M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

Effect of the ion nitriding surface hardening process on fatiguebehavior of AISI 4340 steel

Sule Yildiz Sirina, Kahraman Sirinb, Erdinc Kalucc,⁎aVocational School of Asim Kocabiyik, Kocaeli University, 41800 Hereke- Kocaeli, TurkeybNoksel Steel Pipe Industries Company, 54300 Hendek- Sakarya, TurkeycDepartment of Mechanical Engineering, Engineering Faculty, Kocaeli University, 41040 Kocaeli, Turkey

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +90 262 335 11 4E-mail address: ekaluc@kou.edu.tr (E. Kalu

1044-5803/$ – see front matter © 2007 Elsevidoi:10.1016/j.matchar.2007.01.019

A B S T R A C T

Article history:Received 12 June 2006Received in revised form24 January 2007Accepted 30 January 2007

Ion nitriding is a glow discharge surface modification technique, which is primarily used toincrease the fatigue strength, wear, corrosion resistance and surface hardness of steels.Because of the formation of high compressive residual stresses in the case region, increasingsurface hardness and case depth cause remarkable improvement in fatigue properties ofsteels. In this study, the ion nitrided properties of quenched and tempered AISI 4340 lowalloy steel were investigated under different process parameters including time andtemperature. It has been found that the ion nitriding surface treatment improves the fatiguestrength and increases the fatigue limit depending on the case depth. Up to 91%improvement in fatigue strength of the steel has been attained by ion nitriding. It wasdetermined that, the subsurface ‘fish eye’ type formation is the dominant fatigue crackinitiation mechanism in ion nitrided AISI 4340 steel in high cycle region and its origin wasnonmetallic inclusions.

© 2007 Elsevier Inc. All rights reserved.

Keywords:Ion nitridingFatigue strengthMechanical propertiesAISI 4340 steel

1. Introduction

Plasma or ion nitriding is amethod of surface hardening usingd.c. glow discharge technology to introduce elemental nitro-gen to the surface of metal part for subsequent diffusion intothematerial [1,2]. The ion nitriding process which is applied toferrous materials has been extensively used to improve thesurface properties, wear, fatigue, corrosion and frictionproperties and also load bearing capacity of dynamicallyloaded components [3–6]. During the ion nitriding process,the nitriding reaction not only occurs at the surface but also inthe subsurface owing to the long distance diffusion of nitrogenatoms from the surface towards the core. Diffused nitrogeninto a steel surface is combinedwith alloying elements to forma fine dispersion of alloy nitrides. As a result, a thin ironnitride layer, which consists of γ-Fe4N and/or ε-Fe2,3N inter-metallic phases, referred to as the ‘white layer’ because it isnot attacked by alcoholic nitric acid etch [7,8] is produced on

8x1147; fax: +90 262 335c).

er Inc. All rights reserved

the surface together with a relatively thick and strongdiffusion zone in the subsurface of a steel component, whichgradually reduces the hardness and nitrogen concentrationtowards the core, resulting in a diffuse case–core interface [9].The hardness and the depth of the diffusion layer dependmainly on the amount of nitride-former elements in the steels[10–12], the time and the temperature of the ion nitridingtreatment [12–14], the specific gas composition used in an ionnitriding system [15] and the initial microstructure of thematerial to nitriding [16,17]. Ion nitriding has recently receivedconsiderable industrial interest owing to its characteristic offaster nitrogen penetration, short treatment time, low processtemperature,minimal distortion, clean specimens, low energyuse and easier control of compound and diffusion layer forma-tion compared with conventional techniques such as gas andliquid nitriding [8,18–21]. Nitriding has grown especially fromthe fact that, through nitriding very hard surface layersmay beobtainedwithout substantialmodification of the bulkmaterial

28 80.

.

Fig. 1 –Fatigue test specimen, dimensions in mm.

352 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

properties [12]. This led to numerous applications of theprocess in industries such as the manufacture of machineparts for plastics and food processing, packaging and tooling,as well as pumps and hydraulic machine parts, crankshafts,rolls and heavy gears, motor and car construction, cold andhot working dies and cutting tools [1,2].

