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Formation of nitrogen bubbles during the solidification of 16–18% Cr high nitrogen austenitic stainless steels
Intermetallics 11 (2003) 1335–1338 www.elsevier.com/locate/intermet
Formation of nitrogen bubbles during the solidification of 16–18% Cr high nitrogen austenitic stainless steels M.R. Ridolfi, O. Tassa* Centro Sviluppo Materiali, S.p.A., Rome, Italy
Abstract Nitrogen alloyed austenitic stainless steels (NAASS) exhibit attractive properties as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitisation. However, slabs produced by these steels with nitrogen content above about 0.2 mass% show the presence of small pores located just below the slab surface, with consequent downgrading of the product. The origin of the pores is to be attributed to the formation of gas bubbles due to segregation/supersaturation phenomena occurring during solidification of gas forming solutes. Thermodynamic methods and mathematical analysis of solidification have been used to predict the composition adjustments, still remaining in the NAASS grade, which permit to avoid the formation of bubbles. Predictions have been tested against results of laboratory scale solidification trials and found satisfactory. # 2003 Published by Elsevier Ltd. Keywords: C. casting (including segregation); E. phase stability, prediction
1. Introduction Nitrogen alloyed austenitic stainless steels (NAASS), high nitrogen steels (N concentration above about 0.2 mass%) with Cr content ranging between 16–18 mass% have received great attention in recent years due to their excellent mechanical properties and corrosion resistance [1–4]. However, slabs produced by these steels with nitrogen content above about 0.2 mass% show the presence of small, almost spherical, pores located just below the slab surface, and appearing as discontinuities on the surface of the strip obtained from slab rolling, with consequent downgrading of the product. The form of the cavities suggests them to be gas bubbles, presumably formed by the following mechanism: during solidification in a cooled mould, stainless steel produces a dendritic structure with segregation of solutes in the dendrite arm spacing (Fig. 1); enrichment of solutes is counterbalanced by diffusion and convection but, if the transport of solutes is not fast enough, new phases, by reaction among segregated solutes, can nucleate in the liquid. According to this, gas bubbles should result from segregation of solutes able to react to produce gas. In the case of NAASS, the low carbon and oxygen contents permit us to exclude the CO bubbles; on the * Corresponding author. E-mail address: [email protected] (O. Tassa). 0966-9795/$ - see front matter # 2003 Published by Elsevier Ltd. doi:10.1016/S0966-9795(03)00176-6
other hand, the increasing severity of the problem, once the threshold limit of N concentration has been overcome, suggests that bubbles can be of N2, formed through the reaction N ðsolutionÞ ! 1=2N2 gas ðin bubbleÞ:
ð1Þ
It is interesting to remark that, in most of the bubbles examined no third phase (oxides or other) has been found accompanying the bubble, suggesting, together with the almost spherical shape of the bubble, that a homogeneous nucleation mechanism can be invoked to explain the bubble formation. The present work had the objective of exploring the possibility that, acting on the NAASS composition, but still remaining in the typical range of this grade of steels, the activity coefficient of solute N could be decreased to a level such that, even in segregating conditions, the reaction forming bubbles is suppressed.
2. Thermodynamic basis From the thermodynamic point of view, the solution of the problem consists of finding the steel composition, still remaining in the NAASS grade, which prevents the occurrence of reaction (1); in turn, the problem consists of computing the free energy change of reaction (1) as a function of the activity of solute nitrogen in the
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The dependence of log fN, according to the same author, is represented by the equation log fN ¼
Fig. 1. Schematisation of gas porosity formation during the solidification of steel in a cooled mould.
interdendritic liquid, and the pressure of N2 in the nucleating bubble. Henry’s activity of nitrogen dissolved in liquid steel can be represented by the expression [5] N;solute ¼ fN ½%N;
ð2Þ
where fN, activity coefficient of nitrogen in solution, is a function of concentrations of all solutes, and can be represented through the interaction coefficient formalism [51]. In the present case, the dependence of fN activity on composition has been expressed, according to Tarboton [6], as function of the parameter %Creq, defined as:
n X %Creq ¼ cxNi ½%xi
ð3Þ
ul ¼
ð4Þ
Table 1 Steel compositions (%) for which calculations have been performed No.
