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The effect of inclusions on the fatigue behavior of fine-grained high strength 42CrMoVNb steel
International Journal of Fatigue 26 (2004) 959–966 www.elsevier.com/locate/ijfatigue
The effect of inclusions on the fatigue behavior of fine-grained high strength 42CrMoVNb steel Z.G. Yang a,, G. Yao a, G.Y. Li a, S.X. Li a, Z.M. Chu b, W.J. Hui c, H. Dong c, Y.Q. Weng c a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b Beijing Research Institute of Mechanical and Electrical Technology, Beijing 100083, PR China c Central Iron and Steel Research Institute, Beijing 100081, PR China Received 23 April 2003; received in revised form 8 October 2003; accepted 28 January 2004
Abstract The fatigue properties of high strength 42CrMoVNb steel with three fine grain sizes produced by different heat treatment procedures are studied in this paper. The experimental results illustrate that there is no horizontal asymptote in fatigue S–N curves for smooth specimens at 106–107 cycles regime, hence the conventional fatigue limit eliminates. The fatigue crack initiation sites in smooth specimens were related closely to fatigue life, and most of the fracture origins initiated at the inclusions. The diameters and locations of these inclusions were measured, and their morphology was observed in SEM. The critical size of inclusion and the appropriate grain size of prior austenite, which should be controlled in practical production, are obtained by analyzing the experimental data. # 2004 Elsevier Ltd. All rights reserved. Keywords: High strength 42CrMoVNb steel; Fine grain; Inclusion; Fatigue strength; Fatigue crack initiation site
1. Introduction In order to improve the properties of the steels, a surge to seek ultra fine grained technology, purified technology and uniformity technology took place recently [1]. It is known that grain refinement is a promising way to enhance the tensile strength and toughness of steels. It is demonstrated that the Hall– Petch relation holds for yield strength and ultimate tensile strength when the grain size of steels decreases down to 1 lm [2–4]. Generally speaking, the higher the tensile strength, the higher the fatigue strength will be. However, the inclusion plays a very crucial role in the fatigue fracture process for high strength steels. The application of secondary refining techniques and nonmetallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of inclusions in steels. The inclusions in clean
steels consist of a few large ones and clouds of small ones. But these few large inclusions, which are hard to avoid in clean steel, can prove to be catastrophic [5]. Therefore, in order to get the best comprehensive properties of high strength steel, how to harmonize the relation of grain size and inclusion size to achieve higher tensile strength and fatigue strength, is currently a hot topic of steel research [6]. The materials studied in this work are fine-grained high strength 42CrMoVNb steel to be used for automotive structures and components. The fatigue property of the steel is one of the key issues for improvements of the mechanical performance. In this paper, the fatigue behaviors and micro-fracture characteristics of these steels are studied in detail, and the relations between the properties and microstructures (inclusions and grains) of these steels are discussed. 2. Materials, specimens and experimental method
Corresponding author. Tel.: +86-24-8397-8023; fax: +86-24-23891320. E-mail address: [email protected] (Z.G. Yang). 0142-1123/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2004.01.009
Based on 42CrMo, the high strength 42CrMoVNb steel has been designed, and its main chemical
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Z.G. Yang et al. / International Journal of Fatigue 26 (2004) 959–966
composition consists of (wt%) [7]: 0.42 C, 0.17 Si, 0.31 Mn, 0.001 S, 0.004 P, 1.10 Cr, 0.98 Mo, 0.32 V, 0.04 Nb, 0.01 Al, and the balance Fe. Steel of 18 mm diameter were produced by hot rolling after smelting in electric furnace, then machined to rough specimens, and heat treated. Finally, standard rotating bending fatigue specimens (Fig. 1) were prepared for testing, and correspondingly the circularly notched rotating bending fatigue specimens were also prepared for testing of the notch sensitivity. Both the depth of the notch and the radius of the notch tip are 0.75 mm. The high strength steels used in the present work are designated as ADF1-880, ADF1-920 and ADF1-940 in terms of various quenching temperatures. Table 1 shows the heat treatment procedures, prior austenite grain sizes, and tensile properties. The prior austenite grain sizes and appearances are shown in Fig. 2. All fatigue tests for smooth and notched specimens were carried out under rotating bending at a rotation rate of 5000 rpm, and the compressed air was used to eliminate the possible ‘‘self-heating’’ in specimens during testing.
3. Results 3.1. S–N curves and fatigue strength The S–N curves of smooth specimens are shown in Fig. 3, for which no horizontal asymptote but gradient exists at 106–107 cycles regime, hence the conventional fatigue limits cannot be determined. At 107 cycles, the fatigue strengths of smooth and notched specimens, r1 and r1N, were figured out by the staircase method in order to raise the confidence (Fig. 4). Notch sensitivity coefficient q could be calculated from the formula [8] q¼
Kf 1 Kt 1
ð1Þ
here Kf ¼ r1 =r1N , and Kt ¼ 1:89 is a notch stress concentration factor in the present experiment. The fatigue properties of three steels are shown in Table 2. The fatigue strength of smooth specimens does not change monotonically with prior austenite grain size altering but increases monotonically with tensile strength (Fig. 5). The ADF1-920 with the highest tensile strength has the highest fatigue strength, however, the ADF1-940 with the lowest tensile strength has the highest fatigue strength of notched specimens. 3.2. Fatigue crack initiation
Fig. 1.
Specimen configuration for rotating bending test.
