Surface & Coatings Technology 200 (2006) 6073–6078www.elsevier.com/locate/surfcoat

Effect of carburizing on notch fatigue behaviour in AISI 316austenitic stainless steel

Masayuki Akita a, Keiro Tokaji b,⁎

a Technical Section, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japanb Department of Mechanical and Systems Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan

Received 11 July 2005; accepted in revised form 19 September 2005Available online 14 November 2005

Abstract

The present paper describes the fatigue behaviour of carburized notched specimens in AISI 316 austenitic stainless steel. Cantilever-type rotarybending fatigue tests have been performed using carburized notched specimens with three different stress concentration factors, Kt, of 2.08, 3.55and 6.50 and the effects of carburizing on fatigue strength and notch sensitivity were discussed. Carburizing was performed at a temperature below773 K for 35 h in a CO and H2 gas mixture and the resulting carburized case depth was approximately 40 μm. As normally observed, the fatiguestrengths of both the untreated and carburized specimens decreased with increasing Kt. All carburized notched specimens showed higher fatiguestrength than the untreated ones, and the extent of increase in fatigue strength decreased with increasing Kt and then saturated at high Kt values. Inthe carburized notched specimens with Kt =2.08 and 3.55, fatigue cracks initiated at the surface of the notch root when applied stress was high, butunderneath the carburized case (subsurface) when applied stress was low. On the contrary, the carburized notched specimens with Kt =6.50showed surface crack initiation at the notch root regardless of applied stress level. Furthermore, it was indicated that both the untreated andcarburized specimens had significantly low notch sensitivity, with a slight increase by carburizing.© 2005 Elsevier B.V. All rights reserved.

Keywords: Notch fatigue behaviour; Carburizing; Austenitic stainless steel; Crack initiation; Notch sensitivity

4050

φ10

R27 51

φ5.5φ1

4

1. Introduction

In recent years, various properties such as high strength,excellent corrosion and wear resistance have been stronglyrequired for structural materials because of the demands suchas high performance and use in severe environments of ma-chine components and structures. Austenitic stainless steelshave excellent corrosion resistance, but they possess relativelylow strength and poor wear resistance. To improve those dis-advantages, it is believed that application of various surfaceengineering techniques would be effective. When surface-mod-ified materials are applied to load-bearing components, thefatigue properties become critical.

Until now, the fatigue behaviour of austenitic stainless steelmodified by various surface engineering techniques such as shotpeening [1–5], laser [6], dynamic ion mixing [7] and coating [8]

⁎ Corresponding author. Tel.: +81 58 293 2500; fax: +81 58 230 1892.E-mail address: tokaji@cc.gifu-u.ac.jp (K. Tokaji).

0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.09.018

has been reported. In addition to those techniques, a modifiedcarburizing technique has been developed, which can apply toaustenitic stainless steels without any loss of their advantagessuch as corrosion resistance and ductility [9]. Wear resistance andstrength can be improved by this method [9], but fatigue proper-ties have not been known. In previous reports [10,11], therefore,the authors studied the fatigue behaviour of carburized smoothspecimens in AISI 316 austenitic stainless steel and indicated thatfatigue strength was increased significantly by the carburizing.

In structural components, many geometric changes, i.e.,notches such as groove, shoulder and hole inevitably exist

115

Fig. 1. Configuration of smooth specimen.

6742

115R

7

51

A

60 60 60

1

R0.

40

1

R0.

10

1

R0.

03(b) Kt=3.55 (c) Kt=6.50 (a) Kt=2.08

φ14

φ6 φ8Fig. 2. Configuration of notched specimens.

0 20 40 60 80 100

200

400

600

800

1000

Distance from surface d (μm)

Vic

kers

har

dnes

s H

V

Carburized AISI 316 steel

Kt=2.08Kt=3.55Kt=6.50

Untreated

Fig. 4. Vickers hardness profiles for carburized notched specimens.