Fatigue strength can be significantly improved by nitriding,and this improvement attributes to increasing surface hard-ness and residual stress distribution [22–28] or increasing casedepth [9,14,24,25,29]. At the first model, improvement infatigue behavior attributes to alterations of the surfaceproperties and some other causes, as follows. The formationprecipitate particles can hinder dislocation motion and,therefore, slip band penetration through the nitrided layer. Adiffusion layer, which is formed during ion nitriding process,causes compressive residual stresses on or near the surface ofmachine parts, as well as low tensile residual stress in thecore. Both increased residual stress and surface hardnessresult in improvements in the fatigue life because the thin,hard layer prevents plastic flow. Therefore, the slip bands canonly be activated by very high stresses in this region. Thismeans that plastic deformation, which is the source of crackinitiation, probably appears in the subsurface. The suppres-sion of slip band formation at the surface delays crackinitiation and propagation, thus being responsible for theimprovement of fatigue life [27]. At the other model, the effectof increasing the case depth on the fatigue limit can be viewedas effectively moving the fatigue crack initiation site furtherinto the core. This means that a greater applied bending stresswill be required at the surface to create a sufficiently high levelof stress at the case–core interface to initiate failure [24]. In thehigh cycle regime, it was also determined that the subsurfacefatigue crack formation was dominant in failure initiationmechanism in ion nitrided steels [8,9,14,24–30]. Not only thelevel of applied stress and the thickness of nitrided case ofspecimens, but also the inclusion size, type and orientationare very effective in the determination of crack origins[14,31,32].

There have been studies of the effects of plasma nitridingon the rotating bending fatigue strength of AISI 1045 [30,33],AISI 4140 [14,28], AISI 5140 [34], EN40B [9,24], 34CrNiMo6 [16],AISI 15B21H steel [35,36] and AISI 304L [37], AISI 316 [38], 3CR12stainless steel [38]. However, there has not been any informa-tion about fatigue behavior of ion nitrided AISI 4340. There-fore, in the present research AISI 4340 low-alloy steel was ionnitrided in NH3 under different conditions including time (2, 4,8, 16 h) and temperature (500, 520, 540 °C) and their effect onthe rotating bending fatigue strength was investigated.

2. Experimental Work

A standard medium carbon, CrMoNi low alloy AISI 4340 steel,widely used in the production of automotive crankshafts, rear

Table 1 – Chemical composition of AISI 4340 steel

C Si Mn P S Cr Mo Ni Fe

0.38 0.26 0.69 0.016 0.004 0.80 0.22 0.69 Bal.

axle shafts, aircraft crankshafts, connection rods, propellerhubs, gear, drive shafts, landing gear parts and heavy dutyparts of rock drills, has been used in this study. The chemicalcomposition of the steel is given in Table 1. The 22 mmdiameter steel was received in normalized condition andaustenitized at 840 °C for 30 min, quenched in oil and thantempered at 600 °C for 1 h. The hourglass shaped, rotatingbending fatigue test specimens were machined according toDIN 50113 (Fig. 1) at CNC turning lathe with very sensitivedimension. The surface of the reduced section was groundthrough 200, 400, 600 and 1200 grades emery papers and thenmechanically polished by using alumina slurry. The surfaceroughness within the gage length of the samples wasmaintained below approximately 0.3 μm.

Ion nitriding treatment was applied at industrial ionnitriding furnace, AN-50 Plus and specimens were employedas the cathode. Specimens were ion nitrided for 2, 4, 8, 16 h at500, 520, 540 °C with NH3 as treatment gas. The process timewas accounted after treatment temperature was reached.After ion nitriding, the specimens were cooled to roomtemperature in the vacuum chamber. Microhardness mea-surements were used to determine diffusion layer case depthaswell as to aid in characterizing the physical properties of thecase as a function of depth [12,13]. The microhardness of theion-nitrided surface was evaluated using a FutureTech micro-hardness tester with a load of 200 g. The hardness at 20 μmdepth was chosen for comparison of surface hardness, so thatany possible effects from a compound layer would benegligible. The case depth was defined as the depth at whichthe hardness is 10% Vickers above the core hardness. For eachspecimen's hardness, profile was determined by three mea-surements at the cross section and the average was plotted.Optical microscopy was used for the determination of thewhite layer thickness and also phase analysis was conductedwith a Shimadzu WXRD-6000 X-ray diffractometer using CuX-ray tube (λ=1.5405 Å). The fracture surfaces of specimenswere examined under optical microscopy and scanningelectron microscopy in order to evaluate the crack initiationand growth characteristics of the materials in the core andnitrided case regions of specimens. The main purpose of mostengineering fatigue tests is to determine the relationshipbetween the applied load and the number of load applicationsto cause failure, and to obtain some estimate of the probabilityof failure under specified loading conditions. Fatigue strengthwas determined by using a rotating bending fatigue machine.Rotating bending fatigue test were conducted by being use asinusoidal load of frequency 95 Hz (5700 rpm) in laboratory airatmosphere and load ration R=− 1. Specimens ranging from14 to 20 were used to obtain the fatigue strength for each

Fig. 2 –The microstructure of the nitrided material at 500 °Cfor 4 h. Fig. 3 –The relationship between compound layer thickness

and treatment time at 500 and 540 °C.

