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Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification: the Role of Nitrogen Content
J. Mater. Sci. Technol., 2012, 28(5), 446–452.
Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification: the Role of Nitrogen Content Rong Jiang, Bo Chen, Xianchao Hao, Yingche Ma† , Shuo Li and Kui Liu Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received April 19, 2011, in revised form August 19, 2011]
The segregation and precipitation behavior of Alloy 690 containing 0.001–0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 ◦ C have been investigated using optical microscopy (OM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The results indicate that the volume fraction of TiN-type nitride formed during isothermal solidification increases with the nitrogen content of Alloy 690. Segregation of Ti and Cr exists in samples solidified at 1370 and 1355 ◦ C. The Ti content in the residual liquid markedly decreases and the concentration of Cr increases when the nitrogen content of Alloy 690 increases. Furthermore, N and S also show segregation to some extent in the residual liquids at 1355 ◦ C. Accompanying by the segregation of Cr, Ti, C, N and S, sulfides and chromium nitrides form. In a low nitrogen content Alloy 690, sulfur segregates and precipitates in the form of Ti4 C2 S2 and (Cr, Ti)S, but in the form of (Cr, Ti)S or CrS in a high nitrogen content Alloy 690. (Cr, Ti)N-type nitrides with an fcc crystal structure have been identified in a sample with 0.11 wt% nitrogen. KEY WORDS: Alloy 690; Nitrogen content; Solidification; Micro-segregation; Precipitate
1. Introduction Micro-segregation formed during solidification from the melt is of practical importance because it is detrimental to the mechanical properties and workability of ingots. It can be carried over into the wrought products and particularly affects the transverse properties of wrought material[1–3] . The literature[4–7] suggests that controlling the content of trace elements (such as P, S, Si, Zr) could minimize the microsegregation in some Fe-Ni-Cr and Ni-based superalloys and thereby lead to an improvement in the mechanical properties of the cast or wrought materials at room and high temperatures. Nitrogen is an inevitable element in both steels and nickel-based alloys[8,9] . Alloying with nitrogen has been proposed as a method to enhance the mechanical properties and corrosion resistance of stainless steels[10–12] . Because of its low solubility in nickel† Corresponding author. Ph.D.; Tel.: +86 24 23971986; Fax: +86 24 23906716; E-mail address: [email protected] (Y.C. Ma).
based alloys, nitrogen has a great tendency to form TiN-type nitrides during solidification[13,14] . Though Liu et al.[15] and Fuchs and Hayden[16] have reported that N has a positive effect on enhancing the mechanical properties, limited information is available regarding the effect of nitrogen on the solidification characteristics and the segregation of Ti, Cr, S and C. High alloying with Cr has been employed in the past to improve the corrosion resistance of stainless steels and nickel-based alloys. But the negative effects of Cr have also been reported because it exhibits certain positive or negative segregation behavior during solidification depending on the constitution of the material[17–19] . Zheng et al.[20] reported that Cr segregated severely in the interdendrite region, and formed a Cr-rich type α-phase in a nickel-based superalloy containing approximately 34 wt% Cr. The α-phase with a high Cr content is hard and brittle, which is detrimental to the mechanical properties and workability of this material. Alloy 690 is another highchromium content nickel-based alloy of superior resis-
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Table 1 Chemical compositions of Alloy 690 (wt%) 690N10 690N110 690N200 690N300 690N1100
N 0.001 0.011 0.020 0.030 0.110
C 0.020 0.024 0.023 0.024 0.021
Ti 0.18 0.18 0.18 0.19 0.18
Cr 29.85 29.6 29.91 29.98 29.57
S 0.0007 0.0003 0.0004 0.0006 0.0005
Al 0.19 0.17 0.18 0.18 0.17
O 0.0032 0.005 0.0046 0.0044 0.0062
Fe 10.2 10.8 10.0 10.2 10.2
Ni Bal. Bal. Bal. Bal. Bal.
