MME 291: Lecture 07 Iron Iron Carbide Phase Diagram Prof. A.K.M.B. Rashid 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, 2 nd Ed., Ch. 7. Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 02 1
Allotropic Transformation of Pure Iron Cooling Curve of Pure Iron Te emperature, C 1535 1401 910 768 Liquid δ (delta) iron (BCC) γ (gamma) iron (FCC) nonmagnetic α (alpha) iron (BCC) nonmagnetic α (alpha) iron (BCC) magnetic Time Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 03 Iron Carbon Phase Diagram Iron Carbide An interstitial compound,, containing 6.67 wt.% carbon. The compound dissociates into iron and carbon. A metastable compound, because such decomposition takes a very long time at room temperature. This phase diagram is not a true equilibrium diagram Iron Carbon Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 04 2
1535 1492 1401 910 1130 723 Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 05 0.10 0.50 0.18 0.025 0.80 2.0 4.3 0.008 6.67 Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 06 3
Delta (δ) iron Liquid Austenite (γ ) rrite (α) Cemenite Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 07 ferrite + ferrite L + L L + cementite + cementite cemenite ferrite + cementite Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 08 4
Invariant Reactions Te emperature, C δ -iron + L Peritectic Point The Peritectic Reaction @ 1492 C cooling Delta-iron 0.10%C + Liquid 0.50%C Austenite 0.18%C heating Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 09 Invariant Reactions Te emperature, C liquid ledeburite ( + cementite ) Eutectic ti Point The Eutectic Reaction @ 1130 C cooling Liquid 4.3%C ( Austenite 2.0%C + Cementite 6.67%C ) heating Ledeburite (an eutectic mixture) Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 10 5
Invariant Reactions pearlite ( ferrite + cementite ) Eutectoid Point The Eutectoid Reaction @ 723 C cooling Austenite 0.8%C wt. %( carbon rrite 0.025%C + Cementite 36.67%C C ) heating Pearlite (an eutectoid mixture) Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 11 delta iron liquid ferrite pearl lite ledeburite cemen nite Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 12 6
ferrite ferrite + pearlite + ferrite + liquid pe earlite liquid + ledeburite liquid + cementite ledeburite pearlite + cementite ledeburite + cementite cem menite Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 13 rrite (α) Definition and Properties of Structures An interstitial solid solution of carbon dissolved in BCC α-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. Austenite (γ) An interstitial t solid solution o of carbon dissolved in FCC γ-iron. o 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. Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 14 7
Definition and Properties of Structures Cementite () An interstitial intermetallic compound of iron carbide with an orthorhombic structure. Its chemical formula is and contains 6.67 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. Pearlite The eutectoid mixture of fine plate-like lamellar mixture of ferrite and cementite. Formed from that contains 0.80 wt.% carbon during slow cooling at 723 C. Average properties: 120,000000 psi TS, 20 % elong. in 2 inch, RC 20 hardness. Ladeburite The eutectic mixture of and cementite. Formed from liquid that contains 4.30 wt.% carbon during slow cooling at 1130 C. Not stable below 723 C, where of ladeburite transformed into pearlite. The structure is then called transformed ladeburite. Carbon Solubility in Iron Austenite FCC structure 4 iron atoms/unit cell Dense atomic packing (74 %) Radius of the max. void 0.52 (x10-8 ) cm rrite BCC structure 2 iron atoms/unit cell Loose atomic packing (68 %) Radius of the max. void 0.36 (x10-8 ) cm Iron expands when changes from to ferrite during cooling. Radius of an average carbon atom is 0.70 (x10-8 ) cm. The distortion of ferrite lattice by carbon atom is much greater than in case of Austenite has a greater solubility of carbon atom than ferrite Rashid, DMME, BUET. 2009 MME 291, Lec 07: P 16 8
Classification of Iron Carbon Alloys Steels are -C alloys that pass through zone during cooling ferrite ferrite + pearlite hypo - eutectoid eutectoid 0.8 STEELS pearlite + cementite hyper - eutectoid 2.0 liquid hypo - eutectic eutectic pearlite + cementite 4.3 CAST IRONS hyper - eutectic Next Class MME 291: Lecture 08 Structural Changes in -C Alloys 9