Effect of Cooling Rate on the Precipitation Mechanism of Primary Carbide During Solidification in High Carbon-Chromium Bearing Steel
LI Shanshan1,2, CHEN Yun1(), GONG Tongzhao1,2, CHEN Xingqiu1, FU Paixian1, LI Dianzhong1
1.Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
Cite this article:
LI Shanshan, CHEN Yun, GONG Tongzhao, CHEN Xingqiu, FU Paixian, LI Dianzhong. Effect of Cooling Rate on the Precipitation Mechanism of Primary Carbide During Solidification in High Carbon-Chromium Bearing Steel. Acta Metall Sin, 2022, 58(8): 1024-1034.
Bearing is one of the most technologically important engineering components in machines. With the development of several advanced steel-refining technologies to suppress the detrimental effect of nonmetallic inclusions on the mechanical properties of materials, the impact of carbides on the service life of bearings has gradually highlighted. The carbides have become a key factor in determining the performance of a bearing, particularly for primary carbides formed during the solidification of high carbon-chromium bearing steel. Therefore, exploring the formation mechanism of primary carbides and their control strategies is vital to improve the manufacturing process of bearing steel as well as the service life and reliability of bearings. To clarify the formation mechanism of primary carbides and the effects of the processing technique, as well as the addition of rare earth elements, a modified type of GCr15 high carbon-chromium bearing steel with and without rare earth elements was remelted and solidified at different cooling rates. After solidification, the quantity, area, average size, and chemical composition of the primary carbide in the as-cast bearing steel were characterized and analyzed via OM, EPMA, SEM, and XRD. The results show that the type of carbide in GCr15 series bearing steel is M3C cementite with high Cr content (more than 15%, mass fraction). The nucleation rate of M3C cementite increased with the increase in the cooling rate; thus, the number of carbides increased considerably. However, at very high cooling rates, the primary austenite was refined and the diffusion time of C and Cr elements required to form carbides declined; therefore, the size of carbides was reduced significantly, resulting in more uniform dispersion of the carbides. Moreover, the addition of rare earth elements could refine the primary austenite, and subsequently, refine the carbide to some extent. Considering the properties of the primary carbides at different cooling rates, the kinetic formation mechanism for the primary carbide in high carbon-chromium bearing steel during solidification is proposed.
Fund: National Natural Science Foundation of China(52031013);Strategic Priority Research Program of the Chinese Academy of Sciences(XDC04040202);Youth Innovation Promotion Association, Chinese Academy of Sciences
About author: CHEN Yun, professor, Tel: (024)83970106, E-mail: chenyun@imr.ac.cn
Table 1 Chemical compositions of the steels without RE (GCr15SiMn) and with RE (GCr15SiMn(RE))
Fig.1 As-received cylindrical samples of GCr15SiMn and GCr15SiM(RE) steels before remelting (a) and the samples after remelting and solidification and corundum crucibles (b)
Fig.2 Pseudo-binary phase diagram of GCr15SiMn steel calculated by Thermo-Calc software (The dashed line represents the phase change sequence from liquid to solid at 1.0%C (mass fraction))
Fig.3 Heating and cooling curves of the sample in the remelting and solidification experiments
Fig.4 Low (a) and locally high (b) magnified OM images of primary carbides found in the center zone of a continuous cast GCr15SiMn bearing steel ingot with a diameter of 600 mm
Fig.5 OM images of as-cast GCr15SiMn steel under different cooling rates (Arrows indicate the isolated primary carbides) (a) 0.1oC/min (b) 1oC /min (c) furnace cooling (d) air cooling
Fig.6 OM images of as-cast GCr15SiMn(RE) steel under different cooling rates (Arrows indicate the isolated primary carbides) (a) 0.1oC/min (b) 1oC/min (c) furnace cooling (d) air cooling
Fig.7 Effects of cooling rate on isolated primary carbides (a) number of primary carbides (b) total area (c) average diameter
Steel
Cooling rate / (oC·min-1)
˃ 40 μm
30-40 μm
20-30 μm
10-20 μm
˂ 10 μm
GCr15SiMn
0.1
75
0
0
25
0
1
10
30
30
20
10
10
4.35
13.04
21.74
34.78
26.09
40
0
0
10.81
32.43
56.76
190
0
0
0
1.94
98.06
GCr15SiMn(RE)
0.1
0
0
66.37
33.33
0
1
0
22.22
44.45
33.33
0
10
0
0
8.33
83.34
8.33
40
0
4.35
17.39
21.74
56.52
190
0
0
0
6.93
93.07
Table 2 Average diameter distributions of isolated primary carbides at different cooling rates
Steel
0-10 μm
10-20 μm
20-30 μm
30-40 μm
40-50 μm
50-60 μm
60-70 μm
70-80 μm
˃ 80 μm
GCr15SiMn
6
24
95
62
49
31
19
18
10
GCr15SiMn(RE)
15
170
123
33
4
1
-
-
-
Table 3 Austenite grain size distributions (quantity) of GCr15SiMn and GCr15SiMn(RE) samples solidified with a cooling rate of 1oC/min
Fig.8 Effects of cooling rate on the area of eutectic carbide
Fig.9 SEM images (a, b) and XRD spectra (c, d) of primary carbides obtained by electrolytic extraction in GCr15SiMn (a, c) and GCr15SiMn(RE) (b, d) steels
Fig.10 Effects of cooling rate on the chemical composition of primary carbide (a) Cr content (b) Fe content (c) Mn content (d) C content
Fig.11 Schematic of the formation mechanism of primary carbide (a) nucleation and growth of primary austenite at the early solidification (b) primary carbides precipitate randomly among the austenite grains after solidification for a while and then grow in size via solute diffusion (c) austenite grains stop growing and two typical morphologies of carbides form eventually at the end of solidification
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