Precipitation Mechanism and Thermal Stability of Primary Carbide in Ce Microalloyed H13 Steel
HUANG Yu,CHENG Guoguang(),LI Shijian,DAI Weixing
State Key laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
HUANG Yu, CHENG Guoguang, LI Shijian, DAI Weixing. Precipitation Mechanism and Thermal Stability of Primary Carbide in Ce Microalloyed H13 Steel. Acta Metall Sin, 2019, 55(12): 1487-1494.
Ce microalloyed H13 hot die steel is widely used in manufacturing hot extrusion and die casting mold of magnesium-aluminium alloy because of its excellent combination of hot strength and impact toughness. The C content in Ce microalloyed H13 steel is approximately 4% (mass fraction), and the alloy elements content such as Cr, Mo, V et al are about 8%. Therefore, it is easy for primary carbide to precipitate during the solidification of the molten steel due to the segregation of alloy elements. Most researchers study the precipitation mechanism of primary carbide in the two-dimensional perspective. A few people are involved in the three-dimensional morphology of the primary carbide, especially the thermal stability of the primary carbide in the three-dimensional perspective in the Ce microalloyed H13 steel. Therefore, the precipitation mechanism and thermal stability of the primary carbide were systematically studied in this work. First, the SEM and inclusion automatic analysis system were used to analyze the morphology, number density and size of the inclusions in Ce microalloyed H13 steel. The three-dimensional morphology of the primary carbide in sample was observed after electrolyzed in a non-aqueous solution. The voltage was 20 V, the electrolysis time was about 3 min and the electrolyte was composed of 1% tetramethylammonium chloride, 10% acetylacetone, and 89% methanol (volume fraction). Three samples were heated to 1150, 1200 and 1250 ℃ for 1 h to investigate the thermal stability of the primary carbide. Finally, Factsage 7.2 software was used to calculate the precipitation mechanism and thermal stability of the primary carbide. Elemental Ce can effectively react with O, S, P and As elements to form the corresponding Ce-O, Ce-S and Ce-P-As inclusions. There is a huge difference between the two-dimensional and three-dimensional morphologies of the primary carbide, the two-dimensional morphology is strip and the three-dimensional morphology is irregular flake. Ti-V-rich carbide precipitates first, and then acts as the nucleation core of V-rich carbide. When the heating temperature reaches 1250 ℃, the V-rich carbide has completely dissolved, and the Ti-V-rich carbide begins to dissolve. The three-dimensional morphology of the wrapped Ti-V-rich carbide is completely exposed after the V-rich carbide disappears completely. The Ce-O inclusion is formed before solidification, and the primary carbide precipitates at the end of the solidification of molten steel. As the Ce content in molten steel increases, the stability diagram of Ce2O2S and Ce-S increases gradually. The precipitation temperature of Ti-V-rich carbide is approximately 1360 ℃, and the V-rich carbide starts to precipitate at about 1200 ℃. The calculated results are keeping well with the experimental observations. The damage of primary carbide in Ce microalloyed H13 steel can be partly reduced by higher heating temperature, but cannot be completely removed.
Fig.1 SEM images of main inclusions in Ce microalloyed H13 steel(a) Ce-O (b) Ce-P-As (c) Ce-O+Ti-V-rich carbide (d) Ce-P-As+V-rich carbide
Primary carbide
1~3 μm
3~5 μm
5~10 μm
>10 μm
Total
V-rich
3.24
1.20
2.32
1.60
8.36
Ti-V-rich
0.68
0.08
0.04
0
0.80
Table 1 Number densities of primary carbide in Ce microalloyed H13 steel (mm-2)
Fig.2 Composition of V-rich carbide
Fig.3 Aspect ratio of V-rich carbide
Fig.4 Low (a) and high (b~d) magnified 3D-morphologies of primary carbide in Ce microalloyed H13 steel
Fig.5 EDS maps of primary carbide in 3D-morphology of Fig.4aColor online
Fig.6 Thermal stabilities of primary carbide in 3D-morphology(a) 1150 ℃ for 1 h (b) 1200 ℃ for 1 h (c) 1250 ℃ for 1 h
Fig.7 Number densities and sizes of primary carbide in 3D-morphology after heat treatment at 1150, 1200 and 1250 ℃ for 1 h
Fig.8 EDS maps of primary carbide in 3D-morphology after heat treatment at 1150 ℃ (a) and 1250 ℃ (b) for 1 h
Fig.9 Equilibrium solidification of Ce microalloyed H13 steel
Fig.10 Stability diagram of the Ce-O-S in Fe-C-Cr-Mo-Ce-O-S system at 1600 ℃ (W—mass fraction of element)
Fig.11 Solidification process of primary carbide
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