Fig. 1 Microstructures of the solidified samples (a) 690N110 and (b) 690N1100 quenched at 1355 ◦ C
tance to intergranular stress corrosion cracking (IGSCC)[21–23] . But Alloy 690 might also be put at risk of micro-segregation during solidification from the melt considering the presence of trace elements(Ti, S, C) and high content of Cr. In the present study, a series of isothermal solidification experiments have been carried out at 1370 and 1355 ◦ C to investigate the effect of N additions on the solidification process of Alloy 690 with a view to understanding the micro-segregation of Ti, Cr, S, C and any subsequent phase transformation. 2. Experimental A master ingot of Alloy 690 used in this study was prepared by vacuum induced melting (VIM). The ingot was then divided into 5 parts and re-melted with the addition of high-purity CrN to produce 5 experimental materials. The re-melted ingots were then forged and hot-rolled into bars. The compositions of 5 materials is listed in Table 1. Samples for the isothermal solidification experiments were cut from the bars with a size of Φ16 mm×12 mm, and then placed into alumina crucibles. The samples were surrounded by alumina powder to minimize the contact area between samples and air environment, which is favorable for protecting melts from oxidation and nitrogen absorption during melting and solidification. The packaged samples were put in the furnace at 1250 ◦ C, and then heated to 1450 ◦ C and held for 5 min, followed by cooling at 10 ◦ C/min to different isothermal solidification temperatures. The samples were then held
at these temperatures for 15 min followed by water quenching. The isothermal temperatures selected in this study were 1370 and 1355 ◦ C. The microstructures of the solidified samples were observed by optical microscopy (OM), and the volume fraction of TiN was measured according to ASTM E1245-03. Electron probe microanalysis (EPMA) was employed to investigate the element concentrations and distribution in solidified samples. To quantitatively measure the Cr and Ti content in residual liquid, five points were analyzed for each sample. Transmission electron microscopy (TEM) specimens were prepared by jet-beam electropolishing using an electrolyte of 10% perchloric acid and 90% ethanol solution maintained between −25 and −20 ◦ C, and the voltage was 15 V. The microstructure and composition analysis of the precipitates were performed using an FEI Tecnai G2 20 TEM equipped with an energy disperse X-ray spectrometer (EDS). 3. Results 3.1 Metallography of Alloy 690 The microstructures of the isothermally solidified samples at 1370 and 1355 ◦ C were examined by OM. Fig. 1 illustrates the microstructural features of the samples 690N110 and 690N1100 quenched at 1355 ◦ C. It shows that N had an obvious effect on the solidification characteristics of Alloy 690. Golden yellow TiN of several microns in side length and with a typical blocky shape morphology was formed. The amount of TiN in sample 690N1100 was much more than that
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Fig. 2 Qualitative elemental mapping images of Cr, Ti, S, C and N in the samples 690N10, 690N200 and 690N1100 quenched at 1370 ◦ C
Fig. 3 Qualitative elemental mapping images of Cr, Ti, S, C and N in the samples 690N10, 690N200 and 690N1100 quenched at 1355 ◦ C
Table 2 Volume fraction of TiN in Alloy 690 samples with different N content Temp./◦ C 1370 1355 690N10 NP NP 690N110 NP 0.1% 690N200 NP 0.11% 690N300 NP 0.18% 690N1100 0.16% 0.52% Note: NP means that there is no TiN precipitation under the observation of OM Sample
in sample 690N110. Table 2 gives the quantitative analysis of the volume fraction of TiN. Clearly, the volume fraction of TiN increased gradually as the N content increased. At 1370 ◦ C, TiN was only observed in the sample with 0.11 wt% nitrogen. When the isothermal temperature was lowered to 1355 ◦ C, TiN formed in the samples containing 0.011–0.11 wt% nitrogen, i.e., TiN precipitates at higher temperature
in high N content samples. 3.2 Micro-segregation in Alloy 690 A qualitative analysis of Cr, Ti, S, C, N segregation at 1370 and 1355 ◦ C using mapping analysis technology is presented in Figs. 2 and 3, respectively. The colour in the figures represents the amount of an element qualitatively, and the colour contrast represents the degree of segregation. As is apparent in Fig. 2, Cr significantly enriched in the residual liquids in the samples isothermally solidified at 1370 ◦ C. Titanium had a similar segregation behavior except for the sample containing 0.11 wt% nitrogen, in which Ti showed a smaller segregation tendency than those containing 0.001 wt% and 0.02 wt% nitrogen. Additionally, some slight segregation of S and C in residual liquid was observed, and N was distributed homogeneously.