3.2.1. Fractographic observation Observed by SEM, three kinds of fatigue crack initiation sites in the fractured specimens were found: internal inclusion (I), surface inclusion (SI) and surface
Table 1 Heat treatment procedures, prior austenite grain sizes and tensile properties of 42CrMoVNb Steel code ADF1-880
ADF1-920 ADF1-940
Heat treatment procedure v
920 C vacuum furnace preheating ð30 minÞ þ oil cooling þ electricity rapid v v v heating (70–100 C/s) to 880 C þ oil quenching three times þ 600 C tempering (2 h) v 920 C vacuum furnace preheating ð30 minÞ þ oil cooling þ electricity rapid v v v heating (70–100 C/s) to 920 C+oil quenching þ 600 C tempering (2 h) v v 940 C salt furnace preheating ð30 minÞ þ oil quenching þ 600 C tempering (2 h)
d (lm)
rs (MPa)
rb (MPa)
rs/rb
1.7
1370
1440
0.95
4.0
1465
1540
0.94
8.0
1315
1400
0.94
Fig. 2. The appearances of prior austenite grain boundary networks of the steels: (a) ADF1-880, (b) ADF1-920, (c) ADF1-940.
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Table 2 Fatigue properties (at 107 cycles) Materials code
r1 (MPa)
r1N (MPa)
q
ADF1-880 ADF1-920 ADF1-940
746 20 767 19 738 22
421 20 417 10 446 20
0.87 0.94 0.75
Fig. 3. S–N curves of smooth specimens of 42CrMoVNb after three heat treatments.
Fig. 5. Monotonic relationship of r-1 to rb for smooth specimens of three steels.
Fig. 4. Experimental result of smooth specimens of ADF1-920 by the staircase method.
matrix (S). Most fracture origins are internal inclupffiffiffiffiffiffiffiffiffi sions, and their diameters / (calculated from area) and depths D were measured, as listed in Tables 3–5. The appearance of an inclusion can be seen in Fig. 6. These inclusions, containing Al, Mg, Ca and O, most likely consist of Al2O3, MgO and CaO. In addition, all
these inclusions were divorced from the matrix and the most of them were cracked and broken. There are several radial ledges around the inclusion (Fig. 7), which were formed by propagation and linkage of primary micro-cracks at different planes, meanwhile the tire patterns emanating from the inclusions were found in a few specimens, the direction of which is parallel to the specimen surface (Fig. 8). 3.2.2. Factors affecting the type of fatigue crack initiation site As seen in Tables 3–5, when rmax 4r1 and Nf < 106 cycles, fatigue cracks mostly initiated at the specimen
Table 3 Fatigue cracking data of ADF1-880 Specimen no.
Applied stress, rmax (MPa)
Fatigue life, Nf (cycle)
Stress ratio, rmax/r1
Fatigue crack initiation sitea
Distance, D (lm)
ADF1-880-09 ADF1-880-08 ADF1-880-19 ADF1-880-18 ADF1-880-07 ADF1-880-06 ADF1-880-11 ADF1-880-12 ADF1-880-17 ADF1-880-15 ADF1-880-20 ADF1-880-22 ADF1-880-23 ADF1-880-21
725 725 725 750 750 750 775 775 775 800 850 850 900 900
5:85 106 1:027 107 1:28 107 5:14 106 6:60 106 9:14 106 1:68 106 6:40 106 8:05 106 1:24 106 1:90 105 2:00 105 1:27 104 9:53 104
0.97 0.97 0.97 1.01 1.01 1.01 1.04 1.04 1.04 1.07 1.14 1.14 1.21 1.21
I I SI I I I I SI I I S S S SI
260 33 146 25 0 18 322 37 66 18 520 45 145 40 0 8 66 35 50 45 – – – – – – (0.17 17):(35.15 15)
a
I, internal inclusion; SI, surface inclusion; S, surface matrix.
Inclusion diameter, / (lm)
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Table 4 Fatigue cracking data of ADF1-920 Specimen no.
Applied stress, rmax (MPa)
Fatigue life, Nf (cycles)
Stress ratio, rmax/r1
Fatigue crack initiation sitea
Distance, D (lm)
Inclusion diameter, / (lm)
ADF1-920-06
750 775 775 775 800 800 800 850 875 900 900 950 950 1000
9:97 106 3:025 105 7:79 106 9:91 106 8:93 105 1:84 106 2:72 106 2:27 106 1:70 106 1:03 105 1:77 105 1:37 105 1:40 105 8:70 104
0.98 1.01 1.01 1.01 1.04 1.04 1.04 1.11 1.14 1.17 1.17 1.24 1.24 1.30
I I I I I I I I I SI SI SI SI SI
247 24 215 253 36 68 105 91 180 0 0 0 0 0
40 44 29 36 17 23 22 38 29 27 17 21 38 10
ADF1-920-08 ADF1-920-05 ADF1-920-10 ADF1-920-01 ADF1-920-04 ADF1-920-12 ADF1-920-14 ADF1-920-16 ADF1-920-15 ADF1-920-17 ADF1-920-19 ADF1-920-18 ADF1-920-20 a
I, internal inclusion; SI, surface inclusion; S, surface matrix.
Table 5 Fatigue cracking data of ADF1-940 Specimen no.
