Effects of Nitrogen Segregation and Solubility on the Formation of Nitrogen Gas Pores in 21.5Cr-1.5Ni Duplex Stainless S
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DUPLEX stainless steels (DSSs) are characterized by their mixed microstructures which exhibit approximately equal proportions of austenite and ferrite.[1] They exhibit an attractive combination of excellent mechanical properties and corrosion resistance,[2] have approximately twice the strength of conventional austenitic steels, and have excellent pitting- and stress-corrosion resistance in chloride environments.
HONG-CHUN ZHU, ZHOU-HUA JIANG, HUA-BING LI, HAO FENG, SHU-CAI ZHANG, GUO-HAI LIU, JUN-HUI ZHU, PENG-BO WANG, and BIN-BIN ZHANG are with the School of Metallurgy, Northeastern University, Shenyang 110819, P.R. China. Contact e-mails: [email protected], [email protected] GUANG-WEI FAN and GUO-PING LI are with the Technology Center of Taiyuan Iron and Steel Croup Co. Ltd., Taiyuan 030001, P.R. China. Manuscript submitted May 26, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The modified chemical composition of DSSs (low Ni and Mo) renders them relatively inexpensive.[3,4] These steels have been used extensively in nuclear, chemical, and petroleum industries, as well as offshore and marine applications, and under other harsh service environments.[5–7] As an important alloying element, nitrogen is typically used to replace expensive Ni in DSSs. In particular, its pitting corrosion resistance is equivalent to 16 times that of Cr, and its austenite-stabilizing effect is equivalent to 30 times that of Ni.[8] With this excellent performance, nitrogen-enhanced DSSs (N ‡ 0.2 wt pct), such as S32101, S32304, and S32750, have been widely developed.[9] In general, DSSs solidify in a fully ferritic mode, and the austenite phase forms during the later stages of solidification, at temperatures less than the solidus temperature.[7] Compared to the austenite and liquid phases, nitrogen is not very soluble in the ferrite phase.[10] Therefore, among the liquid, ferrite, and austenite phases, there are considerable differences in nitrogen solubility, which result in significant nitrogen
Table I.
Chemical Compositions and Creq/Nieq of DSSs, Weight Percent
Test
Solidification Pressure (MPa)
Cr
Ni
Mo
Cu
Mn
N
Si
C
P
S
Fe
Creq/Nieq*
D1 D2 D3 D4 D5
0.04 0.10 0.10 0.10 0.13
21.01 21.52 20.86 21.73 21.68
1.46 1.45 1.45 1.44 1.51
0.487 0.497 0.491 0.507 0.500
0.454 0.434 0.455 0.433 0.450
4.14 4.00 4.12 4.24 3.95
0.23 0.25 0.26 0.29 0.28
0.564 0.485 0.522 0.480 0.403
0.040 0.045 0.041 0.037 0.044
0.0091 0.0084 0.0083 0.0085 0.0094
0.0041 0.0036 0.0035 0.0042 0.0045
bal. bal. bal. bal. bal.
1.92 1.85 1.77 1.72 1.73
*Nieq ¼ ½pct Ni] + 30 ½pct C] + 30 ½pct N] þ 0:5 ½pct Mn], Creq ¼ ½pct Cr] + [pct Mo] + 1:5 ½pct Si] þ 0:5 ½pct Nb].
segregation and the formation of nitrogen gas pores during the solidification of DSSs.[11,12] Such gas pore defects are considered a major problem concerning the manufacture of the nitrogen-alloyed DSSs.[7,10,13–15] Recently, several investigations have been conducted with regard to gas pore formation, focusing on the effects of the solidification mode, pressure, and
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