19, 1994
Solidification of Metais and
Metali i Stopów
PL ISSN 0208-9386
Ałłoys
Krzepnięcie
INFLUENCE OF THE CHEMICAL COMPOSITION
OF CAST STEEL ON THE CRACKING FORMA TION
IN THE SOLIDIFICATION RANGE
ANDRZEJ CHOJECKI, IRENEUSZ TELEJKO
Academy of Mining and Metallurgy, C raco w
The paper presents the thermodynamic study of Fe-C-S and
Fe-Mn-S systems. The influence of the chernica l composition o f steel on
the hot-tear formation is elucidated. In steel the cracking susceptibility
is determined by sułphur behaviour. Carbon itsełf has a minor direct
effect, but through its influence on the sulphur activity it changes
the liquid miscibility gap and the quantity of sulfides. Manganese forms
the sulfides at the solidification front, controlling the enrichment of the
liquid in sulphur.
Introduction
The int1uence of components on the hot cracking tendency of cast steel may be
by their effect on the length of the liquid film separating the crystals in the
solidiftcation range. Below a certain length, the material becomes duetile and crack
formation is no longer possibłe. This łength determines the brittle-ductile transition
temperatuce [1,2] .
Sulphur is aJways presented in the iron aJloys. Its segregation leads to create the lowest
melting point phase. The range of brittlness and hence hot cracking susceptibility is
determined by the sulphur behaviour. The other ełements have little direct int1uence, but
affect the hot tearing tendency changing the partition coefficient of sulphur.
In the present work the thermodynamic anaJysis of the Fe-C-S and Fe-Mn-S
diagrams have been presented to define the creation of sulfide incłusions .
expłained
104
A. Chojecki, l. Telejko
Fe-C-S System
The temary diagram for Fe-C-S system was studied experimentally by several works
[3, 4, 5]. On the basis of thermodynamic analysis of the binary system Fe-S and Fe-C,
the phase equilibria in Fe-C-S was computed, by Ohtani and Nakanishi (6], in U1e iron
rich comer of ternary system . Carbon and sulphur atoms are strongly repulsive in iron
melts. They mutually increase their activity. The presence of carbon increases the liquid
miscibility gap existing in U1e binary Fe-S system and strongly influences tlle distribution
of sulphur between tlle liquid and solid phases. To describe tlle ternary system tlle
two-sublattice model is applied. Ohtani and Nakanishi [6] have conclude tllat S as well
as C may be considered as interstitial atoms in fcc, bcc and Iiquid phases. Then tlle Gibbs
free energy (for tlle l mole of metal atom) o f the liquid or fcc phase may be calculated as
+ yc( l- Yc- Ys)L~e +Y s (l- Ys - Yc)L~e + YcYsLtc+
+ RT[y c In Yc+ Ys In Ys+ (1- Yc- ys) In (1-yc- ys)]
(l)
where:
yi = a/c xi l (l - xc - x),
a/c - number of ferrous atoms per interstitial sublattice site; ale = l for U1e liquid
or fcc phases,
°CFeX - tlle Gibbs energy of l mol of Fe atoms, provided tllat all t11e host lattice sites are
occupied by X atoms,
L - tlle interaction coefficient.
The tllermodynamic parameters in tlle binary Fe-C and Fe-S systems are listed in Table
I [6]. Parameter Lr·c· very important for the ternary system, was calculated by Ohtani
and Nakanishi [6), based upon U1e interaction coefficient, determined experimentally by
Neumann and Schenck [7] , for T = 1600°C and X s~ =O as E~= 4.96. Ohtani and
Nakanishi applied the Ltc value for U1e analysis of iron corner of tlle ternary system. To
study U1e solidification of liquid tllat is rich in sulphur it is indispensabl e to determine U1e
new value of parameter Lr~c · and tlle Iiquidus surface of U1e system.
For U1is aim. U1e partial molar free energy of components have been calculated from
equation (l) , by standard metllod. According to tlle data presented by Schunnann et al [3)
U1e monotectic is fonned at the temperature equal to 1373°C. The tie line describing U1e
equ ilibrium between two liquids (Ll nad L2) and U1e fcc phase joint U1e points 1.45 % S,
3.45 o/c C and 27 .6 '7c S. O.OS o/c C. From U1ese data U1e parameter L~~E equal to
9 1 000 J/mol at l l 00°C have been calcu lated. Assuming t11e proportional dependence for
U1c temperature range ll 00-1600°C one obta.in
L~e~ = -88640 + 130.~4 T · J/mol
(2)
105
Influence of the Chemical Composition ...
