Effect of different Mn contents on tensile and corrosion behavior of CD4MCU cast duplex stainless steels

Effect of different Mn contents on tensile and corrosion behavior of CD4MCU cast duplex stainless steels

Materials Science and Engineering A 396 (2005) 302–310 Effect of different Mn contents on tensile and corrosion behavior of CD4MCU cast duplex stainl...

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Materials Science and Engineering A 396 (2005) 302–310

Effect of different Mn contents on tensile and corrosion behavior of CD4MCU cast duplex stainless steels Y.H. Jang a , S.S. Kim a,∗ , J.H. Lee b a b

Division of Materials Science and Engineering, Engineering Research Center, Gyeongsang National University, 900 Gazwa-dong, Chinju 660-701, Republic of Korea Department of Metallurgical Engineering, Changwon National University, Changwon 641-773, Republic of Korea Received 12 October 2004; received in revised form 18 January 2005; accepted 18 January 2005

Abstract Despite the significant use in the industry, the effect of Mn on the tensile and corrosion behavior of cast duplex stainless steels has not been well established. In the present study, the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mn contents of 0, 0.8 and 2%, respectively, was therefore examined. The polarization and the in situ slow strain rate tests were conducted in 3.5% NaCl + 5% H2 SO4 aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking with different Mn contents. The addition of Mn, which stabilized ferrite in the present study, affected the microstructure of the present alloy, and eventually the tensile and corrosion behaviors in a complex manner. Tensile properties of CD4MCU cast duplex stainless steel, for example, was found to be determined by the volume fraction of hard ferritic phase and the shape of austenitic phase. The addition of 0.8% Mn was detrimental to both pitting corrosion and stress corrosion cracking properties of CD4MCU cast duplex stainless steel due to the significant increase in contact area between the less-noble ferritic and the noble austenitic phases. With the addition of 2% Mn, the resistance to pitting corrosion and stress corrosion cracking in 3.5% NaCl + 5% H2 SO4 aqueous solution is recovered. The resistance to stress corrosion cracking of the specimen with 2% Mn was still greatly inferior to that of the 0% Mn counterpart. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mn contents was discussed based on the micrographic and fractographic observations. © 2005 Elsevier B.V. All rights reserved. Keywords: Mn; Tensile behavior; Corrosion behavior; CD4MCU cast duplex stainless steels

1. Introduction The industrial use of duplex stainless steel is rapidly increasing due to the combined advantages of better mechanical and corrosion properties [1–5]. Since the development of first-generation duplex stainless steels in 1930s, considerable research efforts have been conducted to improve both mechanical and corrosion properties, particularly by controlling alloying elements, such as N, Cr, W and Mo [3,6–16]. Charles, for example, reported that the addition of Cr and/or Mo improved the resistance to pitting corrosion and stress corrosion cracking of duplex stainless steels [4]. He fur∗

Corresponding author. Tel.: +82 55 751 5309; fax: +82 55 759 1745. E-mail address: [email protected] (S.S. Kim).

0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.046

ther proposed that the addition of such elements needs to be done with caution since they can promote detrimental sigma phases at elevated temperatures. Despite the extensive research works on the mechanical and corrosion behavior of duplex stainless steels, most of the researches have been conducted on the wrought products, and only a limited number of studies are available on the cast products of duplex stainless steels. Previously, the authors examined the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different N, Cr and Mo contents, and reported that the mechanical and corrosion properties were strongly influenced by the changes in the shape and the volume fraction of austenitic phase with different amount of N, Cr and/or Mo contents [4,5,7]. Mn is often added to duplex stainless steel to increase the solubility of N to maximize the beneficial effect

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of N [17,18]. According to the work of Gunn [19] and Kemp et al. [20] however, the effect of Mn on the microstructural evolution, as well as the mechanical and corrosion properties, of duplex stainless steels has not been well established. The objective of present study was therefore to examine the effect of Mn on the tensile and corrosion behavior of CD4MCU cast duplex stainless steels. The in situ slow strain rate tests were conducted in air and 3.5% NaCl + 5% H2 SO4 aqueous solution to quantify the resistance to stress corrosion cracking with different Mn contents. The changes in the volume fraction and the shape of austenitic phase with different Mn contents were correlated with the tensile and corrosion behavior of CD4MCU cast duplex stainless steel, based on the optical and the SEM micrographic and fractographic observations.

