Materials Science and Engineering A 527 (2009) 212–217

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Materials Science and Engineering A

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Very high cycle fatigue mechanism of carbide-free bainite/martensite steelmicro-alloyed with Nb

Y. Yua,∗, J.L. Gua, B.Z. Baia, Y.B. Liub, S.X. Lib

a Lab of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, PR Chinab Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China

a r t i c l e i n f o

Article history:Received 15 April 2009Received in revised form 25 July 2009Accepted 11 August 2009

Keywords:

a b s t r a c t

In this study, the carbide-free bainite/martensite (CFB/M) steel was micro-alloyed with Nb (CFB/M-Nb).The very high cycle fatigue (VHCF) behaviors of this CFB/M-Nb steel were investigated by ultrasonicfatigue test. Especially, different microstructures of the CFB/M-Nb steel were deliberately designed todisclose the VHCF failure mechanism. In addition, the effect of hydrogen in the formation of the opticaldark area (ODA) was analyzed, which was thought to control the formation of ODA by concentratingaround interior inclusions. Results show the hydrogen cannot be considered to dominate the formationof ODA. Mostly important, “soft or coarsely soft structure induced fatigue crack” should be responsible

Carbide-free bainite/martensiteVery high cycle fatigueHSM

for the primary VHCF failure mechanism of CFB/M-Nb steel.

1

a(oeh(tcwdtbmtm1iabvip

0d

igh strength steel–N curveechanism

. Introduction

In recent years many components of vehicles, bridges, railways,nd space shuttles made by steels requires not only high strength>1200 MPa) but also excellent fatigue properties to endure 108

r even longer life time when exposed to cyclic stress. How-ver, the theory of traditional fatigue limit is not applicable forigh strength steels under the condition of very high cycle fatigueVHCF). Generally speaking, the fatigue limit would increase withhe increasing of their ultimate tensile strength for low or mediumarbon steels with lower strength level. But for high strength steelshose ultimate tensile strength exceeds 1200 MPa, the fatigue limitata is scattered. In addition, lots of researchers [1–3] proved thathe stress–cycle numbers (S–N) curve would continuously declineeyond 107 cycles in many alloys through a great deal of experi-ents, and the fatigue strength at 108 cycles was more reliable than

hat at 107 cycles for the safe-life design. However, it remains argu-entative whether there is another plateau beyond 108 or even

09 cycles. During the course of investigations on VHCF behav-ors, the fish-eye mark around the crack initiation site is graduallycknowledged. Murakami et al. [4,5] considered the fish-eye would

uild when the fatigue crack initiated from interior inclusions atery high cycles region, and he firstly named the white rough zonen the initiation site for optical dark area (ODA). Murakami alsout forward “Hydrogen embitterment” (HE) model, considering the

∗ Corresponding author. Tel.: +86 10 62772976; fax: +86 10 62771160.E-mail address: yang.yu.yuy@gmail.com (Y. Yu).

921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2009.08.024

© 2009 Elsevier B.V. All rights reserved.

formation of ODA as the effect of hydrogen trapped by the inte-rior inclusions cooperating with the cyclic stress. However, otherresearchers had different opinions on the formation of ODA. Forexample, Furuya et al. [6] claimed there was a relation betweenthe formation of ODA and hydrogen while the mechanism was dif-ferent from the HE model. Although the formation mechanism ofODA remains unclear, it is widely accepted that the ODA plays animportant role in the whole VHCF life. For the factors causing thefatigue crack initiation, generally speaking, surface defects of spec-imens as well as interior inclusions or fine inclusion clusters arethe main factors. In recent years, interior non-inclusions-inducedfatigue crack were discovered [7,8], which gradually becomes therising investigation field.

Consequently, this study concentrated on the VHCF mechanismof interior non-inclusions-induced fatigue crack and the effect ofhydrogen during the formation of ODA. A novel high strength steelmicro-alloyed with Nb was used to investigate the VHCF proper-ties, and different microstructures were deliberately designed andobtained with different heat treatments to disclose the VHCF fail-ure mechanism. The predecessor of the steel was invented andapplied in many industrial fields such as frog of railway, kingboltof dock, etc., which is of the strength of 1500 MPa grade. In addi-tion, the steel has carbide-free bainite/martensite (CFB/M) complexmicrostructure and there is filmy retained austenite in and between

laths of lower bainite ferrite and martensite, which can effectivelyimprove the toughness of the steel and enhance the combination ofstrength and toughness. In order to ulteriorly enhance the tough-ness and consequently improve fatigue properties, the CFB/M steelwas micro-alloyed with Nb (CFB/M-Nb) to refine the grain. The

Y. Yu et al. / Materials Science and Engineering A 527 (2009) 212–217 213

Table 1Contents of main elements of CFB/N-Nb.

