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12 Journal of Mineral, Metal and Material Engineering, 2019, 5, 12-21

E-ISSN: 2414-2115/19 © 2018 Scientific Array

Effect of Bainite Volume Fraction on Fatigue Properties of Bainite/Martensite Dual Phase EA4T Steel

Jian-Zhi Chen and Bin Zhang*

Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, School of Materials Science and Engineering, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, P. R. China

Abstract: Bainite/martensite dual phase EA4T steel with bainite volume fraction ranging from 21% to 70% was prepared by a process of isothermal quenching with the variation of isothermal holding time. Tensile tests and tension-compression fatigue tests were conducted to investigate the effect of bainite volume fraction on tensile properties and fatigue damage behavior of the steel. The results show that fatigue strength varied in a quadratic relation with its tensile ultimate strength, and the specimens with bainite volume fraction of 49% have the highest fatigue strength and the best synergy of strength and ductility, which is contributed by the smaller hardness difference between bainite and martensite, better coordination of plastic deformation and higher fatigue crack propagation resistance of the specimens.

Keywords: EA4T steel, Bainite/martensite, Dual phase, Bainite volume fraction, Fatigue properties.

1. INTRODUCTION

As an important load-bearing component of high-speed railway, train axles are subject to cyclic loading during their serving process. Apart from the synergy of strength and ductility of the axle materilas, fatigue performance is also very crucial for the long-term serving reliability of the axles. EA4T steel is a type of standardized European low alloy steel, which is widely used in the axle manufacturing [1]. Normally, microstructure of EA4T steel is sorbite obtained by quenching and high-temperature tempering, which enables the steel to have a good synergy of strength and ductility [1, 2]. Unfortunately, strength of EA4T steel with microstructure of sorbite is very limited, which is not beneficial to improving fatigue strength and realizing the 30-year service life of the axles, corresponding to 6×109 fatigue cycles [3-5]. Therefore, it has become a research focus around how to improve fatigue performance of EA4T steel and maintain a good synergy of strength and ductility at the same time.

As a main type of advanced high strength steel, dual phase (DP) steels have better formability and strength-ductility combination compared with those of traditional steels [6-11]. Steels with DP microstructures usually refer to ferrite/austenite (F/A), ferrite/martensite (F/M) and bainite/martensite (B/M) steels, in which the harder phase usually distributes in the soft phase matrix. Strength, ductility and fatigue properties of the DP steels might be controlled by adjusting volume

*Address correspondence to this author at the Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, School of Materials Science and Engineering, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, P. R. China; Tel: +86-24-8369 1585; E-mail: [email protected]

ratios of the two phases [12-15]. When fatigue loading is applied to a DP steel, stress concentration tends to occur in the softer phase and induce crack initiation, especially the strength difference between the two phases is larger [16-19]. According to the study on very high cycle fatigue properties of a carbide-free bainite/martensite (CFB/M) DP steel [17], mechanical-property difference between bainite and martensite would lead to uncoordinated deformation of the two phases during the fatigue loading process, and the bainite bearing plastic deformation primarily and becoming crack initiation sites in the steel. Research on fatigue behavior of ferrite/bainite steels also indicated that the strength difference between ferrite and bainite had an important effect on the initiation and propagation of fatigue cracks [20]. Aside from the difference of mechanical properties between the two phases, volume ratio of the two phases in a DP steel also plays an important role in fatigue properties. It was found that both fatigue strength and fatigue crack propagation threshold of the CFB/M DP steel were higher than those of the full martensite steels [21]. An investigation about the effects of martensite contents on high cycle fatigue properties of F/M DP steels indicated that the content increase of martensite was beneficial to the improvement of fatigue performance [22]. When volume percent of martensite in a DP steel was less than 30%, fatigue strength increased with the enhancement of martensite, while it decreased as martensite contents exceeds 30 vol.% [23]. In addition, phase morphology of the DP steels also affects the fatigue performance according to the report of a DP steel with network and fibrous martensite [24]. Study on fatigue crack growth behavior of a DP steel with two different microstructures, martensite islands encapsulated by ferrite and ferrite encapsulated by