2. Experimental procedures In the present study, CD4MCU cast duplex stainless steels with different Mn contents of 0, 0.8 and 2%, respectively, were used. Table 1 represents the measured chemical compositions of the alloys used in the present study. The alloys were designated as CD4MCUMn0, CD4MCUMn1

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and CD4MCUMn2, respectively, depending on the Mn contents. Each alloy specimen was melted at 1620 ◦ C using an induction furnace and subsequently sand cast into an Y-block mold. They were subsequently solution heat treated at 1050 ◦ C for 2 h and water-quenched. The microstructures of the alloy with different Mn contents were documented by using an optical microscope after electro-etching using a solution mixture of 100 ml H2 O and 10 g CrO3 at an applied potential of 3–6 V. The tensile specimens were prepared from the central portion of each Y-block cast with a gauge length of 20 mm and a diameter of 4 mm. Tensile tests were carried out at a nominal strain rate of 1 × 10−3 s−1 on an R&B (Daejun, Korea) model S2 universal testing system. For the study of general corrosion behavior, polarization tests were conducted in 3.5% NaCl + 5% H2 SO4 aqueous solution using a PAR (Oak Ridge, TN) model Versastat II potentiostat at a scan rate of 1 mV s−1 . The in situ slow strain rate (SSR) tests were conducted at a nominal strain rate of 1 × 10−6 s−1 in air and 3.5% NaCl + 5% H2 SO4 aqueous solution at an anodically applied potential of 1100 mV versus Ag/AgCl on an R&B (Daejun, Korea) model V2 constant extension rate tester (CERT). The micrographic and the fractographic analyses were conducted on the tested specimens by using scanning optical microscope (SEM).

Table 1 Chemical compositions of CD4MCU cast duplex stainless steels in wt.% Specimen

C

Si

Mn

P

S

Ni

Cr

Mo

Cu

N

Fe

CD4MCUMn0 CD4MCUMn1 CD4MCUMn2

0.04 0.03 0.04

0.86 0.75 0.86

0.0 0.8 2.0

0.027 0.029 0.027

0.006 0.004 0.006

5.32 5.36 5.32

25.19 25.37 25.19

1.84 1.99 1.84

2.8 2.8 2.8

0.13 0.13 0.13

Balance Balance Balance

Fig. 1. Typical optical micrographs of CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively.

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Fig. 2. SEM micrographs of CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, showing the second precipitates of fine austenitic phase.

3. Experimental results Fig. 1 represents the typical optical micrographs of CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively. This figure demonstrates that the microstructure of CD4MCU cast duplex stainless steel varied significantly with different Mn contents. In CD4MCUMn0 alloy, for example, the colonies of austenitic phase with the volume fraction of 44% and the diameter ranging from 80 to 200 ␮m were observed. With increasing Mn content to 0.8%, the volume fraction of austenitic phase was reduced to 40%. The shape of austenitic phase became a needle-shape with the Mn addition of 0.8%. With the addition of Mn up to 2%, the volume fraction of austenitic phase was further reduced to 35%, and the size of austenitic phase became slightly larger than that of the 0.8% Mn added counterpart. The aspect ratio of austenitic phase in CD4MCUMn2 was noted to become slightly larger compared to that in CD4MCUMn1 alloy. Regardless of Mn content, a substantial amount of the second precipitates of fine austenitic phase were observed in the ferritic matrix. Fig. 2 shows the SEM micrographs of CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, showing the second precipitates of fine austenitic phase. It was shown that the shape and the size of these precipitates were substantially different in CD4MCUMn1 from those observed in the other two alloys. Most of these precipitates in CD4MCUMn1 alloy were, for example, needle-shaped, while they were fine globular shape in the other two alloys.