Gradients (Wt.%) C Si Mn S P Cr Nb

CFB/M steel 0.22 1.8 2.3 ≤0.010 ≤0.010 0.7 0.04

Table 2General mechanical properties of the CFB/M-Nb steel.

Microstructure Temperingtemperature (◦C)

Tensile strength(MPa)

Yield strength(MPa)

Elongation ı5 Reduction in Impact

CFB/M 280 1640 1330CFB/M/F 280 1628 1318

swrns

cctsO

2

Cmcio

Ff

Fig. 1. Specimen designed for VHCF test.

trength of CFB/M-Nb steel reaches to approximately 1600 MPahile the Charpy impact energy (AKU at room temperature) still

emains about 80 J, which demonstrates more outstanding combi-ation of strength and toughness than that of other kinds of hightrength steels [9,10].

In this paper, we concentrated on the CFB/M-Nb steel and dis-losed the fatigue mechanism by analyzing the differences of S–Nurves as well as the general mechanical properties and microstruc-ures. Furthermore, on the basis of the characteristic of the CFB/Mteel, we discussed the probability of hydrogen dominating theDA.

. Materials and experimental

The CFB/M-Nb steel was produced by BaoShan Iron & Steel

o., Ltd. The steel possesses favorable purity and uniformity oficrostructure and thimbleful inclusions under industrial smelting

onditions and its oxygen and hydrogen concentration are lim-ted to 7 ppm and 1 ppm, respectively. The oxide inclusion size isnly about 2–5 �m. The CFB/M microstructure was obtained after

ig. 2. Microstructures of CFB/M-Nb steel (B: bainite; M: martensite; F: ferrite). (a) Typerrite.

(%) area � (%) energy AKU(J)

12.7 54.7 7613.3 55.2 81

austenitizing at 900 ◦C then simply air-cooling to room tempera-ture, and finally tempering at 280 ◦C for 2 h. The composition of thisCFB/M-Nb steel is shown in Table 1. The design of the fatigue speci-men is shown schematically in Fig. 1. The fatigue test was conductedon ultrasonic fatigue testing equipment at a frequency of 20 kHz upto 109 cycles at room temperature. The stress ratio R is chosen as−1. Compressed air was introduced to eliminate the possible “self-heating” in specimens during testing. The fracture surfaces and themicrostructures were observed with scanning electron microscope(SEM) and transmission electron microscope (TEM).

After austenitization, two different microstructures wereobtained by controlling the cooling rate from the variation of theround billet diameters. One is of typical CFB/M complex microstruc-ture resulting from the smaller diameter and higher cooling rate,while the other is of a very small quantity of ferrite (CFB/M/F) plusCFB/M complex microstructure due to bigger diameter and lowercooling rate.

3. Results and discussion

3.1. Mechanical properties and microstructure

The general mechanical properties of two kinds of steels withdifferent microstructures are listed in Table 2. It is obviously thatthe strength and toughness of the steels with different microstruc-tures are quite close. Fig. 2 is the SEM images showing theirmicrostructures. Fig. 2a shows the typical microstructure of CFB/Mmultiphase high strength steel which consists of two constituents:

low carbon lath martensite (M) and carbide free bainite (CFB). Thiskind of microstructure is further illustrated by TEM observationsas shown in Fig. 3. Both Fig. 3a (bright field) and Fig. 3b (dark field)exhibit filmy retained austenite exists in and between laths of lowerbainite and martensite. During the course of cooling, the bainite

ical CFB/M-Nb complex microstructure; (b) CFB/M-Nb microstructure containing

214 Y. Yu et al. / Materials Science and Engineering A 527 (2009) 212–217

Brigh

ftlaaCg

an

3

ls(Trt

m

Fig. 3. Filmy retained austenite in the CFB/M-Nb steel. (a)

errite plates formed earlier separate the original austenite grainhen subsequently reduce the size of the later formed martensiteaths. Fig. 2b shows another microstructure of CFB/M/F containing

small quantity of ferrite. The microstructures of both steels arelmost the same on the whole except the negligible ferrite in theFB/M/F steel. In addition, the appearance of several microns ferriterain is occasional.