C
Cr
Cu
Ni
Mn
N
Si
Mo
Creq
1 2 3 4 5 6 7 8
0.045 0.05 0.025 0.04 0.037 0.026 0.1 0.03
18.19 15 16 18 17.30 18.6 19.5 16.50
0.95 2 0.1 3
0.99 1.7 2.5 1.5 0.2 0.42 3.5 1.7
9.9 11 8 7.5 10.5 10.5 9 11
0.41 0.3 0.25 0.21 0.5 0.4 0.3 0.3
0.31 0.15 0.3 0.2
0.23
21.37 19.09 18.49 20.23 21.61 22.49 21.27 20.64
0.7 2
0.04 1 0.15
0.2 0.05 2
1
K gradp:
ð6Þ
For the dendritic front the empirical relation used for K is [10]: K¼
2 log fN ¼ 0:048 %Creq þ 3:6 104 %Creq :
ð5Þ
To actually perform numerical computations, the local concentrations of all solutes must be known. The evolution of the solute concentration in the liquid in dendritic arm spacing during solidification, for given solidification rates, have been computed using the IDS commercial code, dedicated to the solidification analysis [7,8], integrated with home made post processing codes. The original IDS code takes into account the effects of cooling rate and of dendrite arm radius while completely neglecting the transport of solutes in the dendrite axis direction due to convection in the liquid ahead; to take into account the effect of removal of segregated interdendritic liquid steel operated by the convection in the liquid pool, IDS generated data have been post processed by the following procedure: a) the value of the interdendritic concentration computed by IDS is given as input to a simplified model describing the dilution operated by the liquid convection; b) the velocity field in the dendritic zone has been represented by the velocity field through a porous medium, characterised by a permeability factor K [9] which connects, together with , the dynamic viscosity of the liquid, the velocity field in the liquid, ul, and the pressure field across the porous medium.
i¼1
with the coefficients cxNi listed in Table 1. The relationship between log fN and %Creq at 1600 C is given by equation 3 and represented by Fig. 2:
2600 700 0:39 log fNð1600 CÞ 0:37 : T T
SDAS 2 fl3 180ð1 fl Þ2
;
ð7Þ
where SDAS is the value of the local secondary dendrite arm spacing, defined as the distance between two adjacent secondary branches developed during the dendrite growth, and fl is the local value of liquid fraction at a given point in the dendritic front. The parameters describing this relation have been used as fitting parameters to obtain a good agreement with experimental data. For the pressure of N2 in nucleating bubble the simple homogeneous nucleation theory has been used. Due to the extreme physical and chemical complexity of the system in the arm spacing, some approximations have been used, namely: (a) diameter of the nucleus is assumed equal to the dendrite arm spacing experimentally measured, (b) interface tension between the gas and liquid phase has been set at fixed value, kept constant independent from the liquid composition (because
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M.R. Ridolfi, O. Tassa / Intermetallics 11 (2003) 1335–1338
For the equilibrium constant of reaction (1) the following expression of the standard free energy variation has been used [52] DG0 ¼ 3600 23:90 T
J=g:atom:
ð8Þ
3. Comparison between calculated and experimental results
Fig. 2. Curve of activity coefficient fN as function of % Creq, at T=1600 C.
of the lack of information on the solute concentrations effect); the value used has been determined by tuning the model on the experimental results; the tuned value is 0.5 N/m. Ferrostatic pressure corresponding to a liquid steel head of 0.1 m has been used.
Table 2 Coefficients to compute %Creq xi
cxi N
xi
cxi N
xi
cxi N
Ti Zr V Nb Cr Ta Mn Mo
+19.4 +13.13 + 2.05 + 1.05 +1 + 0.69 + 0.5 + 0.27
Cu Sn Co Ar Sb Ni Al Si
0.12 0.16 0.2 0.2 0.2 0.22 0.85 0.9
P B C N
1 1.73 2.46 2.7
Some experimental solidification trials of small steel ingots (diameter 40 mm, height 300 mm) have been run in laboratory under controlled solidification rates detected through thermocouples. Different steel compositions have been investigated. Table 2 reports the compositions and corresponding %Creq values. Occurrence of bubbles has been detected through cutting of the ingot, and microscopic and visual inspection; this inspection allowed us to determine the typical arm spacing and consequently the nucleus diameter. The experimental solidification rates have been fed to the IDS code, which computed the concentration of all elements in the interdendritic arm spacing, permitting us to evaluate the activity of Nitrogen and, finally, the occurrence of reaction (1). The results of the calculations are shown in Fig. 3, where it is shown the free energy variation of reaction (1) for different compositions of steel, at different positions in the sample. Complete absence of cavities is obtained using low nitrogen content (for ex. Curves 1 and 2). High %Creq values hinder gas formation for the same nitrogen concentration as it is evident from comparison of curves 2 and 3 and curves 4 and 5. %Creq can be increased by increasing concentrations of Mn, Ti and Cr itself.
