International Journal of Fatigue 33 (2011) 500–506

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International Journal of Fatigue

journal homepage: www.elsevier .com/locate / i j fa t igue

Technical note

Competition mechanism between microstructure type and inclusion levelin determining VHCF behavior of bainite/martensite dual phase steels

Y. Yu a,⇑, J.L. Gu a, F.L. Shou a, L. Xu a, B.Z. Bai a, Y.B. Liu b

a Lab of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR Chinab 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 3 May 2010Received in revised form 23 September2010Accepted 5 October 2010Available online 25 October 2010

Keywords:Bainite/martensiteVery high cycle fatigueS–N curvesMicrostructure typeInclusion level

0142-1123/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.ijfatigue.2010.10.004

⇑ Corresponding author. Tel.: +86 10 62772976; faxE-mail address: yang.yu.yuy@gmail.com (Y. Yu).

a b s t r a c t

The very high cycle fatigue (VHCF) behaviors of bainite/martensite (B/M) dual phase high-strength low-alloy (HSLA) steels were investigated using ultrasonic fatigue technology. Both non-inclusion and inclu-sion induced crack initiations occurred in the designed steels under VHCF test. It is shown that the VHCFbehaviors are determined by the competition between the inclusion level of steels and the microstructuretype consisted of different kinds of phases, microstructure homogeneity and refinement degree. Thephysical and chemical metallurgy methods could be coordinated to achieve excellent VHCF propertiesof high strength steels through microstructure optimization and inclusion size reduction.

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1. Introduction

In recent years, the very high cycle fatigue (VHCF) properties ofmany metallic materials, especially high strength steels, have beenstudied extensively driven by the requirement of both highstrength (>1200 MPa) and excellent fatigue properties to endure108 cycles or even longer life when exposed to cyclic stress. In theprevious research [1–3], fatigue cracks tended to initiate in the inte-rior of specimens after 106 cycles in VHCF test, which means the fa-tigue property in this very high cycle case is more closely related tothe intrinsic characteristic of material microstructure than in highcycle fatigue (HCF) and low cycle fatigue (LCF) cases. In general,inclusion induced crack initiation is considered as the main fatiguefailure mechanism, and based on this assumption lots of efforts aremade to improve VHCF properties of steels, including studying theinfluences of inclusions on the VHCF properties as well as improv-ing chemical metallurgy technology to diminish the size and quan-tity of inclusions in steels [4–6].

In recent years, the effect of non-inclusion induced crackinitiation on the VHCF behaviors comes into view of researchers[6–8]. Different from the inclusion induced fatigue failure, thecrack initiates directly from the substrate in this case, instead fromthe inclusions or inclusion clusters.

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Chai [7] first reported this phenomenon (termed as SubsurfaceNon-Defect Crack Origins-SNDFCO by the author) in ferrite/mar-tensite and austenite/martensite dual phase steels (termed astwo-phase steels by the author). The author thought there are tran-sitions of fatigue crack initiation from surface defects, subsurfaceinclusions to SNDFCO with decreasing applied stress or increasingfatigue life in two-phase steels, where the two phases have very dif-ferent strengths. However, the author did not gather the statistics ofthe inclusion size and did not focus on the relation between theinclusion and non-inclusion induced crack initiation. Zhang et al.[6] found this phenomenon in clean 54SiCr6 steel in which theinclusion size is smaller than 1 lm. The author found the crackinitiation region is enriched with carbon. Furuya et al. [8,9] also ob-served a small number of specimens failed because of non-inclusioninduced crack initiation in studying the VHCF behaviors of JIS-SUJ2and SUP7 steels. Our previous study in Mn–Si series B/M dual phaseHSLA steels [10,11] also demonstrated that non-inclusion inducedcrack initiation is one of the most important fatigue failure mecha-nisms in VHCF regime. Soft-structure induced crack initiation isproposed in connection with the mechanism of non-inclusioninduced crack initiation [7,10]. Furthermore, we confirmed thesoft-structure induced crack initiation mechanism in a series ofcomposition ranges by comparing the general mechanical and VHCFproperties of designed B/M dual phase steels [12]. Studies showthat, the more different the yield strengths of the two phases are,the easier the softer phase is to induce the crack initiation.

Table 1Chemical compositions of studied steels.