Table l. Thennodynamical data for Fe-S, Fe-C and Fe-C-S systems [6]
System
Phase
o o Y _ o Gy _o G gr+ L Fey
FeC
Fe
C
C
fe c
Fe-C
Data
For
L~er
-28 700-5 .6 T
ooL
o GL o osr +L FeL
FeCFeC
2400-16.4 T
L ~eL
-26-25.3 T
oo L _ o Gy
Fe
Fe
1190+14.336 T-1.018 T In T
-0. 0041; T"2
liquid
oGY , - o Gy Fe
FeS
fcc
45 360-18.4 T
l/2
oo gaz
S2
-47 000+42 T
o
LFey
s
ooL _ o GL FeS
Fe
1iquid
Fe-S
112
oog•s +L FeL
s2
S
-120 700+50.8 T
L~eL = L fys +L 2 (l- ys)
59 800-35 .9 T
Lf
s
28 100
L;·~
147 000
L2
Fe-S-C
liquid
..
1300
u
1-
1200
u!
~;
-.
tn!
l
l
l
ul
~·
o '
..... oU! t
noo
1XX)
o
~··-··-
l
l
'' ''
4
..
'
l
(J
o\.o
/!]
o·
l
iv
jo\o .
....
''
'
'
\.
·&
.'o·
l
8
12
'6
20
24
/o S
Fig. l. The liquidus surface and the miscibility gap of Fe-C-S system
0
28
A. Choj ecki, J. Telejko
106
Using this parameter the liquidus surface for Fe-C-S system was calculated. Figure l
presents the cross-section for 0.001 ; 0.05; 0.5 and 1.5 % of carbon . Figure 2a and b
presents tl1e partition coefficient kc and k5 for the fcc phase. For the iron rich melt
l----
a)
31
l
l
2L
V
i
~ 1~
l
oL
i
8
16
12
20
14
18
·)r-----·----------------------------,
b)
•A,
s
Fi g. 2 . Infl uence of the sulphur and carbon contents on the partiti on coefficien t kc (a ) and ks (b) in iron alloys
the sulphur partition coefficient descends with increase of either sulphur or carbon.
The carbon effect is however very little. This tendency changes for sulphur rich Iiquid
(L2). Carbon partition coefficient kc grows with sulphur concentration for both LI and L2
melts. Because of comparati vely high diffusion rate of carbon in fcc phase, its segregation
during solidification may be neg lected and the process may be considered at tl1e
cross- ~ec ti o n of the ternary system. Enriching the liquid with sulphur results, in every
fnjlu ence of the Chemical Composition .. .
107
case, to reach tbe monotectic composition. Then tbe liquid L2 appears. With furtber
cooling it changes composition: sulphur eoncentrarion grows up to 31 % while tbe carbon
descends to 0.05 %. Tbis melt finalty solidifies at 950°C as tbe ternary Fe y-FeS-Fe 3C
eutectic. Brittlness of tbe cast steel in tbe solidification range results from tbe presence
of liquid tilm at tbe grain boundries. The L2 melt creates tbis tilm . !ts length depends on
the quantity and on tbe composition o f L l liquid wben i t re ach tbe monotectic lin e. l f the
fcc phase is formed as tbe primary structure, the carbon int1uence is ratber little. Change
of the initial sulphur eoncentralian changes the quantity of the L2 liquid, but does not
change the temperature of effective solidus, because the L2 formation in Fe-S-C alloys is
unavoiJable.
Fe-Mn-S System
To raise the brittle-ductile transltlon temperature, Iimiting the hot tearing tendency
of steel it is indispensable to introduce tbe element eliminating tlle L2 liquid formation
Juring the solidification process. This is the case of manganese. To describe its effect on
the brittle-ductile transition temperatuce the Fe-Mn-S system has been studied.
A two-sublattice model yields the following expression for tbe Gibbs free energy per mol
of metalatomsof the Fe-Mn-S alloy
+Y s( l -Y s )( YFe L sFe - YMn LMn
s )
(2)
(3)
GMs represents the molar free energy of hypothetical cases of the solution with all
interstitial sites occupied by sulphur.
The free energy of the suitide may be desribed by Redlich-Kister potynominat
where Y~e is the site fraction of iron in sulflde, EG represents the excess free energy
ofmixing.
The partia! molar free energy for liquid, fcc phase and sulfide were obtained
by standard method. lt was assumed tbat interaction energy in tbe ternary system L~eMn
is equal L~e. As the FeS-MnS solution is nearly idea! it was assumed that EG = O.
