UDK 669.14.018.8:621.785
Original scientific article/lzvirni znanstveni članek
ISSN 1580-2949 MTAEC9, 45(2)111(2011)
THERMODYNAMIC CONDITIONS FOR THE NUCLEATlON OF BORON COMPOUNDS DURING THE
COOLING OF STEEL
TERMODINAMIČNI POGOJl ZA NUKLEAClJO BOROVlH SPOJlN
PRl OHLAJANJU JEKLA
Zdenek Adolf, Jiri Bažan, Ladislav Socha
VŠB-Technical University of Ostrava, Department of Metallurgy, 17. listopadu 15/2172, 708 33 Ostrava-Poruba, Czech Republic
zdenek.adolf @ vsb.cz
Prejem rokopisa - received: 2010-11-08; sprejem za objavo - accepted for publication: 2011-01-31
The higher the value of the product of boron and oxygen concentrations, or of boron and nitrogen concentrations, than the value corresponding to the balance for the given temperature, is the thermo-dynamic criterion for the nucleation of a new phase (B2O3 or BN). lt follows from the calculations that the theoretical temperature of the beginning of B2O3 nucleation is higher than the temperature of the beginning of BN nitride nucleation. During the solidification and cooling down of steel the boron oxide will be formed preferentially before the boron nitride.
Keywords: boron steel, nucleation, non-metallic inclusions
Večji produkt vsebnosti bora in kisika ter bora in dušika kot ravnotežna vrednost pri izbrani temperaturi je termodinamični pogoj za nukleacijo nove faze (B2O3 oz. BN). lzračuni so pokazali, daje teoretično temperatura začetka nukleacije oksida B2O3 višja kot pri nitridu BN. Zato bo pri ohlajanju jekla borov oksid nastal pred nitridom.
Ključne besede: bor, jeklo, nukleacija, nekovinski vključki
1 INTRODUCTION
This paper presents a thermodynamic analysis of the probability of the formation of boron oxide and nitride in boron- and nitrogen-microalloyed stainless steels. The steels are designated for forgings for the production of valves of nuclear power plants' primary circuits and their chemical composition is given in Table 1.
Table 1: Chemical composition of the steel (in mass fractions w/%) Tabela 1: Kemična sestava jekel (v masnih deležih w/%)
Element. (w/%)
C	0.04	S	0.001	V	0.05	Nb	0.017
Mn	1.57	Cr	17.5	W	0.02	B	0.004
Si	0.6	Ni	10.5	Al	0.050	N	0.0126
P	0.020	Mo	0.07	Ti	0.40		
The objective of this work is to determine at which temperatures the B2O3 or BN is formed during the cooling and solidification of the steel. The formation of a new phase (inclusion) is related to the content of boron, nitrogen and oxygen in the steel.
2 THERMODYNAMIC BALANCE
The temperature dependencies of the Gibbs energy for the formation of B2O3 and BN were derived from table data1,2 with the use of the equations:
[B] + 3/2 [O] = 1/2 B2O3(,
[B] + [N] = BN(s) AGj0 =-411 990 + 143,585 T AG ° = -227 737 + 97,95 T
(2)
(3)
(4)
Due to the fact that the melting temperature of B2O3 is 450 °C, this oxide is at the temperatures of steel solidification in the liquid state2.
lt is possible to derive from equations (3) and (4) the following relations for the temperature dependencies of the equilibrium constants:
21517
lg K,	7,50	(5)
T
11894 lg K2 ^—^-5,116
(6)
The following is valid for the equilibrium constants of the reactions (1) and (2):
K, =
{a [B] •a [O]2)
equilibrium
K, =
{a[B] • a[N] )
(7)
(8)
equilibrium
(1)
Assuming that pure boron oxide and boron nitride are formed, it is possible to consider their activities to be equal to one. lt is similarly possible to assume unequivocally that the solutions of boron, oxygen and nitrogen in steel are diluted and that the activities of these elements
a
a
BN
are equal to a mass percentage. It is then possible to adjust the equations (7) and (8) to these forms:
([B] • [O]3/2)	(9)
([B] • [N] )
equilibrium K 1
equilibrium K
(10)
Due to the fact that the equilibrium constants K1 and K2 are a function of temperature only in accordance with equations (5) and (6), the equilibrium products of the concentrations of boron and oxygen, as well as boron and nitrogen, depend only on the temperature (see equations (11) and (12)) and can be calculated from the temperature dependencies.