Effect of Bainite Volume Fraction on Fatigue Properties Bainite/Martensite Journal of Mineral, Metal and Material Engineering, 2019, Vol. 5 13

martensite islands, respectively, revealed that the crack growth resistance in the near threshold region of the steel with the former microstructure is higher that of the steel with latter microstructure [13]. What is more, for the B/M DP steels, bainite and martensite possess more similar crystallographic characteristics [25, 26] and smaller difference of mechanical properties comparing with those of ferrite and martensite in the F/M DP steels. However, few reports on the effects of bainite volume fraction on the high cycle fatigue properties of B/M DP steels.

In this work, B/M DP EA4T steel with different volume fractions of bainite (VB) were prepared through the process of isothermal quenching, and the effects of VB on the high cycle fatigue (HCF) properties of EA4T steel were investigated systematacially.

2. EXPERIMENTAL PROCEDURE

Chemical composition of EA4T steel is shown in Table 1. The as-received materials are cylindrical bars with 18 mm-diameter and 170 mm-length, and isothermal heat treatment regime is presented schematically in Figure 1. All the specimens were heated to 910 ℃ for 120 min holding in a furnace, and then soaked in a salt bath at 440 ℃ for different duration time of 0.5 min, 2 min, 5 min and 30 min, respectively, to alter the VB before rapidly cooling (quenched in water) to room temperature. Finally, the tempering treatment was conducted at 300 ℃ for 180 min followed by air cooling.

Macro-hardness of four types of specimens with different VB was investigated by VH-5ACL Vickers hardness tester at a load of 294 N for 20 s duration. Micro-hardness of different phases were measured by LECO AMH43 micro-hardness tester at a load of 0.245 N for 20 s.

The tensile specimens were processed in accordance with ISO 6892:1998 standard, and the gauge dimension of the specimens is 10 mm×4 mm×2.5 mm. Tensile tests were conducted on INSTRON 5982 mechanical testing machine with the nominal strain rate of 3.3×10-3 s-1 at room temperature. Fatigue specimens were designed based on the ISO

1099:2006 standard, and the gauge dimension of the specimens is 10 mm×4 mm×2.5 mm. All the specimens were grinded and electro-polished to obtain smooth surfaces. The conventional tension-compression HCF tests were performed on MTS 858 A-T testing machine at a stress ratio of R = -1 with a sine waveform and test frequency of 50 Hz. All the fatigue tests were conducted at room temperature.

Microstructures, tensile and fatigue fracture surfaces and side surfaces near the fatigue fracture zones of the specimens with different VB were observed and analyzed by using scanning electron microscopy (SEM, Leo Supra 35).

Figure 1: Heat treatment regime for as-received EA4T steel.

3. RESULTS

3.1. Microstructure

Typical SEM images of specimens being austempered in 440 ℃ for different durations are shown in Figure 2. It can be observed that DP EA4T steel is composed of bundles of bainite laths and some blocks of martensite, and the plate-like bainite distributes along austensite grain boundaries and across the entire primary austenite grains (see Figure 2a-d). Figure 2e shows a typical TEM image of the specimens austempered in 440 ℃ for 2 min. One can see that bainite is plate-like, and carbides distribute in the bainite plates, precipitating at an angle of 60° to the

Table 1: Main Chemical Compositions of EA4T Steel (wt.%)

C Si Mn S P Cr Cu Mo Ni V

0.22-0.29 0.15-0.40 0.50-0.80 0.015 0.020 0.90-1.20 0.30 0.15-0.30 0.30 0.06

14 Journal of Mineral, Metal and Material Engineering, 2019, Vol. 5 Chen and Zhang

bainite axial direction. With the increase of isothermal time, VB increases from 21% to 70% according to the statistical results, and the width of bainite plates (WB) also increases accordingly but tending to be stable finally, which can be seen clearly in Figure 2f.