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0.35 wt% in the samples containing 0.001–0.03 wt% nitrogen which had no TiN precipitation at 1370 ◦ C, but was extremely low in the sample 690N1100. The solidification temperature had the same effect on the segregation of Ti as that of Cr. The enrichment of Ti in the residual liquid was more obvious at 1355 ◦ C than that at 1370 ◦ C. 3.3 Precipitates in Alloy 690
Fig. 4 Variation of mean (a) Cr and (b) Ti concentration in the residual liquid with the N content at 1370 and 1355 ◦ C
When Alloy 690 was isothermally solidified at 1355 ◦ C, the segregation of S, C and N became severe (Fig. 3). It was found that the segregation of Cr was slighter in sample 690N10 than in samples 690N200 and 690N1100. Sulfur was inclined to segregate together with Ti in the sample 690N10, but co-segregated with Cr in the sample 690N1100. The mean Cr and Ti contents in the residual liquid in all the samples were studied by the point analysis technology using EPMA (Fig. 4). The results showed that Cr was a strong positive segregation element, and was severely enriched in the liquid during the final stages of solidification. The degree of Cr segregation became severe when the isothermal solidification temperature decreased from 1370 to 1355 ◦ C. And at the same isothermal temperature, the segregation of Cr was influenced by the nitrogen content. As shown in Fig. 4(a), at 1355 ◦ C the concentration of Cr rose from approximately 35.3 wt% to 39.7 wt% in the remaining liquid when the content of N increased from 0.001 wt% to 0.11 wt%. By contrast, the concentration of Ti (Fig. 4(b)) decreased dramatically with the increase of N content at 1355 ◦ C, though it still enriched in the remaining liquid as shown in Figs. 2 and 3. The variation of Ti content was also closely related to the amount of TiN precipitate as shown in Table 2. It is worth noting that the concentration of Ti fluctuated around the level of
Accompanied by the solidification segregation of alloying elements in Alloy 690, nitrides and sulfides formed in the residual liquid and in the vicinity of solid/liquid interfaces. As illustrated in Fig. 5, the lath-like precipitate with an fcc crystal structure in the sample 690N1100 was identified as (Cr, Ti)N. The lattice constant of this nitride was 0.419 nm, which fell intermediately between that of TiN (0.424 nm) and CrN (0.415 nm), as determined from the analysis of its selected area diffraction patterns (SADP) shown in Fig. 5(b). Moreover, it was clearly observed that this type of nitride had a {100}nitride //{100}matrix orientation relationship with the matrix. The EDS analysis revealed that the precipitate contained a very high Cr concentration. As shown in Fig. 6(a), the granular carbosulfide with a hexagonal crystal structure in the sample 690N10 was identified as Ti4 C2 S2 . The lattice constants of the carbosulfide were a=0.325 nm, and c=1.196 nm. Fig. 6(b) shows the EDS spectrum for the precipitate. These titanium-rich carbosulfides were not found in any other samples. Sulfides with a short-bar or spherical morphology shown in Fig. 7 were identified as primitive hexagonal (Cr, Ti)S or CrS by analyzing the corresponding SADP. The composition of the precipitates depended on the nitrogen content of the samples and the local element distribution. The short-bar precipitate shown in Fig. 7(a) in sample 690N200 was (Cr, Ti)S with lattice constants of a=0.340 nm, c=0.602 nm, while the spherical precipitate shown in Fig. 3(b) in the sample 690N1100 was CrS with lattice constants of a=0.341 nm, c=0.577 nm. The variation of the lattice constants is induced by the different compositions of the precipitates. Fig. 7(c) and (d) shows the EDS spectra for precipitates of (Cr, Ti)S and CrS, respectively. 4. Discussion 4.1 Effect of nitrogen on solidification segregation The segregation and precipitation of nitrides and sulfides during solidification have been investigated in this study. The results of EPMA and OM show that the segregation of Ti and Cr is an important microstructural feature during isothermal solidification. The concentration of Ti and Cr in the residual liquid are related to the temperature and effects of nitrogen on the solidification characteristics and the precipita-
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Fig. 5 TEM characterization of nitrides in the sample 690N1100 quenched at 1355 ◦ C. (a) the morphology and (b) SADP of (Cr,Ti)N precipitated during solidification; and (c) EDS spectrum of (Cr,Ti)N shown in (a)
Fig. 6 TEM characterization of carbosulfides in the sample 690N10 quenched at 1355 ◦ C. (a) the morphology and SADP (inset) of Ti4 C2 S2 precipitated during solidification, and (b) EDS spectrum of Ti4 C2 S2 shown in (a)
tion of titanium nitrides. It is well known that the positive segregated elements enrich in residual liquid as the volume fraction of liquid decreases caused by temperature decreasing, and the negative segregated elements act conversely[4] , which are in accordance with present experimental results. From the results of EPMA (Figs. 2 and 3), we have found that both Cr and Ti are rejected into the residual liquid as solidification proceeds, which indicate that both Cr and Ti are positive segregated elements, and their concentration increase with the decreasing temperature when the content of nitrogen is not greater than 0.03 wt% as shown in Fig. 4. But there exists competitive segregation between them in the residual liquid. An increase of Cr content in the residual liquid proceeds along with a de-
crease of the Ti content according the results shown in Fig. 4. Sung and Poirier[24] summarized the equilibrium partition coefficients of Cr and Ti in nickel-based superalloys and suggested that Ti was a stronger positive segregation element than Cr, i.e., Ti will preferentially segregate in the residual liquid or the interdendritic region during solidification as compared with Cr. Furthermore, the degree of Ti segregation in the residual liquid should also be connected with the TiN-type nitride precipitation which is the function of temperature and nitrogen content according to the results of this study. At 1370 ◦ C, the quenched samples with 0.001–0.03 wt% nitrogen contained no nitrides as shown in Table 2. Ti, therefore, was not consumed and its concentration fluctuated slightly at a concentration of 0.35 wt% as shown in Fig. 4(b).