21517
lg([B] • [O]3/2)
equilibrium
lg([B]- [N])
equilibrium
T
11894 " T
- + 7,50
-+5,116
(11)
(12)
It is subsequently possible to affirm logically that the formation of boron trioxide or boron oxide at the temperature T is conditioned by a higher value of the real product of boron and oxygen, or a boron and nitrogen concentration that would correspond to the equilibrium.
([B]- [O]3/2 ) real >([B]- [O]3^2 )
equilibrium
([B]- [N]) >([B]- [N])
equilibrium
(13)
(14)
Figure 1: Dependence of the beginning temperature of formation of B2O3 and the content of oxygen in steel
Slika 1: Za~etna temperatura tvorbe B2O3 v odvisnosti od vsebnosti kisika v jeklu
It follows from equations (11) and (12) that with decreasing temperature the value of equilibrium products (13) and (14) also decreases, and therefore the probability of the formation of the inclusions B2O3 and BN increases, since the real products (13) and (14) remain constant.
3 DISCUSSION OF THE RESULTS
The derived relationships were applied to the steel microalloyed with boron and nitrogen of required chemical composition - see Table 1.
The theoretical dependencies of the temperatures of the beginning of formation of the B2O3 or BN on the content of oxygen and nitrogen are given in Figures 1 and 2.
These dependencies were calculated from equations (5) and (6), adjusted for mass fractions 0.001 %, 0.006 %, 0.03 % and 0.05 % of boron.
It follows from Figure 2 that, for example, the theoretical temperature of the beginning of nucleation of BN nitride is for the nitrogen content of 100 ^g/g (ppm) in the interval 903 °C, 1001 °C, 1104 °C and 1040 °C, or for 200 ^g/g of nitrogen in the interval 939 °C, 10043 °C, 1154 °C and 1193 °C. The theoretical temperature for the start of nucleation of the oxide B2O3 is for the achieved oxygen contents (10 ^g/g) higher, i.e., 1161 °C,
Figure 2: Dependence of the starting temperature on the formation of BN and the content of nitrogen in steel
Slika 2: Za~etna temperatura tvorbe B2O3 v odvisnosti od vsebnost dušika v jeklu
Figure 3: Equilibrium temperatures and equilibrium contents of nitrogen corresponding to the oxygen content in steel of 10-100 ^g/g Slika 3: Ravnotežne temperature in ravnotežne vsebnosti dušika pri vsebnosti 10-100 ^g/g kisika v jeklu
1240 °C, 1318 °C and 1345 °C, and for 20 ^g/g of oxygen it is 1206 °C, 1290 °C, 1373 °C and 1402 °C. It follows, therefore, that boron oxide will be formed preferentially before boron nitride during the cooling of the steel.
For the steels with the above-mentioned boron contents the equilibrium temperatures and equilibrium
nitrogen contents corresponding to 10-100 ^g/g of oxygen in steel are shown in Figure 3. It is evident from the figure that B2O3 oxides can be formed only at higher contents of oxygen than would correspond to an equilibrium (right to the curve). Similarly, boron nitrides can be formed only at higher nitrogen contents than would correspond to an equilibrium (above the curve), since inequalities (13) and (l4) are fulfilled.
4 CONCLUSIONS
The thermodynamic balance of probability for the formation of oxide B2O3 and nitride BN in boron-microalloyed steels was calculated. The balance proved that the oxide is more stable than the boron nitride and,
therefore, during the cooling of steels it is formed preferentially.
The work was prepared at the conclusion of the projects FR-TI1/477 and FR-TI1/222 under the financial support of the Ministry of Industry and Trade (MPO ČR).
5 REFERENCES
1	J. Fruehan, et al. The Making Shaping and Treating of Steel. Pittsburgh 1998, 767 pp., ISBN 0-930767-02-0
2	J. Leitner. Database of thermodynamic data for admixtures in iron based melts. VŠCHT Praha, 2002, 23 pp.