The calculus of the thermodynamic activity of carbon in silicon alloyed austenitic stainless steels.

Ghiban, Brandusa ; Ghiban, Nicolae ; Serban, Nicolae 等

1. INTRODUCTION

The rapid development of austenitic stainless steels (A.S.S.) was stopped in the 70's due to the susceptibility of these materials towards intergranular corrosion (I.C.). The acting way of this corrosion form leads to selective destructions of the grain without the matrix being attacked. The amazing phenomena of I.C. which have been found in the exploitation process were rapid destroyed with the help of metallurgical means. One of the metallurgical methods of diminishing the intergranular corrosion of stainless steels in aggressive media is silicon alloying. The present paper reveals by thermodynamic considerations the influence of silicon content in presence of carbon in the austenitic stainless steels.

2. MATERIALS AND METHODS OF INVESTIGATION

There were elaborated eight stainless steels with the chemical composition given in table 1.

From the corrosion behavior of the experimental austenitic stainless steels, not only in nitric acid agents, but also in chlorine agents it has been proven mainly the positive role of the silicon annihilated by the presence of the carbon content. A possible explanation of the operation mode on the behavior of the studied stainless steels is given by the influence of the different alloying elements on the thermodynamic activity of the carbon in the alloyed austenite.

Considering as constant the nickel and chromium contents for all the experimental austenitic stainless steels (respectively 18%Cr and 15%Ni) and the carbon contents for the three experimental stainless steels categories (respectively of 0.02%, 0.03% and 0.05%) it has been determinate the thermodynamic activities of the carbon in the unalloyed austenite ([a.sub.C.sup.[gamma]]), in the nickel alloyed austenite ([a.sub.C,Ni.sup.[gamma]]), in the chromium alloyed austenite ([a.sub.C,Cr.sup.[gamma]]) and also in the complex alloyed austenite ([a.sub.C,Complex.sup.[gamma]]). The formulas used for calculating the carbon's thermodynamic activities, in the unalloyed austenite and also in the alloyed austenite, are the following:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (1)

where: [N.sub.c]--the carbon concentration, in atomic %; T--the temperature, in Kelvin degrees (K).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where: % gr.Ei--the weight concentration of the i element; Ai--the atomic mass of the i element.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

3. RESULTS AND INTERPRETATIONS

The results regarding the calculated values of the carbon's thermodynamic activities corresponding to the experimental stainless steels are given in figure 1 (a and b). Similar behavior was found also by other researchers (Vilenskaya et al., 2004).

From the analysis of the data shown in figure 1 considering thermodynamic activities of the complex alloyed austenite in the experimental stainless steels dependent on the temperature and on the silicon and carbon contents, it results that the silicon is even able to double, at the same carbon content, its activity in the austenite alloyed with chromium and nickel. This fact has major consequences regarding the behavior of the experimental steels under different stress conditions. If in the austenitic stainless steels with a relatively high carbon content (of 0.03%, respectively 0.05%) the silicon presence is positive, from the corrosion resistance under stress point of view, in case of loading the same steels in nitric acid environments the influence of the silicon is neutralized by the carbon contents. The effect of the silicon on the intergranular corrosion resistance appears only if the carbon content is very low ([less than or equal to] 0.02%).

[FIGURE 1 OMITTED]

Considering the dissolution temperature (the precipitation of the first carbides from the stainless steels) given by the formula:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

the only carbides that precipitate in the area of 550-850[degrees]C are the [Cr.sub.23][C.sub.6] type, and in the austenitic stainless steels with [approximately equal to] 20%Cr and 9 - 40%Ni we have found the formula:

1/[T.sub.s] = 1/5500 (2.92 - 0.010%Ni - ln%C) (7)

in which %Ni and %C are the weight percentage of the mentioned elements (nickel, respectively carbon).

