Fatigue of 0.55C-1.72Si Steel with Tempered Martensitic and Carbide-Free Bainitic Microstructures

doi:10.1007/s40195-013-0020-4

摘要/Abstract

Abstract:

High-Si spring steel was heat treated in three different ways: Quenching and tempering at 460 °C to obtain a tempered martensite microstructure, and austempering at 300 and 350 °C, respectively, to obtain two different carbide-free bainitic microstructures. In the steel austempered at 350 °C, both the bainite lath thickness and retained austenite content were higher than those of the steel austempered at 300 °C. Rotating-bending fatigue tests were done in order to evaluate the effect of each heat treatment on the high-cycle fatigue behavior of the steel. When the austempering temperature was 300 °C, the endurance limit was increased by 25% despite a 5% reduction in tensile strength when compared with that of the quenched and tempered steel. The relationship between endurance limit [R_fat (50%)] and ultimate tensile strength (R_m) was higher for the austempered samples in comparison with that of the quenched and tempered material. Therefore, it is believed that the presence of retained austenite affects the relationship between endurance limit and tensile strength.

Key words: Steels, Fatigue, Carbide-free bainite, TRIP effect

. [J]. 金属学报英文版, 2014, 27(1): 55-62.

Alejandro Leiro, Arash Roshan, Karl-Gustaf Sundin, Esa Vuorinen, Braham Prakash. Fatigue of 0.55C-1.72Si Steel with Tempered Martensitic and Carbide-Free Bainitic Microstructures[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(1): 55-62.

图/表 12

Fig. 1 Schematic representation of the austempering treatment at 300 °C. The TTT data correspond to 0.55C-1.62Si-0.78Mn-0.77Cr steel [16]

Fig. 2 Schematic of rotating-bending fatigue setup showing specimen dimensions

Fig. 3 Microstructures of experimental steel after different heat treated: a austempered at 300 °C; b austempered at 350 °C; c quenched tempered at 460 °C

Table 1 Summary of microstructural parameters and mechanical properties

Sample	d (μm)	Χ_γ	Hardness (HV_0.5)	R_p0.2 (MPa)	R_m (MPa)	A (%)	R_fat(50%) (MPa)	R_fat(50%)/R_m
QT	0.73 ± 0.71	–	462	1,530	1,640	8	770	0.47
A1	0.29 ± 0.13	19	532	1,370	1,550	–	980	0.63
A2	0.46 ± 0.14	31	415	1,150	1,290	15	750	0.58

Fig. 4 S–N diagrams for all heat treatments

Fig. 5 Fractograph of a fractured fatigue specimen of the QT sample, the applied stress is 740 MPa

Fig. 6 Fractograph of a fractured fatigue specimen austempered at 300 °C (A1) and the corresponding single point EDX analysis results of the observed inclusion, applied stress is 975 MPa

Fig. 7 Fractograph of a fractured fatigue specimen austempered at 350 °C (A2) showing multiple initiation zones, applied stress is 740 MPa

Fig. 8 Detail view of the of crack initiation zone which is indicated by the arrow in Fig. 7, notice multiple initiation sites and radial ridges

Fig. 9 Fractographs of final fracture zone of sample QT a, sample A2 b, both showing a mixture of brittle and ductile fracture

Fig. 10 XRD patterns in the fracture surface and the bulk of A1 and A2 samples, note the disappearance of the austenite peak in the bulk

Fig. 11 Comparison between endurance limit (Rfat), crack surface hardness (HV Fat) and bulk hardness (HV Bulk) for the samples with different heat treatments

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