6074 M. Akita, K. Tokaji / Surface & Coatings Technology 200 (2006) 6073–6078

and fatigue failure takes place very frequently at the notch rootdue to stress concentration. Hence, the application of surfaceengineering techniques is expected to be more effective forstrengthening the notch root. Although the fatigue strength ofnotched specimens in austenitic stainless steels has beenreported [12–15], no studies on the fatigue behaviour of sur-face-modified notched specimens exist. Therefore, it is veryimportant to understand mechanisms of fracture resulting fromthe surface-modified notch root, because it is assumed that theeffect of surface modification on fatigue behaviour would bedifferent from smooth specimens and the notch sensitivitywould also be affected by surface modification. In the presentstudy, cantilever-type rotary bending fatigue tests were per-formed using carburized notched specimens with three differentstress concentration factors, Kt, of 2.08, 3.55 and 6.50 in AISI316 austenitic stainless steel, and the effects of carburizing onnotch fatigue behaviour were discussed.

2. Experimental details

2.1. Materials and specimens

The material used in the present study is the same AISI 316austenitic stainless steel of 16 mm diameter as in previousreports [10,11] whose chemical composition (wt.%) is C 0.05,Si 0.35, Mn 1.35, P 0.033, S 0.025, Ni 10.1, Cr 16.9, Mo 2.11.

50 μm

(a)

Fig. 3. Microstructures: (a) untreated

The material was solution treated at 1353 K for 1 h followed byoil cooling, from which fatigue samples were machined.

The configurations of fatigue specimens are shown in Figs. 1and 2. The smooth specimen has an hourglass shape with aminimum diameter of 5.5 mm (Fig. 1). This gave a stressconcentration factor, Kt, of 1.03 under cantilever-type rotarybending. In the notched specimens (Fig. 2), a circumferentialnotch with a depth of 1 mm and three different notch radii, ρ, of0.40 mm, 0.10 mm and 0.03 mm was introduced, whose Kt

values are 2.08, 3.55 and 6.50, respectively. After machining,the following surface treatment was applied to the fatiguespecimens.

2.2. Surface modification technique

A modified gas-carburizing technique, which is called pio-nite treatment [9], was performed at a temperature below 773 Kfor 35 h in a CO and H2 gas mixture. During this process,carbon is diffused into the material, and thus, a carbon-diffusedzone is formed at the surface region without any Cr carbideformation where hardness is remarkably increased. It is be-lieved that this treatment can improve wear resistance andstrength without any loss of corrosion resistance, ductility andtoughness of austenitic stainless steels.

2.3. Procedures

Fatigue tests were carried out using cantilever-type rotarybending fatigue testing machines operating at a frequency of 19

Car

buri

zed

case

(b)

50 μm

material, (b) carburized material.

Table 1Mechanical properties

Material Proof stressσ0.2 (MPa)

Tensile strengthσB (MPa)

Elongationϕ (%)

Reduction ofarea ψ (%)

Untreated 299 576 67 77Carburized 581 57 72

6075M. Akita, K. Tokaji / Surface & Coatings Technology 200 (2006) 6073–6078

Hz in laboratory air at ambient temperature. After experiment,fracture surfaces were examined in detail by a scanning electronmicroscope (SEM).

3. Results

3.1. Microstructure characterization

SEM micrographs of the microstructures after etching areshown in Fig. 3. As well known, the untreated material has amicrostructure consisting of austenitic grains (Fig. 3a). In thecarburized materials, the surface region, which is clearly dis-tinguished from the core material, can be recognized, which isthe case formed by carburizing (Fig. 3b). As can be seen in thefigure, the carburized case depth is approximately 40 μm. It hasbeen indicated that no Cr carbides were formed in the carbu-rized case and the microstructure underneath the carburizedcase was the same austenitic structure as in the untreatedmaterial [9].

