma

nitrided at different treatment temperatures. The obtained

metallurgical structures as well as microhardness profiles

and corrosion behaviour were analysed and discussed.

P. Corengia et al. / Surface & Coatings Technology 187 (2004) 63–6964

2. Experimental

The alloy studied here (AISI 410) possesses the follow-

ing composition (wt.%): C, 0.13; P, 0.022; S, 0.005; Mn,

0.74; Cr, 12.93; Mo, 0.170; Al, 0.015; V, 0.030; Si, 0.43; Ni,

0.28; Fe, balance. Two different treatments were applied to

the substrate before nitriding: annealed (AN) and quenched-

tempered (QT).

The hardness of AN samples was 212 HV with a

structure of carbide precipitates in an a-ferritic matrix; some

delta ferrite grains were also observed. QT samples were

heat treated according to the NACE MR0175-2001 standard

[15] (austenization at 1243 K and tempered at 943 K)

obtaining a hardness of 250 HV and a tempered martensitic

structure.

Samples, consisting of discs with a diameter of 45 mm

and 8 mm thick, were fine-ground using wet SiC emery

papers to a 600-grit finish, and cleaned with acetone in an

ultrasonic cleaner before being placed in the chamber.

Previous to ion nitriding process, the samples were

sputter cleaned with a gas mixture composed of 50%

Ar+50% H2 during 2 h to remove the passive film formed

on stainless steel [16]. This mixture combines the mechan-

ical removal properties of Ar and the reducing chemical

effect of hydrogen on Cr2O3.

Treatments were carried out in a plasma nitriding indus-

trial equipment described previously [17], and the process

parameters used in the nitriding of the steel can be found in

Table 1.

Dual heating with electric resistances and ion bombard-

ment have been used to reduce dimensional distortions of

the specimens [17]. After the nitriding process, the samples

were slowly cooled down to room temperature in a nitrogen

atmosphere.

In order to retain the edges during metallographic

preparation, the nitrided samples were nickel electroplated

and sectioned for metallographic examination and hardness

profile determination. The microstructure was revealed

using Vilella and Marble reagents, and was examined with

a Zeiss Axiotech optical microscope (OM) and a Philips

SEM 505 scanning electron microscope (SEM). Micro-

hardness was measured with a Vickers microhardness

tester (Akashi, MVK-H2) using a load of 25 g and three

readings were taken to determine the exact value. The

thickness of the nitrided case was determined from SEM

Table 1

Main nitriding parameters

Parameter Value

Nitriding time 20 and 28 h

Voltage between electrodes 750 V

Current density 1.03 mA cm�2

Pressure 6 hPa

Temperature 623, 673 and 773 K

Plasma chamber atmosphere 75% H2+25%N2

ton�toff 70–200 As

images. The glancing angle X-ray diffraction measure-

ments (GIXRD) were carried out with an RX Philips

model PW3710 with two vertical goniometers; Co Ka

radiation was used to determine the phases present in the

modified layer.

The corrosion behaviour was evaluated by electro-

chemical methods. The measurements were performed

with an EG&G PAR 273A potentiostat, using a conven-

tional three-electrode cell. The test solutions of 3% NaCl

were made from analytical grade reagent and distilled

water. The counter electrode was a Pt foil and all the

potentials were measured against a saturated calomel

electrode (SCE). The potentiodynamic curves were mea-

sured at a potential scan rate of 1 mV/s, starting from a

potential of �1.00 V toward the anodic direction up to a

current density value of 1 mA cm�2. Electrochemical

measurements were also performed on the unnitrided

back of the samples to evaluate the sensitization effect

due to the heat treatment associated to the nitriding

process.

3. Results

3.1. Microhardness

Fig. 1 shows microhardness profiles of AN and QT

DC-pulsed ion nitrided samples at 623, 673 and 773 K

during 20 h. Microhardness profiles obtained from cross

sections of processed specimens show the presence of a

sharp interface between the case (nitrided layer) and the

core (matrix). All samples show high surface microhard-

ness values ( > 1000 HV) that drop abruptly at the case/

core interface to substrate microhardness values. It can

be observed in Fig. 1 that higher surface hardness

values are obtained for nitriding temperatures of 623

K and 673 K.