Fig. 3 shows the effect of Mn content on the tensile property of CD4MCU cast duplex stainless steels. Each data point represents the average of at least five test results. This figure shows that the yield strength of CD4MCU cast duplex stainless steel tended to increase slightly with increasing Mn content from 0 to 2%. The ultimate tensile strength, on the other hand, decreased from 777 to 753 MPa with increasing Mo content from 0 to 0.8%, while it increased to 768 MPa with the addition of 2% Mn. The tensile elongation decreased from 33.0 to 19.8% with increasing Mn content from 0 to 0.8%. With the addition of 2% Mn, the tensile elongation became greatly improved to the level of CD4MCUMn1. Fig. 4 represents the typical SEM fractographs of tensile-tested CD4MCU cast duplex stainless steels with different Mn con-

Fig. 3. The effect of Mn contents on the tensile property of CD4MCU cast duplex stainless steels.

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Fig. 4. SEM fractographs of tensile-tested CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively.

tents of (a) 0%, (b) 0.8% and (c) 2%, respectively. Depending on the Mn content, the tensile fracture mode of CD4MCU alloy showed a subtle difference. In CD4MCUMn0 alloy, for example, relatively large dimples with an average diameter of 18 ␮m were observed with throughout the matrix. The specimens with 0.8 and 2% Mn, on the other hand, showed a bimodal distribution of 3.7 ␮m diameter fine and 19 ␮m diameter large dimples, while the overall size of dimples in CD4MCUMn2 alloy appeared to be approximately 1 ␮m larger than that in CD4MCUMn1 alloy. In order to understand the effect of Mn addition on the corrosion behavior of CD4MCU alloy, the polarization tests were conducted in 3.5% NaCl + 5% H2 SO4 aqueous solution. Fig. 5 shows the representative polarization curves

Fig. 5. Polarization curves for CD4MCU alloys with different Mn contents in 3.5% NaCl + 5% H2 SO4 .

for CD4MCU alloys with different Mn contents in 3.5% NaCl + 5% H2 SO4 . Interestingly, the pitting corrosion potential decreased from 1033 to 955 mV versus Ag/AgCl with the addition of 0.8% Mn. With the further addition of Mn to 2%, on the other hand, the pitting corrosion potential increased to 1020 mV versus Ag/AgCl. Fig. 6 shows the SEM micrographs of the corroded surfaces of CD4MCU alloys with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, after the polarization test in 3.5% NaCl + 5% H2 SO4 . The surface examination clearly reflected the trend observed in Fig. 5, such that CD4MCUMn0 and CD4MCUMn2 alloys had better resistance to pitting corrosion than CD4MCUMn1 alloy. Indeed, Fig. 6 demonstrates that the number and the size of pittings were substantially greater in CD4MCUMn1 alloy compared to those in CD4MCUMn0 and CD4MCUMn2 alloys. In the present study, the in situ slow strain rate tests (SSRTs) were conducted on CD4MCU alloys with different Mn contents at a nominal strain rate of 1 × 10−6 s−1 either in air or 3.5% NaCl + 5% H2 SO4 aqueous solution at an anodically applied potential of 1100 mV versus Ag/AgCl to quantify the resistance to stress corrosion cracking (SCC). Fig. 7 shows the tensile strength and the tensile elongation of SSRTed specimens in air and 3.5% NaCl + 5% H2 SO4 aqueous solution as a function of Mn contents. Each data represents the average of at least three test results. In air, the general trend was similar to that observed at a strain rate of 1 × 10−3 s−1 , as shown in Fig. 3, although the absolute values were somewhat different due to the strain rate effect. The resistance to SCC is often correlated to either the percent change in tensile elongation or the percent change in total time to failure with exposure to

