High-Temperature Decomposition Mechanism of M2C Primary Carbide in M50 Steel
MA Fang1,2, LU Xingyu3, ZHOU Lina2, DU Ningyu3, LEI Chengshuai3(), LIU Hongwei3(), LI Dianzhong3
1 School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China 2 AECC Harbin Bearing Co. Ltd., Harbin 150025, China 3 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
MA Fang, LU Xingyu, ZHOU Lina, DU Ningyu, LEI Chengshuai, LIU Hongwei, LI Dianzhong. High-Temperature Decomposition Mechanism of M2C Primary Carbide in M50 Steel. Acta Metall Sin, 2024, 60(7): 901-914.
M50 steel is primarily used for manufacturing the main shaft bearings of aero engines. However, the fatigue property of M50 steel affects the service life of shaft bearings owing to their operation in the environment with high temperature, high rotation speed, and high contact stress. Inclusions and large-sized carbides are proved to be the primary reasons that cause fatigue cracking. Nevertheless, inclusions in M50 steel and fatigue failure due to inclusions are substantially reduced with the rapid development of metallurgical technology and metallurgical equipment in recent years. M50 steel contains high fractions of Cr, Mo, and V elements, which are easily enriched and can form primary carbides. The primary carbides in M50 steel are hard and brittle and cause stress concentration under external load, thereby accelerating the initiation and propagation of fatigue cracks. Currently, the large-sized primary carbides in M50 steel play an important role in reducing the service life of bearings and have attracted substantially research attention. The present investigation focuses on the decomposition mechanism of large-sized M2C primary carbide in M50 steel to reveal the carbide-refinement mechanism during high-temperature heat treatment. In addition, M2C primary carbide in M50 steel was systematically characterized by SEM, EPMA, and TEM, and its decomposition mechanism at 1160-1250oC was studied. The difference in chemical composition of different M2C primary carbides and its effect on the decomposition mechanism were also explored. Three forms of M2C carbides in M50 steel were revealed: the rod-like carbide, the lamellar-like carbide, and the block-like carbide. In these three M2C carbides, the content of Fe increased, while the content of Mo decreased successively. The difference in chemical composition and morphology of these three M2C carbides led to the different microstructure-evolution process when heat-treated at elevated temperature. When the steel was heat-treated at 1160-1180oC, only the M2C carbides with high Fe content decomposed and a few of the carbides transformed to MC carbide. The growth rate of MC carbide was extremely low at this temperature. When the steel was heat-treated at 1210oC, most of the M2C carbides decomposed after 20 h. The growth rate of MC carbide also increased rapidly, and a large amount of large-sized MC carbides were found. Further heat treatment of steel at 1250oC resulted in the decomposition of all M2C carbides and the absence of large-sized primary carbides in the microstructure. However, a large amount of newly born M2C carbide, formed due to the melting of the matrix and re-solidification, were found in the microstructure.
Fund: National Key Research and Development Program(2018YFA0702900);National Natural Science Foundation of China(52031013);Special Fund Project for Self-innovation of Aero Engine Corporation of China(ZZCX-2020-027)
Corresponding Authors:
LEI Chengshuai, Tel: 17824032796, E-mail: cslei@imr.ac.cn;
Fig.1 SEM-BSE images of the as-cast M50 steel, EPMA mappings of elements, and Kikuchi diffraction patterns of primary carbide (a, b) SEM-BSE images of the as-cast M50 steel (a), and M2C and MC primary carbides (b) (c, d) EPMA distribution maps of Mo (c) and V (d) elements (e, f) Kikuchi diffraction patterns of M2C (Point A in Fig.1b) (e) and MC (Point B in Fig.1b) (f)