Fig. 3. Calculated values of G at T=1500 C; of the reaction: N!0.5 N2 at 10 cm below the liquid level, for six different steel compositions. Curves are traced as function of the distance from the external surface.
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smaller the SDAS measure, the higher the N2 pressure requested for pore formation.
These results are in good agreement with the experimental situations observed in samples.
4. Conclusions The influence of chemical and process parameters on Nitrogen bubble formation during in mould solidification of an austenitic stainless steel has been investigated. Defectiveness due to porosity formation in high nitrogen steels in the solidification process can be reduced, operating on two main factors: 1. steel composition: it has been shown that Nitrogen solubility in liquid steel is increased by some elements, such as Cr, Mn, Ti, Mo. From another point of view, nitrogen segregation from the solid into the liquid is favoured by ferritic solidification, and, as a consequence, an increase in ferritising elements, such as Cr, has negative effects on interdendritic N content. 2. solid growth rate: one of the main parameters affecting the possibility of pore formation is represented by the dendrite arm spacing. The
A method to evaluate the effects of the combined effects of composition and solidification conditions has been set up in order to design bubble-free steel compositions.
References [1] [2] [3] [4]
[5]
[6] [7] [8] [9] [10]
Stein G. Int J Mater Sci Prod Tech 1995;10:290. Balachandran G, et al. ISIJ Int 2000;40:501. Uggowidzer PJ, et al. ISIJ 1996;36:901. Holzgrubber W. Proc Int Conf on High Nitrogen Steels HNS - 88 Inst. of Met., London & Soc. France: Francaise Metallurgie; 1989. Elliott JF, et al. Thermochemistry for steelmaking, vol II. 491; Reading, MA: Addison Wesley Publishing Company; 491; 51, ibidem: 562; 52, ibidem: 618. Tarboton JN, et al. Mechanical Technology 1996;6:9. Miettinen J. Metall Trans A 1992;23A:1155. Miettinen J. Metall Trans B 1997;28 B:281. Fleming MC. Solidification processing. New York: Mac Graw Hill; 1974. Kubo K, Pehlke RD. Met Trans 1985;16B:359.
Formation of nitrogen bubbles during the solidification of 16–18% Cr high nitrogen austenitic stainless steels M.R. Ridolfi, O. Tassa* Centro Sviluppo Materiali, S.p.A., Rome, Italy
Abstract Nitrogen alloyed austenitic stainless steels (NAASS) exhibit attractive properties as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitisation. However, slabs produced by these steels with nitrogen content above about 0.2 mass% show the presence of small pores located just below the slab surface, with consequent downgrading of the product. The origin of the pores is to be attributed to the formation of gas bubbles due to segregation/supersaturation phenomena occurring during solidification of gas forming solutes. Thermodynamic methods and mathematical analysis of solidification have been used to predict the composition adjustments, still remaining in the NAASS grade, which permit to avoid the formation of bubbles. Predictions have been tested against results of laboratory scale solidification trials and found satisfactory. # 2003 Published by Elsevier Ltd. Keywords: C. casting (including segregation); E. phase stability, prediction
1. Introduction Nitrogen alloyed austenitic stainless steels (NAASS), high nitrogen steels (N concentration above about 0.2 mass%) with Cr content ranging between 16–18 mass% have received great attention in recent years due to their excellent mechanical properties and corrosion resistance [1–4]. However, slabs produced by these steels with nitrogen content above about 0.2 mass% show the presence of small, almost spherical, pores located just below the slab surface, and appearing as discontinuities on the surface of the strip obtained from slab rolling, with consequent downgrading of the product. The form of the cavities suggests them to be gas bubbles, presumably formed by the following mechanism: during solidification in a cooled mould, stainless steel produces a dendritic structure with segregation of solutes in the dendrite arm spacing (Fig. 1); enrichment of solutes is counterbalanced by diffusion and convection but, if the transport of solutes is not fast enough, new phases, by reaction among segregated solutes, can nucleate in the liquid. According to this, gas bubbles should result from segregation of solutes able to react to produce gas. In the case of NAASS, the low carbon and oxygen contents permit us to exclude the CO bubbles; on the * Corresponding author. E-mail address: [email protected] (O. Tassa). 0966-9795/$ - see front matter # 2003 Published by Elsevier Ltd. doi:10.1016/S0966-9795(03)00176-6
other hand, the increasing severity of the problem, once the threshold limit of N concentration has been overcome, suggests that bubbles can be of N2, formed through the reaction N ðsolutionÞ ! 1=2N2 gas ðin bubbleÞ:
ð1Þ
It is interesting to remark that, in most of the bubbles examined no third phase (oxides or other) has been found accompanying the bubble, suggesting, together with the almost spherical shape of the bubble, that a homogeneous nucleation mechanism can be invoked to explain the bubble formation. The present work had the objective of exploring the possibility that, acting on the NAASS composition, but still remaining in the typical range of this grade of steels, the activity coefficient of solute N could be decreased to a level such that, even in segregating conditions, the reaction forming bubbles is suppressed.