3.2. Hardness profile

Vickers hardness profiles measured on the minimum crosssection of the carburized notched specimens are represented inFig. 4, where the applied load was 0.098 N. It can be seenthat hardness at or near the surface attains more than approx-imately 940 HV. Such high hardness cannot be obtained bymechanical surface treatments such as shot peening, whichresults in the maximum value of approximately 300 HV [2–5]. Hardness rapidly decreases with increasing the distancefrom the surface and then reaches a constant value of approx-imately 220 HV that is the hardness of the core material, i.e.,the untreated material. Regardless of notch geometry, theregion of the increased hardness is 40–50 μm that is nearly

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108

150

200

250

300

350

400

450

500

550

Number of cycles to failure Nf

Str

ess

ampl

itud

e

(M

Pa)

Kt=3.55Kt=6.50

AISI 316 steel

Open: untreatedSolid: carburized

SmoothKt=2.08

σ

Fig. 5. S–N diagram for untreated and carburized specimens characterized interms of nominal stress amplitude.

equal to the case depth established by the microstructurecharacterization shown in Fig. 3.

3.3. Mechanical properties

Tensile tests were conducted using smooth specimens andthe obtained mechanical properties are listed in Table 1. It canbe seen that in the carburized specimen, tensile strengthincreases and ductility decreases compared with the untreatedspecimen, but the differences in tensile properties between theuntreated and carburized specimens are small, which is due tovery thin case depths of 40 μm.

From residual stress measurement by the X-ray diffractionmethod under the conditions of the characteristic X-ray ofCrKα, the tube voltage of 35 kV, the tube current of 200 mAand (220) plane reflection, it was found that the compressiveresidual stress of approximately − 1500 MPa was measured atthe surface of a carburized smooth specimen [10].

3.4. Fatigue behaviour

3.4.1. Fatigue strengthThe S–N diagram characterized in terms of nominal stress

amplitude, σ, for the untreated and carburized specimens isshown in Fig. 5 and the obtained fatigue limits are also listedin Table 2. As normally observed, fatigue strength decreaseswith increasing Kt in both the untreated and carburized speci-mens, but the differences in fatigue strength between thenotched specimens with Kt =3.55 and 6.50 become small. Inthe carburized specimens, the fatigue strengths increase signif-icantly compared with the untreated specimens. As indicated inprevious reports [10,11], this is due to suppression of slipdeformation at the notch root surface because of remarkablehardness increase, i.e., the resistance to crack initiation is sig-nificantly enhanced in the carburized case. It should be notedthat the extent of increase in fatigue strength is largest in thesmooth specimen and decreases with increasing Kt, then tendsto saturate at Kt =3.55 (see Table 2). It is also worth noting thatno non-propagating cracks were seen in all the run-out notchedspecimens in both the untreated and carburized conditions.

3.4.2. Crack initiationIn the untreated notched specimens, it was found that fatigue

cracks initiated at the notch root surface due to cyclic slipdeformation, because a stage I-like facet was seen at the crackinitiation site regardless of applied stress level.

Figs. 6–8 reveal SEM micrographs of fracture surfaces nearthe crack initiation site in the carburized notched specimens. In

Table 2Fatigue limits in untreated and carburized specimens

Specimen Untreatedσwo (MPa)

Carburizedσwc (MPa)

Increasedratio (%)

Smooth 305 390 27.9Kt =2.08 230 280 21.0Kt =3.55 200 230 15.0Kt =6.50 190 220 15.8

(b)

50 μm

(a)

50 μm

Fig. 6. SEM micrographs of fracture surfaces near crack initiation site in carburized notched specimens with Kt =2.08: (a) surface (σ=370 MPa, Nf =1.7×104), (b)

subsurface (σ=300 MPa, Nf =5.3×105). Arrow indicates the crack initiation site.

6076 M. Akita, K. Tokaji / Surface & Coatings Technology 200 (2006) 6073–6078

the carburized notched specimens with Kt =2.08 (Fig. 6) and3.55 (Fig. 7), the crack initiation site depends on applied stresslevel, where the cracks initiated at the notch root surface at highapplied stresses (surface crack initiation, Figs. 6a and 7a), whileunderneath the carburized case at low applied stresses (subsur-face crack initiation, Figs. 6b and 7b). On the contrary, in thecarburized notched specimens with Kt =6.50 (Fig. 8), the cracksgenerated at the notch root surface regardless of applied stresslevel.

