Fatigue cracks analysis in high-strength bainitic steel

M.C. MARINELLI; M. BALBI; U. KRUPP

Darmstadt

Congreso; Materials Science and Engineering; 2018

DGM · Deutsche Gesellschaft für Materialkunde e. V.

Nowadays, the mostmodern bainitic steels are designed with much reduced carbon and other alloyingelement concentrations exhibiting excellent mechanical properties and are widely applied inthe automobile industry as crash reinforcement bars to protect against sidewiseimpact and for injection lines (under pulsating loads) in common rail dieselengines [Ca09]. Although most applications of high-strength bainitic steels asstructural materials are exposed to dynamic cyclic loading, only a limitedamount of research has been reported concerning the fatigue induced damage inthese types of steels. According to some authors [Zh15,Bh12, So14],fatigue microcracks nucleate at the interface between the bainitic ferrite andthe retained austenite regardless of the strain amplitude imposed. Otherauthors [Zh16, Br12, Re15, Ma17] reported that microcracks initiate along slipbands and show transgranular propagation and eventually form a coalescence ofshort cracks. In order to understand the behavior of microcracks,it is proposed a gradual monitoring of them during low cycle fatigue atdifferent plastic strain amplitudes by means of optical and electron microscopyalong with EBSD data analysis.The study wascarried out on the bainitic steel 16CrMnV7-7 with 5 Vol. % of retained austenite.With the aim of study the initiation microcracks, low cycle fatigue tests onshallow notched cylindrical specimens were performed at room temperature underconstant plastic strain amplitude. The central part of the notch was monitoredand digitally recorded during the fatigue test. In order to correlate slipsystems with the initiation and propagation of microcraks, Schmid and Taylorfactors were analyzed in the bainitic blocks. Additionally, the dislocationstructure developed during cycling was also analyzed. From main results, it ishighlighted that there are two modes of initiation of microcracks according tothe plastic strain applied. At low plastic strain amplitude the microcracks preferablyinitiate along slip systems {110} <111> with high Schmid factor and lowTaylor factor, while at high plastic strain amplitude the microcracks mostly are present on high-angle block boundaries.  References[Ca 09] Caballero FG, Santofimia MJ, Garcia-Mateo C,Chao J, Garcia de Andres C. Theoretical design and advanced microstructure insuper high strength steels. Mater. Des. 30 (2009) 2077?83. [Zh15] Q. Zhou, L. Qian, J. Meng, L. Zhao, F. Zhang,Low-cycle fatigue behavior and microstructural evolution in a low-carboncarbide-free bainitic steel, Mater. Des. 85 (2015) 487?496.[Bh12] H.K.D.H. Bhadeshia, Steels for bearings, Prog.Mater. Sci. 57 (2012) 268?435[So14] W. Solano-Alvarez, E.J. Pickering, H.K.D.H.Bhadeshia, Degradation of nano structured bainitic steel under rolling contactfatigue, Mater. Sci. Eng. A 617 (2014) 156?164