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Fig. 6. SEM micrographic observations of the corroded surfaces of CD4MCU alloys with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, after polarization test in 3.5% NaCl + 5% H2 SO4 .

the SCC-causing environments with respect to the reference environment (i.e. air in the present study) [21]. Fig. 8 demonstrates that the SCC resistance, as represented by the percent change in tensile elongation with exposure to the aggressive environment, was the highest for CD4MCUMn0 alloy, followed by CD4MCUMn2 and CD4MCUMn1 alloys. Indeed, CD4MCUMn0 alloy showed almost no reduction in tensile elongation with exposure to the aggressive environment. Fig. 9 shows the SEM fractographs of CD4MCU alloys with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, after slow strain rate tests in 3.5% NaCl + 5% H2 SO4 . Compared to the fractographs shown in Fig. 4, the tensile fracture mode of SSRTed CD4MCU cast duplex stain-

Fig. 7. Tensile strength and the tensile elongation of SSRTed specimens in air and 3.5% NaCl + 5% H2 SO4 aqueous solution as a function of Mn contents.

less steel was significantly altered with exposure to the aggressive environment. It was noted that the tensile fracture mode of CD4MCIMn0 was almost unaffected with exposure to 3.5% NaCl + 5% H2 SO4 aqueous solution at an anodically applied potential of 1100 mV versus Ag/AgCl. In CD4MCUMn1 alloy, on the other hand, the dimpled rupture mode in air changed to the quasi-cleavage mode in an SCC-causing environment. CD4MCUMn2 alloy also showed a substantial portion of cleavage mode along with the dimpled rupture mode. Fig. 10 represents the SEM micrographs of the surface area of SSRTed CD4MCU alloys with differ-

Fig. 8. Percent change in tensile elongation of CD4MCU cast duplex stainless steels with different Mn contents.

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Fig. 9. SEM fractographs of CD4MCU alloys with different Mn contents of (a) 1%, (b) 0.8% and (c) 2%, respectively, after slow strain rate tests in 3.5% NaCl + 5% H2 SO4 .

ent Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, in 3.5% NaCl + 5% H2 SO4 aqueous solution, documented in the vicinity of fracture location. This figure shows that the crack morphology of SSRTed specimens in 3.5%

NaCl + 5% H2 SO4 was quite different with each other depending on the Mn contents. In CD4MCUMn1 alloy, for example, relatively well-developed sharp cracks were observed throughout the surface with the long axis perpendicular to

Fig. 10. SEM micrographs of surface areas of CD4MCU alloys with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, close to the fractured surfaces documented after slow strain rate test in 3.5% NaCl + 5% H2 SO4 .

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Fig. 11. SEM cross-sectional micrographs of CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, after slow strain rate test in 3.5% NaCl + 5% H2 SO4 .

the tensile direction. The stress corrosion cracks observed in CD4MCUMn2 alloy were substantially smaller compared to those in CD4MCUMn1 alloy. In CD4MCUMn0 alloy, even though the specimen surface was severely corroded, any sharply developed stress corrosion cracks were not observed. Fig. 11 shows the SEM cross-sectional micrographs of CD4MCU cast duplex stainless steels with different Mn contents of (a) 0%, (b) 0.8% and (c) 2%, respectively, after slow strain rate tests in 3.5% NaCl + 5% H2 SO4 . It was demonstrated that the stress corrosion cracks were mainly confined in the ferritic region and propagated through the ferritic phase.

4. Discussion In the present study, a notable microstructural evolution in CD4MCU cast duplex stainless steel was observed with different Mn contents, including the volume fraction, the size and the shape of austenitic phase. There is a controversy on which phase Mn stabilizes [19,20]. The present study suggests that Mn is a ferrite stabilizing element in CD4MCU cast duplex stainless steel, since the volume fraction of ferritic phase increased from 56 to 65% with increasing Mn content from 0 to 2%. The size and shape of austenitic phase also varied with different Mn contents, as shown in Fig. 1. With increasing Mn content from 0 to 0.8%, the size of austenitic phase was substantially reduced giving a needle-shaped appearance. With the addition of Mn up to 2%, the volume frac-