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Fig. 7 TEM characterization of sulfides in the samples 690N200 and 690N1100 quenched at 1355 ◦ C. (a) the morphology and SADP (inset) of (Cr,Ti)S precipitated in the sample 690N200; (b) the morphology and SADP (inset) of CrS precipitated in the sample 690N1100; (c) EDS spectrum of (Cr,Ti)S shown in (a); and (d) EDS spectrum of CrS shown in (b)
With the amount of nitrides increasing at 1355 ◦ C, the Ti concentration in the residual liquid clearly decreased, and even fell to an unexpected low value when the nitrogen content was up to 0.11 wt%, which made the effect of temperature on Ti segregation less significant. Therefore, it can be confirmed that alloying with nitrogen for Alloy 690 will cause consumption of Ti due to the precipitation of TiN (Fig. 1 and Table 2). This will break the previous equilibrium between Cr and Ti at the solid/liquid interface and make Cr enrichment in remaining liquid more significant. Additionally, nitrogen has a high affinity for Cr; the segregation of N in the remaining liquid (Fig. 3) may contribute to the severe Cr segregation as well. The concentration of Cr in the residual liquid, thus, increases with the increasing N content. 4.2 Effect of nitrogen on phase precipitation The other important microstructural feature observed in the isothermal solidification of Alloy 690 was the precipitation of nitrides and sulfides. The nitrides include TiN and (Cr, Ti)N and the sulfides are Ti4 C2 S2 , CrS and (Cr, Ti)S. The precipitation behavior of these phases could be connected to the solidification micro-segregation which is a function of N content. As is known, sulfur is an extremely strong surface
active element and always enriches in the residual liquid, free surfaces or grain boundaries[1,25,26] . Titanium is used in steels to eliminate the detrimental effect of sulfur segregation on the microstructure and its properties[27] because of its high affinity for S and the formation of sulfides. According to the research of Liu et al.[28] , nitrogen has a greater affinity for Ti as compared with S and C in Ti-bearing steels. The solubility product of the precipitates which are often observed in Ti-bearing steels increase in the order: TiN
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cipitate was Ti4 C2 S2 because little Ti was consumed during isothermal solidification at 1355 ◦ C. With the segregation of Cr and S becoming severe in the high nitrogen content Alloy 690, (Cr, Ti)S precipitation occurred and became common in the samples 690N200 and 690N1100. That is, sulfur was inclined to segregate with Ti in low-nitrogen content Alloy 690 samples, while it had the tendency of segregating with Cr in samples with high-nitrogen contents. Although Liu et al.[15] and Fuchs and Hayden[16] reported the positive influence of nitrogen on the mechanical property of Alloy 690, its potential deficiency should be noticed. There is an increasing amount of micron-scale TiN precipitates formed during solidification as the N content increases in Alloy 690 based on the present study. It is generally believed that this kind of TiN will be detrimental to the pitting corrosion and IGSCC resistance of Alloy 690 by acting as preferred sites for crack initiation[29–31] . Furthermore, the results of the present study also confirm that the addition of nitrogen will decrease the concentration of Ti in the residual liquid and promote the segregation of Cr, which will cause undesired phase precipitation. In light of the significant differences between isothermal solidification and the solidification process used in industrial practice, we will focus our further work on the solidification process of commercial Alloy 690 ingots prepared by VIM and electroslag remelting in order to obtain a better understanding of the effect of nitrogen on the solidification segregation and the precipitation reactions. 5. Conclusions (1) Micro-segregation of Cr, Ti, C, S and N are observed in the solidified microstructure of Alloy 690, and the degree of segregation is related to nitrogen content. (2) Nitrogen has great effect on the segregation of Ti and Cr. With the nitrogen content increasing from 0.001 wt% to 0.11 wt%, the Ti content in the residual liquid decreases markedly, but the enrichment and segregation of Cr become severe. (3) The addition of nitrogen increases the volume fraction of TiN-type nitrides. (Cr, Ti)N-type nitride is also identified in samples with 0.11 wt% nitrogen. (4) Sulfur has a great tendency to segregate in the solidifying liquid even though its content is less than 0.001 wt%. In a low nitrogen content Alloy 690, sulfur segregates in the form of Ti4 C2 S2 and (Cr, Ti)S, while in the form of (Cr, Ti)S or CrS in a high nitrogen content Alloy 690. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 50901076). REFERENCES [1 ] W. Wallace, R.T. Holt and T. Terada: Metallography,
