Phase Diagram Evaluations: Section II The C-Fe (Carbon-Iron) System by Ho Okamoto ASM International Equilibrium Diagram The stable equilibrium phases of the Fe-C system at ambient pres- sure are (1) the gas, g; (2) the liquid, L; (3) bcc (~iFe); (4) fcc (yFe), or austenite; (5) bcc (~tFe), or ferrite; and (6) hexagonal (C), or The number of experimental and theoretical publications on the graphite. Orthorhombic Fe3C, or cementite, is a metastable phase. Fe-C phase diagrams and related subjects is virtually unlimited However, the metastable Fe-Fe3C system is discussed in this sec- because of the unqueslionable importance of Fe-C alloys in all as- tion together with the stable Fe-C system. The alphabetical desig- peas of human activities. The details of the stable and metastable nations at special points in the phase diagrams are adopted from phase diagrams of the Fe-C system, especially on the Fe-rich side, [Hansen] forA to S except for/and R, and from [79Sch2] forR to are known much better than any other binary systems with similar Wexcept forS. Figure 1 shows a conventional Fe-C and Fe-Fe3C complexity. However, there are still substantial areas where the double diagram, and Table 1 summarizes the reaction types and phase diagram has not been well established---in the temperature, coordinates of special points. composition, and pressure ranges not related directly to iron and steel making. In the present evaluation, the assessed stable Fe-C (graphite) and Experimental Methods metastable Fe-Fe3C (cementite) equilibrium phase diagrams for 0 The experimental methods used to determine the phase bounda- to 25 at.% C are based on thermodynamic calculations reported ries are not cited here in complete detail because the sources are by [79Schl] and [84Oht]. Comparison of these calculated results numerous. The primary methods used are thermal analysis with experimental data indicates that the uncertainty of the dia- [1897Rob, 1899Rob, 00Bak, 04Car, 04Hey, 12Meu, 13Hon, grams is approximately • 2 ~ and • 0.1 at.%. The phase dia- 14Rue, 14Rue, 14Rum, 16Bar, 20Rue, 21Mau, 21Rue, 26E11, grams in the remaining composition range and at pressures other 29And, 30Har, 34Piw, 37Adc, 38Wel, 60Buc, 64Ver, 82Chi, than 1 atm are less certain. One of the reasons for this uncertainty 83Chi], chemical analysis of equilibrated samples [07Cha, is that even the transition temperatures of pure Fe are not well de- 09Gut, 11Han, llRue, llRuf, 12Wit, 14Rue, 29Sch, 31Soh, termined. Although the reference temperature scale varied among 42Gur, 49Sta, 52Chi, 52Kit, 55Lin, 55Mat, 55Tur, 59Smil, quoted reports, the experimental data are cited in this evaluation 61Ben, 61Schl, 61Sch2, 62Smi, 63Mor, 64Cab, 66Nak, 71Wad, as given in the original papers; attempts to correct temperatures 85Has], metallography [1897Rob, 1899Rob, 09Gut, 10Goe, would have been meaningless in many instances due to more l lWar, 14Sal, 17Tsc, 37Meh], composition dependence of physi- grave problems, such as unknown impurities in alloys, cool- cal properties such as magnetism and thermodynamic activities ing/heating rate effect, and uncertainty in the chemical composi- [49Dij, 51Dar, 52Fal, 58Gen, 58Pet, 59Heu, 59Sin, 60Sin, tion of specimens. 67Swa, 69Swa, 70Ban], temperature dependence of physical Table 1 Special Points of the Assessed Fe-C Phase Diagram Composition of the Temperature, Reaction respective phases, at.% C *C Reaction type Stable Fe-C (graphite) system g ~. L ....................................................................... 0 2862 Boiling L *~, 8Fe ................................................................... 0 1536 Melting bFe ~, ~,Fe................................................................ 0 1392 Allotropic yFe ~ ctFe ............................................................... 0 911 Allolropic L + (6Fe) ~ (yFe)..................................................... 2.43 0.40 0.74 1493 Peritectic (?Fe) ** (o.Fe) + (C) ................................................. 2.97 0.096 100 740 Eutectoid L ** (yFe)+ (C) ........................................................ 17.1 9.06 100 1153 Eutectic g ~ C (graphite)....................................................... 100 3826 Sublimation Metastable Fe-Fe3C (cementite) system (~Fe) ~ (aVe) + Ff3C ............................................... 3.46 0.104 25 727 Eutectoid L ~ (yFe) + Fe3C ..................................................... 17.3 9.23 25 1147 Eutectic L,,* Fe3C ................................................................. 25 1252 Congruent Journal of Phase Equilibria Vol. 13 No. 5 1992 543 Section II: Phase Diagram Evaluations Weight Percent Carbon 0 1 2 3 4 5 6 1600J I I .... -,. J,, r ..... I .... ,' ,I 4A 1536"C~.~/0.4~ 14oo-~, .... "3~lt. ~ . la02"c1~ ~ -"-.