In the case of surface crack initiation, the cracks seem to beinitiated due to brittle fracture of the hard carburized case. Onthe other hand, in the case of subsurface crack initiation, it isbelieved that the cracks initiated due to cyclic slip deformationunderneath the carburized case and then immediately propagat-ed to the surface. This is supported by the presence of a smoothfacet in the carburized case, which can be clearly recognized ina carburized notched specimen with Kt =3.55 at σ=300 MPashown in Fig. 7.

4. Discussion

4.1. Subsurface crack initiation in carburized notchedspecimens

The S–N diagram for the carburized notched specimenscharacterized in terms of the maximum stress, σmax=Ktσ, isshown in Fig. 9. As described previously, in the carburizednotched specimens with Kt =2.08 and 3.55, the cracks initiatedat the notch root surface at high applied stresses, and under-neath the carburized case at low applied stresses. On the con-trary, in the notched specimen with Kt =6.50, the cracks

(a)

50 μm

Fig. 7. SEM micrographs of fracture surfaces near crack initiation site in carburizedsubsurface (σ=300 MPa, Nf =1.2×10

5). Arrow indicates the crack initiation site.

generated at the notch root surface regardless of applied stresslevel. It should be noted that there are no changes in morphol-ogy of the S–N curves depending on the crack initiation site,surface or subsurface. This may be due to the very shallowcarburized case depth of 40 μm.

The authors have indicated previously that in the smoothspecimens, crack initiation took place underneath the carbu-rized case independent of applied stress and case depth,which could be understood reasonably by a simple stress–strength model [10]. The hardness of the carburized case isincreased remarkably by carburizing; thus, the strength of thecarburized case increases significantly compared with thecore material. In the case of the smooth specimens, thestrengths of the carburized case were considerably higherthan applied stresses; thus, subsurface crack initiation alwaysoccurred.

In the carburized notched specimens with Kt =6.50, it isbelieved that the maximum stresses play a significant role incrack initiation, which are extremely high in the carburizedcase; thus, the surface crack initiation would be attributed tobrittle fracture of the carburized case itself or caused by theincompatibility of deformation due to large plastic deformationat the soft core material underneath the carburized case. On theother hand, in the carburized notched specimens with Kt =2.08and 3.55, subsurface crack initiation took place at low appliedstresses. In those specimens, the stress gradient at or near thenotch root would be significant. The maximum stresses at thesurface are considerably lower than in the notched specimenswith Kt =6.55, but the applied stresses underneath the carbu-rized case are still relatively high compared with those at thesurface due to gentle slopes of the stress gradient; thus, cyclic

(b)

50 μm

notched specimens with Kt =3.55: (a) surface (σ=340 MPa, Nf =2.5×104), (b)

(b)

50 μm 50 μm

(a)

Fig. 8. SEM micrographs of fracture surfaces near crack initiation site in carburized notched specimens with Kt =6.50: (a) surface (σ=300 MPa, Nf =1.2×105), (b)

surface (σ=280 MPa, Nf =3.4×105).

6077M. Akita, K. Tokaji / Surface & Coatings Technology 200 (2006) 6073–6078

plastic deformation occurs there, then leading to crack initiationprior to brittle fracture of the carburized case.

4.2. Relationship between fatigue limit and stress concentra-tion factor

Fig. 10 illustrates the relationship between fatigue limit,σwk/σwo, and Kt, where σwk and σwo are the fatigue limits forthe notched and smooth specimens, respectively. As described

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0

500

1000

1500

2000

2500

Kt=2.08

Number of cycles to failure Nf

Max

imum

str

ess

m

ax (

MPa

)σ

Kt=3.55Kt=6.50

Carburized AISI 316 steel

Open: surfaceSolid: underneath carburized case

Crack initiation site:

Fig. 9. S–N diagram for carburized notched specimens characterized in terms ofmaximum stress.

