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Surface & Coatings Technolog

Effect of nitriding time on the nitrided layer of AISI 304

austenitic stainless steel

Liang Wang *, Shijun Ji, Juncai Sun

Institute of Metals and Technology, Dalian Maritime University, Dalian 116024, PR China

Received 19 October 2004; accepted in revised form 19 May 2005

Available online 11 July 2005

Abstract

The effect of plasma nitriding time on the microstructure and phase composition of nitrided layers on AISI 304 stainless steel was

investigated. The phase composition and structure of the nitrided layer have been studied by X-ray diffraction (XRD) and scanning electron

microscopy (SEM). The XRD analysis of samples treated at 420 -C showed the presence of gN phase in the nitrided layers for all nitriding

times involved in this study. The lattice parameters calculated based on gN(111) and gN(200) were different and became larger with time for

up to 5 h of nitriding treatment. The surface hardness of nitrided layer was also increased with nitriding time. The maximum thickness of the

nitrided layer reached 27 Am after 44 h of treatment in this study.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Austenitic stainless steel; Plasma nitriding; Expanded austenite

1. Introduction

Plasma nitriding is commonly used to increase the

materials’ hardness, wear or corrosion resistance. Conven-

tionally, the nitriding temperature is above 500 -C in order to

get a relatively thick hard layer, but, for austenitic stainless

steels nitrided at temperatures above 450 -C, precipitation ofchromium nitrides occurs which depletes Cr from solid

solution. With the lack of Cr in solid solution, stainless steels

lose their corrosion resistance. Following this, the objective

of any nitriding process for austenitic stainless steel is both

to create a hard layer on the surface and to retain corrosion

resistance. Nitriding of the stainless steels by different

plasma processes at temperatures of about 400 -C generally

has no adverse effect on corrosion performance. A variety of

processing techniques have been developed to form a hard

nitrided layer composed of gN phase with fcc structure

(expanded austenite) on austenitic stainless steel surfaces at

low temperatures. These techniques include plasma nitriding

[1–3], low energy ion implantation [4,5] and plasma source

0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.05.036

* Corresponding author. Tel.: +86 411 4727975.

E-mail address: wlimt@yahoo.com (L. Wang).

ion implantation [6–8]. Among them, plasma nitriding

offers unique advantages for practical applications. The

structure of gN layers has been intensively studied during the

last 10 years to explain their nature and properties. Previous

work [9,10] has shown that the nitrogen content in the layers

can be varied over a wide range from 0 to over 40 at.%

nitrogen. The maximum solubility limit of nitrogen in

expanded austenite is not known exactly and the structural

stability of the nitrogen supersaturated layer is an open

question. Since a close relationship exists between the

material’s properties and microstructure, altering processing

parameters can affect structural, compositional and hence

property changes in nitrided layer.

In this article, we report on low-temperature nitriding of

AISI 304 austenitic stainless steel using plasma nitriding and

study the influence of processing time on the microstructure,

surface hardness and phase composition in the nitrided layer.

2. Experimental

The substrates were made of AISI 304 stainless steel with

the composition (wt.%): C 0.05, Cr 18.9, Ni 9.20, Si 0.80,

y 200 (2006) 5067 – 5070

Fig. 2. The thickness of the nitrided layer versus processing time.

L. Wang et al. / Surface & Coatings Technology 200 (2006) 5067–50702068

Mn 2.00, S 0.02, P 0.02 and Fe balance. The plasma

nitriding of austenitic stainless steels was performed using a

commercial furnace equipped with a mechanical pump

yielding a base pressure of 1�10�1 Pa. For the nitriding

process, ammonia was fed into the vacuum chamber. The

working pressure during nitriding was 100 Pa in all cases.

The voltage applied between cathode and anode was 600–

650 V and the current density on sample surface was 0.5–

1.0 mA/cm2. During nitriding, the substrate temperature was

controlled to a value of about 410–420 -C using a

thermocouple. Cross-sectional micrographs were studied

by scanning electron microscopy (SEM). The surface

hardness was measured using a MH-1 Vickers microhard-

ness tester with loads of 25 g, 50 g and 100 g. The layers

were investigated by X-ray diffraction using Bragg–

Brentano gonionmeter with Co-Ka radiation in a Rigaku

Dmax-3A X-ray diffractometer. This XRD geometry exam-

ines the diffraction peaks from crystallographic planes

parallel to the specimen surface. The dependence of the

X-ray structural characteristics on the layer thickness is

described in the present paper with the aim of observing

structural development with the nitriding time.

3. Results and discussion

Fig. 1 shows the cross-sectional morphology of plasma

nitrided layer obtained at 420 -C for different nitriding times

Fig. 1. Micrographs of nitrided layer formed by plasma nitriding of austenitic stainl

(c) 22 h and (d) 44 h, respectively.

of (a) 2 h, (b) 12 h, (c) 22 h and (d) 44 h. The layer was

featureless when nitriding time was within 8 h. Some

defects or precipitations appeared in the nitrided layers for

long time nitriding (>12 h). Measurements of the thickness

of the nitrided layer on AISI304 stainless steel samples

confirmed that the growth of layer with the t1/2 law about a

temperature of approximately 420 -C. The thickness of the

layers prepared with different times is shown in Fig. 2. The

growth of the nitride layer takes place mainly by nitrogen

diffusion according to the expected parabolic rate law. The

ess steel AISI 304 at 420 -C for different processing times of (a) 2 h, (b) 12 h,

(a)

γ(200)

γ(111)

γN(200)

γN(200)

γN(111)

γ

2h

0.5h

Substrate

Inte

nsity

(a.

u.)