The data needed for the calculation are listed in Table 2. The liquidus for the Fe-Mn-S
has been calculated. The results are presented on Figure 3.
A. Chojecki, I. Telejko
108
Table 2. Thermodynamic data fortheMn-S and Fe-Mn systems [8, 9]
System
Phase
Value, J/rnol
For
o G~ _ o GL
Mn
Mn
0
L
- 18 640 + 12.55 T
G~nS_o G~n -1/2
oGs
-281 123 + 67 .65 T
2
0 L~nL
ILMnL
LMnL
s = o LMnL(l-y
s
s )+y s
s
1
M n-S
_ o G~
Mn
Mn
L~ns = 74 057
-21 072/T+2004.2-1 9,9331T-
o G 'Y
fcc
= 34 656
-2.4634E-03 l- +3.0385 Tln T
oG S _ o G~ - 1/2 oGgas
MnS
Mn
S2
L
Fe-M
fcc
-273 655 + 66.95 T
L~eMn = o LFeMn+ 1 LFeMn(YFe - YMn)
"L MoL= -3800
Ls
Lv
FeMn = FeMn
'LM'L = 1785
OL V
o Ls
FeMn =
FeMn
- 8178 + 3.788 T
_lLS
ILV
FeMn FeMn
340 + 0.0871 T
~~--~-----L----~----L---J
0001
001
Q1
•;.s
Fig. 3. Liquidus of tbe Fe-Mn-S system
10
Influence of the Chemical Composition ...
109
Manganese limits the process of enrichment of the liquid in sulphur, at the
crystal lization front, because of precipit.ation of the MnS . The Mn/Fe ratio in suitide
Jepends on temperature and the manganese eontent in the liquid phase. Increase in Mn
raises the temperature of the suitide formation, inhibiting the formation of low-melling
liquid.
Conclusions
Brittlness of an alloy in the solidification range results from the presence of the Iiquid
tilm on the grain boundries. At a given temperature its length in steel depends on the
activity of sulphur in the liquid. Carbon itself has a little direct effect, but through its
intluence on the sulphur activity change the liquids miscibility gap and the partion
coeft'ic ient of sulphur. The segregation of sulphur resulls, in every case, that the chemical
composition meets the monotectic line, during the solidit'ication. Then the sulphur rich
liquid L2 appears. It creats the film at the grain boundries. Change of the initial sulphur
concentration changes the quantity of the L2, but does not change the temperatuce
of etlective solidus because the L2 formation in Fe-C-S alloys is unavoidable.
Manganese limits the process of sulphur segregation at the solidit'ication front, because
o f precipit.ation of the sultide. lncrease in Mn eontent raises its melting temperature,
inhibiting t11e formation of the low-melting liquid.
References
l . A. CHOJEC KI. I. TELEJKO: Cast Met. 1992, 5. 3, 163-168.
2. I. TELEJKO. A. CHOJECKI: Trans. [ Int. Symp. of [ran. [ran Sc. and T ech. Univ . Publ. Avhaz
199 1.
.1. E. SCHURMANN. H. D. KUNZE: Giesserei-Forsch. 1969. 21. 65.
4. E. SCHU RMANN. V. NEUBERT: Giesserei-Forsch. 1980. 32, l.
5. E. SCHURMANN, E. SCHAFER: Giesserei-Forsch. 1968, 20. 21.
6. H. OHTANI, T. NAKANISHI: Trans. ISIJ 1986, 26, 665.
7. F. NEUMANN, H. SCHENCK: Arch. Eisenh . 1959, 30, 477
R. L.J. STAFFANSSON: Metali. Trans . 1976, 78 , 131.
9. B.J. LEE, D.N. LEE: CALPHAD 1989, 13, 345.
Streszczenie
WPLYW SKŁADU CHEMICZNEGO STALIWA NA TWORZENIE PĘKNIĘĆ
W ZAKRESIE TEMPERA TUR KRZEPNIĘCIA
chemicznego stałiwa na jego
na gorąco przeprowadzono analizę
termodynamiczną układów równowagi Fe-C-S i Fe-Mn-S. Skłonność
do tworzenia pęknięć jest uzależniona od zachowania siarki. Wpływ
W celu
s kłonność
wyjaśnienia wpływu składu
do tworzenia
pęknięć
!lO
A. Chojecki. J. Telejko
węgla jest pośredni. Oddziałuje on zmieniając aktywność siarki, a przez
to zakres braku mieszałoości fazy ciekłej i ilość siarczków. Mangan
tworzy wysokotopliwe siarczki przed frontem krystalizacji, ograniczając wzbogacenie ciekłej fazy w siarkę.