Iron – Iron Carbide Phase Diagram MME 293: Lecture 08 Iron – Iron Carbide Phase Diagram Department of MME BUET, Dhaka

Today’s Topics  The iron – carbon phase diagram  Details of iron – iron carbide phase diagram  Classification of iron – carbon alloys Reference: 1. SH Avner. Introduction to physical metallurgy, 2nd Ed., Ch. 7.

Allotropic Transformation of Pure Iron Liquid d (delta) iron (BCC) g (gamma) iron (FCC) a (alpha) iron (BCC) nonmagnetic magnetic 1535° 1401° 910° 768° Temperature, °C Time Cooling Curve of Pure Iron

Iron –Carbon Phase Diagram Iron Carbide, Fe3C  An interstitial compound, containing 6.67 wt.% carbon.  The compound dissociates into iron and carbon, if given the opportunity: Fe3C = Fe + 3C  Fe3C is a metastable compound, because such decomposition takes a very long time at room temperature.  Because of the presence of Fe3C, this diagran is not a true equilibrium diagram. Fe3C 6.67 Temperature, °C Iron Carbon wt. % carbon

Iron – Iron Carbide Phase Diagram Temperature, °C 1535° 1401° 1492° 910° 723° 1130° Fe wt. % carbon Fe3C

Temperature, °C wt. % carbon 0.10 0.50    0.18   2.0 4.3   0.025 0.80 0.008 6.67   Fe wt. % carbon Fe3C

Temperature, °C wt. % carbon Delta (d) iron Liquid Austenite (g ) Ferrite (a) Cemenite Fe Fe3C

Temperature, °C wt. % carbon L austenite + L L + cementite austenite austenite + ferrite austenite + cementite ferrite cemenite ferrite + cementite Fe Fe3C

The Invariant Reactions Temperature, °C wt. % carbon d -iron + L Peritectic Point  austenite Fe Fe3C The Peritectic Reaction @ 1492 C Delta-iron 0.10%C + Liquid 0.50%C Austenite 0.18%C cooling heating

 Temperature, °C wt. % carbon liquid austenite + cementite Fe Fe3C Eutectic Point liquid  austenite + cementite Fe Fe3C The Eutectic Reaction @ 1130 C Liquid 4.3%C ( Austenite 2.0%C + Cementite 6.67%C ) Ledeburite (an eutectic mixture) cooling heating

 Temperature, °C wt. % carbon austenite ferrite + cementite Eutectoid Point  ferrite + cementite The Eutectoid Reaction @ 723 C Austenite 0.8%C ( Ferrite 0.025%C + Cementite 6.67%C ) Pearlite (an eutectoid mixture) cooling heating Fe Fe3C

The Major Phases Temperature, °C wt. % carbon Fe Fe3C delta iron liquid lede-burite austenite cemenite pearlite ferrite Fe Fe3C

Iron – Iron Carbide Phase Diagram Temperature, °C wt. % carbon liquid austenite + liquid liquid + cementite austenite austenite + ferrite austenite + cementite pearlite cemenite ferrite ferrite + pearlite pearlite + cementite Fe Fe3C

Characteristics of Major Phases Temperature, °C wt. % carbon Ferrite (a)  An interstitial solid solution of carbon dissolved in BCC a-iron.  Carbon solubility – 0.025 wt.% max. at 723 C, 0.008 wt.% min. at 0 C.  The softest structure that appears on the iron – iron carbide diagram.  Average properties: 40,000 psi TS, 40 % elong. in 2 inch, < RC 0 or < RB 90 hardness. lede-burite ferrite Fe Fe3C

Austenite (g) Temperature, °C wt. % carbon Fe Fe3C  An interstitial solid solution of carbon dissolved in FCC g-iron.  Carbon solubility – 2.00 wt.% max. at 1130 C, 0.80 wt.% min. at 723 C.  Not stable at room temperature; can be made stable under certain conditions.  Average properties: 150,000 psi TS, 10 % elong. in 2 inch, RC 40 hardness, high toughness. austenite Fe Fe3C

Cementite (Fe3C) Temperature, °C wt. % carbon Fe Fe3C  An interstitial intermetallic compound of iron carbide with an orthorhombic structure.  Its chemical formula is Fe3C and contains 6.67 wt.% carbon.  The hardest and brittle structure that appears on the iron – iron carbide diagram.  Average properties: 5,000 psi TS, high compressive strength. cemenite Fe Fe3C

Pearlite Temperature, °C wt. % carbon Fe Fe3C  The eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite.  Formed from austenite that contains 0.80 wt.% carbon during slow cooling at 723 C.  Average properties: 120,000 psi TS, 20 % elong. in 2 inch, RC 20 hardness. pearlite Fe Fe3C

Ledeburite Temperature, °C wt. % carbon Fe Fe3C lede-burite  The eutectic mixture of austenite and cementite.  Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C.  Not stable below 723 C, where austenite of ladeburite transformed into pearlite. The structure is then called “transformed ladeburite.” Fe Fe3C

Carbon Solubility in Iron Austenite  FCC structure  4 iron atoms/unit cell  Dense atomic packing (74 %) Ferrite  BCC structure  2 iron atoms/unit cell  Loose atomic packing (68 %)  Iron expands when changes from austenite to ferrite during cooling. Why does austenite absorb more carbon than ferrite ?  Radius of an average carbon atom is 0.70 (x10-8) cm.  Radius of maximum void in FCC austenite is 0.52 (x10-8) cm, while that in BCC ferrite is 0.36 (x10-8) cm.  The distortion of ferrite lattice by carbon atom is much greater than in case of austenite.  Austenite has a greater solubility of C atoms than ferrite

Classification of Iron –Carbon Alloys Temperature, °C liquid Steels are Fe-C alloys that pass through austenite zone during cooling austenite eutectic eutectoid ferrite ferrite + pearlite pearlite + cementite 0.8 2.0 4.3 STEELS CAST IRONS hypo - eutectoid hyper - eutectoid hypo - eutectic hyper - eutectic