353M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

condition of nitriding. In each test, the number of cycles tofatigue failure was noted; the tests were terminated if nofailure had occurred after 106 cycles and at least two speci-menswere tested for one stress level. Statistical analysis usingfour or five stress levels was done after the fatigue test results.ASTM E739-91 standard was employed to determine fatiguestrength.

3. Results and Discussion

The microstructure of the nitrided material is shown in Fig. 2which is ion nitrided at 500 °C for 4 h. The compound layerthicknesses, case depth, surface hardness, and fatiguestrength of ion nitrided specimens are given in Table 2 withsome dimensionless parameters like relative case area AR,relative case depth t/D and D/(D−2t), together with details ofapplied heat treatments.

Table 2 – Structural and fatigue properties of ion nitrided AISI 4

Treatment details Surfacehardness,

HV0,2

Comp.layer,μm

Casedepth,t μm

Recas

Austenitized 840 °C, 30 m/oilquenched tempered at 600 °Cfor 1 h (Q.T.)

– – –

Q.T. and 2 h, 500 °C ion nitrided 510 2.6 280Q.T. and 4 h, 500 °C ion nitrided 526 3.0 300Q.T. and 8 h, 500 °C ion nitrided 634 4.0 360Q.T. and 16 h, 500 °C ion nitrided 609 4.8 470Q.T. and 4 h, 520 °C ion nitrided 537 3.9 395Q.T. and 2 h, 540 °C ion nitrided 553 4.8 315Q.T. and 4 h, 540 °C ion nitrided 580 5.4 420Q.T. and 8 h, 540 °C ion nitrided 610 6.9 450Q.T. and 16 h, 540 °C ion nitrided 549 8.8 510

The thickness of compound layer increases parabolicallywith time [5] and the growth rate of compound layer ischanged by temperature [21,34]. For example, increasing thetreatment time from 2 to 16 h resulted in an increase incompound layer thickness from 2.6 to 4.8 μm at 500 °C; thisincrease in compound layer thickness was 4.8 to 8.8 μm at540 °C (Fig. 3). Time and temperature can influence not onlythe thickness of the compound layer, but also which phasesit consists of. Fig. 4 shows the diffraction patterns obtainedfor ion nitrided AISI 4340 low alloy steel at different temper-ature and time. The X-ray analysis shows the existence of aγ-Fe4N phase in all compound zones on the surface at alltemperature and time. The compound layer consisted ofγ-Fe4N and ε-Fe2,3N phases, and proportion of γ-Fe4N toε-Fe2,3N and the thickness of this layer increased with in-creased time at 500 °C. The compound layer consisted ofonly γ-Fe4N phase at 540 °C and the thickness of this layerincreased with increased time.

340 steels

lativee depth,(t/D)

D/(D−2t) Relativecase area,

AR

Fatiguestrength,σ MPa

% improvement

– – – 534 –

0.070 1.163 0.352 822 540.075 1.176 0.384 838 570.090 1.220 0.487 912 700.118 1.307 0.709 924 730.099 1.246 0.553 902 690.079 1.187 0.409 830 550.105 1.266 0.602 922 720.113 1.290 0.665 954 780.127 1.342 0.802 1024 91

Fig. 4 –X-ray diffraction patterns from the surface of specimens ion nitrided for (a) 500 °C, 2 h; (b) 500 °C, 16 h; (c)540 °C, 2 h; (d)540 °C, 16 h.

354 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

The hardness profiles of nitrided cases are given in Fig. 5.The hardness decreases from surface to core, since theconcentration of metal nitrides decreases towards core. Thesurface hardness of specimens is in the range of 560–634 HV0,2,while the core hardness remains unchanged, 315 HV0,2. The

Fig. 5 –Microhardness profiles obtained on AI

surface hardness is a strong function of temperature [39]. Thehardness decreases can be explained in terms of precipitatesize and density. Although the surface hardness of nitridedspecimen increases with increasing time and temperature,the reduced surface hardness was observed at longer nitriding

SI 4340 after various nitriding treatments.

Fig. 7 –The relationships between case depth and treatmenttime at 500 and 540 °C.