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Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification: the Role of Nitrogen Content Rong Jiang, Bo Chen, Xianchao Hao, Yingche Ma† , Shuo Li and Kui Liu Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received April 19, 2011, in revised form August 19, 2011]
The segregation and precipitation behavior of Alloy 690 containing 0.001–0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 ◦ C have been investigated using optical microscopy (OM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The results indicate that the volume fraction of TiN-type nitride formed during isothermal solidification increases with the nitrogen content of Alloy 690. Segregation of Ti and Cr exists in samples solidified at 1370 and 1355 ◦ C. The Ti content in the residual liquid markedly decreases and the concentration of Cr increases when the nitrogen content of Alloy 690 increases. Furthermore, N and S also show segregation to some extent in the residual liquids at 1355 ◦ C. Accompanying by the segregation of Cr, Ti, C, N and S, sulfides and chromium nitrides form. In a low nitrogen content Alloy 690, sulfur segregates and precipitates in the form of Ti4 C2 S2 and (Cr, Ti)S, but in the form of (Cr, Ti)S or CrS in a high nitrogen content Alloy 690. (Cr, Ti)N-type nitrides with an fcc crystal structure have been identified in a sample with 0.11 wt% nitrogen. KEY WORDS: Alloy 690; Nitrogen content; Solidification; Micro-segregation; Precipitate
1. Introduction Micro-segregation formed during solidification from the melt is of practical importance because it is detrimental to the mechanical properties and workability of ingots. It can be carried over into the wrought products and particularly affects the transverse properties of wrought material[1–3] . The literature[4–7] suggests that controlling the content of trace elements (such as P, S, Si, Zr) could minimize the microsegregation in some Fe-Ni-Cr and Ni-based superalloys and thereby lead to an improvement in the mechanical properties of the cast or wrought materials at room and high temperatures. Nitrogen is an inevitable element in both steels and nickel-based alloys[8,9] . Alloying with nitrogen has been proposed as a method to enhance the mechanical properties and corrosion resistance of stainless steels[10–12] . Because of its low solubility in nickel† Corresponding author. Ph.D.; Tel.: +86 24 23971986; Fax: +86 24 23906716; E-mail address: [email protected] (Y.C. Ma).
based alloys, nitrogen has a great tendency to form TiN-type nitrides during solidification[13,14] . Though Liu et al.[15] and Fuchs and Hayden[16] have reported that N has a positive effect on enhancing the mechanical properties, limited information is available regarding the effect of nitrogen on the solidification characteristics and the segregation of Ti, Cr, S and C. High alloying with Cr has been employed in the past to improve the corrosion resistance of stainless steels and nickel-based alloys. But the negative effects of Cr have also been reported because it exhibits certain positive or negative segregation behavior during solidification depending on the constitution of the material[17–19] . Zheng et al.[20] reported that Cr segregated severely in the interdendrite region, and formed a Cr-rich type α-phase in a nickel-based superalloy containing approximately 34 wt% Cr. The α-phase with a high Cr content is hard and brittle, which is detrimental to the mechanical properties and workability of this material. Alloy 690 is another highchromium content nickel-based alloy of superior resis-
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Table 1 Chemical compositions of Alloy 690 (wt%) 690N10 690N110 690N200 690N300 690N1100
N 0.001 0.011 0.020 0.030 0.110
C 0.020 0.024 0.023 0.024 0.021
Ti 0.18 0.18 0.18 0.19 0.18
Cr 29.85 29.6 29.91 29.98 29.57
S 0.0007 0.0003 0.0004 0.0006 0.0005
Al 0.19 0.17 0.18 0.18 0.17
O 0.0032 0.005 0.0046 0.0044 0.0062
Fe 10.2 10.8 10.0 10.2 10.2
Ni Bal. Bal. Bal. Bal. Bal.