Applied stress, rmax (MPa)
Fatigue life, Nf (cycles)
Stress ratio, rmax/r1
Fatigue crack initiation sitea
Distance, D (lm)
Inclusion diameter, / (lm)
ADF1-940-15 ADF1-940-17 ADF1-940-14 ADF1-940-19 ADF1-940-03 ADF1-940-13 ADF1-940-05 ADF1-940-02 ADF1-940-01 ADF1-940-12 ADF1-940-11 ADF1-940-10
725 725 750 750 750 775 775 800 800 800 825 850
3:60 106 5:70 106 5:10 105 7:19 105 7:04 106 3:59 105 2:19 106 3:85 105 9:98 105 2:50 106 8:12 104 1:20 103
0.98 0.98
I I I I I SI I S S I S S
38 75 34 27 170 0 187 – – 318 – –
10 17 46 37 10 18 45 – – 41 – –
a
1.02 1.02 1.02 1.05 1.05 1.08 1.08 1.08 1.12 1.15
I, internal inclusion; SI, surface inclusion; S, surface matrix.
surface of three steels. Surface cracks started usually at surface inclusions for ADF1-920 and at surface matrix for other two lower strength steels, except two specimens of ADF1-940, in which cracks initiated at inclusions inside with extraordinary large sizes and at short distances from the surface. But when rmax r1 and Nf > 106 cycles, the fatigue cracks for all the fractured specimens initiated at interior inclusions. According to the stress distribution in the cross section of a specimen under rotating bending, we can assume, instead of rmax, the local stress at inclusion center, rL, D rL ¼ rmax 1 ð2Þ R where rmax is the maximum stress applied, MPa; D, the shortest distance from an inclusion center to the speci-
men surface, lm; R, the diameter of working section of specimen and R ¼ 4000 lm. When only the interior inclusions are considered and rL =r1 < 1, we can find the fatigue life of all specimens is above 106 cycles. So, the ratio of local stress to fatigue strength is an important factor affecting the type of crack initiation. Moreover, under the present experiment conditions, a simple linear relationship between the internal inclusions’ size / and location D can be found / ¼ 12:5 þ 0:076D
ð3Þ
among filled symbols that represent the local stress rL =r1 < 1, as shown in Fig. 9. This means that the diameter / of the cracking inclusions should increase with increase in the distance D. But the open symbols
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963
Fig. 9. Relationship between / and D of interior inclusions as fatigue crack initiation sites at two stress ranges (filled symbols: rL =r1 < 1; open symbols: rL =r1 > 1).
that represent the local stress rL =r1 > 1 were all above the line (fitted by solid symbols).
4. Discussion Fig. 6. An internal inclusion as fracture origin on the fracture surface observed by SEM, showing its global and broken appearance. (a) Location of an inclusion; (b) appearance of an inclusion (specimen ADF1-920-06#: rmax ¼ 750 MPa, Nf ¼ 9:97 106 cycles, D ¼ 247 lm, / ¼ 40 lm).
4.1. Elimination of fatigue limit for high strength steel with fine grain
Fig. 7. Radial ledges around an inclusion served as crack initiation site (specimen ADF1-880-17#: rmax ¼ 775 MPa, Nf ¼ 8:05 106 cycles, D ¼ 66 lm, / ¼ 35 lm).
4.2. Influence of grain size on the tensile and fatigue properties of high strength steel
Fig. 8. Tire patterns from an inclusion served as crack initiation site (specimen ADF1-880-06#: rmax ¼ 750 MPa, Nf ¼ 9:14 106 cycles, D ¼ 520 lm, / ¼ 45 lm).
Morris et al. [13] have pointed out that in the case of martensitic steel the adequate have measure of grain size was the size of the crystallographically coherent block or packet of tempered martensites, which was regarded as the effective size affecting the strength. In our studies, grain refinement does effectively reduce the lengths of the laths, so the size of blocks decreases. According to Hall–Petch’s equation, the strengths of steels should have increased with decrease in grain sizes, however, the highest strength is of ADF1-920 with blocks of medium size, not of ADF1-880 with the smallest sized blocks (Fig. 10), which means there should be other factors influencing the strength. The factors affecting the strength may, at the least, include:
The results presented above show that all three steels cracked from inclusions when rmax r1 . Their fatigue lives are within 3:6 106 1:28 107 cycles. There is no conventional fatigue limit at 107 cycles. Miller and O’Donnell [9] and Murakami et al. [10–12] have discussed possible factors which may cause fatigue failure in the regime of Nf > 107 cycles. According to their viewpoint, it is not so easy to identify the crucial mechanism, among many possible factors for eliminating the fatigue limit. However, special attention should be paid to the influence of the hydrogen trapped by nonmetallic inclusions in high strength steels.
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Fig. 10. Relation between austenite grain size and tensile strength.
(1) For present low alloy steels with microalloy elements V and Nb, the smaller size of laths leads to the increase of the specific surface of laths, which can decrease the amount of microalloy element in the interior of the laths, then precipitation carbides reduce and the strength of matrix reduces. As compared with ADF1-940, no obvious carbide precipitated in ADF1880 after tempering, and acicular carbide in ADF1-940 perpendicular to lath longitudinal direction can resist the cleavage fracture along the direction of lath, as shown in Fig. 11. (2) On increasing the times of quenching and tempering, the dislocation density becomes greater. Such a condition can enhance the strength but at the sametime can effectively obstruct the precipitation of carbides, which then reduces the strength. More quenching and tempering times will bring more vacancies in the steel that can also reduce the strength. 2 more vacancies in steel can cause the premature fragmentation of inclusion and decohesion between one inclusion and the surrounding grains [14]; this is a favorable factor for the early formation of microcracks. Thus, the strength of ADF1-880 is not improved when compared to that of ADF1-940. Besides, the grain size may affect micro-crack initiation during tensile testing by other mechanisms, as
Fig. 12. Schematic diagram illustrating variant crack initiation sites in finer or coarser grains (a) Finer grains and (b) Coarser grains.
schematically given in Fig. 12. Under larger tensile stress, the smaller grains can withstand less strain in terms of their strength and more uniform plastic deformation, so the strain will be imposed on the inclusion, which can be so serious that micro-cracks form finally in this inclusion and cause its breakage. Comparatively, because the localized plastic deformation will occur in the coarser grains rather easily, the slip lines and slip steps are able to impinge the interface between the matrix and inclusion. Then the micro-cracks form directly at the interface rather easily. In both the cases cracking inclusions will lead to lower tensile strength of the steels. The appropriate grain size may not cause the micro-cracks to initiate in the interior of the inclusion and at the interface between the inclusion and the metal matrix as easily as in finer or coarser grains. 4.3. Inclusion and grain size control
Fig. 11. The appearances of precipitates in: (a) ADF1-940 and (b) ADF1-880.