355M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

time because of the formation of larger nitride precipitateswhich are less effective in increasing hardness than smallerones [5]. At the lower temperature (500 °C) and shortertreatment time (2 h), nitrogen uptake and precipitate densityare low; therefore the amount of hardening is also low. At thehigher temperature (540 °C) and longer treatment time (16 h),precipitate growth and tempering of matrix become impor-tant. The decrease in surface hardness is related to bothprecipitate density and size. The maximum surface hardnessis obtained at temperature of 500 °C and a time of 8 h owing tooptimum precipitate density and size. The reduction ofsurface hardness related to longer treatment time was moreeffective at high temperature (Fig. 6).

The effective cases depth of nitrided specimens based on340 HV0,2 are given in Table 2. It can be seen from Table 2 thatthe case depth increased linearly with increased treatmenttime at both temperatures as indicated in some earlier studies[5,9,12,13] and the generation of nitrided case wasmuch fasterat 540 °C than 500 °C (Fig. 7) as expected for diffusion-controlled growth. It can be seen from Fig. 8 that highertreatment temperatures produce deeper cases at the sametreatment times. While the maximum case depth obtained attemperature of 540 °C and a time of 16 h is 510 μm, theminimum case depth obtained at temperature of 500 °C and atime of 2 h is 280 μm. While the nitrided case depth increasedstrongly with temperature, the surface hardness decreased atlonger nitriding time because of the formation of larger nitrideprecipitates [12,13]. It is interesting that the maximumhardness and the maximum case depth do not coincide [8,40].

The S–N curves obtained from rotating bending fatiguetests are given in Fig. 9. Different fatigue limit properties wereobserved for all types of materials considered in this study,and an absolute progress in fatigue limit with increasing casedepth was observed like previous investigations[9,14,24,25,29]. The effect of increasing the case depth on thefatigue limit can be viewed as effectively moving the fatiguecrack initiation site further into the core. This means that agreater applied bending stresswill be required at the surface tocreate a sufficiently high level of stress at the case–coreinterface to initiate failure [24]. Table 2 gives the fatiguestrength improvements as a result of the ion nitriding treat-

Fig. 6 –Effect of nitriding temperature and time on surfacehardness.

ment. Increase in case depth caused higher fatigue resistance,and an improvement of 91% in fatigue strengthwas developedfor 16 h, 540 °C ion nitrided specimens, which had the heaviestnitrided case in the study, when the fatigue performance ofquenched and tempered steel was assumed to be thereference. It was seen that a linear relationship was obtainedbetween fatigue strength of steel and case depth (Fig. 10) [41].

The section size is one of the most important factorsdetermining the fatigue performance ofmachine parts. Somedimensionless parameters are defined to express the pro-portion of nitrided case zone in the whole cross-section suchas, relative case area AR, which is defined as the ratio of casearea to core area, the ratio of specimen diameter to corediameter D/(D−2t) and the ratio of specimen diameter tospecimen diameter t/D for round specimens [14,24,25]. Thechange in fatigue performance and the improvement infatigue strength, as functions of dimensionless parametersD/(D−2t), (t/D) and AR are given in Table 2, referring to the

Fig. 8 –The relationships between nitriding temperature andcase depth at 4 h.

Fig. 9 –Fatigue curves of heat treated and ion nitrided AISI 4340 steel for various process conditions.

Fig. 10 –Relationship between case depth and fatigue strength of ion nitrided AISI 4340 steel [41].

Fig. 11 –SEM fractograph of a typical subsurface fish eye crackformation.

356 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

fatigue performance in quenched and tempered condition ofsteel. There is a relationship between fatigue strength anddimensionless parameters.

Macroscopic examinations of the fracture surface offatigued ion nitrided samples have shown that the fracturesurface can be characterized into two groups. In the firstgroup, the fatigue cracks initiated at the surface in the low-cycle/high stress region and in the second group cracksinitiate at the subsurface, case–core transition zone in thehigh-cycle/low stress region. All the fractured specimens wereobserved to have failed by the ‘fish eye’ phenomenon withfatigue crack originating from nonmetallic inclusion in thehigh-cycle/low stress region. In this type, the cracks areoriginated at inclusions in the core and at positions near thecase/core interface [14,24–27,29]. The fish eye cracks arecircular in shape, which is a direct consequence of purerotating bending type of cyclic stressing [14]. Scanningelectron micrograph (SEM) fractograph of a typical subsurfacefish eye crack formation is given in Fig. 11. The specimen wasion nitrided at 540 °C for 16 h and loaded under 1100MPa cyclicstress up to complete fracture at the failure cycle of 1.87·105.