3.2. Hardness

Testing values of hardnesses of bainite (HVB), martensite (HVM) and B/M DP (HVBMT) are shown in Figure 3a, which decrease with the increase of VB. It is well known that the phase transformation from austenite to martensite results in martensite volume expansion and production of transformation stress leading to hardening of the surrounding bainite, the softer phase in B/M DP steels. With the increase of isothermal time, VB increases and martensite volume fraction (VM) decreases, the interaction between martensite and bainite decreases accordingly, weakening the hardening effect of the steels [7, 27, 28]. In addition, WB increases with prolongation of the

isothermal time (see Figure 2f), leading to the decrease of bainite hardness according to the Hall-Petch relation. For the hardness of martensite in B/M DP steels, it reduces with the increase of bainite contents since the transformation stress would be easily transferred to the surrounding softer phase bainite comparing with that of the full martensite steels [27, 29, 30]. The calculated value of macro-hardness of DP (HVBMC) as a function of VB and VM is obtained according to the following rule-of-mixtures [27, 31, 32],

HVBMC = HVB !VB +HVM !VM (1)

As shown in Figure 3a, values of HVBMC are close to those of the HVBMT. Difference between HVM and HVB

vs VB is shown in Figure 3b. It can be seen that the hardness difference decreases with the increase of VB, and it is the largest when the VB is 21%. Besides, hardness differences of the specimens with VB of 49 %, 60% and 70% are close, much lower than that of the specimen with 21% VB.

Figure 2: SEM images of B/M DP EA4T steel austempered in 440 ℃ for holding times of (a) 0.5 min, (b) 2 min, (c) 5 min and (d) 30 min, respectively; (e) TEM image of a specimen, holding 2 min, (f) variation of VB and WB with austempering duration.


3.3. Tensile Properties

Tensile properties of the four types of specimens with different VB are shown in Table 2. With the increase of VB, the tensile strength and yield strength decrease, while total elongation increases. Normally, product of tensile strength and total elongation (PSE) is a reflection of combination of strength and ductility [33-35]. Therfore, specimens with 49% VB have the best synergy of strength and ductility due to the highest value of PSE. Moreover, normalized strain hardening rate (Θ) as a function of true strain (εt) of specimens with different VB are shown in Figure 4. The Θ value of all specimens decreases with the increase of εt, and it increases with the increase of VB at the same εt. It is known that necking would occur once Θ≤1[36]. When necking starts, the corresponding strain values of specimens with VB of 21%, 49%, 60% and 70% are εt=4.34%, 5.30%, 5.50% and 5.71%, respectively (Figure 4), which indicates that strain hardening rate and uniform deformation ability of B/M DP EA4T steels increase with the increase of VB.

Figure 4: Normalized strain hardening rate (Θ) as a function of true strain of specimens with different VB.

SEM images of tensile fracture surfaces of the specimens with different VB are shown in Figure 5. For the specimen with 21%VB, brittle tendency with characteristic of well-defined facets, cleavage steps and tearing ridges can be observed clearly, and numbers of dimples is very limited (see Figure 5a). Dimple number increases with the enhancement of VB,

Figure 3: (a) Variations of hardness of bainite, martensite and B/M DP, and (b) hardness difference between martensite and bainite as a function of VB.

Table 2: Mechanical Properties of EA4T Steel with Different VB

VB (vol.%) σy (MPa) σb (MPa) δ (%) PSE (GPa·%)

21 1136 1401 11.0 15.4

49 722 1114 15.3 17.0

60 719 1012 16.6 16.8

70 715 969 16.2 15.7

σy: yield strength, σb: tensile strength, δ: total elongation, PSE: the product of tensile strength and total elongation.


and the distribution of dimples becomes denser and more uniform, as seen from Figure 5a to Figure 5d. The dominant fracture mode of the specimens is ductile when the VB increases to 70% (see Figure 5d).