No. C Si Mn Cr Ni Mo Nb

1 0.21 0.86 2.21 0.96 0.65 0.35 –2 0.21 1.74 2.20 0.62 – – –3 0.22 1.80 2.3 0.7 – – 0.04

Y. Yu et al. / International Journal of Fatigue 33 (2011) 500–506 501

More efforts are required to reveal the relation between inclu-sion and non-inclusion induced crack initiation. In the traditionalfatigue theory, if the inclusion level of steels is high, which meansthe steels contain many and big inclusions, then the fatigue crackwould certainly initiate from the inclusions. However, in our on-going studies, it is shown that non-inclusion induced crack initia-tion can occur even if the inclusion level is very high, which meansthe location of crack initiation is more determined by the substrateproperties in this case. Previous studies on the VHCF behaviors ofB/M steels suggested that the crack initiation is significantly re-lated to the microstructure type and inclusion level [10–12]. Thusin the present work, we aimed at the VHCF properties of a series B/M steels with different microstructure types and inclusion levels.The microstructure types introduced here not only identify the dif-ference between the lower bainite and granular bainite, but alsodescribe the microstructures with different homogeneity andrefinement degree. The inclusion level includes the size and shapeas well as the deformability of inclusions. Considering the differentimportance of the varied factors in fatigue study, in this work wewill only focus on the size influence from spherical non-deform-able inclusions to qualitatively evaluate the negative influenceson the VHCF properties from the inclusions.

It will also be shown that the VHCF properties of steels could beimproved by the coordinations between physical and chemicalmetallurgies, where the former aims at the optimization of micro-structure and the latter focuses on the reduction of inclusion levels.

2. Materials and experimental

In this study, we chose three representative chemical composi-tions in six kinds of B/M steels with different production and heattreatment procedures. The compositions (wt.%), production condi-

Table 2Production conditions and heat treatment procedures of the studied steels.

No. Productioncondition

Heat treatment procedures

1–1 Lab 900 �C (20 min) – air cooling to 350 �C – annealing to room1–2 Lab 900 �C (20 min) – air cooling to room temperature – 280 �2–1 Lab 900 �C (20 min) – air cooling to room temperature – 280 �2–2 Industrial 900 �C (20 min) – air cooling to room temperature – 280 �3–1 Industrial 900 �C (20 min) – air cooling to room temperature – 280 �

3–2 Industrial 900 �C (20 min) – air cooling to room temperature – 280 �

Table 3General mechanical properties of the studied steels.

No. Tempering temperature (�C) Tensile strength (MPa) Yield strength (M

1–1 280 1400 9561–2 280 1487 13422–1 280 1570 13602–2 280 1581 13103–1 280 1640 13843–2 320 1528 1411

tions and heat treatment procedures as well as the generalmechanical properties are listed in Tables 1–3 respectively. The1–1#, 1–2# and 2–1# samples were produced in lab conditionsand the others were all produced in steelworks. The microstructureof 1–1# sample is granular bainite/martensite, and other samplesexhibit lower bainite/martensite microstructures.

All the six types of B/M steels have excellent combination ofstrength and toughness, as shown in Table 3. To compare the effectof different microstructure types on the VHCF behaviors, samplesare artificially designed with a series of different phases, micro-structure homogeneities and prior austenite grain (PAG) sizes.

The No. 1 series steels have the worst microstructure homoge-neity and largest average PAG size. Fig. 1a shows the PAG morphol-ogy of 1–1# and 1–2# steels. Serious mischcrystal and extremelycoarse austenite grains demonstrate the poor microstructurehomogeneity. As shown in Fig. 1b and c, the main difference be-tween the 1–1# and 1–2# samples in microstructure is the granu-lar (1–1#) and lower bainite (1–2#). The lower bainite/martensitemicrostructure of 1–2# steel has higher strength than the granularbainite/martensite microstructure of 1–1# steel, which results inthe different tensile strengths and yield strengths of the two steels.

The No. 2 series steels are of the common PAG size of about20 lm, which is shown in Fig. 2. The typical dual phase microstruc-tures of lower bainite/martensite are shown in Fig. 3. The mainmicrostructure difference between 2–1# and 2–2# steel is themicrostructure homogeneity, since the greater forging ratio isintroduced into 2–2# steel under industrial processing conditions.However, this is a kind of macroscopic difference of homogeneity,so it could hardly be recognized in microtopography analysis. As aresult, there is almost no difference between Fig. 2a and b. Never-theless, the amount and distribution of lower bainite in Fig. 3bdemonstrate better homogeneity than that in Fig. 3a.

The No. 3 series steels are produced by Nb microalloying withdifferent PAG sizes: the 3–1# steel is of the common PAG size of20 lm, while in the 3–2# steel the PAG size was reduced to5 lm by grain refining treatment. The PAG morphology and typicallower bainite/martensite microstructures of No. 3 series steels areshown in Figs. 4 and 5 respectively. Compared with the 3–1# steel,the yield ratio and charpy impact energy of 3–2# steel is greatlyimproved while the tensile strength remains at the original level,which exhibits the effect of refining treatment.