Fig. 1 Microstructures of the solidified samples (a) 690N110 and (b) 690N1100 quenched at 1355 ◦ C
tance to intergranular stress corrosion cracking (IGSCC)[21–23] . But Alloy 690 might also be put at risk of micro-segregation during solidification from the melt considering the presence of trace elements(Ti, S, C) and high content of Cr. In the present study, a series of isothermal solidification experiments have been carried out at 1370 and 1355 ◦ C to investigate the effect of N additions on the solidification process of Alloy 690 with a view to understanding the micro-segregation of Ti, Cr, S, C and any subsequent phase transformation. 2. Experimental A master ingot of Alloy 690 used in this study was prepared by vacuum induced melting (VIM). The ingot was then divided into 5 parts and re-melted with the addition of high-purity CrN to produce 5 experimental materials. The re-melted ingots were then forged and hot-rolled into bars. The compositions of 5 materials is listed in Table 1. Samples for the isothermal solidification experiments were cut from the bars with a size of Φ16 mm×12 mm, and then placed into alumina crucibles. The samples were surrounded by alumina powder to minimize the contact area between samples and air environment, which is favorable for protecting melts from oxidation and nitrogen absorption during melting and solidification. The packaged samples were put in the furnace at 1250 ◦ C, and then heated to 1450 ◦ C and held for 5 min, followed by cooling at 10 ◦ C/min to different isothermal solidification temperatures. The samples were then held
at these temperatures for 15 min followed by water quenching. The isothermal temperatures selected in this study were 1370 and 1355 ◦ C. The microstructures of the solidified samples were observed by optical microscopy (OM), and the volume fraction of TiN was measured according to ASTM E1245-03. Electron probe microanalysis (EPMA) was employed to investigate the element concentrations and distribution in solidified samples. To quantitatively measure the Cr and Ti content in residual liquid, five points were analyzed for each sample. Transmission electron microscopy (TEM) specimens were prepared by jet-beam electropolishing using an electrolyte of 10% perchloric acid and 90% ethanol solution maintained between −25 and −20 ◦ C, and the voltage was 15 V. The microstructure and composition analysis of the precipitates were performed using an FEI Tecnai G2 20 TEM equipped with an energy disperse X-ray spectrometer (EDS). 3. Results 3.1 Metallography of Alloy 690 The microstructures of the isothermally solidified samples at 1370 and 1355 ◦ C were examined by OM. Fig. 1 illustrates the microstructural features of the samples 690N110 and 690N1100 quenched at 1355 ◦ C. It shows that N had an obvious effect on the solidification characteristics of Alloy 690. Golden yellow TiN of several microns in side length and with a typical blocky shape morphology was formed. The amount of TiN in sample 690N1100 was much more than that
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Fig. 2 Qualitative elemental mapping images of Cr, Ti, S, C and N in the samples 690N10, 690N200 and 690N1100 quenched at 1370 ◦ C
Fig. 3 Qualitative elemental mapping images of Cr, Ti, S, C and N in the samples 690N10, 690N200 and 690N1100 quenched at 1355 ◦ C
Table 2 Volume fraction of TiN in Alloy 690 samples with different N content Temp./◦ C 1370 1355 690N10 NP NP 690N110 NP 0.1% 690N200 NP 0.11% 690N300 NP 0.18% 690N1100 0.16% 0.52% Note: NP means that there is no TiN precipitation under the observation of OM Sample
in sample 690N110. Table 2 gives the quantitative analysis of the volume fraction of TiN. Clearly, the volume fraction of TiN increased gradually as the N content increased. At 1370 ◦ C, TiN was only observed in the sample with 0.11 wt% nitrogen. When the isothermal temperature was lowered to 1355 ◦ C, TiN formed in the samples containing 0.011–0.11 wt% nitrogen, i.e., TiN precipitates at higher temperature
in high N content samples. 3.2 Micro-segregation in Alloy 690 A qualitative analysis of Cr, Ti, S, C, N segregation at 1370 and 1355 ◦ C using mapping analysis technology is presented in Figs. 2 and 3, respectively. The colour in the figures represents the amount of an element qualitatively, and the colour contrast represents the degree of segregation. As is apparent in Fig. 2, Cr significantly enriched in the residual liquids in the samples isothermally solidified at 1370 ◦ C. Titanium had a similar segregation behavior except for the sample containing 0.11 wt% nitrogen, in which Ti showed a smaller segregation tendency than those containing 0.001 wt% and 0.02 wt% nitrogen. Additionally, some slight segregation of S and C in residual liquid was observed, and N was distributed homogeneously.