Higher magnification of Fig. 11 reveals that such nuclei havetheir origin at a non-metallic inclusion, but it could not bedetermined what it was. On the other hand, the fatigue cracks

Fig. 12 –SEM fractograph of quenched and temperedstructure.

357M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

were developed at inclusions on the surface of specimen inquenched and tempered structure (Fig. 12) and ion nitridedcomponents bending low-cycle fatigue region.

4. Conclusions

1. The thickness of the compound layer increased withincreasing treatment time and temperature. The structureof the compound layer formed during the ion nitriding ofAISI 4340 steel shown a transformation from ε-Fe2,3Nnitride to γ-Fe4N nitride with increasing treatment timeand temperature. While γ-Fe4N phase was formed at alltemperatures and times, ε-Fe2,3N was not formed at hightemperatures.

2. The case depth increased with increasing time andtemperature for volume diffusion controlled growth. Forthis reason, the progress of case depth in ion nitriding canbe expressed by a power function of process time.

3. The surface hardness increased with increasing time andtemperature of ion nitriding treatment. But for extendednitriding periods, a significant loss of surface hardness wasobserved at both nitriding temperatures as a result of theformation of large nitride precipitates.

4. The surface hardness of AISI 4340 steel was increased byion nitriding. It is interesting that the maximum hardnessand the maximum case does not coincide. While themaximum surface hardness was obtained at 500 °C for 8 h,the largest case depth thickness formed at 540 °C for 16 h.

5. The fatigue strength of AISI 4340 steel was increased by upto 91% by ion nitriding process. The improvement of 91% infatigue strength was developed for 16 h, 540 °C ion nitridedspecimens, which had the heaviest nitrided case in thestudy, when the fatigue performance of quenched andtempered steel was assumed to be the reference. It wasseen that a linear relationship was obtained betweenfatigue strength of steel and case depth.

6. In the high cycle fatigue regime, fish eye crack initiation isone of the main characteristics of fractured ion nitridedspecimens. The subsurface crack initiation may take placein the core region from nonmetallic inclusions.

R E F E R E N C E S

[1] O'Brien JM, Goodman D. Plasma (ion) nitriding. MetalsHandbook. Heat Treating, 10th ed.Metals Park OH: AmericanSociety for Metals; 1991. p. 420–4.

[2] Edenhofer B. The ion nitriding process-thermochemicaltreatment of steel and cast iron materials. Metall MaterTechnol 1976;8/8:421–6.

[3] Karamış MB. Tribological behaviour of plasma nitrided722M24 material under dry sliding conditions. Wear1991;147:385–99.

[4] Karamış MB. Some effects of the plasma nitriding process onlayer properties. Thin Solid Films 1992;217:38–47.

[5] Karamış MB, Staines AM. An evaluation of the response of722M24steel tohigh-temperatureplasmanitriding treatments.Heat Treat Met 1989;3:79–82.

[6] Bewley TJ. Ion nitriding. Metall Met Form 1974:227–9.[7] Çelik A, Karadeniz S. Investigation of compound layer formed

during ion nitriding of AISI 4140 steel. Surf Coat Technol1996;80:283–6.

[8] Jones CK. Ion nitriding. Heat Treatment '73; 1975. p. 71–7.[9] Sun Y, Bell T. Plasma surface engineering of low alloy steel.

Mater Sci Eng A 1991;140:419–34.[10] İnal OT, Robino CV. Structural characterization of some ion

nitrided steels. Thin Solid Films 1982;95:195–207.[11] Spalvins T. Tribological andmicrostructural characteristics of

ion-nitrided steels. Thin Solid Films 1983;108:157–63.[12] Robino CV, İnal OT. Ion nitriding behaviour of several low

alloy steels. Mater Sci Eng 1983;59:79–90.[13] Özbaysal K. Ion nitriding behaviour of several tool steels.

Mater Sci Eng 1986;78:179–91.[14] Genel K, Demirkol M, ÇapaM. Effect of ion nitriding on fatigue

behaviour of AISI 4140 steel. Mater Sci Eng A 2000;279:207–16.[15] Cho KS, Lee CO. The effects of carbon on ion-nitriding. J Eng

Mater Technol 1980;102:229–33.[16] Costa JP, Ferreira JM, Ramalho AL. Fatigue and fretting fatigue

of ion-nitrided 34CrNiMo6 steel. Theor Appl Fract Mech2001;35:69–79.