The fact that the two steels have almost the same strengthnd toughness indicates that the small quantity of ferrite haseglectable effects on the general mechanical properties.

.2. S–N curve characteristics

The S–N curve of CFB/M/F steel is illustrated in Fig. 4. The hol-ow square dots stand for the broken specimens. The unbrokenpecimens are represented by hollow circle dots. The highest stress675 MPa) was again applied to reload on the unbroken specimens.

he results of reloading are shown in the S–N curve with hollowhombic dots. The number of specimens is marked inside or besidehe hollow dots, as shown in Fig. 4.

The VHCF strength of CFB/M/F steel at 108 cycles is approxi-ately 525 MPa from the S–N curve, which is comparatively low

Fig. 4. S–N diagram of CFB/M/F-Nb steel.

t field; (b) dark field; (c) diffraction of retained austenite.

and unsatisfactory. For the six unbroken specimens, the higheststress of 675 MPa was reloaded. Results show that three of theunbroken specimens (No.7, No.11, and No.12) broke and theirfatigue life is much lower than that of specimens of No.21 andNo.22 which broke in the first stress loading with 675 MPa, whileanother three specimens (No.13, No.15, and No.17) broke withslightly higher fatigue life than that of No.21 and No.22.

Fig. 5 shows the S–N curve of CFB/M steel. The square and cyclehollow dots stand for the broken and unbroken specimens, respec-tively. Specimen number is also marked inside or beside of thehollow dots. The VHCF strength at 108 cycles of CFB/M steel is about630 MPa which is evidently enhanced than that of CFB/M/F steel.Furthermore, there is an observable transition of surface fatiguefracture and interior fatigue fracture around the 106 cycles in theS–N curve as shown in Fig. 5, which is consistent to the results ofother researchers [2,3].

3.3. Observations of fracture surface

It is necessary to distinguish the difference of fracture surfacesbetween the CFB/M/F and CFB/M steel, since the S–N curves exhibitremarkable difference. The fracture surface was observed by SEM.

Fig. 5. S–N diagram of CFB/M-Nb steel.

Y. Yu et al. / Materials Science and Engineering A 527 (2009) 212–217 215

18E8

FsOcinfTOio

wsob

sssclfmttl

it

Fig. 6. Interior non-inclusion induced fracture (�a = 500 MPa, Nf = 2.2

or the steel of CFB/M/F microstructure, almost all the initiationites of the fatigue crack are located at the interior of the specimens.nly one initiation site is located at the surface (No.25) and can beonsidered to be occasional. There are no visible inclusions foundn the initiation sites in all broken specimens, which means interioron-inclusion-induced fatigue fracture is the main model of fatigue

ailure. Fig. 6 shows the typical fracture surface of the CFB/M/F steel.he fish-eye mark can be observed in Fig. 6b and the details ofDA are shown in Fig. 6c. In the center of ODA, it is not an interior

nclusion but a piece of flat area, which is evidently different withther steels whose fatigue crack are induced by inclusions.

In the course of reloading, specimens of No.7, No.11, and No.12,hose fatigue life decreased remarkably, failed because of the

urface-induced fatigue crack. However, another three specimensf No.13, No.15, and No.17, whose fatigue life increased slightly,roke because of the interior non-inclusion-induced fatigue crack.

It is suggested that the second stress reloading test discloseome important facts. Firstly, the loading of long time with lowtress and high cycles worsens the surface quality of specimens toome extent. The specimen proportion of surface-induced fatiguerack in reloading specimens is much higher than that in the firstoading. Secondly, except the surface-induced fatigue crack, theatigue life of reloading specimens is observably improved, which

eans the microstructure is hardened under the loading of longime with low stress and high cycles. Lastly, it is also suggested

hat there is no crack nucleation even after long time loading withow stress and high cycles.

