Development of Low Segregation Technology

doi:10.11900/0412.1961.2016.00312

Acta Metall Sin

2017, Vol. 53

Issue (5): 559-566 DOI: 10.11900/0412.1961.2016.00312

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Development of Low Segregation Technology

Yutuo ZHANG^1,²,Bo CHEN^2,³,Kui LIU^2,³,Dianzhong LI^2,³,Yiyi LI^2,³(

)

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

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Yutuo ZHANG,Bo CHEN,Kui LIU,Dianzhong LI,Yiyi LI. Development of Low Segregation Technology. Acta Metall Sin, 2017, 53(5): 559-566.

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Abstract

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.

Key words: segregation low segregation technology minor element solidification interval

Received: 20 July 2016

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	Yutuo ZHANG
	Bo CHEN
	Kui LIU
	Dianzhong LI
	Yiyi LI

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00312 OR https://www.ams.org.cn/EN/Y2017/V53/I5/559

Fig.1 Schematic diagram showing alloy solidification interval (w_s1, ΔT₁, Δw₁—equilibrium solid phase composition, solidus-liquidus temperature interval and composition range; w_s2, ΔT₂, Δw₂—actual solidification solid phase composition, temperature interval and composition range of alloy with minor element; w_s3, 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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