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0.35 wt% in the samples containing 0.001–0.03 wt% nitrogen which had no TiN precipitation at 1370 ◦ C, but was extremely low in the sample 690N1100. The solidification temperature had the same effect on the segregation of Ti as that of Cr. The enrichment of Ti in the residual liquid was more obvious at 1355 ◦ C than that at 1370 ◦ C. 3.3 Precipitates in Alloy 690
Fig. 4 Variation of mean (a) Cr and (b) Ti concentration in the residual liquid with the N content at 1370 and 1355 ◦ C
When Alloy 690 was isothermally solidified at 1355 ◦ C, the segregation of S, C and N became severe (Fig. 3). It was found that the segregation of Cr was slighter in sample 690N10 than in samples 690N200 and 690N1100. Sulfur was inclined to segregate together with Ti in the sample 690N10, but co-segregated with Cr in the sample 690N1100. The mean Cr and Ti contents in the residual liquid in all the samples were studied by the point analysis technology using EPMA (Fig. 4). The results showed that Cr was a strong positive segregation element, and was severely enriched in the liquid during the final stages of solidification. The degree of Cr segregation became severe when the isothermal solidification temperature decreased from 1370 to 1355 ◦ C. And at the same isothermal temperature, the segregation of Cr was influenced by the nitrogen content. As shown in Fig. 4(a), at 1355 ◦ C the concentration of Cr rose from approximately 35.3 wt% to 39.7 wt% in the remaining liquid when the content of N increased from 0.001 wt% to 0.11 wt%. By contrast, the concentration of Ti (Fig. 4(b)) decreased dramatically with the increase of N content at 1355 ◦ C, though it still enriched in the remaining liquid as shown in Figs. 2 and 3. The variation of Ti content was also closely related to the amount of TiN precipitate as shown in Table 2. It is worth noting that the concentration of Ti fluctuated around the level of
Accompanied by the solidification segregation of alloying elements in Alloy 690, nitrides and sulfides formed in the residual liquid and in the vicinity of solid/liquid interfaces. As illustrated in Fig. 5, the lath-like precipitate with an fcc crystal structure in the sample 690N1100 was identified as (Cr, Ti)N. The lattice constant of this nitride was 0.419 nm, which fell intermediately between that of TiN (0.424 nm) and CrN (0.415 nm), as determined from the analysis of its selected area diffraction patterns (SADP) shown in Fig. 5(b). Moreover, it was clearly observed that this type of nitride had a {100}nitride //{100}matrix orientation relationship with the matrix. The EDS analysis revealed that the precipitate contained a very high Cr concentration. As shown in Fig. 6(a), the granular carbosulfide with a hexagonal crystal structure in the sample 690N10 was identified as Ti4 C2 S2 . The lattice constants of the carbosulfide were a=0.325 nm, and c=1.196 nm. Fig. 6(b) shows the EDS spectrum for the precipitate. These titanium-rich carbosulfides were not found in any other samples. Sulfides with a short-bar or spherical morphology shown in Fig. 7 were identified as primitive hexagonal (Cr, Ti)S or CrS by analyzing the corresponding SADP. The composition of the precipitates depended on the nitrogen content of the samples and the local element distribution. The short-bar precipitate shown in Fig. 7(a) in sample 690N200 was (Cr, Ti)S with lattice constants of a=0.340 nm, c=0.602 nm, while the spherical precipitate shown in Fig. 3(b) in the sample 690N1100 was CrS with lattice constants of a=0.341 nm, c=0.577 nm. The variation of the lattice constants is induced by the different compositions of the precipitates. Fig. 7(c) and (d) shows the EDS spectra for precipitates of (Cr, Ti)S and CrS, respectively. 4. Discussion 4.1 Effect of nitrogen on solidification segregation The segregation and precipitation of nitrides and sulfides during solidification have been investigated in this study. The results of EPMA and OM show that the segregation of Ti and Cr is an important microstructural feature during isothermal solidification. The concentration of Ti and Cr in the residual liquid are related to the temperature and effects of nitrogen on the solidification characteristics and the precipita-
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Fig. 5 TEM characterization of nitrides in the sample 690N1100 quenched at 1355 ◦ C. (a) the morphology and (b) SADP of (Cr,Ti)N precipitated during solidification; and (c) EDS spectrum of (Cr,Ti)N shown in (a)
Fig. 6 TEM characterization of carbosulfides in the sample 690N10 quenched at 1355 ◦ C. (a) the morphology and SADP (inset) of Ti4 C2 S2 precipitated during solidification, and (b) EDS spectrum of Ti4 C2 S2 shown in (a)
tion of titanium nitrides. It is well known that the positive segregated elements enrich in residual liquid as the volume fraction of liquid decreases caused by temperature decreasing, and the negative segregated elements act conversely[4] , which are in accordance with present experimental results. From the results of EPMA (Figs. 2 and 3), we have found that both Cr and Ti are rejected into the residual liquid as solidification proceeds, which indicate that both Cr and Ti are positive segregated elements, and their concentration increase with the decreasing temperature when the content of nitrogen is not greater than 0.03 wt% as shown in Fig. 4. But there exists competitive segregation between them in the residual liquid. An increase of Cr content in the residual liquid proceeds along with a de-
crease of the Ti content according the results shown in Fig. 4. Sung and Poirier[24] summarized the equilibrium partition coefficients of Cr and Ti in nickel-based superalloys and suggested that Ti was a stronger positive segregation element than Cr, i.e., Ti will preferentially segregate in the residual liquid or the interdendritic region during solidification as compared with Cr. Furthermore, the degree of Ti segregation in the residual liquid should also be connected with the TiN-type nitride precipitation which is the function of temperature and nitrogen content according to the results of this study. At 1370 ◦ C, the quenched samples with 0.001–0.03 wt% nitrogen contained no nitrides as shown in Table 2. Ti, therefore, was not consumed and its concentration fluctuated slightly at a concentration of 0.35 wt% as shown in Fig. 4(b).