Fig.2 SEM images of M2C primary carbides (a) lamellar-like carbide (b) rod-like carbide (c) block-like carbide
Point
Fe
Cr
Mo
V
C
1
26.19
12.19
40.49
9.71
11.42
2
22.82
10.18
45.05
10.15
11.80
3
17.17
11.32
47.99
11.37
12.15
4
22.41
10.96
44.75
10.30
11.58
5
21.41
11.71
44.40
11.16
11.32
Table 1 EDS results of the lamellar-like carbide in Fig.2a
Point
Fe
Cr
Mo
V
C
1
26.06
14.12
41.12
9.17
9.41
2
34.51
10.07
38.97
8.23
8.23
3
33.48
9.38
39.61
7.99
9.32
4
36.25
9.73
38.17
7.70
8.15
5
22.93
10.34
45.77
11.22
8.64
Table 2 EDS results of the rod-like carbide in Fig.2b
Point
Fe
Cr
Mo
V
C
1
8.02
11.75
55.09
12.62
12.52
2
8.73
11.76
54.51
12.59
12.41
3
6.29
12.71
56.14
12.46
12.40
4
9.04
12.03
54.35
12.47
12.11
5
10.28
12.47
53.46
11.61
12.18
Table 3 EDS results of the block-like carbide in Fig.2c
Fig.3 SEM-BSE images of the microstructure of M50 steel after heat-treating at 1160oC for 20 h (a) macroscopic morphology of the microstructure (b) block-like carbide (c, d) rod-like carbides (e, f) lamellar-like carbides
Fig.4 Morphologies of retained M2C carbide and newborn MC carbide in M50 steel after heat-treating at 1160oC for 20 h
Carbide
Point
Fe
Cr
Mo
V
C
M2C
1
5.61
9.07
58.87
14.72
11.73
2
6.68
9.49
55.82
14.54
13.47
3
5.90
10.52
57.07
13.57
12.94
4
6.87
9.53
56.20
14.60
12.80
5
5.84
8.96
57.77
14.10
13.33
MC
6
3.03
5.56
38.34
33.54
19.53
7
2.79
5.61
38.34
37.49
15.77
8
5.27
5.50
35.33
38.32
15.58
Table 4 EDS results of M2C and MC carbides in Fig.4
Fig.5 Interface of M2C primary carbide and new-born MC carbide in M50 steel after heat-treating at 1160oC for 20 h (a), and EDS line scanning of V (b) and Mo (c) elements along line 1 in Fig.5a (Dash lines in Figs.5b and c show the interface)
Fig.6 Low-magnification high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the interface of M2C and MC (a); fast Fourier transform (FFT) patterns of the microstructure in the rectangle of Fig.6a (b); atomic-resolution HAADF-STEM image of the interface of M2C and MC (c); EDS mappings of Fe (d), Mo (e), and V (f)
Area
C
V
Cr
Fe
Mo
1
3.95
17.20
5.66
3.77
69.42
2
6.39
39.74
4.16
2.07
47.64
Table 5 EDS results of M2C (Area 1)与MC (Area 2) in Fig.6c
Fig.7 SEM-BSE images of the microstructure of M50 steel after heat-treating at 1180oC for 20 h (a) macroscopic morphology of the microstructure (b) retained M2C carbide (c, d) high magnified images of positions 1 (c) and 2 (d) in Fig.7b (Arrows in Fig.7d show newborn MC carbides transformed from M2C)
Fig.8 SEM-BSE images of the microstructure of M50 steel after heat-treating at 1210oC for 20 h (a) macroscopic morphology of the microstructure (b, c) large sized MC carbides (d) the growth of MC carbide
Fig.9 Low (a) and high (b) magnified SEM-BSE images of the microstructure of M50 steel after heat-treating at 1250oC for 20 h, and EDS analysis of point 1 in Fig.9b (c)
Fig.10 SEM-BSE image of the newly-formed primary M2C carbide after heat-treating at 1250oC for 20 h; and the EDS mappings of Mo, V, and C elements
Fig.11 SEM-BSE images of the large-scaled carbides in forging M50 steel bars after heat-treating at different temperatures for 20 h (a) 1160oC (b) 1180oC (c) 1210oC (d) 1250oC
Fig.12 Schematics of the decomposition and transition mechanism of M2C primary carbide when heat-treating at 1160-1180oC (a1-a4) rod-like M2C primary carbides (b1-b4) lamellar-like M2C primary carbides (c1-c3) block-like M2C primary carbides
Fig.13 Schematics of the decomposition and transition mechanism of M2C primary carbide when heat-treating at 1210oC (a-d)
Fig.14 Schematics of the decomposition and transition mechanism of M2C primary carbide when heat-treating at 1250oC (a-e)
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