The size and location of inclusions in high strength steel components under rotating and bending loading should be controlled strictly in terms of their important influence on the fatigue properties. Fig. 13 represents the relation between sizes and locations of inclusions as crack initiation sites with fatigue life above 106 cycles, in which we can find three approximately parallel lines. Each line denotes the critical size of inclusion as the crack initiation site, above which the larger the distance to the line the shorter the fatigue life of speci-
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965
that the appropriate prior austenite grain size for ADF1 steel is around 4 lm. 5. Conclusions
Fig. 13. Relationship between / and D of inclusions (Nf > 106 cycles).
mens, and below which the fatigue failure is very difficult to take place. We can obtain the safe critical values of initiations in these high strength steels from Fig. 13: (1) when D ¼ 0 100 lm, / ¼ 8 14 lm; (2) when D ¼ 100 300 lm, / ¼ 14 28 lm; and (3) when D > 300 lm, / > 30 lm. For ADF1-880, ADF1-920 and ADF1-940, if fatigue crack initiation site is not from the inclusion, the maximum size of inclusion will be less than 8, 15.5 and 7.5 lm, respectively. ADF1-880 and ADF1-940 have more rigorous requirement of inclusion size than ADF1-920. In fatigue testing, the micro-crack initiation may be divided into two stages. At first the inclusion or the interface between the inclusion and the matrix cracks, and then the micro-crack is initiated in the adjacent grains. ADF1-880 has the finest grains (1.7 lm) and most of its fatigue life is consumed during the period of crack initiation in the matrix near the inclusion due to uniform deformation occurring in the finer grains. However, once the fatigue crack is formed in the matrix, it will lead to failure quickly because of its smaller fatigue crack growth threshold and higher crack growth rate [15]. ADF1-940, which possesses the coarsest grains (8 lm) can bring about more localized slip activities relatively, which causes the fatigue crack, to initiate in the matrix grains that are close to the inclusion rather easily. Although it has larger fatigue crack growth threshold and higher resistance to crack growth, the fatigue strength is still close to ADF1-880. For ADF1-920 with a medium grain size (4 lm), the more difficult micro-crack initiation than ADF1-940 (8 lm) in the matrix adjacent to the inclusion and the larger crack growth threshold than ADF1-880 (1.7 lm) made it the sample with the highest fatigue strength. It is obvious that fatigue crack initiates more easily in coarser grains but grow faster in smaller grains. Assuredly, there is an appropriate grain size that makes both the initiation and the growth of fatigue crack relatively difficult. The highest fatigue strength may be reached around this grain size. In the present study, it seems
1. The S–N curves of smooth specimens of three steels have no horizontal asymptotes in the 106–107 cycles regime, and the conventional fatigue limits cannot be determined. The fatigue strength of smooth specimens does not change monotonically with alteration in the prior grain size but increases monotonically with tensile strength. 2. Three kinds of fatigue crack initiation sites in the fractured specimens were found: internal inclusion (I), surface inclusion (SI) and surface matrix (S). When rmax r1 , fatigue cracks of the most fractured smooth specimens initiated at interior inclusions; when rmax 4r1 , the cracks generally occurred in the surface matrix. 3. Grain refinement has complex influence on fatigue strength of high strength steels. There is an appropriate grain size that increases the fatigue strength. The inclusion plays a very important role in fatigue strength of high strength steels, and there is a critical size of inclusions, below which the fatigue fracture origins will not be initiated from the inclusions. Acknowledgements This work was supported by the project ‘‘Fundamental research on new generation of iron and steel in China’’ (No. G19980615). The authors wish to thank Professor W.C. Yu, Dr. Z.X. Yin, Ms. H. Zhou, Senior engineers W.K. Gong and H.H. Su for their experimental supports.
References [1] Weng YQ. Development of ultrafine grained steels in China. In: Proceedings of the Workshop on New Generation Steel (NG STEEL’2001), 13–16 November. Beijing (China): The Chinese Society for Metals; 2001. p. 1–7. [2] Niikura M, Yokota Y, Sato K, Shirota T. Ultra refinement of grain size and its effect on mechanical properties. In: Proceedings of the Workshop on New Generation Steel (NG STEEL’2001), 13–16 November. Beijing (China): The Chinese Society for Metals; 2001. p. 100–3. [3] Takaki S, Kawasaki K, Kimura Y. Mechanical properties of ultra fine grained steels. J Mater Process Technol 2001;117:359. [4] Shin DH, Kim DJ, Choo WY. Grain refinement of a commercial 0.15% C steel by equal-channel angular pressing. Scripta Mater 1999;41:259. [5] Zhou S, Murakami Y, Beretta S, Fukushima Y. Experimental investigation statistics of extremes for three-dimensional distribution of non-metallic inclusions. Mater Sci Technol 2002;18: 1535–1543.