3.4. Fatigue Properties

S-N curves of specimens with different VB are shown in Figure 6a. The curves intersect at the stress amplitude (σa) of 372 MPa, from which two zones are divided, the low and high cyclic fatigue zone, respectively. From the comparsion of the curves in the low cyclic fatigue zone, the blue zone in Figue 6a, one can find that specimens with lower VB have better fatigue performance. However, fatigue performance of specimens with 21% VB deteriorates sharply with the decrease of σa. As σa is lower than 372 MPa, fatigue

performance of the specimens with VB of 21% is much worse than those of the other specimens, and specimens with VB of 49% have the highest fatigue limit (σ-1). Relationship between σ-1 and ultimate tensile strength (σb) of the specimens with different VB is shown in Figure 6b. One can see that σ-1 increases with the enhancement of σb first, and then decreases. A quadratic fitting curve is obtained from the relation between σ-1 and σb, which can be expressed as follows,

!-1 = -1567.70 + 3.26!b -0.0014!b2 (2)

The maximum fatigue strength could be obtained by

differentiating Eq. 2, i.e. d!-1

d!b

"#$

%&'= 0 , as σb=1164 MPa.

Figure 5: SEM images of tensile fracture surfaces of specimens with VB of (a) 21%, (b) 49%, (c) 60% and (d) 70%, respectively.

Figure 6: (a) S-N curves, and (b) relationship between fatigue strength and tensile strength of different VB specimens.


Thus, it is expected that the specimen with tensile strength of 1164 MPa should have the maximum fatigue strength. For the present four types of specimens whose values of σb are listed in Table 2, in which σb = 1114 MPa corresponding with the specimens of 49%VB is very close to σb=1164 MPa. Therefore, specimens with VB of 49% has the highest fatigue strength among the four types of specimens, which is consistent with the results shown in Figure 6a.

3.5. Fatigue Crack Initiation

In order to explore fatigue crack initiation of the specimens with different VB, the side surfaces close to

the fatigue fracture zone were observed carefully. From the SEM observation shown in Figure 7, most of fatigue cracks initiate in bainite areas (see Figure 7a), and some cracks initiate in bainite areas near the boundaries of bainite and martensite (see Figure 7b). In addition, only a few fatigue cracks locate at the austenite boundaries (see Figure 7c) or in the martensite areas (see Figure 7d). As shown in Figure 6a, fatigue performance of the specimens with different VB changes a lot when σa varies from higher to lower. Thus, fatigue crack initiation behavior of the four types of specimens at higher stress amplitude (σa = 450

Figure 7: SEM images of fatigue cracks initiation (a) in bainite (VB=49%), (b) at interface of bainite and martensite (VB=49%), (c) at grain boundaries (VB=21%), and (d) in martensite (VB=49%) in specimens at σa=450 MPa.

Figure 8: SEM images of fatigue cracks initiation on side surfaces of specimens with different VB (a) 21%, (b) 49%, (c) 60%, and (d) 70%, at σa=450 MPa.


MPa) and lower stress amplitude (σa = 325 MPa) were compared. At the higher σa of 450 MPa, crack numbers increase with the enhancement of VB (see Figure 8). Gaps between crack planes in the specimens of 70% VB (see Figure 8d) are obviously wider than those in other specimens (see Figure 8(a-c)), indicating more serious fatigue damage occurred in the specimens[37]. Furthermore, coalescence of fatigue cracks shown in the elliptic region of Figure 8d indicates the severity of fatigue damage in the specimens with 70%VB. Specimens fatigued at σa = 325 MPa, only few cracks can be observed on the side surfaces of the specimens, and almost all cracks initiate in bainite areas (see Figure 9).

The statistical results of fatigue crack density of the four types of specimens at σa of 450 MPa and 325 MPa, respectively, are shown in Figure 10a. At σa =

450 MPa, numbers of cracks in bainite areas account for the majority of the total numbers of cracks. Fatigue crack density increases with the increase of VB when VB increases from 21% to 60%, while it drops as VB increases to 70%, which may be related to the coalescence of fatigue cracks shown in Figure 8d. At σa =325 MPa, cracks in all specimens mainly initiate in bainite areas, and the crack density of the specimens with 21% VB is higher than that of other specimens.

4. DISCUSSION

4.1. Effect of Volume Fraction of Phase on Fatigue Strength

From the perspective of microstructure, the interaction between martensite and bainite becomes stronger with the decrease of VB. In addition, WB of specimens also decreases with the lowering of VB (see

Figure 9: SEM images of fatigue cracks initiated on surfaces of specimens with different VB (a) 21%, (b) 49%, (c) 60% and (d) 70%, at σa= 325 MPa.