Characteristic of microstructure

temperature – 280 �C tempering (120 min) Granular bainite/martensiteC tempering (120 min) Lower bainite/martensiteC tempering (120 min) Lower bainite/martensiteC tempering (120 min) Lower bainite/martensiteC tempering (120 min) Lower bainite/martensite

Average grain size: 20 lmC tempering (120 min) Lower bainite/martensite

Average grain size: 5 lm

Pa) Elongation d5 (%) Reduction in area, W (%) Impact energy, AKU (J)

13.5 54.3 71.311.2 55.4 67.017.2 64.1 92.810.0 53.6 88.0

9.6 58.0 80.814.0 50.0 114.4

Fig. 1. Microstructures of No. 1 series steels. (a) PAG morphology; (b) microstructures of 1–1# steels; (c) microstructures of 1–2# steels.

Fig. 2. PAG morphology of No. 2 series steels. (a) PAG morphology of 2–1# steels; (b) PAG morphology of 2–2# steels.

502 Y. Yu et al. / International Journal of Fatigue 33 (2011) 500–506

In addition, Nos. 1–3 series steels are of different inclusion lev-els, and their degrees of purity increase from 1 to 3, which will bedescribed in next section.

The fatigue test was conducted on ultrasonic fatigue testingequipment at a frequency of 20 kHz up to 109 cycles at room tem-perature, and the fatigue specimen designed for VHCF test isshown in Fig. 6. The stress ratio R was chosen as �1. Compressedair was introduced to eliminate the possible ‘‘self-heating” in spec-imens during testing. The fracture surfaces were observed withScanning Electron Microscope (SEM).

3. Results and discussion

3.1. VHCF behaviors of B/M steels

The S–N curves of six types of B/M steels are illustrated in Fig. 7.From Fig. 7a, we found non-inclusion induced crack initiations oc-curred in failure specimens of 1–1# steel whose axial tension andcompression fatigue strength at 108 cycles is about 410 MPa. Thefatigue strength of 1–2# steel is about 435 MPa which is improvedby about 6% compared with 1–1# steel, while the dispersion de-gree of the data points becomes very severe. It is interesting thatmore than half of the specimens failed due to the inclusion inducedcrack initiations in addition to the non-inclusion induced crack ini-tiations, which resulted in the high data dispersion degree.

Fig. 7b shows the S–N curves of 2–1# and 2–2# steel. In both S–N curves, there exists a transition from surface crack initiation tointerior crack initiation between 106 and 107 cycles, which is con-sistent with previous studies [1,2,13]. However, the behaviors ofcrack initiation of the two steels are totally different when thecrack initiates in the interior of the specimens. In lab-scale pro-duced 2–1# steel, the dominant mechanism is non-inclusion in-duced crack initiation while in industry-scale produced 2–2#steel is inclusion induced crack initiation. Although the crack initi-

ates from the interior inclusions in 2–2# steel instead of the sub-strate in 2–1# steel, the fatigue strength of 2–2# steel is620 MPa, which is about 10% higher than that of 2–1# steel.

Similar to the No. 2 series steels, both non-inclusion and inclu-sion induced crack initiations occurred in 3–1# and 3–2# steel, asshown in Fig. 7c. In 3–2# steel, the PAG size was reduced to about5 lm to obtain better overall mechanical properties comparedwith 3–1# steel. As an example, the fatigue strength at 108 cyclesof 3–2# steel is about 760 MPa due to grain refinement, which isabout 20% higher than that of 3–1# steel.

The typical fracture surfaces of the six steels observed by SEMare shown in Fig. 8. Fig. 8a, c and e exhibits the fracture surfacesof non-inclusion induced crack initiation in the 1–1#, 2–1# and3–1# steels respectively. The initiation sites are white rough areawith large roughness and obvious plastic deformation, which indi-cates that the substrate is easier to generate stress concentrationand develop into crack initiation sites than the inclusions. For com-parison, the fracture surfaces of inclusion induced crack initiationin 1–2#, 2–2# and 3–2# steel are shown in Fig. 8b, d and f. Themicrostructure of initiation sites is common and there are typicalnon-metallic inclusions in the center of initiation sites. As shownin the statistical results, the average inclusion diameters of 1–2#,2–2# and 3–2# steel are 45.1 lm, 23.0 lm and 13.1 lm respec-tively, which demonstrates the increasing purity degrees fromNos. 1 to 3 steels.

