1 College of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China 2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 3 School of Materials Science and Engineering, University of Science and Technology of China,Shenyang 110016, China
Cite this article:
Yutuo ZHANG,Bo CHEN,Kui LIU,Dianzhong LI,Yiyi LI. Development of Low Segregation Technology. Acta Metall Sin, 2017, 53(5): 559-566.
The minor element in alloy greatly aggravate the segregation of main elements and formation of harmful phase, resulting the deterioration of mechanical properties. Low segregation technology of cast superalloy was pioneered by Prof. Shi Changxu and co-workers in the early eighties. The technology is to control the content of minor element, such as P, Si, B and Zr, to lower the solidification segregation in the super-alloy. The working temperature and mechanical properties of superalloy can be increased greatly by using the low segregation technology. A series of alloys, such as M17 and GH738 with low segregation and excellent properties, had been developed. This study extends low segregation technology to 30Cr2Ni4MoV steel of large shaft for thermal power equipment, 690 alloy for steam generator tube in nuclear power plant, and uranium alloy for nuclear fuel. The solidification and segregation behaviour in the 30Cr2Ni4MoV steel was investigated, it is found that the minor elements of O and Al are essential for the formation of serious solidification segregation in the steel. Moreover, the solidification behavior of 690 alloy has been studied. S and N increases solidification interval, and the effect of S is greater than that of N. The solidification segregation of 690 alloy can be alleviated by controlling the contents of the S and N. Finally, the solidification temperature interval of high carbon uranium is calculated. With the car bon content increasing from 0.01% to 0.03%, the solidification interval is from 40 ℃ to 75 ℃. Thus, for the radioactive uranium alloys, minor elements show segregation to some extent in the residual liquids of final solidification zone. The minor elements in U-6Nb alloy are C, N and O. For uranium with high carbon content, the minor elements are C and O.
Fig.1 Schematic diagram showing alloy solidification interval (ws1, ΔT1, Δw1—equilibrium solid phase composition, solidus-liquidus temperature interval and composition range; ws2, ΔT2, Δw2—actual solidification solid phase composition, temperature interval and composition range of alloy with minor element; ws3, ΔT3, Δw3—solidification solid phase composition, temperature interval and composition range of alloy by using low segregation technology; w0—initial composition of alloy)
Fig.2 The first 100 t 30Cr2Ni4MoV steel ingot produced by the original process[21]
Fig.3 The second and the third 100 t 30Cr2Ni4MoV steel ingots by deoxidization smelting[20] (a) 15×10-6 O (b) 12×10-6 O
Fig.4 Effects of S and N contents on solidification temperature[23]
Fig.5 Isothermal solidification microstructures of 690 alloy with different S contents quenched at 1310 ℃[23] (a) 30×10-6 S (b) 50×10-6 S (c) 100×10-6 S (d) 1200×10-6 S
Fig.6 SEM images and qualitative elemental mapping images of Cr, Ti, S, C and N in the samples 690 alloys quenched at 1355 ℃[23] (a) 10×10-6 N (b) 200×10-6 N
Fig.7 Solidification microstructures of the 690 alloy with 7×10-6 S and 10×10-6 N quenched at different temperatures[24] (a) 1390 ℃ (b) 1385 ℃ (c) 1380 ℃ (d) 1375 ℃ (e) 1370 ℃ (f) 1365 ℃
Fig.8 U-Nb alloy phase diagram and solidification mode[27]
Fig.9 Calculated equilibrium solidus and liquidus of U-Nb alloy
Fig.10 Differential interference contrast (DIC) micrographs of arc-cast U-6Nb[28](a) low magnification showing the dendritic structure(b) higher magnification showing an inclusion cluster at the center of a dendrite(c) an oxide particle in the center of a U(N, C)/Nb2C inclusion cluster
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