PSI - Issue 28

Avanish Kumar et al. / Procedia Structural Integrity 28 (2020) 93–100 Avanish et al. / Structural Integrity Procedia 00 (2019) 000–000

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It is evident from the d a /d N - Δ K plots that the crack growth rate becomes unstable eventually and specimens fail due to overload. The critical crack length that leads to catastrophic failure is higher for steel transformed at higher austempering temperature. The maximum value of stress intensity factor K max (indicated by arrow marks in Fig. 3) at which specimens failed due catastrophic crack growth were calculated using the relation: Δ K = (1-R) K max , where R = 0.1 are given in Table 3 . At this stage of crack growth, the applied values of K max were found in well agreement with the plane-strain fracture toughness, K 1C of the respective specimens (Table 2). Detailed fractography of tested specimens has been reported in our prior work (Kumar and Singh, 2019). 4. Conclusions Nano-structured bainitic steel transformed at higher austempering showed better damage tolerance in spite of having a lower strength. The threshold stress intensity factor range (Δ K th ) where growth rate of a long crack is considered negligible is higher for steel with coarser microstructure and higher content of retained austenite. This has been attributed to roughness-induced crack closure as well as phase-transformation-induced crack closure. The sub critical fatigue crack growth in stage-II was also slower in steel with coarser microstructure and higher retained austenite content. This has been attributed to the higher amount of input mechanical energy absorbed in phase transformation of retained austenite to martensite and also the compressive stress caused by volume expansion associated with this phase-transformation in steel with coarser and higher retained austenite content. References Antolovich, S. D. and Singh, B. (1970) ‘Observations of martensite formation and fracture in TRIP steels’, Metallurgical Transactions , 1(12), pp. 3463–3465. doi: 10.1007/BF03037885. Antolovich, S. D. and Singh, B. (1971) ‘On the toughness increment associated with the austenite to martensite phase transformation in TRIP steels’, Metallurgical and Materials Transactions B , 2(8), pp. 2135–2141. doi: 10.1007/BF02917542. Bhadeshia, H. K. D. H. (2010) ‘Nanostructured bainite’, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences , 466(2113), pp. 3–18. doi: 10.1098/rspa.2009.0407. Cheng, X. et al. (2008) ‘Fatigue crack growth in TRIP steel under positive R-ratios’, Engineering Fracture Mechanics , 75(3–4), pp. 739–749. doi: 10.1016/j.engfracmech.2007.01.019. Conshohocken, W. (2016) ‘Standard Test Method for Measurement of Fatigue Crack Growth Rates 1’, ASTM International , pp. 1–49. doi: 10.1520/E0647-15E01.2. Diego-Calderón, I. de et al. (2015) ‘Effect of microstructure on fatigue behavior of advanced high strength steels produced by quenching and partitioning and the role of retained austenite’, Materials Science and Engineering A . Elsevier, 641, pp. 215–224. doi: 10.1016/j.msea.2015.06.034. Garcia-Mateo, C. et al. (2016) ‘Analyzing the scale of the bainitic ferrite plates by XRD, SEM and TEM’, Materials Characterization , 122(October), pp. 83–89. doi: 10.1016/j.matchar.2016.10.023. Hornbogen, E. (1978) ‘Martensitic transformation at a propagating crack’, Acta Metallurgica , 26(1), pp. 147–152. doi: https://doi.org/10.1016/0001-6160(78)90211-0. Huo, C. Y. and Gao, H. L. (2005) ‘Strain-induced martensitic transformation in fatigue crack tip zone for a high strength steel’, Materials Characterization , 55(1), pp. 12–18. doi: 10.1016/j.matchar.2005.02.004. Kaneshita, T., Miyamoto, G. and Furuhara, T. (2017) ‘Variant selection in grain boundary nucleation of bainite in Fe-2Mn-C alloys’, Acta Materialia . Elsevier Ltd, 127, pp. 368–378. doi: 10.1016/j.actamat.2017.01.035. Kong, D., Liu, Q. and Yuan, L. (2014) ‘Effect of Austenitizing Temperature on Formation of Hard Bainite’, Metal Science and Heat Treatment , 56(7–8), pp. 444–448. doi: 10.1007/s11041-014-9779-9. Kumar, A. and Singh, A. (2018a) ‘Improvement of Strength-Toughness Combination in Nanostructured Bainite’, Procedia Structural Integrity . Elsevier B.V., 13, pp. 548–553. doi: 10.1016/j.prostr.2018.12.090. Kumar, A. and Singh, A. (2018b) ‘Toughness dependence of nano-bainite on phase fraction and morphology’, Materials Science & Engineering A . Elsevier B.V., 729, pp. 439–443. doi: 10.1016/j.msea.2018.05.106. Kumar, A. and Singh, A. (2019) ‘Microstructural effects on the sub-critical fatigue crack growth in nano- bainite’, Materials Science & Engineering A . Elsevier B.V., 743(October 2018), pp. 464–471. doi: 10.1016/j.msea.2018.11.114. Kumar, A. and Singh, A. (2020) ‘Deformation mechanisms in nanostructured bainitic steels under torsion’, Materials Science and Engineering A .