An Investigation on Rotating Bending Fatigue Behavior of Nanostructured Low-Temperature Bainitic Steel

doi:10.1007/s40195-014-0044-4

Abstract

Abstract:

The aim of this work is to investigate the fatigue behavior of a new class of nanostructured, low-temperature bainitic steels, where rotating bending fatigue tests were conducted on samples isothermally transformed at 300, 250, and 200 °C, after which, fracture surfaces were examined using scanning electron microscopy. Results showed that, fatigue limits of about 820, 945, and 1,005 MPa were achieved for the samples transformed at each transformation temperature, respectively. Moreover, according to the SEM micrographs, secondary crack initiation was observed in the high carbon retained austenite blocks for samples transformed at 300 °C and at the interface of blocky austenite-bainitic ferrite for samples transformed at 250 and 200 °C.

Key words: Fatigue, Bainitic ferrite, Retained austenite, Fractography

B. R. Shendy, M. N. Yoozbashi, B. Avishan, S. Yazdani. An Investigation on Rotating Bending Fatigue Behavior of Nanostructured Low-Temperature Bainitic Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(2): 233-238.

Figures/Tables 9

Fig. 1 Dimension of fatigue samples

Fig. 2 Typical TEM micrographs of the samples transformed at 300 °C a, 250 °C b, 200 °C c

Table 1 Bainitic ferrite plate thickness measured by TEM, volume fraction of retained austenite and carbon content in retained austenite of the samples austempered at different transformation temperatures

Temperature (°C)	Bainitic ferrite plate thickness (nm)	Volume fraction of high carbon retained austenite (vol%)	Carbon content in austenite (wt%)
300	110	27	1.7
250	90	21	1.4
200	65	15	1.2

Table 2 Mechanical properties of the samples austempered at different transformation temperatures

Temperature (°C)	YS (MPa)	UTS (MPa)	EL (%)
300	961	1,602	13.6
250	1,138	1,801	10.5
200	1,288	1,963	7.6

Fig. 3 Fatigue limit as a function of the maximum stress for the specimens with different transformation temperatures of 300 °C a, 250 °C b, 200 °C c

Fig. 4 Typical SEM micrographs of crack initiation regions on fracture surfaces of different samples: a transformed at 300 °C, maximum stress 900 MPa and 400,465 cyc to failure; b transformed at 250 °C, maximum stress 980 MPa and 537,817 cyc to failure; c transformed at 200 °C, maximum stress 1,030 MPa and 513,000 cyc to failure

Fig. 5 Typical SEM micrographs of crack propagation region in fatigue fracture of different samples: a transformed at 300 °C, maximum stress 900 MPa and 400,465 cyc to failure; b transformed at 250 °C, maximum stress 980 MPa and 537,817 cyc to failure; c transformed at 200 °C, maximum stress 1,030 MPa and 513,000 to failure

Fig. 6 Typical SEM micrographs of final fatigue fracture surfaces of different samples: a transformed at 300 °C, maximum stress 900 MPa and 400,465 cyc to failure; b transformed at 250 °C, maximum stress 980 MPa and 537,817 cyc to failure; c transformed at 200 °C, maximum stress 1,030 MPa and 513,000 cyc to failure

Fig. 7 Typical SEM micrographs showing secondary cracks on fracture surfaces of different samples: a transformed at 300 °C, maximum stress 900 MPa and 400,465 cyc to failure; b transformed at 250 °C, maximum stress 980 MPa and 537,817 cyc to failure; c transformed at 200 °C, maximum stress 1,030 MPa and 513,000 cyc to failure

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