2. Thermodynamic basis From the thermodynamic point of view, the solution of the problem consists of finding the steel composition, still remaining in the NAASS grade, which prevents the occurrence of reaction (1); in turn, the problem consists of computing the free energy change of reaction (1) as a function of the activity of solute nitrogen in the
1336
M.R. Ridolfi, O. Tassa / Intermetallics 11 (2003) 1335–1338
The dependence of log fN, according to the same author, is represented by the equation log fN ¼
Fig. 1. Schematisation of gas porosity formation during the solidification of steel in a cooled mould.
interdendritic liquid, and the pressure of N2 in the nucleating bubble. Henry’s activity of nitrogen dissolved in liquid steel can be represented by the expression [5] N;solute ¼ fN ½%N;
ð2Þ
where fN, activity coefficient of nitrogen in solution, is a function of concentrations of all solutes, and can be represented through the interaction coefficient formalism [51]. In the present case, the dependence of fN activity on composition has been expressed, according to Tarboton [6], as function of the parameter %Creq, defined as:
n X %Creq ¼ cxNi ½%xi
ð3Þ
ul ¼
ð4Þ
Table 1 Steel compositions (%) for which calculations have been performed No.
C
Cr
Cu
Ni
Mn
N
Si
Mo
Creq
1 2 3 4 5 6 7 8
0.045 0.05 0.025 0.04 0.037 0.026 0.1 0.03
18.19 15 16 18 17.30 18.6 19.5 16.50
0.95 2 0.1 3
0.99 1.7 2.5 1.5 0.2 0.42 3.5 1.7
9.9 11 8 7.5 10.5 10.5 9 11
0.41 0.3 0.25 0.21 0.5 0.4 0.3 0.3
0.31 0.15 0.3 0.2
0.23
21.37 19.09 18.49 20.23 21.61 22.49 21.27 20.64
0.7 2
0.04 1 0.15
0.2 0.05 2
1
K gradp:
ð6Þ
For the dendritic front the empirical relation used for K is [10]: K¼
2 log fN ¼ 0:048 %Creq þ 3:6 104 %Creq :
ð5Þ
To actually perform numerical computations, the local concentrations of all solutes must be known. The evolution of the solute concentration in the liquid in dendritic arm spacing during solidification, for given solidification rates, have been computed using the IDS commercial code, dedicated to the solidification analysis [7,8], integrated with home made post processing codes. The original IDS code takes into account the effects of cooling rate and of dendrite arm radius while completely neglecting the transport of solutes in the dendrite axis direction due to convection in the liquid ahead; to take into account the effect of removal of segregated interdendritic liquid steel operated by the convection in the liquid pool, IDS generated data have been post processed by the following procedure: a) the value of the interdendritic concentration computed by IDS is given as input to a simplified model describing the dilution operated by the liquid convection; b) the velocity field in the dendritic zone has been represented by the velocity field through a porous medium, characterised by a permeability factor K [9] which connects, together with , the dynamic viscosity of the liquid, the velocity field in the liquid, ul, and the pressure field across the porous medium.
i¼1
with the coefficients cxNi listed in Table 1. The relationship between log fN and %Creq at 1600 C is given by equation 3 and represented by Fig. 2:
2600 700 0:39 log fNð1600 CÞ 0:37 : T T
SDAS 2 fl3 180ð1 fl Þ2
;
ð7Þ
where SDAS is the value of the local secondary dendrite arm spacing, defined as the distance between two adjacent secondary branches developed during the dendrite growth, and fl is the local value of liquid fraction at a given point in the dendritic front. The parameters describing this relation have been used as fitting parameters to obtain a good agreement with experimental data. For the pressure of N2 in nucleating bubble the simple homogeneous nucleation theory has been used. Due to the extreme physical and chemical complexity of the system in the arm spacing, some approximations have been used, namely: (a) diameter of the nucleus is assumed equal to the dendrite arm spacing experimentally measured, (b) interface tension between the gas and liquid phase has been set at fixed value, kept constant independent from the liquid composition (because
1337
M.R. Ridolfi, O. Tassa / Intermetallics 11 (2003) 1335–1338
For the equilibrium constant of reaction (1) the following expression of the standard free energy variation has been used [52] DG0 ¼ 3600 23:90 T
J=g:atom:
ð8Þ
3. Comparison between calculated and experimental results
Fig. 2. Curve of activity coefficient fN as function of % Creq, at T=1600 C.