3.2. Competition mechanism between microstructure type andinclusion level in VHCF

Now we move to the discussion of the relation between theVHCF property, microstructure types and the behaviors of crackinitiation. First of all, in order to describe whether the fatigue crackwill initiate from the substrate or not, we classify the microstruc-ture type as ‘weak’ or ‘strong’. The microstructure type is decided

Fig. 3. Microstructures of No. 2 series steels (B: lower bainite; M: martensite). (a) Microstructure of 2–1# steels; (b) microstructure of 2–2# steels.

Fig. 4. PAG morphology of No. 3 series steels. (a) PAG morphology of 3–1# steels; (b) PAG morphology of 3–2# steels.

Fig. 5. Microstructures of No. 3 series steels (B: lower bainite; M: martensite). (a) Microstructure of 2–1# steels; (b) microstructure of 2–2# steels.

Fig. 6. Specimen designed for VHCF test.

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by the phase composition, the microstructure homogeneity andmicrostructure refinement degree. For example, granular bainite,serious mischcrystal and extremely coarse PAG can markedly de-crease the overall mechanical properties of microstructures, whichmakes the substrate easier to induce stress concentration anddamage accumulation. Finally the substrate develops into fatigue

crack initiation sites. In brief, the microstructure type is weak.On the contrary, in the strong microstructure type, the inclusionis easier to induce stress concentration and become initiation sites.In general, the more homogeneous and more refined the micro-structures are, the stronger the microstructures are.

The above experimental observations have already demon-strated that inclusion/non-inclusion induced crack initiation tendsto occur selectively in steels with different microstructure typesand inclusion levels. Now we begin to discuss the relation betweeninclusion and non-inclusion induced crack initiation.

The No. 1 series steels are of the worst microstructure homoge-neity and highest inclusion level, so their VHCF properties are thelowest in all the studied steels. Besides these shortages, the granu-lar bainite in 1–1# steel makes its microstructure even weaker, be-cause the yield strength of granular bainite is much less than lowerbainite, which can be demonstrated by the yield strength of 1–1#and 1–2# steel in Table 3. As a result, the microstructure of 1–1#steel is even more disadvantageous than the inclusion, whichdetermines the crack initiates from the substrate. When thegranular bainite is changed into lower bainite, the microstructure

Fig. 7. S–N curves of six types of B/M steels. (a) No. 1 series steels; (b) No. 2 series steels; (c) No. 3 series steels.

504 Y. Yu et al. / International Journal of Fatigue 33 (2011) 500–506

of 1–2# steel becomes stronger than 1–1# steel. Consequently, thenegative effect of inclusions on fatigue property begins to emergeand inclusion induced crack initiation comes into being. However,it is very difficult to eliminate non-inclusion induced crack initia-tion in No. 1 series steels only through heat treatment because ofthe serious mischcrystal and extremely coarse PAG. Therefore bothinclusion and non-inclusion induced crack initiation occur in 1–2#steel.

The No. 2 series steels are of ordinary microstructure homoge-neity and inclusion level, so their VHCF properties are also inter-mediate in all the studied steels. Though the 2–1# steel hasalmost the same phase composition (lower bainite/martensite) asthe 1–2# steel, the crack chooses to initiate from the substrate in2–1# steel while from the inclusion in 1–2# steel. The reason isthat the inclusion level of No. 2 steels is greatly reduced, and theformer strong microstructure type in No. 1 series steels now shouldbe regarded as weak microstructure type under the inclusion levelof No. 2 series steels. Consequently, the fatigue crack again choosesto initiate from the substrate of 2–1# steel. Subsequently, macro-scopic microstructural homogeneity is introduced into 2–2# steelthrough industrial producing condition to optimize the micro-structure, so the microstructure type is upgraded, and again theinclusion begins to control the VHCF behaviors.

The No. 3 series steels are of the same microstructure homoge-neity as 2–2# steel and lowest inclusion level in all the studiedsteels, so their VHCF properties are the best. Similar with 1–2#and 2–1# steel, the 2–2# and 3–1# steel are of almost the same

phase composition, microstructure homogeneity and averagePAG size, while inclusion and non-inclusion induced crack initia-tion occur respectively in 2–2# and 3–1# steel because of the re-duced inclusion level. Grain refinement treatment is introducedto obtain 3–2# steel. Refinement also further optimizes the micro-structure and makes it strong, so again the inclusion predominatesthe VHCF behaviors of 3–2# steel.

Obviously, both the microstructure type and the inclusion levelcan affect the behaviors of crack initiation and VHCF properties.Though the conception of weak and strong microstructure typewould be different according to different inclusion level, on thewhole the fatigue crack tends to initiate from the more disadvan-tageous aspect between the microstructure and inclusion. Conse-quently, it is suggested that the inclusion level is in competitionwith the microstructure type to dominate the VHCF behaviors.