Supposing that [DELTA][H.sup.0] and [DELTA][S.sup.0] are not depending on the temperature, from the calculation of the thermodynamic activity of the carbon in the alloyed austenite, we can calculate [T.sub.S] and therefore we can estimate TM. (Cihal, 1970) and (Gustavsson et al., 2005) have showed in their work the same behavior.

According to our own data and the information from the literature it has been found, for the determination of the [T.sub.M] temperature the following formula, considering as constant the nickel content:

[10.sup.4]/[T.sub.M] = 2 - 41g [a.sup.g.sub.C,Complex] (8)

[FIGURE 2 OMITTED]

From the figure 2, in which it's shown the dependency of the maximum temperature at which the intergranular corrosion for the experimental austenitic stainless steels appears, we can clearly see that the silicon moves the [T.sub.M] temperature to higher values for the low carbon content steels. The temperature is between 500-550[degrees]C. This is perfectly according to the experimental results. Therefore the silicon influence on the intergranular corrosion behavior is positive only in the presence of a low carbon content (<0.02%). Similar behavior was illustrated also by (Josh & Sern, 1972), (Million et al., 1995).

4. CONCLUSIONS

This present paper presents the corrosion processes from the thermodynamic point of view. It has been shown that the silicon doubles the carbon's thermodynamic activity in the complex alloyed austenite with chromium and nickel. Based on the existent correlation between the chromium carbides separation domain and the domain of susceptibility at intergranular corrosion, it was determinated the maximum temperature at which the intergranular corrosion appears ([T.sub.M]) in the silicon stainless steels only from thermodynamic considerations. So, the silicon, by raising the thermodynamic activity of the carbon, it also moves the [T.sub.M] temperature to higher values for low carbon content steels.

5. REFERENCES

Cihal, V. (1970). Sur la corrosion intergranulaire des aciers inoxydables Cr-Ni (About the intergranular corrosion of the Cr-Ni stainless steels). Corrosion Trait. Protecion. Finisation, vol. 18, no. 7, p. 441;

Gustavsson, J.; Andersson, A.M.T. & Joensson, P.G. (2005). A thermodynamic study of silicon containing gas around a blast furnace Raceway. ISIJ International, Vol. 45 (2005), 5, pp. 662-668, ISSN 0915-1559;

Josh, A. & Sern, D.F. (1972). Chemistry of Grain Boundaries and It's Relation to Intergranular Corrosion of Austenitic Stainless Steels. Corrosion, vol. 28, nr. 8;

Million, B.; Kucera, J. & Michalicka, P. (1995). The influence of silicon on carbon redistribution in steel weldments. Materials Science & Engineering (1995), ISSN 0921-5093;

Vilenskaya, T.N.; Lifshits, A.E. & Mikhailov, L.A. (2004). Calculating the interaction of controlled atmospheres with alloyed steels. Chemistr, and Materials Science, Springer, december 2004, ISSN 0026-0673.

Tab. 1. The chemical composition of the experimental
stainless steels

 Chemical compound, %

Steel C Mn S P

A 0.020 0.87 0.007 0.017
B 0.015 0.73 0.008 0.015
C 0.012 0.85 0.009 0.014
D 0.031 1.48 0.008 0.011
E 0.030 1.23 0.006 0.011
F 0.012 1.10 0.006 0.011
G 0.039 0.94 0.011 0.017
H 0.050 0.91 0.011 0.015

 Chemical compound, %

Steel Cr Ni Si

A 17.9 15.6 0.94
B 18.7 14.9 3.39
C 17.2 15.6 3.83
D 17.1 15.6 4.11
E 17.3 14.8 5.34
F 18.3 14.8 5.62
G 17.3 15.2 4.60
H 18.00 15.2 5.03

 N
Steel ppm [E.sub.Cr] [E.sub.Ni]

A 60 20.1 16.4
B 60 25.5 15.6
C 268 23.7 16.5
D 300 24.1 17.2
E 280 26.1 16.3
F 108 26.7 15.6
G 128 25.0 16.6
H 100 26.3 16.8