tion of austenitic phase further decreased, and the colonies of austenitic phase became slightly larger and rounder compared to those of CD4MCUMn1. Regardless of Mn content, a significant amount of the second precipitates of fine austenitic phase were observed in the ferritic matrix. Previously, the authors reported that high N content in CD4MCU cast duplex stainless steel promotes this second precipitates of fine austenitic phase in the ferrite matrix [6,7]. However, the shape and size of these second precipitates varied depending on Mn content, as demonstrated in Fig. 2. Mn is often added to stainless steels for the partitioning effect of N [17]. In the present study, it was demonstrated that different Mn contents significantly affected the tensile behavior of CD4MCU cast duplex stainless steel, as well as the corrosion behavior. With increasing Mn content from 0 to 0.8%, the strength level was not notably affected, while the tensile elongation was greatly reduced from 33 to 19.8%. With the addition of 2% Mn, the tensile elongation became similar to that of the specimen without Mn. Such a complex trend observed in tensile properties with varying Mn contents may be largely related to the microstructural evolution in the present alloy, including the volume fraction of each phase and the shape and size of austentic phase, both primary and secondary, along with an intrinsic solid solution hardening effect of Mn. The ferritic phase is a harder phase than the austentic phase, and the increase in the volume fraction of ferritic phase would improve the strength level of CD4MCU cast duplex stainless steel. The slight increase in the yield strength value of CD4MCU alloy with increasing Mn con-

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tent from 0 to 2% was therefore attributable to the increase in the volume fraction of ferritic phase from 56 to 65%, as well as the solid solution hardening effect of Mn. The ultimate tensile strength value appeared to be related to the tensile ductility which represents the capability of plastic deformation. The complex trend associated with the ultimate tensile strength values observed in Fig. 3 was therefore believed to be due to the intrinsic hardening effect of Mn and the change in tensile ductility with Mn content. The decrease in the tensile elongation with increasing Mn content from 0 to 0.8% appeared to be dependent on the change in the shape of primary and secondary austenitic phase. As shown in Fig. 1, the shape of austenitic phase, both primary and secondary, in CD4MCUMn1 alloy was sharp needle-shape compared to the round-shape in CD4MCUMn0 and CD4MCUMn2 alloys. The fractographic studies in Fig. 4 suggested that large dimples were originated at the boundary between austenitic and ferritic phases, while fine dimples were formed at the second precipitates of austenitic phases. Therefore, the microvoid would be formed more easily at the needle-shaped austenitic phase compared to the round-shaped counterpart. With the 2% Mn addition, the austenitic phase became round again, and the tensile elongation recovered to that of CD4MCUMn0 alloy. It has often been reported that the addition of Mn is beneficial to the pitting corrosion resistance of stainless steels in chloride environment [22]. The present observation on the corrosion behavior of CD4MCU alloys with different Mn contents in 3.5% NaCl + 5% H2 SO4 , however, demonstrates that this notion is not always true. The corrosion resistance of CD4MCU cast duplex stainless steel appeared to be strongly dependent on the microstructure, particularly the contact area between the ferritic and the austenitic phases rather than the intrinsic Mn effect. It has been reported that the ferritic phase is slightly less-noble to austentic phase in duplex stainless steel [3]. The polarization test results demonstrated that the corrosion resistances of CD4MCUMn0 and CD4MCUMn2 alloy in 3.5% NaCl + 5% H2 SO4 were similar with each other. Among the alloy systems studied, the CD4MCUMn1 alloy showed the least resistance to corrosion. Even though the addition of Mn is beneficial, the shape of austenitic phase became needle-like with 0.8% Mn, which would increase the contact area between the noble and the less-noble phases. In the present study, the contact area was quantified by measuring the line length along the boundary between ferrite and austenite in unit area. Considering the line length in CD4MCUMn0 as 1, the line length of CD4MCUMn1 and CD4MCUMn2, respectively, was 1.5 and 1.2, respectively. The increase in the contact area with the addition of 0.8% Mn would therefore reduce the resistance to corrosion of CD4MCU cast duplex stainless steel. With increasing Mn content from 0.8 to 2%, the contact area between noble and less-noble phases decreased due to the globulization of primary austenitic phases. Eventually, the corrosion resistance of the 2% Mn added specimen became similar to that of the no Mn-containing counterpart.

