Low-Cycle Fatigue Behavior of 1100 MPa Grade High-Strength Steel
ZHOU Hongwei1,2, BAI Fengmei3,4(), YANG Lei1,2, CHEN Yan1,2, FANG Junfei1,2, ZHANG Liqiang3, YI Hailong4, HE Yizhu1,2
1. Key Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials, Ministry of Education, Anhui University of Technology, Maanshan 243032, China 2. School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243032, China 3. School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243032, China 4. State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
Cite this article:
ZHOU Hongwei, BAI Fengmei, YANG Lei, CHEN Yan, FANG Junfei, ZHANG Liqiang, YI Hailong, HE Yizhu. Low-Cycle Fatigue Behavior of 1100 MPa Grade High-Strength Steel. Acta Metall Sin, 2020, 56(7): 937-948.
The low-cycle fatigue (LCF) behavior of 1100 MPa grade tempered high strength steel under symmetrical strain control conditions was studied at the strain amplitude ranges of 0.4%~1.2% in this work. The LCF properties of quenching and tempering high strength steel were examined by means of OM, SEM and TEM. The microstructure changes, fracture morphology, crack propagation characteristics and inclusion morphology were studied in detail. The results show that the cyclic hardening and cyclic softening depend on strain amplitude. At the low strain amplitude of 0.4%, rapid cyclic hardening occurs in initial 10 cyc, and then the stress remains almost unchanged until the sample breaks. While at the strain amplitude ranges of 0.5%~1.2%, the cyclic hardening reaches a peak at the first few cycles, followed by the remarkable cyclic softening until the sample fails. The cyclic softening is mainly due to the recovery of some martensite lath under low-cycle fatigue loading and the decrease of dislocation density in the laths. 1100 MPa grade high-strength steel is found to obey LCF Manson-Coffin relationship. The high-strength steel has excellent LCF performance for two main reasons, which is related with the shape and size of inclusions. One is that the shape of the inclusion is nearly circular, and the diameter is 2~5 μm, which is lower than the critical dimension of the inclusions causing fatigue crack initiation. The crack is initiated on the surface of the sample. This increases fatigue crack initiation life. The other one is that the original austenite grain boundary, the martensite packet/block boundary and the inclusions or cavities can induce the crack deflection, reducing the crack propagation rate and increasing fatigue crack propagation life.
Fund: Joint Fund of National Natural Science Foundation of China(U1760108);National Natural Science Foundation of China(51674079);National Natural Science Foundation of China(51874001);Natural Science Foundation of Anhui Provincial Education Department(KJ2018A0062);Natural Science Foundation of Anhui Provincial Education Department(KJ2019A0059)
Fig.1 Specimen geometry used in low-cycle fatigue (LCF) tests (unit: mm)
Fig.2 Cyclic stress response curves of high-strength steels under different strain amplitudes (Nf—the number of cycles to failure)
Fig.3 The low-cycle fatigue curves of high-strength steels (a) Manson-Coffin equation fitting curve for 1100 MPa grade steel(b) comparison of the LCF curves of several high-strength steels
High-strength steel
σ0.2 / MPa
σb / MPa
δ / %
Microstructure
Ref.
1100 MPa grade
1160
1355
17.8
Tempered lath martensite
-
ASTM A723
1170
1262
13.0
Tempered lath martensite
[25]
34CrNiMo6
967
1035
18.0
Martensite, lower bainite
[7]
HSLA
893
914
15.3
Lath bainite
[24]
10CrNiMo
830
925
19.0
Tempered lath martensite
[5]
Table 1 Mechanical properties for several high-strength steels
Fig.4 TEM images of high-strength steel (The dotted circle indicates triangular grain boundaries and the boxes highlight the position of precipitates) (a) martensitic lath and nanoscale precipitate (b) prior austenite grain boundary (PAGB) and nanoscale precipitate (c, d) bright (c) and dark (d) field TEM images of precipitates, respectively
Fig.5 TEM images, SAED pattern and EDS analyses of high-strength steel (a) spherical precipitate (Inset is the SAED pattern of the carbide) (b) short rod-shaped precipitate (c, d) EDS of precipitates denoted by cycles in Figs.5a (c) and b (d), respectively
Fig.6 TEM images of fatigue fracture specimens (a, b) lath structure under the strain amplitude of 0.4% at low (a) and high (b) magnifications (c) low dislocation density, precipitate, PAGB, lath under the strain amplitude of 1.0% (d) crack propagation path indicated by arrows under the strain amplitude of 1.0%
Fig.7 Fatigue cracks initiated from the surface of specimens under the strain amplitudes of 0.4% (a), 0.6% (b), 0.8% (c) and 1.2% (d)
Fig.8 Main crack fracture morphology and secondary crack under strain amplitude of 1.0% (a) whole view of fracture including crack initiation stage I, crack propagation stage II and stage III (b~d) locally magnified images indicated by boxes in Fig.8a, showing striation (b), secondary crack and striation (c) and dimple fracture (d)
Fig.9 Inclusions in the fractured specimens (a) inclusion and EDS during stage II (b) inclusion and striation during stage II (c) tyre pattern and striation during stage II (d) inclusion during stage III
Fig.10 Fatigue crack propagation morphology (a) and local magnifications (b, c) of high-strength steel at 1.0% strain amplitude (The inserted EDS indicates the composition of the inclusion, and arrows in Figs.10b and c show crack propagation directions)
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