3.2. Microstructures

Fig. 2 shows a SEM image of an AN sample DC-pulsed

ion nitrided during 20 h at 623 K. An incipient precipitation

in certain directions can be observed close to the surface.

Images Fig. 3a,b show cross sections of AN and QT

samples ion nitrided during 20 h at 673 K. In Fig. 3a, a

white nitrided case of approximately 30 Am thick can be

observed, with some dark spots and voids, apparently along

grain boundaries. Delta ferrite grains, lined up following the

direction of the rolling process, can be observed below the

nitrided case (Fig. 3b).

Fig. 4 shows an optical micrograph of an AN sample

DC-pulsed plasma nitrided during 20 h at 773 K. Three

regions can be distinguished: a dark near surface zone,

followed by a white one and the unnitrided core. Some

specimens were nitrided for a longer time with the aim of

studying the dark front evolution.

Fig. 2. SEM image of an AN sample AISI 410 steel DC-pulsed ion nitrided

at 623 K for 20 h.

Fig. 1. Microhardness profiles of the samples DC-pulsed plasma nitrided for 20 h at: 623 K (a), 673 K (b) and 773 K (c). QT: () and AN:(o).

P. Corengia et al. / Surface & Coatings Technology 187 (2004) 63–69 65

Fig. 5 shows a SEM image of an AN sample ion nitrided

at 773 K during 28 h, where the dark zone occupies the

complete nitrided case. It is possible to observe a lamellae-

like structure inside the dark zone.

Fig. 6a shows the GIXRD pattern obtained from an AN

unnitrided sample and a DC-pulsed ion nitrided during 20

h at 623 K specimen of AISI 410 martensitic stainless steel

with AN structure. The figure evidences the presence of an

a-Fe peak for the unnitrided sample. For the nitrided

sample, in addition to the a-Fe peak, the presence of a

minor phase of gV-Fe4N was detected; it is worth noting that

no evidence of CrN was found. It can also be observed that

the a-Fe peak is shifted towards lower diffraction angles and

broadened considerably.

Fig. 6b,c show the diffractograms of samples ion nitrided

during 20 h at 673 K and 773 K, respectively. The images

show that in both cases, the main components of the nitrided

layer are gV-Fe4N, q-Fe2–3 N, CrN and a-Fe. It is also

observed that the a-Fe peak sharpens and returns to the

unnitrided higher diffraction angle (Fig. 6c).

Fig. 3. OM and SEM images of AISI 410 martensitic stainless steel DC-pulsed ion nitrided at 673 K for 20 h. (a) QT; and (b) AN.

P. Corengia et al. / Surface & Coatings Technology 187 (2004) 63–6966

3.3. Electrochemical measurements

Fig. 7 shows anodic polarization curves for an AN steel

sample in chloride media, in the presence of oxygen. The

anodic branch presents a complex shape, where three mixed

potentials are apparent. This behaviour can be attributed to

the oscillation of the system between the active and passive

states, resulting in the presence of multiple steady states

[18]. At more positive potentials, a well-defined passive

region is observed up to approximately 0 V, where the

current increases abruptly indicating pitting. For the sake of

comparison, the electrochemical measurements on AN

samples, which were tempered at 673 and 773 K, are

included in Fig. 7. The sample tempered at 673 K shows

a passive zone followed by a steady increase of the current

beyond a well-defined breakdown potential, at approximate-

ly �150 mV vs. SCE. The sample tempered at 773 K

present an active/passive transition, followed by a poorly

Fig. 4. Optical micrograph of an AN sample of AISI 410 steel DC-pulsed

ion nitrided at 773 K for 20 h.

defined passive region and a steady increase of the current at

potentials positive to approximately �150 mV. The corro-

sion currents for the sample tempered at 773 K are approx-

imately an order of magnitude higher than those for samples

tempered at 673 K.

The polarization curves for AN samples DC-pulsed

plasma nitrided during 20 h at 623, 673 and 773 K are

depicted in Fig. 8. A noteworthy shift of the corrosion

potentials to more positive values is observed as the

nitriding temperature is decreased. The samples nitrided at

773 and 673 K show active dissolution of the metal at

potentials positive to corrosion potential, Ecorr. In the case of

the sample treated at 773 K, the active dissolution is

preceded by an oscillation of the system between multiple

steady states.