For the steel of CFB/M microstructure, about half of the spec-mens broke because of the surface-induced fatigue crack, whoseypical fracture surface is shown in Fig. 7a. The crack initiation sites

). (a) Fracture surface; (b) fish-eye area and ODA; (c) details of ODA.

locate at the interior after the fatigue cycles are beyond 106, andstill there are no visible inclusions in the initiation sites as wellas the CFB/M/F steel. One typical fracture surface of interior crackinitiation is illustrated in Fig. 7b and c. Also the fish-eye mark canbe observed, whereas the shape of ODA is different with that ofCFB/M/F steel, as shown in Fig. 7c.

In both steels with CFB/M and CFB/M/F microstructures, fish-eye marks can be observed around the interior crack initiation sites,whose typical morphology is shown in Figs. 6b and 7b. The initiationsites are white rough area with large roughness and obvious plasticdeformation. There are no manifest inclusions or other defects suchas cavities found in the initiation sites.

Based on the observations, we believe the HE model raised byMurakami is inappropriate for the steel studied in this paper. Firstly,the total amount of hydrogen is very low in the steel because thehydrogen concentration is only 1 ppm. Secondly, the infinitesimalcontent of oxygen element and excellent smelting technology makethe inclusions possess the characteristic of low contents, smallsizes, and scattering uniformity. Consequently, the inclusions arenot harmful enough to induce the initiation of fatigue crack, whichis demonstrated by the fact that there are no inclusions foundin interior initiation sites of all specimens. According to the HEmodel, hydrogen concentrating at the initiation site would be dif-ficult because of the absence of inclusions. Thirdly, the diffusionof hydrogen is difficult due to the trapping effect for hydrogen

in CFB/M steel. The average hydrogen trap binding energy (EB)of CFB/M steel is about 20 kJ/mol which is far higher than that ofother high strength steels such as 30CrMnSiA and 42CrMo whose EBare only about 10 kJ/mol. Especially, the EB of B/M lath boundariesand retained austenite is determined as 28 kJ/mol and 44 kJ/mol,

216 Y. Yu et al. / Materials Science and Engineering A 527 (2009) 212–217

F 5 MP(

rTicsl

t

waacch

X

whfoCfot

ig. 7. Fracture surface of CFB/M-Nb. (a) Surface induced fatigue fracture (�a = 67�a = 625 MPa, Nf = 7.648E6); (c) details of ODA in b.

espectively, which can be seen as irreversible hydrogen traps [11].herefore, it is difficult for the hydrogen to diffuse and concentraten the initiation sites. Fourthly, the time for hydrogen to diffuse andoncentrate to a certain concentration at the initiation site is toohort. When the fatigue failure takes place, the ultrasonic testingoading time can be calculated through Eq. (1),

l = Nf

f, (1)

here Nf is the fatigue life, f is the frequency of the ultrasonic test,nd tl stands for the loading time. In this work, f is 20 kHz, and thectual loading time is 50 s when the specimens failed at about 106

ycles. Accordingly, the loading time of tension stress is 25 s, whichontributes to the hydrogen concentrating. The diffusion length ofydrogen can be estimate through Eq. (2),

=√

DtH, (2)

here X is the diffusion length, D is the diffusion coefficient ofydrogen in CFB/M microstructure, and tH means the virtual dif-

usion time of hydrogen, which can be equal to the loading time

f tension stress. Dr. Chang Kaidi studied diffusion of hydrogen inFB/M steel and confirmed D to be 34.8 �m2/s [11]. So the dif-usion length could be approximately 30 �m. However, the scalef ODA shown in Fig. 7 is about 35–90 �m, which is far morehan the diffusion length of hydrogen. Obviously it is impossi-

a, Nf = 5.9E5); (b) interior non-inclusion induced fracture, fish-eye area and ODA

ble for hydrogen outside ODA to concentrate in ODA throughdiffusing.

As a result, the contribution of hydrogen in the formation of ODAin CFB/M steels could not be considered.

3.4. VHCF mechanism of CFB/M-Nb steel

For the interior non-inclusion-induced crack initiation, softstructure-induced fatigue crack can be considered as a reason-able mechanism of VHCF failure for the CFB/M/F-Nb and CFB/M-Nbsteel through comparing the general mechanical properties, VHCFbehaviors and microstructures of the two steels, since it is difficultto observe the microstructure of initiation sites directly.

