FeS-MnS phase relationships in the presence of excess iron
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400 . ,i . i . f . I . I . FeS 10 20 30 40 50
FeS
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20 30 I , I
Mn
L 40 50 I 9 I
60 I
70 I
MnS
i f i = ~-(Mn.F__e)S
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~"-(Fe,Mn)S
Mn
Fig. 4-Liquidus phase r e l a t i o n s h i p s ( F e - F e S - M n S - M n ) . Binary invariants: ei-989~ (1262 K); e2-1110~ (1383K); e3-1230~ (1503 K);Pl-1474~ (1747 K); m 1 and m2-~1580~ (1583 K). T e r n a r y invariantS and c r i t i c a l points: P1"-997~ (1270 K); P2-~1400~ (1673 K). M t andM2-~1600~ (1873K); K-1370~ (1643 K); R-~1510~ (1783 K). [Pt = 66 wt pct Fe, 0.5 pet Mn, and 34 pct S , v s 36.5 pet S in s t o i c h i o m e t r i e FeS. K = 76 wt pct Fe, 4 pet Mn, 21 pet $4]. METALLURGICAL TRANSACTIONS B
O=
Fe
Mn
)~
Fig. 5-Sulfide e q u i l i b r i a with solid iron. (a) Solid (Mn,Fe)S and liquid, i r o n - r i c h sulfide above the p e r i t e c t i c t e m p e r a ture. (The FeS c o r n e r has been magnifieid.) (b) Solid (Mn,F__ee)S, solid (Fe, Mn._)S, and solid metal. The metal must con- tain 1.0 pct Mn. This also explains why the Mn/S ratio is not a good index of manganese r e quirements. Thus, the common statement that the ratio should be ~20/1 does not hold at high sulfur levels. (Manganese requirements in excess of the amount calculated from the above equation may be r e quired if surface scaling is encountered in reheating operations and an oxide-sulfide liquid is introduced into the subscale region.) The tie lines between solid metal and the MrSr i c h e r solid sulfide are of special interest to the metallurgist since they define subsolidus inclusion compositions in steel. Complete data have been elusive; however, we may note the qualitative relationships in Fig. 6. At 1000~ (1273 K), the tie line of the previous figures connects points a and a'. Resulfurized steels, containing 1 wt pct manganese and quenched from the above temperature, p o s s e s s sulfides close to 95MrS5FeS; 23 and of course, the manganese-xich side equilibrates pure MrS with Mn. Thus, although the solubility of FeS in MrS may be high (up to 70 wt pct), the iron content is expected to be low in the sulfide inclusions of most steels.
474-VOLUME 7B, SEPTEMBER 1976
The following conclusions may be drawn from the present investigations: 1) FeS has a troilite, NiAs (B8t-type of crystal structure in the presence of metallic iron. Therefore, the FeS-Mrs phase diagram containing FeS with the pyrrhotite crystal structure is not applicable to steelmaking situations. 2) The maximum solid solubility limits in the FeSMrs system (with no excess metallic iron) a r e 7 wt pct MnS in FeS and 79 wt pet FeS in Mrs. The invariant temperature of this s y s t e m is at 1110 ~: 3~ (1383 ~= 3 K) when stoichiometric FeS and MrS a r e used as the starting m a t e r i a l s . 3) In the FeS-MrS system (with excess metallic iron), the maximum solid solubility limits are 7.5 wt pct MnS in FeS and 73.5 wt pct FeS in Mrs. The peritectic invariant point is located at 997 + 3~ (1270 K). The equilibrium phases at this temperature are Fe (
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