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Fig. 7 TEM characterization of sulfides in the samples 690N200 and 690N1100 quenched at 1355 ◦ C. (a) the morphology and SADP (inset) of (Cr,Ti)S precipitated in the sample 690N200; (b) the morphology and SADP (inset) of CrS precipitated in the sample 690N1100; (c) EDS spectrum of (Cr,Ti)S shown in (a); and (d) EDS spectrum of CrS shown in (b)
With the amount of nitrides increasing at 1355 ◦ C, the Ti concentration in the residual liquid clearly decreased, and even fell to an unexpected low value when the nitrogen content was up to 0.11 wt%, which made the effect of temperature on Ti segregation less significant. Therefore, it can be confirmed that alloying with nitrogen for Alloy 690 will cause consumption of Ti due to the precipitation of TiN (Fig. 1 and Table 2). This will break the previous equilibrium between Cr and Ti at the solid/liquid interface and make Cr enrichment in remaining liquid more significant. Additionally, nitrogen has a high affinity for Cr; the segregation of N in the remaining liquid (Fig. 3) may contribute to the severe Cr segregation as well. The concentration of Cr in the residual liquid, thus, increases with the increasing N content. 4.2 Effect of nitrogen on phase precipitation The other important microstructural feature observed in the isothermal solidification of Alloy 690 was the precipitation of nitrides and sulfides. The nitrides include TiN and (Cr, Ti)N and the sulfides are Ti4 C2 S2 , CrS and (Cr, Ti)S. The precipitation behavior of these phases could be connected to the solidification micro-segregation which is a function of N content. As is known, sulfur is an extremely strong surface
active element and always enriches in the residual liquid, free surfaces or grain boundaries[1,25,26] . Titanium is used in steels to eliminate the detrimental effect of sulfur segregation on the microstructure and its properties[27] because of its high affinity for S and the formation of sulfides. According to the research of Liu et al.[28] , nitrogen has a greater affinity for Ti as compared with S and C in Ti-bearing steels. The solubility product of the precipitates which are often observed in Ti-bearing steels increase in the order: TiN
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cipitate was Ti4 C2 S2 because little Ti was consumed during isothermal solidification at 1355 ◦ C. With the segregation of Cr and S becoming severe in the high nitrogen content Alloy 690, (Cr, Ti)S precipitation occurred and became common in the samples 690N200 and 690N1100. That is, sulfur was inclined to segregate with Ti in low-nitrogen content Alloy 690 samples, while it had the tendency of segregating with Cr in samples with high-nitrogen contents. Although Liu et al.[15] and Fuchs and Hayden[16] reported the positive influence of nitrogen on the mechanical property of Alloy 690, its potential deficiency should be noticed. There is an increasing amount of micron-scale TiN precipitates formed during solidification as the N content increases in Alloy 690 based on the present study. It is generally believed that this kind of TiN will be detrimental to the pitting corrosion and IGSCC resistance of Alloy 690 by acting as preferred sites for crack initiation[29–31] . Furthermore, the results of the present study also confirm that the addition of nitrogen will decrease the concentration of Ti in the residual liquid and promote the segregation of Cr, which will cause undesired phase precipitation. In light of the significant differences between isothermal solidification and the solidification process used in industrial practice, we will focus our further work on the solidification process of commercial Alloy 690 ingots prepared by VIM and electroslag remelting in order to obtain a better understanding of the effect of nitrogen on the solidification segregation and the precipitation reactions. 5. Conclusions (1) Micro-segregation of Cr, Ti, C, S and N are observed in the solidified microstructure of Alloy 690, and the degree of segregation is related to nitrogen content. (2) Nitrogen has great effect on the segregation of Ti and Cr. With the nitrogen content increasing from 0.001 wt% to 0.11 wt%, the Ti content in the residual liquid decreases markedly, but the enrichment and segregation of Cr become severe. (3) The addition of nitrogen increases the volume fraction of TiN-type nitrides. (Cr, Ti)N-type nitride is also identified in samples with 0.11 wt% nitrogen. (4) Sulfur has a great tendency to segregate in the solidifying liquid even though its content is less than 0.001 wt%. In a low nitrogen content Alloy 690, sulfur segregates in the form of Ti4 C2 S2 and (Cr, Ti)S, while in the form of (Cr, Ti)S or CrS in a high nitrogen content Alloy 690. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 50901076). REFERENCES [1 ] W. Wallace, R.T. Holt and T. Terada: Metallography,
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