Effect of Austempering Route on Microstructural Characterization of Nanobainitic Steel

doi:10.1007/s40195-013-0006-2

Abstract

Abstract:

A low-temperature nanobainitic steel was obtained through one-step and two-step austempering. The effect of austempering temperature, time, and route on bainitic lath width, volume fraction of retained austenite, carbon concentration in retained austenite, and nanohardness of bainitic lath and retained austenite was studied. Results showed that the transformation kinetics was slowed down and the bainitic lath was refined as the austempering temperature decreased from 300 to 250 °C. Both coarser and finer bainitic laths were obtained with the two-step austempering, which was consistent with the lath size at 300 and 250 °C austempering, respectively. X-ray diffraction analysis showed that both volume fraction of retained austenite and its carbon concentration decreased with the decrease of austempering temperature for the one-step austempering, and especially the carbon concentration is obviously increased when the two-step austempering is adopted. The nanohardness of the bainitic lath in the sample after two-step austempering treated lies between that of the samples after 300 and 250 °C austempering treated. The product of tensile strength and total elongation of the two-step austempered sample is the highest, which increases monotonously with the product of retained austenite fraction and its carbon concentration. Higher strength–ductility balance may be resulted by the fine bainitic lath, high volume fraction, and high stability of retained austenite in the sample after two-step austempered.

Key words: Nanobainitic steel, Stability, Retained austenite, Nanohardness, Strength–ductility balance

Huifang Lan, Linxiu Du, Na Zhou, Xianghua Liu. Effect of Austempering Route on Microstructural Characterization of Nanobainitic Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(1): 19-26.

Figures/Tables 11

Table 1 Chemical composition of experimental steel (wt%)

C	Si	Mn	P	S	Cr	Mo	V	Fe
0.83	2.0	2.38	0.028	0.015	1.24	0.25	0.10	Bal.

Fig. 1 Optical and SEM micrographs of different samples: a, e No. 1; b, f No. 2; c, g No. 3; d, h No. 4

Fig. 2 TEM micrograph of samples No. 1, No. 3, and No. 4 and the SAD pattern: a No. 1; b No. 3; c coarser laths of No. 4; d finer laths of No. 4

Fig. 3 TEM micrographs of sample No. 3 and the corresponding SAD pattern: a, b TEM micrographs; c K–S relationship in a; d N–W relationship in b

Table 2 Mechanical properties of samples No. 1, No. 2, No. 3, and No. 4

Sample no.	YS (MPa)	TS (MPa)	TEL (%)	TS × TEL (MPa%)
1	1,010 ± 7	1,697 ± 8	12.9 ± 0.1	21,891 ± 273
2	1,278 ± 9	1,955 ± 6	7.3 ± 0.2	14,272 ± 347
3	1,290 ± 5	1,885 ± 4	10.5 ± 0.1	19,793 ± 231
4	1,125 ± 7	1,778 ± 5	13.1 ± 0.1	23,292 ± 112

Fig. 4 XRD patterns of samples No. 1, No. 3, and No. 4

Table 3 Volume fraction and carbon concentration of retained austenite calculated from XRD

Sample no.	V_γ	C_γ (wt%)	V_γ × C_γ (wt%)
1	0.283 ± 0.017	1.551 ± 0.019	0.439 ± 0.033
3	0.207 ± 0.018	1.263 ± 0.022	0.261 ± 0.028
4	0.291 ± 0.022	1.627 ± 0.017	0.473 ± 0.042

Fig. 5 T0′ curve and measured carbon concentration in retained austenite

Fig. 6 Mechanical properties versus product of volume fraction and carbon concentration of retained austenite. a TEL, b product of tensile strength and TEL

Fig. 7 AFM morphology of indentation and load–displacement curves of different samples: a, d No. 1; b, e No. 3; c, f No. 4

Table 4 Nanohardness of bainite and retained austenite

No.	Measured mixed nanohardness (GPa)	Measured nanohardness of block austenite (GPa)	V_B/V_A	Calculated nanohardness of bainitic lath (GPa)	Calculated average nanohardness of bainitic lath (GPa)
1	6.39 ± 0.23	4.50 ± 0.08	1.5:1	7.65 ± 0.35	–
3	8.66 ± 0.39	4.14 ± 0.09	2.5:1	10.47 ± 0.55	–
4	7.83 ± 0.16	5.01 ± 0.13	2.5:1	8.96 ± 0.24	8.33 ± 0.15
4	6.58 ± 0.20	5.01 ± 0.13	1.5:1	7.64 ± 0.22	8.33 ± 0.15

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