1 2 3 4 5 6 70

0.2

0.4

0.6

0.8Solid: carburizedOpen: untreated

Stress concentration factor Kt

AISI 316 steel

Fatig

ue li

mit

wk/

w0

σσ

1/Kt

1.0

Fig. 10. Relationship between fatigue limit and stress concentration factor.

previously, no non-propagating cracks were seen in all the run-out notched specimens in both the untreated and carburizedconditions; this implies that once cracks initiate, they continueto grow and fatigue failure always occurs. Hence, the observedfatigue limits are the threshold stresses for crack initiation. Asestablished in the present study, it has been indicated that non-propagating cracks were not recognized in austenitic stainlesssteels such as AISI 304 and 316 [12–15].

It is well known that in various materials, the σwk/σwo valuesnecessary for failure become independent of Kt beyond somecritical value of Kt, because of the existence of non-propagatingcracks, while the σwk/σwo values for crack initiation decreasecontinuously with increasing Kt, very close to the curve givenby 1/Kt. As can be seen in the figure, the σwk/σwo values for theuntreated and carburized conditions decrease with increasing Kt

and then tend to be constant at Kt≥3.55 [15], which aresituated considerably above the 1/Kt curve, indicating lownotch sensitivity.

4.3. Effect of carburizing on notch sensitivity

The relationship between fatigue strength reduction factor,Kf and Kt, is represented in Fig. 11, where Kf is defined as theratio of the fatigue limit for the smooth specimen, σwo, to thatfor the notched specimens, σwk. As can be seen in the figure,the Kf values for the untreated condition are considerably

1 2 3 4 5 6 71

2

3

4

5

6

7

Kt=Kf

Stress concentration factor Kt

Fatig

ue s

tren

gth

redu

ctio

n fa

ctor

Kf AISI 316 steel

Open: untreatedSolid: carburized

Present results

Awatani et al. [12]Hatanaka and

AISI 304 steel

Shimizu [13]

Itatani et al. [15]AISI 316 steel

Fig. 11. Relationship between fatigue strength reduction factor and stressconcentration factor.

6078 M. Akita, K. Tokaji / Surface & Coatings Technology 200 (2006) 6073–6078

lower than Kt and the difference between both increases withincreasing Kt, then tends to saturate at high Kt values [15].This implies that the present material has very low notchsensitivity. Similar results have been reported on AISI 304and 316 austenitic stainless steels [12–15]. On the other hand,the Kf values for the carburized condition have the same Kt

dependence as observed in the untreated condition but areslightly larger in the entire Kt range. This indicates that thenotch sensitivity of the present material is only slightly in-creased by carburizing.

5. Conclusions

In the present study, rotary bending fatigue tests were per-formed in laboratory air at ambient temperature using carbu-rized notched specimens with three different stressconcentration factors, Kt, of 2.28, 3.55 and 6.50 in AISI 316austenitic stainless steel. The effects of carburizing on fatiguestrength and notch sensitivity were discussed. The main con-clusions can be made as follows.

1. The surface hardness in the carburized case attained to morethan 940 HVand hardness decreased rapidly with increasingthe distance from the surface and then reached the corehardness at 40–50 μm from the surface regardless of notchgeometry. The case depth established by the microstructurecharacterization was 40 μm.

2. In both the untreated and carburized specimens, fatiguestrength decreased with increasing Kt, but the differencesin fatigue strength between the notched specimens withKt =3.55 and 6.50 became small.

3. Fatigue strength was increased by carburizing and the extentof increase was largest in the smooth specimen and de-creased with increasing Kt, then saturated at Kt =3.55. Inthe run-out specimens in both the untreated and carburizedconditions, no non-propagating cracks were seen.

4. In the carburized notched specimens with Kt =2.08 and 3.55,crack initiation occurred at the notch root surface at highapplied stresses, while underneath the carburized case at low

applied stresses. On the contrary, in the carburized notchedspecimens with Kt =6.50, cracks initiated at the notch rootsurface regardless of applied stress.

5. Both the untreated and carburized specimens indicated sig-nificantly low notch sensitivity, with a slight increasing bycarburizing.

Acknowledgments

The authors thank Air Water Inc. for the carburizing offatigue specimens. Thanks are also due to Mr Takenaka forhis experimental assistance.

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