-AA L + C(graphite) ............ 1252"C t (~~47 rc- ........... q.3 % 800. ...................... ~740"C ~q~C .......................... x 49Dij * 55Mat v 61Ben 9 66Nak 600 o 49Sta ! 55Tur 9 61Schl 4 69Swa v 51Bor2 | 58Pet 9 61Sch2 a} 70Ban 13 20Rue [] 51Dar [] 59Heu 9 62Smi @71Wad o 29Sat 9 52Chi $ 59Sin ~ 63Mor ~, 82Chi a 31Seh 9 52Kit a] 59Smil 9 64Cah 9 83Chi # 42Gur 9 55Lin A 60But ~ 64Ver + 85Has 400 ~ ......... ~ r ......... ~ ...... ~ , I 0 5 10 15 20 25 Fe Atomic Percent Carbon Atomic Percent Carbon 0 5 10 15 20 25 1600 ......... i, I ........ I ' I ...... t ...... J' ' F ......... [ I ''I .... i ...... 1536"Ci / (d~Fe): ~4oo~Ir "~ L C(graphite) (m04*1392"C0 " F~ 1252"C 1200 0, '~5~.................................................. cO 1000 911"C // F-~ (912"C)800 t0-62-f . *'/ 0.021N~/ 740oC - - ~7-)-6................................................................... 727~ 600- - (aFe) ....... ~ ......... ~ ......... [ ....... , , r ..... T .~ r ] ...... 0 2 3 4 S 6 Fe Weighk Percent Carbon 1 AssessedFe-C phase diagramfrom 0 to 25 aL% Fe. Stable Fe-C (graphite)and metastableFe-Fe3C (cementite) equilibria. Temperatures in parentheses are recommendedby [Massalsk]2]. properties such as electrical resistance and magnetism [13Hon, Review 15Sal, 16Hon, 18Iit, 23Asa, 23Ber, 23Kon, 25Kay, 26Sta, 27Ess, 27Honl, 27Hon2, 27Hon3, 29Sat, 37Meh, 50Wer, 67Fil], and ki- Toward the end of the 19th century, [1899Rob] and [00Bak] pro- nelies of precipitation [51Bor1, 51Bor2]. Because of the over- posed the first phase diagram representations of the Fe-C system. whelming number of data points, only selected points are shown Although their diagrams are crude by present standards (for ex- in the assessed diagram. ample, the (SFe) phase was unknown), the key features of the 544 Joumal of Phase Equilibria Vol. 13 No. 5 1992 Phase Diagram Evaluations: Section I! Weight Percent Carbon 3ooo~~.-~~~~0 3 4 6 8 10 12 2862'C B.P.!...... ===================== ........... G [] ./ G + (c) 2500 .~" 2380"C ? 2000 L L) o) 1536"CI T + (c) +~ (1538"C)i 1500 ~(la94"c)1392"C I153"C [-~ i000 911"C (TFe) + (C) 740"C 2.97 20Rue * 55Mat 9 61Sehl @ 71Wad 500 O 29Sat ! 55Tur 9 62Smi A 82Chi a 31Sch u 58Pet ~ 63Mor v 83Chi <,,w~ (aFe) + (c) # 42Gut n 59Heu 9 64Cah + 85Has 49Sta ~ 59Sin ~ 64Ver 52Chi ^ 60But 9 66Nak 9 52Kit v 61Ben 9 70Ban o4 o ~ ?o ' ~o ~g 40 Fe Atomic PereenL Carbon r-~2 Assessed Fe-C (graphite) phase diagram. Temperatures in parentheses are recommended by [Massalski2]. Table 2 Calculated Fe-C Special Points [79Sdal] [84Oht] [85Gus] Temperature, Composition, Temperature, Composition, Temperature, Composition, Point ~ at.% C *C at.% C ~ at.%C H ................................................... 1493 0.40 1494 0.38 1495 0.42 J .................................................... 1493 0.74 1494 0.79 1495 0.83 B ................................................... 1493 2.43 1494 2.30 1495 2.41 E'. ................................................. 1153 9.06 1154 8.95 1154 9.36 C'. ................................................. 1153 17.13 1154 17.28 1154 19.87 E ................................................... 1147 9.23 1147 9.03 1148 9.46 C ................................................... 1147 17.29 1147 17.45 1148 20.04 P'. .................................................. 736 0.15 740 0.096 738 0.083 S'. .................................................. 736 3.12 740 2.97 738 3.09 P .................................................... 727 0.16 727 0.104 727 0.088 S .................................................... 727 3.43 727 3.46 727 3.44 double equilibria for the Fe-C and Fe-Fe3C systems were discov- primarily on [72Chi], has been sewing as one of the standard Fe- ered by these investigators. All aspects of the Fe-C diagrams C phase diagrams until now. However, [79Agr] and [84Oht] shown in Fig. 1 had been revealed when [32Goe] reviewed this pointed out errors in the thermodynamic model of [72Chi]. system. Later, the positions of phase boundaries were refined by [82Kub] accepted phase diagrams derived thermodynamically by numerous experimental reports, as mentioned above. Thermody- [79Schl] and [79Sch2]. A more recent review by [86Ban] was l~amic approaches to represent the phase diagrams by [50Wer], based on relatively old information. [530ar], [54Hil], [55Hill, [61Ben], [68Wad], [71Hat], [72Chi], Transformation Temperatures of Fe [74Bas], [78Sch], [79Agr], [79Schll, [84Oht], and [85Gus] clari- fied several uncertain features that experiments had failed to de- The boiling point temperature of Fe is 2862 ~ [Massalski2]. fine. According to IPTS-68 [Melt], the melting point ofbFe is 1535 ~ Earlier literature on the Fe-C system was listed in a review article (secondary reference point), whereas [82Swa] recommended by [36Eps]. [I-lansen]