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[6] Choo W-Y. Present developing status of high performance steel in Korea. In: Proceedings of the Workshop on New Generation Steel (NG STEEL’2001), 13–16 November. Beijing (China): The Chinese Society for Metals; 2001. p. 16–26. [7] Hui WJ, Dong H, Wang MQ, Chen SL, Weng YQ. Effect of heat treatment on mechanical properties of second-hardening high strength 42CrMoVNb steel. In: Proceedings of the Workshop on New Generation Steel (NG STEEL’2001), 13–16 November. Beijing (China): The Chinese Society for Metals; 2001. p. 375–8. [8] Suresh S. Fatigue of materials [Wang ZG, Trans. in Chinese]. Beijing: National Defense Industry Press; 1999. p. 173. [9] Miller KJ, O’Donnell WJ. The fatigue limit and its elimination. Fatigue Fract Eng Mater Struct 1999;22:545–57.
[10] Murakami Y, Namoto T, Ueda T. Factors influencing the mechanism of superlong fatigue failure in steels. Fatigue Fract Eng Mater Struct 1999;22:581–90. [11] Murakami Y, Namoto T, Ueda T, Murakami Y. Fatigue Fract Eng Mater Struct 2000;23:893–902. [12] Murakami Y, Namoto T, Ueda T, Murakami Y. Fatigue Fract Eng Mater Struct 2000;23:903–10. [13] Morris Jr. JW, Guo Z, Krenn CR, Kim YH. The limits of strength and toughness in steel. ISIJ Int 2001;41(6):599–611. [14] Kuhlmann-Wilsdorf D, Thomason PF. The role of vacant lattice sites in the low-amplitude fatigue failure qt inclusions in steel. Acta Metall 1982;30:1243–5. [15] Suresh S. Fatigue of materials [Wang ZG, trans. in Chinese]. Beijing: National Defense Industry Press; 1999. p. 248.
The effect of inclusions on the fatigue behavior of fine-grained high strength 42CrMoVNb steel Z.G. Yang a,, G. Yao a, G.Y. Li a, S.X. Li a, Z.M. Chu b, W.J. Hui c, H. Dong c, Y.Q. Weng c a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China b Beijing Research Institute of Mechanical and Electrical Technology, Beijing 100083, PR China c Central Iron and Steel Research Institute, Beijing 100081, PR China Received 23 April 2003; received in revised form 8 October 2003; accepted 28 January 2004
Abstract The fatigue properties of high strength 42CrMoVNb steel with three fine grain sizes produced by different heat treatment procedures are studied in this paper. The experimental results illustrate that there is no horizontal asymptote in fatigue S–N curves for smooth specimens at 106–107 cycles regime, hence the conventional fatigue limit eliminates. The fatigue crack initiation sites in smooth specimens were related closely to fatigue life, and most of the fracture origins initiated at the inclusions. The diameters and locations of these inclusions were measured, and their morphology was observed in SEM. The critical size of inclusion and the appropriate grain size of prior austenite, which should be controlled in practical production, are obtained by analyzing the experimental data. # 2004 Elsevier Ltd. All rights reserved. Keywords: High strength 42CrMoVNb steel; Fine grain; Inclusion; Fatigue strength; Fatigue crack initiation site
1. Introduction In order to improve the properties of the steels, a surge to seek ultra fine grained technology, purified technology and uniformity technology took place recently [1]. It is known that grain refinement is a promising way to enhance the tensile strength and toughness of steels. It is demonstrated that the Hall– Petch relation holds for yield strength and ultimate tensile strength when the grain size of steels decreases down to 1 lm [2–4]. Generally speaking, the higher the tensile strength, the higher the fatigue strength will be. However, the inclusion plays a very crucial role in the fatigue fracture process for high strength steels. The application of secondary refining techniques and nonmetallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of inclusions in steels. The inclusions in clean
steels consist of a few large ones and clouds of small ones. But these few large inclusions, which are hard to avoid in clean steel, can prove to be catastrophic [5]. Therefore, in order to get the best comprehensive properties of high strength steel, how to harmonize the relation of grain size and inclusion size to achieve higher tensile strength and fatigue strength, is currently a hot topic of steel research [6]. The materials studied in this work are fine-grained high strength 42CrMoVNb steel to be used for automotive structures and components. The fatigue property of the steel is one of the key issues for improvements of the mechanical performance. In this paper, the fatigue behaviors and micro-fracture characteristics of these steels are studied in detail, and the relations between the properties and microstructures (inclusions and grains) of these steels are discussed. 2. Materials, specimens and experimental method
Corresponding author. Tel.: +86-24-8397-8023; fax: +86-24-23891320. E-mail address: [email protected] (Z.G. Yang). 0142-1123/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2004.01.009
Based on 42CrMo, the high strength 42CrMoVNb steel has been designed, and its main chemical
960
Z.G. Yang et al. / International Journal of Fatigue 26 (2004) 959–966
composition consists of (wt%) [7]: 0.42 C, 0.17 Si, 0.31 Mn, 0.001 S, 0.004 P, 1.10 Cr, 0.98 Mo, 0.32 V, 0.04 Nb, 0.01 Al, and the balance Fe. Steel of 18 mm diameter were produced by hot rolling after smelting in electric furnace, then machined to rough specimens, and heat treated. Finally, standard rotating bending fatigue specimens (Fig. 1) were prepared for testing, and correspondingly the circularly notched rotating bending fatigue specimens were also prepared for testing of the notch sensitivity. Both the depth of the notch and the radius of the notch tip are 0.75 mm. The high strength steels used in the present work are designated as ADF1-880, ADF1-920 and ADF1-940 in terms of various quenching temperatures. Table 1 shows the heat treatment procedures, prior austenite grain sizes, and tensile properties. The prior austenite grain sizes and appearances are shown in Fig. 2. All fatigue tests for smooth and notched specimens were carried out under rotating bending at a rotation rate of 5000 rpm, and the compressed air was used to eliminate the possible ‘‘self-heating’’ in specimens during testing.
