Notably Accelerated Nano-Bainite Transformation via Increasing Undissolved Carbides Content on GCr15Si1Mo Bearing Steel

doi:10.1007/s40195-023-01652-2

Abstract

Abstract:

In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 °C, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.

Key words: Nano-bainite, Undissolved carbides, Bainitic transformation kinetics, Carbon concentration

Yan-Hui Wang, Hua-Qiang Sun, Wen-Jing Feng, Lei-Jie Zhao, Xiang Chen, Qing-An Chen, Hai-Tao Sun, Jian-Jun Wang, Zhi-Nan Yang. Notably Accelerated Nano-Bainite Transformation via Increasing Undissolved Carbides Content on GCr15Si1Mo Bearing Steel[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(4): 703-712.

Figures/Tables 15

Fig. 1 SEM images after holding for different time at 900 °C and quenching to room temperature in water: a 5 min, b 20 min, c 60 min, d 90 min, e 130 min, f 180 min

Table 1 Volume fraction of undissolved carbides (VUC) after holding different time at 900 °C

Holding time (min)	5	20	60	90	130	180
V_UC (%)	13.53	9.00	5.42	4.51	3.83	4.47

Fig. 2 Undissolved carbides particle equivalent diameter distributions of the specimens held various duration at 900 °C: a 5 min, b 20 min, c 90 min

Table 2 Average equivalent diameter of the undissolved carbides after holding different time at 900 °C

Holding time (min)	Average equivalent diameter (μm)	Diameter ≤ 0.5 μm percentage (%)
5	0.38	79.7
20	0.39	76.9
90	0.40	72.9

Fig. 3 TEM images after austempering at 210 °C and 300 °C of the specimens holding 5, 20 and 90 min at 900 °C: a 5 min-210 °C, b 20 min-210 °C, c 90 min-210 °C, d 5 min-300 °C, e 20 min-300 °C, f 90 min-300 °C

Fig. 4 Size distribution of the bainitic ferrite plate within the microstructure after austempering at 210 °C of the specimens holding different duration at 900 °C: a 5 min, b 90 min

Fig. 5 Size distribution of the bainitic ferrite plate after austempering at 300 °C of the specimens holding different time at 900 °C: a 5 min, b 20 min, c 90 min

Table 3 Average thicknesses of the bainitic ferrite plates under different heat treatment conditions

Austempering temperature (°C)	Average thicknesses under different heat treatment (nm)
Austempering temperature (°C)	900 °C × 5 min	900 °C × 20 min	900 °C × 90 min	980 °C × 180 min
210	56 ± 12	58 ± 10	63 ± 9	34 ± 8
300	49 ± 11	52 ± 9	41 ± 13	-

Fig. 6 TEM image a and size distribution of bainitic ferrite plate b of the sample austenitizing at 980 °C for 180 min before austempering at 210 °C for 48 h

Table 4 Volume fractions of undissolved carbides (VUC), nanostructured bainite (VNB), retained austenite (VRA), and carbon content in it (Cγ) under different heat treatment conditions

Austempering temperature (°C)	Holding time (min)	V_UC (%)	V_NB (%)	V_RA (%)	C_γ (%)
210	5	13.5	69.0	17.5	0.50
	20	9.0	70.2	20.8	0.56
	90	4.5	71.1	24.4	0.73
300	5	13.5	72.1	14.4	1.49
	20	9.0	73.0	18.0	1.49
	90	4.5	73.2	22.3	1.57

Table 5 Variations of Ms temperatures and the total austempering transformation time (Titt) at 210 °C and 300 °C of the specimens holding different time at 900 °C

Holding time (min)	M_s temperature (°C)	Titt (h, 210 °C)	Titt (h, 300 °C)
5	170	7.76	0.97
20	155	10.5	1.24
90	125	17.3	1.49

Fig. 7 Change in length with holding time during austempering at 210 °C a, 300 °C b of the specimens held for different duration at 900 °C

Fig. 8 Dilation rate with holding time during austempering at 210 °C a, 300 °C b of the specimens held for different duration at 900 °C

Fig. 9 SEM image a and its corresponding EPMA line analysis results of carbon distribution b of the specimen held for 5 min at 900 °C and water quenched

Fig. 10 EPMA map analysis results of carbon distribution in the specimen held for a 980 °C × 3 h, b 900 °C × 5 min, c 900 °C × 20 min, d 900 °C × 90 min

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