Figure 10: (a) Statistical results of fatigue crack density for four types of specimens at σa=450 MPa and 325 MPa, respectively, (b) a crack path observed in a specimen with 49%VB.


Figure 2f), corresponding to the increase of bainite hardness. On one hand, increasing the hardness of bainite has a positive effect on raising crack initiation resistance, and at the same time, decreasing of VB is benefitial to reduction of crack initiation since bainite is the location of fatigue cracks preferring to initiate (Figure 10a). So, the decrease of WB and VB is helpful to the increase of fatigue strength. On the other hand, it can be seen from Eq. 2 that there is a quadratic relationship between σ-1 and σb, and the maximum value of σ-1 can be achieved when σb is 1164 MPa, which is close to σb =1114 MPa for the specimens with 49%VB. However, WB and VB of the specimens with 49%VB are not the smallest ones in the four types of specimens, so other factors should be considered to explain the effects of VB on the HCF properties of our DP EA4T steels.

4.2. Effect of Microstructure Homogeneity on Fatigue Properties

It is known that fatigue damage is mainly caused by localized plastic deformation. A higher work-hardening ability is beneficial to improve the deformation uniformity under fatigue loading, and thus localized plastic deformation and necking are delayed [36, 38, 39]. Therefore, work-hardening ability has a key effect on the fatigue properties of materials in addition to the strength of the materials. Higher work-hardening ability is benefitial to fatigue property improvement [38]. As shown in Figure 4, work-hardening abilities of the specimens with VB of 49%, 60% and 70% are higher than that of the specimens with VB of 21%, indicating the specimens with 49%, 60% and 70% VB possess a better deformation uniformity.

Microstructures with better deformation uniformity usually have better fatigue properties [16-18, 38]. When the softer phase in DP steel begins to yield and bears plastic deformation, martensite remains in the stage of elastic deformation, hindering the continuous deformation of bainite. The inhomogeneity and incoordination deformation between bainite and martensite finally makes the soft bainite becomes the main location of fatigue crack initiation. Furthermore, the larger the strength difference between the two phases, the easier the cracks initiate in the soft phase [16-19]. Hardness difference between the two phases in the specimens with 49% and 70%VB are 65.16 HV and 62.24 HV, respectively, which are the smallest two of the four specimens (see Figure 3b). Additionally, higher PSE indicates a better synergy of strength and ductility of the materials, which is also benefitial to the

improvement of fatigue properties [4, 40]. PSE of the 49% VB specimens is 17.0 GPa·%, which is the highest in the four types of specimens. Therefore, it is reasonable to believe that the 49%VB specimens possess the best fatigue properties. Figure 10b shows an SEM image of a fatigue crack path observed on the surface of a 49%VB specimen. The fatigue crack passed through bainite plates and deflected when it encountered martensite packets, indicating the existence of martensite blocked the fatigue crack growth.

Thus, adjusting an appropriate VB for EA4T steel is essential to ensure an excellent synergy of strength and ductility, making the crack propagation path more tortuous to further enhance the fatigue properties of the DP steel. At the same time, the difference of mechanical properties between bainite and martensite should be considered to improve deformation homogeneity and coordination of the steel under fatigue loading.

CONCLUSIONS

In summary, the EA4T DP steel with 49%VB have the best fatigue properties in the four types of specimens with 21%, 49%, 60% and 70%VB, respectively. The PSE of 49%VB specimens is 17.0 GPa·%, and the hardness difference between bainite and martensite is 65.16 HV. Fatigue cracks mainly initiate in the softer bainite areas, and the fatigue crack path can be deflected when the propagating crack encounters the martensite packets. An optimum ratio and suitable hardness difference of bainite and martensite, better microstructure deformation coordination are beneficial to the improvement of the fatigue properties of the EA4T B/M DP steel.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 51671050).

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Received on 18-01-2019 Accepted on 02-02-2019 Published on 04-03-2019 © 2019 Chen and Zhang; Licensee Scientific Array. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

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