Segregation and Solidification Mechanism in Spray-Formed M3 High-Speed Steel
LIU Jihao1,2, ZHOU Jian1(), WU Huibin2, MA Dangshen1, XU Huixia3, MA Zhijun3
1Institute for Special Steels, Center Iron and Steel Research Institute, Beijing 100081, China 2Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China 3Tiangong Aihe Special Steel Co., Danyang 212312, China
Cite this article:
LIU Jihao, ZHOU Jian, WU Huibin, MA Dangshen, XU Huixia, MA Zhijun. Segregation and Solidification Mechanism in Spray-Formed M3 High-Speed Steel. Acta Metall Sin, 2023, 59(5): 599-610.
Spray forming of various steels and iron-based alloys has been investigated since the 1960s. The microstructure and properties of spray-formed steels are superior to those of cast material, typically resembling those of equivalent powder metallurgy steels. While the wight of the deposits produced in pilot-scale plants is typically less than 100 kg, in some cases, industrial plants are capable of producing preforms that weigh up to several tons. However, in actual industrial production processes, segregation can easily appear in the product structure, especially in large-scale high-speed steel. In this work, M3 high-speed steels with a 250 mm diameter after forging were prepared through spray forming to study their cross-section segregation morphology. An arc-spark direct reading spectrometer, an original position analyzer for metals, OM, SEM, and EDS were used to analyze the distribution of alloy elements and microstructure characteristics of the different parts of a cross-section area. The results show two segregation morphologies in the cross-section of spray-formed M3 high-speed steel: ingot segregation and ring segregation. Carbon and alloy elements are enriched in the ingot segregation, whereas carbon and molybdenum are mainly enriched in the ring segregation, where the degree of segregation is less than that of the ingot segregation. From edge to center, the morphology of carbide changes from plate to massive. In the ring segregation area, there were two morphologies of carbide: one is M6C-wrapped MC composite carbide of the network distribution and the other M6C and MC both nucleating at the carbide/matrix interface of the composite carbide. In the ingot segregation area, carbides were mainly distributed independently of massive M6C and MC, with severe carbide segregation in the macrostructure. On the basis of the above experimental results, the solidification and microstructural evolution of spray forming were discussed, and the slow cooling rate in the deposition stage was the fundamental reason for the above experimental results. It is believed that spray forming loses rapid solidification characteristics in the deposition stage when preparing large-scale products.
Fig.2 Macrostructure image of spray-formed M3 high-speed steel and schematic of sampling for component testing and microstructure analysis
Fig.3 Composition curves of spray-formed M3 high-speed steel (specimen 1#)
Fig.4 OM image of ingot segregation stripe (a) and distributions of elements C (b), W (c), Mo (d), Cr (e), V (f), and Fe (g) (specimen 2#)
Fig.5 OM image of ring segregation stripe (a) and distributions of elements C (b), W (c), Mo (d), Cr (e), V (f), and Fe (g) (specimen 3#)
Fig.6 XRD spectra of specimens 4#-6#
Fig.7 OM images of the microstructures of ring segregation and normal areas in specimen 6# (a, b) microstructures with different magnifications (c) carbide morphology in normal area (d) carbide morphology in ring segregation area (e) carbide strips of ring segregation
Fig.8 SEM images of carbides in specimen 6# (a) network carbide (b) carbide segregation (c) plate-like shape carbide (d) bend-like shape carbide
Position
W
Mo
Cr
V
Fe
Carbide
1
23.49
23.50
6.03
38.50
8.48
MC
2
35.32
30.00
3.38
3.40
27.90
M6C
3
42.53
34.73
5.28
13.20
4.26
M2C
4
31.00
25.00
6.00
4.50
33.50
M6C
5
22.20
17.90
8.30
41.80
9.80
MC
6
34.88
29.28
4.11
2.83
28.90
M6C
7
22.83
23.62
5.01
44.80
3.74
MC
Table 1 EDS analysis results of positions 1-7 in Fig.8
Fig.9 Microstructures of ingot segregation (specimen 5#) (a-e) and central area (specimen 4#) (f, g) (a, b) microstructures of specimen 5# with different magnifications (c) carbide morphology in normal area of specimen 5# (d) carbide morphology in ingot segregation of specimen 5# (e) SEM image of fishbone-like shape carbide of specimen 5# (f) carbide morphology in central area of specimen 4# (g) SEM image of massive-like shape carbide in central area of specimen 4#
Position
W
Mo
Cr
V
Fe
Carbide
1
37.98
26.90
3.77
2.26
29.09
M6C
2
38.62
28.40
3.37
3.01
26.60
M6C
3
34.84
28.47
3.44
3.41
29.84
M6C
4
22.84
16.42
3.22
54.90
2.62
MC
Table 2 EDS analysis results of positions 1-4 in Fig.9
Fig.10 Solidification characteristics of spray forming process (a) atomization and deposition process of spray forming (b) solidification state of droplets with different sizes (c) surface mushy zone (d) formation process of ring segregation
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