The two S–N curves indicate that the VHCF property ofthe steel with CFB/M microstructure is greatly superior to thatwith CFB/M/F microstructure. There are almost no differencesbetween the general mechanical properties and microstructuresexcept the negligible ferrite, while there is a great deal of dif-ference of the VHCF limit at 108 cycles between the two steels.Accordingly, it is reasonable to reach the conclusion that the

existence of the ferrite induces the immense decrease of theVHCF property. The total amount of ferrite is negligible and theappearance of several microns ferrite grain is occasional, but itis obviously that the ferrite plays an important role in the VHCFbehavior.

Y. Yu et al. / Materials Science and Eng

Table 3Vickers hardness at martensite, bainite and ferrite in the CFB/M/F microstructure.

CtotaaAmstfiatmdrytttadostweoT

dbqwoeei

[9] S. Sankaran, V. Subramanya Sarma, K.A. Padmanabhan, et al., Mater. Sci. Eng. A

Martensite Bainite Ferrite

HV 485 382 172

The differences of VHCF limit and microstructure betweenFB/M/F-Nb and CFB/M-Nb steel demonstrate that the existence ofhe ferrite badly affects the VHCF property though the total amountf the ferrite is infinitesimal. Among the phases in the microstruc-ure of the CFB/M/F-Nb, the ferrite is far softer than the bainitend martensite. Table 3 shows the approximate Vickers hardnesst martensite, bainite and ferrite in the CFB/M/F microstructure.ccording to the experiential relation, Vickers hardness of 175eans approximately tensile strength of 560 MPa, and the yield

trength would be much lower than it. During the VHCF test, whenhe specimens endure a load of about 500–600 MPa, the ferriterst begins to yield and endures the plastic deformation, whilet the same time the matrix of bainite and martensite remains inhe stage of elastic deformation. Consequently, the plastic defor-

ation will concentrate on the ferrite continuously. The plasticeformation damage also accumulates in this region and cannot beeleased effectively since the bainite and martensite are too hard toield and make the deformation cooperative. Eventually the plas-ic deformation damage induces the initiation of fatigue crack. Forhe CFB/M-Nb steel, the coarse bainite plate can be considered ashe crack initiation site. The bainite is softer than the martensite,nd especially the coarse bainite plate is inclined to endure plasticeformation firstly and becomes the crack initiation site. The detailsf ODA obtained from Figs. 6c and 7c can confirm the judgment toome extent. The ferrite is idiomorphic in the microstructure andhe shape of the ODA is approximately circular and symmetricalhen the idiomorphic ferrite induces the crack initiation. How-

ver, the bainite plate is of the shape of willow-leaf and the shapef the ODA induced by coarse bainite plate is also like a willow-leaf.he morphology of the ODA greatly accords with the discussion.

The machining quality of the specimen surface, inclusions andefects, and soft or coarsely soft structure all can influence the VHCFehaviors. In the stage of high stress and low cycles, the surfaceuality of specimens predominates over the VHCF property. Other-

ise, the VHCF course will be controlled by the competition result

f inclusions and soft structure. When the inclusions are harmfulnough, the fatigue crack prefer to initiate at the inclusions. How-ver, in the highly clean steel, soft structure induced fatigue cracks a kind of primary VHCF failure mechanism.

[[

ineering A 527 (2009) 212–217 217

4. Conclusions

The VHCF mechanism of CFB/M-Nb steel has been studiedthrough comparing and discussing the differences between thegeneral mechanical properties, VHCF behaviors and microstruc-tures of two steels with different microstructures. Furthermore,based on the characteristic of the CFB/M-Nb steel and the observa-tion of the fracture surfaces, the effect of hydrogen in the formationof ODA is also analyzed. The main conclusions of the study are:

(1) The long time loading with lower stress than fatigue limit andhigh cycles can worsen the surface quality and harden themicrostructure of specimens to some extent, and under thiscondition the crack nucleation even has not begun to form.

(2) Soft or coarsely soft structure induced fatigue crack is a kind ofprimary VHCF mechanism when the inclusions are not harmfulenough to induce the initiation of crack.

(3) It cannot be considered for hydrogen to dominate the formationof the ODA in the CFB/M-Nb steel.

Acknowledgements

The study was sponsored by Chinese on-going national keystonebasic research program that is the National Basic Research Program(also called 973 Program, No. 2004CB619105). The authors wouldalso like to express their appreciation to BaoShan Iron & Steel Co.,Ltd. for supporting this study and supplying the steels used in test.

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