The sample ion nitrided at 623 K showed the more noble

corrosion potential and the lowest corrosion current density

of all the three specimens investigated.

Fig. 5. SEM image of an AN sample AISI 410 steel DC-pulsed ion nitrided

at 773 K for 28 h.

Fig. 7. Anodic polarization curves for: (—) unnitrided AN substrate, ( – – )

AN sample tempered at 673 K, (����) AN sample tempered at 773 K.

P. Corengia et al. / Surface & Coatings Technology 187 (2004) 63–69 67

4. Discussion

The nitrided layer formed in treated specimens presented

microstructures and surface properties dependent on the

process temperature.

Samples ion nitrided at the lower temperature only

showed a-Fe and small amounts of gV-Fe4N in the surface

(Fig. 6a) but, as expected, the presence of CrN nitrides was

not detected because CrN precipitation takes place at

higher temperatures. The shift and broadening of the a-

Fe peaks observed in Fig. 6a are associated with the N

oversaturation and stress caused by the N remaining in

solid solution. Kim et al. [12], who investigated low-

pressure RF ion nitrided AISI 420 stainless steel samples,

called this phase ‘expanded martensite’ by analogy with

the designation ‘expanded austenite’ in previous results

[19]. Following those criteria, we will term this phase as

‘expanded ferrite’, aN. Marchev et al. [10] have studied

samples AISI 410 plasma nitrided treated at different

temperatures and processing times, and reported that sam-

ples treated at 673 K contain one or more undetermined

complex martensitic structures; this would seem to be

consistent with our results. This expanded phase, in addi-

tion to small Fe4N nitrides precipitates, produces the high

surface hardness values observed in Fig. 1.

Fig. 6. Glancing-incident X-ray diffraction patterns of an AN sample: (a)

untreated and plasma nitrided at 623 K; (b) nitrided at 673 K; and (c)

nitrided at 773 K.

At higher temperatures, the nitrogen solid solution in aN

would decompose into a and CrN (Fig. 6b,c). Similar

precipitation has been observed in austenitic stainless steel

samples [20]. The precipitation of CrN depletes the expand-

ed phase of chromium, favouring the formation of ferrite

and CrN in a lamellar structure, observed inside the dark

areas (Fig. 5), resembling pearlite in plain carbon steels.

This precipitation (aN!CrN+a) produces a structure relax-

ation, and consequently the a peaks go back to the original

position as in the untreated sample (Fig. 6b,c). The lamellar

precipitation, detected in the sample nitrided at temperatures

above 673 K, is observed as dark zones at lower magnifi-

cations (Fig. 4).

The dark front evolution depends on the nitriding tem-

perature and time. This front initially grows from grain

boundaries (Fig. 3) in the nitrided case and continues until

the whole matrix has been transformed (Fig. 5). The dark

front is represented by the border of transformed areas while

Fig. 8. Anodic polarization curves for DC-pulsed plasma nitrided AN

samples during 20 h at: (—) 773 K, (– – ) 673 K and 623 K (�����).

P. Corengia et al. / Surface & Coatings Technology 187 (2004) 63–6968

the white front, termed diffusion front, runs parallel to the

nitrided surface [21].

The AN and QT specimens DC-pulsed plasma nitrided

during 20 h at 773 K (Fig. 1c) exhibit in the transformed

zone a decrease in hardness values in comparison with

the untransformed areas, which retain a relative high

hardness, approximately 1300 HV. The difference in

hardness between the transformed and untransformed

zone is approximately DHV=300 HV. The white layer

produced at 773 K is brittle, as evidenced by the many

cracks observed at the original grain boundaries, which

are also the preferred sites for the nucleation of the dark

phases (Fig. 3b). The high hardness values in the un-

transformed area might be a consequence of the mor-

phology of precipitates, which are coherent and of

submicroscopical size. Therefore, an optimum nitriding

temperature exists at which the highest hardness value

may be achieved.

