Multiphase miacrostructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

doi:10.1016/j.jmst.2020.09.034

Abstract

Abstract:

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240, 360, and 400 ℃. The results show that martensite, bainite, and retained austenite (RA) are the main microstructural phases. The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix, and the transformation of ultrafine bainite into coarse lath bainite and granular bainite, respectively. Thick filmy RA was distributed between bainite laths. The polygonal martensite-austenite islands and blocky RA formed along the grain boundaries. The average carbon concentration in the matrix decreased with the temperature increase, while the impact toughness initially increased and then dropped with temperature. The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps. The microcracks disappeared in the RA on the prior austenite grain boundaries. On the other side, the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns. The polygonal martensite-austenite islands or blocky RA constrained the microcracks. After austempered at 400 ℃, the brittle fracture was dominant, showing river patterns, and the microcracks propagated through the granular bainite without any resistance.

Key words: Medium carbon high silicon steel, Austempering, Martensite, Bainite, Retained austenite, Microcracks

Kui Chen, Huabing Li, Zhouhua Jiang, Fubin Liu, Congpeng Kang, Xiaodong Ma, Baojun Zhao. Multiphase miacrostructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel[J]. J. Mater. Sci. Technol., 2021, 72: 81-92.

Figures/Tables 16

Table 1 Chemical composition of the experimental steel (wt.%).

C	Si	Mn	Cr	V	Nb	P	S	O	N	Al	Fe
0.56	1.48	0.70	0.71	0.148	0.011	0.0035	0.0014	0.00085	0.0015	0.011	Bal.

Fig. 1. Dilatometric curves of the samples during austempering heat treatments at 240℃, 360℃ and 400℃: (a) Dilatation-temperature curves, (b) The magnification of the dilatation-temperature curves during the isothermal process, (c) Dilatation-time curves, (d) The magnification of the dilatation-time curves during the isothermal process and (e) Dilatation-time rate curves.

Fig. 2. EPMA map scanning of C distribution and the corresponding SEM images: (a1), (b1) and (c1) are secondary electron microstructures; (a2), (b2) and (c2) are backscattered electron microstructures; (a3), (b3) and (c3) are map scanning of C distribution (PAGB: prior austenite grain boundary; MA: martensite-austenite island; M: martensite; BF: bainite ferrite lath; RAf: filmy retained austenite; RAb: blocky retained austenite; F: ferrite; GB: granular bainite).

Fig. 3. EPMA line scanning showing C concentration evolution between laths and RA or MA (PAGB: prior austenite grain austenite; RAf: filmy retained austenite; B: bainite; MA: martensite-austenite island).

Fig. 4. Results of EBSD: (a), (b) and (c) EBSD inverse pole figure (IPF) maps of 240, 360 and 400 ℃; (d), (e) and (f) Misorientation angle distribution maps of 240, 360 and 400 ℃ (red line: 2°<θ<15°, black line: 15°<θ<65°); (g) Statistics of effective grain size; (h) Statistics of misorientation angle.

Fig. 5. Results of EBSD analysis: (a1), (b1), and (c1) EBSD band contrast (BC) maps, (a2), (b2), and (c2) Gaussian graphs of BCC phase in accordance with band contrast data and (a3), (b3), and (c3) Phase fraction map with EBSD data.

Table 2 Quantitative analysis of martensite, bainite and retained austenite via EBSD and XRD methods.

	Martensite (vol.%)	Bainite(vol.%)	Retained austenite (vol.%)
AT-240	40.8	45.5	13.7
AT-360	43.6	37.2	19.2
AT-400	33.8	51.8	14.4

Fig. 6. Kernel average misorientation (KAM) distribution.

Fig. 7. Geometrically necessary dislocation (GND) densities, calculated from KAM values, of (a) AT-240, (b) AT-360 and (c) AT-400, and (d) the GND density values.

Fig. 8. Volume fraction of retained austenite at different positions from the fracture surface.

Fig. 9. SEM fractographs of macroscopic fracture of the three impact specimens: (a) AT-240, (b) AT-360 and (c) AT-400.

Fig. 10. SEM fractographs of (a1), (b1) and (c1) crack extension zones, (a2), (b2) and (c2) crack sources and (a3), (b3) and (c3) high magnification of crack sources: (a1), (a2) and (a3) AT-240; (b1), (b2) and (b3) AT360; (c1), (c2) and (c2) AT-400.

Fig. 11. The second phase of MnS and NbVC in the impact fracture: (a) MnS in AT-360 and (b) NbVC in AT-400.

Fig. 12. The SEM morphology of primary cracks and secondary cracks: (a1) and (a2) AT-240, (b1) and (b2) AT-360 and (c1) and (c2) AT-400 (B1 and B2 represent carbide-free bainite; LB is the lath of bainite; PAGB is prior austenite grain boundary; RAf is filmy retained austenite; RAb is blocky retained austenite; MA is the island of martensite/austenite; GB is granular bainite).

Fig. 13. EBSD results around the microcracks under the impact fracture surface: BC maps combined with IPF maps of (a1) AT-240, (b1) AT0360 and (c1) AT-400; Phase distribution maps of (a2) AT-240, (b2) AT-360 and (c2) AT-400.

Fig. 14. Schematic illustration of the (a) prior austenite grain and the mechanical evolutions of multiphase microstructure and cracks for the three samples: (b1) and (b2) AT-240, (c1) and (c2) AT-360 and (d1) and (d2) AT-400.

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