3. Results 3.1. S–N curves and fatigue strength The S–N curves of smooth specimens are shown in Fig. 3, for which no horizontal asymptote but gradient exists at 106–107 cycles regime, hence the conventional fatigue limits cannot be determined. At 107 cycles, the fatigue strengths of smooth and notched specimens, r1 and r1N, were figured out by the staircase method in order to raise the confidence (Fig. 4). Notch sensitivity coefficient q could be calculated from the formula [8] q¼
Kf 1 Kt 1
ð1Þ
here Kf ¼ r1 =r1N , and Kt ¼ 1:89 is a notch stress concentration factor in the present experiment. The fatigue properties of three steels are shown in Table 2. The fatigue strength of smooth specimens does not change monotonically with prior austenite grain size altering but increases monotonically with tensile strength (Fig. 5). The ADF1-920 with the highest tensile strength has the highest fatigue strength, however, the ADF1-940 with the lowest tensile strength has the highest fatigue strength of notched specimens. 3.2. Fatigue crack initiation
Fig. 1.
Specimen configuration for rotating bending test.
3.2.1. Fractographic observation Observed by SEM, three kinds of fatigue crack initiation sites in the fractured specimens were found: internal inclusion (I), surface inclusion (SI) and surface
Table 1 Heat treatment procedures, prior austenite grain sizes and tensile properties of 42CrMoVNb Steel code ADF1-880
ADF1-920 ADF1-940
Heat treatment procedure v
920 C vacuum furnace preheating ð30 minÞ þ oil cooling þ electricity rapid v v v heating (70–100 C/s) to 880 C þ oil quenching three times þ 600 C tempering (2 h) v 920 C vacuum furnace preheating ð30 minÞ þ oil cooling þ electricity rapid v v v heating (70–100 C/s) to 920 C+oil quenching þ 600 C tempering (2 h) v v 940 C salt furnace preheating ð30 minÞ þ oil quenching þ 600 C tempering (2 h)
d (lm)
rs (MPa)
rb (MPa)
rs/rb
1.7
1370
1440
0.95
4.0
1465
1540
0.94
8.0
1315
1400
0.94
Fig. 2. The appearances of prior austenite grain boundary networks of the steels: (a) ADF1-880, (b) ADF1-920, (c) ADF1-940.
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Table 2 Fatigue properties (at 107 cycles) Materials code
r1 (MPa)
r1N (MPa)
q
ADF1-880 ADF1-920 ADF1-940
746 20 767 19 738 22
421 20 417 10 446 20
0.87 0.94 0.75
Fig. 3. S–N curves of smooth specimens of 42CrMoVNb after three heat treatments.
Fig. 5. Monotonic relationship of r-1 to rb for smooth specimens of three steels.
Fig. 4. Experimental result of smooth specimens of ADF1-920 by the staircase method.
matrix (S). Most fracture origins are internal inclupffiffiffiffiffiffiffiffiffi sions, and their diameters / (calculated from area) and depths D were measured, as listed in Tables 3–5. The appearance of an inclusion can be seen in Fig. 6. These inclusions, containing Al, Mg, Ca and O, most likely consist of Al2O3, MgO and CaO. In addition, all
these inclusions were divorced from the matrix and the most of them were cracked and broken. There are several radial ledges around the inclusion (Fig. 7), which were formed by propagation and linkage of primary micro-cracks at different planes, meanwhile the tire patterns emanating from the inclusions were found in a few specimens, the direction of which is parallel to the specimen surface (Fig. 8). 3.2.2. Factors affecting the type of fatigue crack initiation site As seen in Tables 3–5, when rmax 4r1 and Nf < 106 cycles, fatigue cracks mostly initiated at the specimen
Table 3 Fatigue cracking data of ADF1-880 Specimen no.
Applied stress, rmax (MPa)
Fatigue life, Nf (cycle)
Stress ratio, rmax/r1
Fatigue crack initiation sitea
Distance, D (lm)
ADF1-880-09 ADF1-880-08 ADF1-880-19 ADF1-880-18 ADF1-880-07 ADF1-880-06 ADF1-880-11 ADF1-880-12 ADF1-880-17 ADF1-880-15 ADF1-880-20 ADF1-880-22 ADF1-880-23 ADF1-880-21
725 725 725 750 750 750 775 775 775 800 850 850 900 900
5:85 106 1:027 107 1:28 107 5:14 106 6:60 106 9:14 106 1:68 106 6:40 106 8:05 106 1:24 106 1:90 105 2:00 105 1:27 104 9:53 104
0.97 0.97 0.97 1.01 1.01 1.01 1.04 1.04 1.04 1.07 1.14 1.14 1.21 1.21
I I SI I I I I SI I I S S S SI
260 33 146 25 0 18 322 37 66 18 520 45 145 40 0 8 66 35 50 45 – – – – – – (0.17 17):(35.15 15)
a
I, internal inclusion; SI, surface inclusion; S, surface matrix.
Inclusion diameter, / (lm)
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Table 4 Fatigue cracking data of ADF1-920 Specimen no.