The corrosion measurements show a distinct shape of the

polarization curves of the untreated samples as compared

with the specimens tempered at 673 and 773 K (Fig. 7). In

the former, pitting corrosion starts at potential positive to 0

V vs. SCE. In the tempered samples, however, the passivity

breakdown is shifted toward less noble potentials and is

associated with intergranular corrosion. This change indi-

cates a typical sensitization effect that can be associated

with the depletion of chromium in the grain boundaries due

to the formation of Cr carbides. This effect is more notice-

able at higher temperature because it is a thermally activated

process [19].

The DC-pulsed nitriding at 673 and 773 K significant-

ly deteriorates the corrosion resistance of the steel samples

(Fig. 8). No passivation is apparent and active dissolution

of the metal occurs at potentials positive to corrosion

potential, Ecorr. From the comparison of Figs. 7 and 8, it

can be concluded that in this case, the corrosion behav-

iour is mainly determined by the depletion of chromium

from the solid solution due to CrN precipitation (e.g. Fig.

6b,c) [1,4]. This effect largely predominates over the

sensitization due to Cr carbides precipitation. The sample

nitrided at 623 K, however, presents the more positive

Ecorr and the lower corrosion current, Icorr, in the set of

samples investigated. This probably indicates a more

noble behaviour of the ‘expanded ferrite’ phase, as

compared to the Cr depleted solid solutions obtained at

higher temperatures.

The samples AN and QT show similar microstructural

aspects and corrosion behaviour. Nevertheless, the differ-

ence in hardness values between samples AN and QT

plasma nitrided during 20 h at 623 and 673 K can be

ascribed to dissimilar matrix Cr content. The AN samples

have a relatively low free Cr content in the ferritic

structure and therefore a small quantity of Cr nitrides is

formed. This implies the existence of an incubation time

to increase the hardness by the precipitation of coherent

Cr–N particles.

5. Conclusions

The DC-pulsed ion nitriding of AN and QT samples of

AISI 410 during 20 h at 623, 673 and 773 K revealed

surface hardness above 1000 HV. A sharp interface between

the case (nitrided layer) and the core (matrix) was observed

due to precipitation of CrN in the nitriding process. Higher

surface hardness values (approx. 1200 HV) are obtained for

treatment temperatures of 623 and 673 K. It is evident that

the nitrided case formed at 673 K is more uniform and has a

higher thickness, which will have a marked influence on its

mechanical performance.

The samples nitrided at lower temperature showed an

incipient precipitation of Fe4N and expanded ferrite.

The specimens treated at temperatures above 673 K were

characterized by the presence of two fronts: transformed

(dark) and untransformed (white) front. The dark front

evolution depends on the nitriding temperature and time.

This front grows from the grain boundaries in the nitrided

case and continues until the whole of the matrix has trans-

formed. Apparently, the structure formed at low temperature

is unstable and, with increasing temperature, it gradually

decomposes forming the dark areas that present a lamellae-

like structure of CrN and a-Fe.

Sensitization to intergranular corrosion can be observed

as a consequence of the tempering associated to ion nitrid-

ing process. Higher sensitization effect was detected in

samples tempered during 20 h at 773 K. Electrochemical

measurements indicate that ion nitriding (at 673–773 K)

reduces the corrosion resistance. This can be attributed to

the depletion in Cr of the steel matrix owing to the CrN

precipitation. The sample nitrided at 623 K shows lower Icorrand more positive Ecorr, probably due to a more noble

behaviour of the ‘expanded ferrite’ phase.

Acknowledgements

This investigation was carried out at the CEMEC and

CIEPS-INTI. The authors want to thank the collaboration

received from Adolfo Rodrigo, Fernando Rodriguez and

Fabian Alvarez from CNEA for discussions and microhard-

ness testing; Horacio De Rosa and Hernan Svoboda from

the University of Buenos Aires for discussions and optical

micrographs; Maria Emilia Rapp for GIXRD measurements;

Clodomiro Alves Jr from Laboratorio de Processamento de

Materiais por Plasma from UFRN (Brazil), Rodrigo Corbari

from UFRGS (Brazil) and Gabriel Lopez from Max-Planck-

Institut fur Metallforschung (Germany) for discussions.

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