[17] İnal OT, Özbaysal K. Mechanism of ion nitriding in Cr, Al, andCr+Al containing 1040 steel. Presented at Proceedings of anInternational Conference on Ion Nitriding, Ohio, USA;September 15–17 1986.

[18] Kruny ASW, Mallya RM, Mohan Rao M. A study on the natureof the compound layer formed during the ion nitriding of EN40B steel. Mater Sci Eng 1986;78:95–100.

[19] Anichkina NL, Bogolyubov VS, Boiko VV, Denisov VE,Dukarevich IS. Comparison of methods of gas, ionic andvacuum nitriding. Met Sci Heat Treat 1989;31/3:170–4.

[20] Kovacs W, Rusell W. An introduction to ion nitriding.Presented at Proceedings of the International Conference onIon Nitriding, Ohio, USA; September 15–17 1986.

[21] Karamış MB. Friction and wear behaviour of plasma-nitridedlayers on 3%Cr–Mo steel. Thin Solid Films 1991;203:49–60.

[22] Podgornik B, Vizintin J, Leskovsek V. Tribological properties ofplasma and pulse plasma nitrided AISI 4140 steel. Surf CoatTechnol 1998;108–109:454–60.

[23] Jones BK, Martin JW. Residual stress distribution in nitridedEN 41 B steel as function of case depth. Met Technol1977:520–3.

[24] Bell T, Loh NL. The fatigue characteristics of plasma nitridedthree-pct. Cr–Mo steel. J Heat Treat 1982;2/3:232–7.

[25] Jones BK, Martin JW. Fatigue failure mechanisms in a nitridedEN 41B steel. Met Technol 1978:217–21.

[26] Heinz JS. Fatigue behaviour of nitrided steels. Mater Technol1993;64-8/9:441–8.

[27] Çelik A, Karadeniz S. Improvement of the fatigue strength ofAISI 4140 steel by an ion nitriding process. Surf Coat Technol1995;72:169–73.

358 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 3 5 1 – 3 5 8

[28] Mittemeijer EJ. Fatigue of case-hardened steels; role ofresidualmacro andmicrostresses. J Heat Treat 1983;3/2:114–9.

[29] Tosic MM, Terzic I, Gligorijevic R, Ognjanovic M. Fatigueimprovements of glow-discharge plasma nitrided steel rotaryspecimens. Surf Coat Technol 1994;63:73–83.

[30] Qian J, Fatemi A. Cyclic deformation and fatigue behaviour ofion nitrided steel. Int J Fatigue 1995;17/1:15–24.

[31] Fowler GF. The influence of non-metallic inclusions on thethreshold behaviour in fatigue. Mater Sci Eng A1976;39:21–126.

[32] Wang Y, Akid R. Role of nonmetallic inclusions in fatigue,pitting and corrosion fatigue. Corros Sci 1996;52/2:92–102.

[33] Ashrafizadeh F. Influence of plasma and gas nitriding onfatigue resistance of plain carbon (Ck45) steel. Surf CoatTechnol 2003;173–174:1196–200.

[34] Alsaran A, Karakan M, Çelik A. The investigation ofmechanical properties of ion-nitrided AISI 5140 low-alloysteel. Mater Charact 2002;48:323–7.

[35] De la Cruz P, Ericsson T. Influence of sea water on the fatiguestrength and notch sensitivity of a plasma nitrided B–Mnsteel. Mater Sci Eng A 1998;247:204–13.

[36] De la Cruz P, OdenM, Ericsson T. Influence of plasma nitridingon fatigue strength and fracture of a B–Mn steel. Mater Sci EngA 1998;242:181–94.

[37] Menthe E, Bulak A, Olfe J, Zimmermann A, Rie KT.Improvement of the mechanical properties of austeniticstainless steel after plasma nitriding. Surf Coat Technol2000;133–134:259–63.

[38] Allen C, Li CX, Bell T, Sun Y. The effect of fretting on thefatigue behaviour of plasma nitrided stainless steels. Wear2003;254:1106–12.

[39] Alsaran A, Çelik A. Structural characterization of ion-nitridedAISI 5140 low-alloy steel. Mater Charact 2001;47:207–13.

[40] Taylor AM, Tookey KM. Ion-nitriding process with a future.Metallurgia 1981:64–6.

[41] Şirin Ş.Y. Investigation of ion nitriding surface hardening heattreatment effect on fatigue strength of AISI 4340 steel, Ph.D.Thesis, Kocaeli University, Kocaeli, Turkey; 2004.