Applied stress, rmax (MPa)
Fatigue life, Nf (cycles)
Stress ratio, rmax/r1
Fatigue crack initiation sitea
Distance, D (lm)
Inclusion diameter, / (lm)
ADF1-920-06
750 775 775 775 800 800 800 850 875 900 900 950 950 1000
9:97 106 3:025 105 7:79 106 9:91 106 8:93 105 1:84 106 2:72 106 2:27 106 1:70 106 1:03 105 1:77 105 1:37 105 1:40 105 8:70 104
0.98 1.01 1.01 1.01 1.04 1.04 1.04 1.11 1.14 1.17 1.17 1.24 1.24 1.30
I I I I I I I I I SI SI SI SI SI
247 24 215 253 36 68 105 91 180 0 0 0 0 0
40 44 29 36 17 23 22 38 29 27 17 21 38 10
ADF1-920-08 ADF1-920-05 ADF1-920-10 ADF1-920-01 ADF1-920-04 ADF1-920-12 ADF1-920-14 ADF1-920-16 ADF1-920-15 ADF1-920-17 ADF1-920-19 ADF1-920-18 ADF1-920-20 a
I, internal inclusion; SI, surface inclusion; S, surface matrix.
Table 5 Fatigue cracking data of ADF1-940 Specimen no.
Applied stress, rmax (MPa)
Fatigue life, Nf (cycles)
Stress ratio, rmax/r1
Fatigue crack initiation sitea
Distance, D (lm)
Inclusion diameter, / (lm)
ADF1-940-15 ADF1-940-17 ADF1-940-14 ADF1-940-19 ADF1-940-03 ADF1-940-13 ADF1-940-05 ADF1-940-02 ADF1-940-01 ADF1-940-12 ADF1-940-11 ADF1-940-10
725 725 750 750 750 775 775 800 800 800 825 850
3:60 106 5:70 106 5:10 105 7:19 105 7:04 106 3:59 105 2:19 106 3:85 105 9:98 105 2:50 106 8:12 104 1:20 103
0.98 0.98
I I I I I SI I S S I S S
38 75 34 27 170 0 187 – – 318 – –
10 17 46 37 10 18 45 – – 41 – –
a
1.02 1.02 1.02 1.05 1.05 1.08 1.08 1.08 1.12 1.15
I, internal inclusion; SI, surface inclusion; S, surface matrix.
surface of three steels. Surface cracks started usually at surface inclusions for ADF1-920 and at surface matrix for other two lower strength steels, except two specimens of ADF1-940, in which cracks initiated at inclusions inside with extraordinary large sizes and at short distances from the surface. But when rmax r1 and Nf > 106 cycles, the fatigue cracks for all the fractured specimens initiated at interior inclusions. According to the stress distribution in the cross section of a specimen under rotating bending, we can assume, instead of rmax, the local stress at inclusion center, rL, D rL ¼ rmax 1 ð2Þ R where rmax is the maximum stress applied, MPa; D, the shortest distance from an inclusion center to the speci-
men surface, lm; R, the diameter of working section of specimen and R ¼ 4000 lm. When only the interior inclusions are considered and rL =r1 < 1, we can find the fatigue life of all specimens is above 106 cycles. So, the ratio of local stress to fatigue strength is an important factor affecting the type of crack initiation. Moreover, under the present experiment conditions, a simple linear relationship between the internal inclusions’ size / and location D can be found / ¼ 12:5 þ 0:076D
ð3Þ
among filled symbols that represent the local stress rL =r1 < 1, as shown in Fig. 9. This means that the diameter / of the cracking inclusions should increase with increase in the distance D. But the open symbols
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Fig. 9. Relationship between / and D of interior inclusions as fatigue crack initiation sites at two stress ranges (filled symbols: rL =r1 < 1; open symbols: rL =r1 > 1).
that represent the local stress rL =r1 > 1 were all above the line (fitted by solid symbols).
4. Discussion Fig. 6. An internal inclusion as fracture origin on the fracture surface observed by SEM, showing its global and broken appearance. (a) Location of an inclusion; (b) appearance of an inclusion (specimen ADF1-920-06#: rmax ¼ 750 MPa, Nf ¼ 9:97 106 cycles, D ¼ 247 lm, / ¼ 40 lm).
4.1. Elimination of fatigue limit for high strength steel with fine grain
Fig. 7. Radial ledges around an inclusion served as crack initiation site (specimen ADF1-880-17#: rmax ¼ 775 MPa, Nf ¼ 8:05 106 cycles, D ¼ 66 lm, / ¼ 35 lm).
4.2. Influence of grain size on the tensile and fatigue properties of high strength steel
Fig. 8. Tire patterns from an inclusion served as crack initiation site (specimen ADF1-880-06#: rmax ¼ 750 MPa, Nf ¼ 9:14 106 cycles, D ¼ 520 lm, / ¼ 45 lm).
Morris et al. [13] have pointed out that in the case of martensitic steel the adequate have measure of grain size was the size of the crystallographically coherent block or packet of tempered martensites, which was regarded as the effective size affecting the strength. In our studies, grain refinement does effectively reduce the lengths of the laths, so the size of blocks decreases. According to Hall–Petch’s equation, the strengths of steels should have increased with decrease in grain sizes, however, the highest strength is of ADF1-920 with blocks of medium size, not of ADF1-880 with the smallest sized blocks (Fig. 10), which means there should be other factors influencing the strength. The factors affecting the strength may, at the least, include:
The results presented above show that all three steels cracked from inclusions when rmax r1 . Their fatigue lives are within 3:6 106 1:28 107 cycles. There is no conventional fatigue limit at 107 cycles. Miller and O’Donnell [9] and Murakami et al. [10–12] have discussed possible factors which may cause fatigue failure in the regime of Nf > 107 cycles. According to their viewpoint, it is not so easy to identify the crucial mechanism, among many possible factors for eliminating the fatigue limit. However, special attention should be paid to the influence of the hydrogen trapped by nonmetallic inclusions in high strength steels.
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Fig. 10. Relation between austenite grain size and tensile strength.