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The overall trend observed in the SCC test results in 3.5% NaCl + 5% H2 SO4 for CD4MCU alloys with different Mn contents was somehow similar to that observed in the polarization test results. CD4MCUMn0 alloy showed almost no susceptibility to SCC in 3.5% NaCl + 5% H2 SO4 . Any notable stress corrosion cracks were not observed, even though the specimen surface was severely corroded. CD4MCUMn1 alloy demonstrated the lowest resistance to SCC, along with the most sharply developed stress corrosion cracks, among the specimens studied, as shown in Fig. 10. In CD4MCUMn1 alloy, the formation of stress corrosion crack would be relatively easy compared to the other two alloys, due to the increase in contact area between less-noble ferritic and noble austenitic phases as a result of needle-shaped austenitic phase. The addition of 2% Mn on CD4MCU cast duplex stainless steel improved the resistance to SCC, probably due to the decrease in the contact area between noble and lessnoble phases, compared to that of the 0.8% Mn added specimen. The resistance to SCC was still greatly inferior to that of the 0% Mn counterpart. As shown in Fig. 11, the stress corrosion cracks were mainly confined in the ferritic matrix. Eventually, the formation and development of stress corrosion cracks would be relatively easier in CD4MCUMn2 alloy, which had the greatest volume fraction of ferrite among the specimens studied, compared to CD4MCUMn0 alloy. Comparing Fig. 1(a) with (c), it was noted that the colony of austenitic phase was smaller in CD4MCUMn2 alloy than in CD4MCUMn0 alloy. The contact area between noble and less-noble phases in CD4MCUMn2 alloy would therefore be greater than that in CD4MCUMn0 alloy. Conclusively, the resistance to SCC in CD4MCUMn2 alloy was inferior to that in CD4MCUMno alloy due to higher volume fraction of ferrite and higher contact area between noble ferritic phase and less-noble austenitic phase.

5. Conclusions The tensile and corrosion behaviors of CD4MCU cast duplex stainless steels with different Mn contents of 0, 0.8 and 2%, respectively, were examined, and the following conclusions are drawn. (1) A substantial microstructural evolution in CD4MCU cast duplex stainless steel was observed with different Mn contents, including the volume fraction and the size and shape of both primary and secondary austenitic phase. (2) With increasing Mn contents from 0 to 2%, the improvement in YS and the UTS values was not significant. The tensile elongation was, on the other hand, greatly impaired with the addition of 0.8% Mn. The trend observed in the present study was believed to be due to the change in shape of austenitic phase, the change in volume fraction of ferritic phase and the intrinsic hardening effect of Mn solute.

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(3) The addition of 0.8% Mn to CD4MCU alloys greatly degraded the resistance to both pitting corrosion and stress corrosion cracking in 3.5% NaCl + 5% H2 SO4 solution, which appeared to be due to the increase in contact area between the less-noble ferritic and the noble austenitic phases as a result of the shape change in austenitic phase. (4) Even though the SCC resistance of CD4MCU cast duplex stainless steel was improved with the addition of 2% Mn, as compared to that of CD4MCU1 alloy, it was still greatly inferior to that of the 0% Mn counterpart. The inferior resistance to SCC in CD4MCUMn2 alloy was believed to be due to higher volume fraction of ferrite and higher contact area between noble ferritic phase and less-noble austenitic phase compared to those of CD4MCUMn0 alloy.

Acknowledgement This work was supported by the Korea Research Foundation Grant (KRF-2003-042-D00201).

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