Effect of N addition on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels
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6/28/03
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Effect of N Addition on Tensile and Corrosion Behaviors of CD4MCU Cast Duplex Stainless Steels JINIL SON, SANGSHIK KIM, JEHYUN LEE, and BYUNGHAK CHOI The effect of N addition on the microstructure, tensile, and corrosion behaviors of CD4MCU (Fe-25Cr5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate tests were also conducted at a nominal strain rate of 1 106/s in air and 3.5 pct NaCl 5 pct H2SO4 solution for studying the stress corrosion cracking (SCC) behavior. It was observed that the volume fraction of austenitic phase in CD4MCU alloy varied from 38 to 59 pct with increasing nitrogen content from 0 to 0.27 wt. pct. The tensile behavior of CD4MCU cast duplex stainless steels, which tended to vary significantly with different N contents, appeared to be strongly related to the volume changes in ferritic and austenitic phases, rather than the intrinsic N effect. The improvement in the resistance to general corrosion in 3.5 pct NaCl 5 pct H2SO4 aqueous solution was notable with 0.13 pct N addition. The further improvement was not significant with further N addition. The resistance to SCC of CD4MCU cast duplex stainless steels in 3.5 pct NaCl 5 pct H2SO4 aqueous solution, however, increased continuously with increasing N content. The enhancement in the SCC resistance was believed to be related to the volume fraction of globular austenitic colonies, which tended to act as barriers for the development of initial pitting cracks in the ferritic phase into the sharp ones.
I. INTRODUCTION
DUPLEX stainless steel is basically a Fe-Cr-Ni alloy system with a microstructure consisting primarily of ferrite and austenite with approximately an 1:1 ratio[1,2,3] This steel is replacing conventional austenitic or ferritic stainless steels in many industrial applications because of its combined advantages of better mechanical and corrosion properties, particularly stress corrosion cracking in chloride-containing environments, over single-phase stainless steels.[1–8] Recent development of duplex stainless steels occurs mainly in the area of alloying Mo, Cu, N, etc. for further improving mechanical and corrosion properties.[2,7,9–11] Particularly, the importance of nitrogen-containing duplex stainless steels is continuously growing due to their unique advantages.[7,9] Nitrogen alloying, however, offers challenges since the solubility of N in Fe-based alloy is extremely limited at atmospheric pressure, and most of the nitrogen-related research on duplex stainless steel has been performed on the wrought products.[3,5–7,9] At present, a limited number of studies have been reported on cast products of duplex stainless steels, despite their significance in the industry. The objective of the present study, therefore, was to examine how the microstructural changes, as a result of N addition, affect the tensile and corrosion behaviors of CD4MCU (Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steels. A particular emphasis was placed on the in-situ slow strain rat
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