30 35 40 45 50 55 60 65 70 75 80 85 90

Inte

nsity

(a.

u.)

(b)N(111)

44h

22h

12h

Two-theta (degrees)

30 35 40 45 50 55 60 65 70 75 80 85 90

Two-theta (degrees)

Fig. 3. XRD patterns for surface nitrided layer after plasma nitriding of

AISI 304 austenitic stainless steel at 420 -C for different processing times

of (a) untreated, 0.5 and 2 h and (b) 12, 22 and 44 h.

Fig. 4. Surface microhardness versus processing time.

L. Wang et al. / Surface & Coatings Technology 200 (2006) 5067–5070 2069

thickness of the nitrided layer is less than 3 Am when the

nitriding treatment was carried out at 420 -C for 30 min,

while its thickness increases up to 27 Am with a 44 h

nitriding treatment.

In Fig. 3 there is a set of typical X-ray diffraction

patterns of the nitrided layers. Broad gN diffraction peaks,

Table 1

Lattice parameters and lattice spacing calculated from (111) and (200) planes for

Nitriding time (h) a(111) (nm) a(200) (nm) d(111) (nm

0.0 3.596 3.594 2.076

0.5 3.731 3.748 2.154

1.0 3.809 3.902 2.199

2.0 3.873 3.942 2.236

3.0 3.884 3.940 2.242

7.0 3.880 3.942 2.240

10 3.895 3.960 2.249

12 3.888 3.942 2.245

30 3.900 3.942 2.251

44 3.911 3.967 2.258

which correspond to the nitrogen expanded austenite and

which are typical of low temperature nitriding of stainless

steel, can be seen in addition to the austenite reflections

from the substrate material. This broadening is probably

due to the gradient of nitrogen, residual stresses, and

possible defect structure of the nitrided layers. It can be

observed that the intensity of the peaks related to the gNphase, in comparison with peak intensity from substrate,

increases as the plasma nitriding time increases. Gradual

peak shift can be observed, and especially the (200)

reflection has moved considerably towards larger d spac-

ing. The layer prepared for 30 min nitriding also showed

evident expanded austenite peaks. Table 1 gives the

estimated lattice parameters and interplanar spacing from

Braggs law as a function of nitriding time. The observation

that the lattice parameter becomes larger when nitriding

time increases is in good agreement with the results of

other authors [9–12]. The most remarkable result of the

present X-ray diffraction study of nitrided layers is the

strong dependence of the lattice parameter values on the

thickness of the layers and the large differences in these

values when they are calculated from different crystallo-

graphic planes parallel to the substrate surface. The lattice

parameter calculated from (200) and (111) planes first

slightly increases as the nitriding time increases becomes

different nitriding times

) d(200) (nm) Dd/do(111) (%) Dd/do(200) (%)

1.797 0.00 0.00

1.874 3.80 3.65

1.951 5.77 7.73

1.971 7.55 8.83

1.971 8.00 8.83

1.971 7.90 8.83

1.980 8.33 10.18

1.971 8.14 8.83

1.971 8.43 8.83

1.983 8.77 10.35

L. Wang et al. / Surface & Coatings Technology 200 (2006) 5067–50702070

almost constant after 3 h. The lattice parameter calculated

from (200) becomes considerably higher than lattice

parameters calculated from (111) planes after 7 h nitriding.

This behavior of the lattice parameter results obviously

from the gradual distortion of the cubic symmetry of the

lattice. This phenomena has also been observed in films

deposited by physical vapor deposition. Previous studies of

TiN films have shown that the lattice parameters of highly

stressed films differ when measured on different lattice

planes [13,14].

The microhardness of nitrided layers as a function of

nitriding time is shown in Fig. 4 where the hardness

increased with the increase of nitriding time. The increase in

hardness with increasing nitriding time is due to the increase

of nitrided layer thickness and high nitrogen content in the

layer. The large increase in the measured values of the

microhardness with increasing nitriding time can be

explained by the known [15] contribution of the substrate

to the measured hardness, which becomes less evident as the

layers become thicker with increasing nitriding time. For the

sample nitrided at temperature 420 -C for 5 h, the hardness

of the sample was increased by approximately a factor of 5

compared with the nitrided substrate. The highest hardness

value obtained in this experiment was about 1400 kg/mm2.

The extremely high values of the microhardness observed

can be explained by large compressive stresses in the layers

[16,17].

4. Conclusions

Austenitic stainless steel nitrided at 420 -C for different

times has been investigated. Both thickness and microhard-

ness measurements indicated that the effect of the nitriding

increased with nitriding time. An expanded austenite layer

was formed on the surface of substrate with the thickness

ranging from 2 Am to 27 Am. The hardness was enhanced by

the formation of nitrided layer due to the nitrogen diffusion

inward substrate. The microhardness increased with time,

while for nitriding times in excess of 7 h at 420 -C the value

of hardness did not change much. Although X-ray

diffraction showed that the nitrided layer was composed

of nitrogen expanded austenite (gN) for all samples in this

study the SEM observation indicated that some defects and

chromium nitride precipitation appeared in the nitrided layer

after longer nitriding time.

Acknowledgments

Financial support of this project by the National Science

Foundation of China under Grant No. 10175012 is grate-

fully acknowledged.

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