(1) For present low alloy steels with microalloy elements V and Nb, the smaller size of laths leads to the increase of the specific surface of laths, which can decrease the amount of microalloy element in the interior of the laths, then precipitation carbides reduce and the strength of matrix reduces. As compared with ADF1-940, no obvious carbide precipitated in ADF1880 after tempering, and acicular carbide in ADF1-940 perpendicular to lath longitudinal direction can resist the cleavage fracture along the direction of lath, as shown in Fig. 11. (2) On increasing the times of quenching and tempering, the dislocation density becomes greater. Such a condition can enhance the strength but at the sametime can effectively obstruct the precipitation of carbides, which then reduces the strength. More quenching and tempering times will bring more vacancies in the steel that can also reduce the strength. 2 more vacancies in steel can cause the premature fragmentation of inclusion and decohesion between one inclusion and the surrounding grains [14]; this is a favorable factor for the early formation of microcracks. Thus, the strength of ADF1-880 is not improved when compared to that of ADF1-940. Besides, the grain size may affect micro-crack initiation during tensile testing by other mechanisms, as
Fig. 12. Schematic diagram illustrating variant crack initiation sites in finer or coarser grains (a) Finer grains and (b) Coarser grains.
schematically given in Fig. 12. Under larger tensile stress, the smaller grains can withstand less strain in terms of their strength and more uniform plastic deformation, so the strain will be imposed on the inclusion, which can be so serious that micro-cracks form finally in this inclusion and cause its breakage. Comparatively, because the localized plastic deformation will occur in the coarser grains rather easily, the slip lines and slip steps are able to impinge the interface between the matrix and inclusion. Then the micro-cracks form directly at the interface rather easily. In both the cases cracking inclusions will lead to lower tensile strength of the steels. The appropriate grain size may not cause the micro-cracks to initiate in the interior of the inclusion and at the interface between the inclusion and the metal matrix as easily as in finer or coarser grains. 4.3. Inclusion and grain size control
Fig. 11. The appearances of precipitates in: (a) ADF1-940 and (b) ADF1-880.
The size and location of inclusions in high strength steel components under rotating and bending loading should be controlled strictly in terms of their important influence on the fatigue properties. Fig. 13 represents the relation between sizes and locations of inclusions as crack initiation sites with fatigue life above 106 cycles, in which we can find three approximately parallel lines. Each line denotes the critical size of inclusion as the crack initiation site, above which the larger the distance to the line the shorter the fatigue life of speci-
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that the appropriate prior austenite grain size for ADF1 steel is around 4 lm. 5. Conclusions
Fig. 13. Relationship between / and D of inclusions (Nf > 106 cycles).
mens, and below which the fatigue failure is very difficult to take place. We can obtain the safe critical values of initiations in these high strength steels from Fig. 13: (1) when D ¼ 0 100 lm, / ¼ 8 14 lm; (2) when D ¼ 100 300 lm, / ¼ 14 28 lm; and (3) when D > 300 lm, / > 30 lm. For ADF1-880, ADF1-920 and ADF1-940, if fatigue crack initiation site is not from the inclusion, the maximum size of inclusion will be less than 8, 15.5 and 7.5 lm, respectively. ADF1-880 and ADF1-940 have more rigorous requirement of inclusion size than ADF1-920. In fatigue testing, the micro-crack initiation may be divided into two stages. At first the inclusion or the interface between the inclusion and the matrix cracks, and then the micro-crack is initiated in the adjacent grains. ADF1-880 has the finest grains (1.7 lm) and most of its fatigue life is consumed during the period of crack initiation in the matrix near the inclusion due to uniform deformation occurring in the finer grains. However, once the fatigue crack is formed in the matrix, it will lead to failure quickly because of its smaller fatigue crack growth threshold and higher crack growth rate [15]. ADF1-940, which possesses the coarsest grains (8 lm) can bring about more localized slip activities relatively, which causes the fatigue crack, to initiate in the matrix grains that are close to the inclusion rather easily. Although it has larger fatigue crack growth threshold and higher resistance to crack growth, the fatigue strength is still close to ADF1-880. For ADF1-920 with a medium grain size (4 lm), the more difficult micro-crack initiation than ADF1-940 (8 lm) in the matrix adjacent to the inclusion and the larger crack growth threshold than ADF1-880 (1.7 lm) made it the sample with the highest fatigue strength. It is obvious that fatigue crack initiates more easily in coarser grains but grow faster in smaller grains. Assuredly, there is an appropriate grain size that makes both the initiation and the growth of fatigue crack relatively difficult. The highest fatigue strength may be reached around this grain size. In the present study, it seems
1. The S–N curves of smooth specimens of three steels have no horizontal asymptotes in the 106–107 cycles regime, and the conventional fatigue limits cannot be determined. The fatigue strength of smooth specimens does not change monotonically with alteration in the prior grain size but increases monotonically with tensile strength. 2. Three kinds of fatigue crack initiation sites in the fractured specimens were found: internal inclusion (I), surface inclusion (SI) and surface matrix (S). When rmax r1 , fatigue cracks of the most fractured smooth specimens initiated at interior inclusions; when rmax 4r1 , the cracks generally occurred in the surface matrix. 3. Grain refinement has complex influence on fatigue strength of high strength steels. There is an appropriate grain size that increases the fatigue strength. The inclusion plays a very important role in fatigue strength of high strength steels, and there is a critical size of inclusions, below which the fatigue fracture origins will not be initiated from the inclusions. Acknowledgements This work was supported by the project ‘‘Fundamental research on new generation of iron and steel in China’’ (No. G19980615). The authors wish to thank Professor W.C. Yu, Dr. Z.X. Yin, Ms. H. Zhou, Senior engineers W.K. Gong and H.H. Su for their experimental supports.
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