According to the VHCF behaviors of the six types of B/M steels, aschematic diagram of the competition mechanism between inclu-sion level and microstructure type is shown in Fig. 9. It could beseen that where the fatigue crack initiates depends on the relativeposition of the microstructure type and the inclusion level. In thiscase, the so-called ‘relative position’ means relative influence onVHCF behaviors. For a certain steel, if the position of its microstruc-ture type is below its inclusion level, then the microstructure factordominates the VHCF behavior and non-inclusion induced crack ini-tiation occurs. Otherwise, if the position of inclusion level is belowthe microstructure type, then inclusion induced crack initiationoccurs.

Fig. 8. Typical microstructure of fracture surfaces. (a) 1–1#, ra = 500 MPa, Nf =4.35E6. (b) 1–2#, ra = 525 MPa, Nf = 1.854E8. (c) 2–1#, ra = 600 MPa, Nf = 4.971E7.(d) 2–2#, ra = 700 MPa, Nf = 8.776E7. (e) 3–1#, ra = 650 MPa, Nf = 1.928E7. (f) 3–2#,ra = 750 MPa, Nf = 5.836E7.

Fig. 9. Schematic diagram of the competition mechanism between inclusion andmicrostructure in VHCF.

Y. Yu et al. / International Journal of Fatigue 33 (2011) 500–506 505

Another important point revealed by Fig. 9 is that both thephysical metallurgy and the chemical metallurgy methods canimprove the VHCF property. Physical metallurgy aims at homoge-nization, refinement, strengthening and toughening of the micro-structure, while chemical metallurgy mainly controls the sizeand distribution of the non-metallic inclusions. The enhancedVHCF property of B/M steels could benefit from both the optimiza-

tion of microstructure and the reduction of inclusion size. How-ever, only by utilizing physical metallurgy or chemicalmetallurgy method we cannot obtain excellent VHCF property, be-cause the microstructure type is in competition with the inclusionlevel in determining the VHCF behaviors. For example, the inclu-sion size of No. 1 series steels can be limited to 15 lm by chemicalmetallurgy, but the VHCF property would remain on the same levelas 1–1# steel if the microstructure has no improvement, becausethe fatigue crack prefers to initiate not from fine inclusions butfrom undesirable substrate microstructure. On the other hand,the microstructure type of No. 1 series steels, including phase com-position, microstructure homogeneity and refinement degree, canbe adjusted to the same level as 3–2# steel while the VHCF prop-erty would remain on the same level as 1–2# steel if the inclusionlevel remains at the original level, because the fatigue crack prefersto initiate from coarse inclusions. Because of the mutual competi-tion and restriction between microstructure type and inclusion le-vel to determine the VHCF behaviors, the physical metallurgy andchemical metallurgy must be coordinated to obtain the optimizedVHCF property. For example, with the benefit of physical metal-lurgy methods, the 3–2# steel gains lower bainite/martensitemicrostructure, excellent microstructure homogeneity and the fin-est PAG size. Furthermore, the improvement of chemical metal-lurgy technology controls the inclusion size around 15 lm. Withthe coordination of physical and chemical metallurgy methods,excellent VHCF property of 3–2# steel is obtained.

4. Conclusions

The competition relation between microstructure type andinclusion level is addressed and proved by studying the VHCFbehaviors of designed B/M steels. Based on this competition rela-tion, in order to improve the VHCF properties of high strengthsteels, the necessary coordination of physical and chemical metal-lurgy is also discussed. The main conclusions are:

(1) Non-inclusion induced crack initiation is one kind of impor-tant VHCF failure mechanism in B/M dual phase steels. Inthis case, the fatigue crack initiates not from inclusions butdirectly from the microstructure of substrate.

(2) Non-inclusion induced crack initiation has nothing to dowith the absolute value of inclusion level but is decided bythe relative influence on VHCF behavior by microstructuretype and inclusion level. The microstructure type and inclu-sion level will compete to dominate the VHCF behaviors, andthe fatigue crack will tend to initiate from the relatively dis-advantageous aspect in the microstructure and inclusion.

(3) The physical and chemical metallurgy must be coordinatedto obtain the optimized VHCF property.

Acknowledgements

The study is sponsored by the Chinese National Basic ResearchProgram under Grant No. 2004CB619105. The authors greatlyappreciate Mr. Guang Sheng at The Pennsylvania State Universityfor helpful discussions and careful reading of this manuscript.

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