of the lack of information on the solute concentrations effect); the value used has been determined by tuning the model on the experimental results; the tuned value is 0.5 N/m. Ferrostatic pressure corresponding to a liquid steel head of 0.1 m has been used.
Table 2 Coefficients to compute %Creq xi
cxi N
xi
cxi N
xi
cxi N
Ti Zr V Nb Cr Ta Mn Mo
+19.4 +13.13 + 2.05 + 1.05 +1 + 0.69 + 0.5 + 0.27
Cu Sn Co Ar Sb Ni Al Si
0.12 0.16 0.2 0.2 0.2 0.22 0.85 0.9
P B C N
1 1.73 2.46 2.7
Some experimental solidification trials of small steel ingots (diameter 40 mm, height 300 mm) have been run in laboratory under controlled solidification rates detected through thermocouples. Different steel compositions have been investigated. Table 2 reports the compositions and corresponding %Creq values. Occurrence of bubbles has been detected through cutting of the ingot, and microscopic and visual inspection; this inspection allowed us to determine the typical arm spacing and consequently the nucleus diameter. The experimental solidification rates have been fed to the IDS code, which computed the concentration of all elements in the interdendritic arm spacing, permitting us to evaluate the activity of Nitrogen and, finally, the occurrence of reaction (1). The results of the calculations are shown in Fig. 3, where it is shown the free energy variation of reaction (1) for different compositions of steel, at different positions in the sample. Complete absence of cavities is obtained using low nitrogen content (for ex. Curves 1 and 2). High %Creq values hinder gas formation for the same nitrogen concentration as it is evident from comparison of curves 2 and 3 and curves 4 and 5. %Creq can be increased by increasing concentrations of Mn, Ti and Cr itself.
Fig. 3. Calculated values of G at T=1500 C; of the reaction: N!0.5 N2 at 10 cm below the liquid level, for six different steel compositions. Curves are traced as function of the distance from the external surface.
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M.R. Ridolfi, O. Tassa / Intermetallics 11 (2003) 1335–1338
smaller the SDAS measure, the higher the N2 pressure requested for pore formation.
These results are in good agreement with the experimental situations observed in samples.
4. Conclusions The influence of chemical and process parameters on Nitrogen bubble formation during in mould solidification of an austenitic stainless steel has been investigated. Defectiveness due to porosity formation in high nitrogen steels in the solidification process can be reduced, operating on two main factors: 1. steel composition: it has been shown that Nitrogen solubility in liquid steel is increased by some elements, such as Cr, Mn, Ti, Mo. From another point of view, nitrogen segregation from the solid into the liquid is favoured by ferritic solidification, and, as a consequence, an increase in ferritising elements, such as Cr, has negative effects on interdendritic N content. 2. solid growth rate: one of the main parameters affecting the possibility of pore formation is represented by the dendrite arm spacing. The
A method to evaluate the effects of the combined effects of composition and solidification conditions has been set up in order to design bubble-free steel compositions.
References [1] [2] [3] [4]
[5]
[6] [7] [8] [9] [10]
Stein G. Int J Mater Sci Prod Tech 1995;10:290. Balachandran G, et al. ISIJ Int 2000;40:501. Uggowidzer PJ, et al. ISIJ 1996;36:901. Holzgrubber W. Proc Int Conf on High Nitrogen Steels HNS - 88 Inst. of Met., London & Soc. France: Francaise Metallurgie; 1989. Elliott JF, et al. Thermochemistry for steelmaking, vol II. 491; Reading, MA: Addison Wesley Publishing Company; 491; 51, ibidem: 562; 52, ibidem: 618. Tarboton JN, et al. Mechanical Technology 1996;6:9. Miettinen J. Metall Trans A 1992;23A:1155. Miettinen J. Metall Trans B 1997;28 B:281. Fleming MC. Solidification processing. New York: Mac Graw Hill; 1974. Kubo K, Pehlke RD. Met Trans 1985;16B:359.