Phase Diagram. Today s Topics. The iron carbon phase diagram Details of iron iron carbide phase diagram Classification of iron carbon alloys

Transcription

1 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, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 02

2 Allotropic Transformation of Pure Iron Liquid δ (delta) iron (BCC) γ (gamma) iron (FCC) nonmagnetic α (alpha) iron (BCC) nonmagnetic α (alpha) iron (BCC) magnetic Time Cooling Curve of Pure Iron Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 03 Iron Carbon Phase Diagram Iron Iron Carbide, An interstitial compound, containing 6.67 wt.% carbon. The compound dissociates into iron and carbon, if given the opportunity: = + 3C is a metastable compound, because such decomposition takes a very long time at room temperature. Because of the presence of, this diagran is not a true equilibrium diagram Carbon Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 04

3 Iron Iron Carbide Phase Diagram Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 06

4 Delta (δ) iron Liquid Austenite (γ ) rrite (α) Cemenite Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 07 ferrite + ferrite L + L L + cementite + cementite cemenite ferrite + cementite Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 08

5 The Invariant Reactions Te emperature, C δ -iron + L Peritectic Point The Peritectic 1492 C cooling Delta-iron 0.10%C + Liquid 0.50%C Austenite 0.18%C heating Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 09 Te emperature, C liquid + cementite Eutectic ti Point The Eutectic 1130 C cooling Liquid 4.3%C ( Austenite 2.0%C + Cementite 6.67%C ) heating Ledeburite (an eutectic mixture) Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 10

6 ferrite + cementite Eutectoid Point The Eutectoid 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, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 11 The Major Phases delta iron liquid ferrite pearl lite ledeburite cemen nite Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 12

7 Iron Iron Carbide Phase Diagram ferrite ferrite + pearlite + ferrite + liquid pe earlite liquid liquid + cementite + cementite pearlite + cementite cem menite Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 13 Characteristics of Major Phases ferrite rrite (α) ledeburite An interstitial solid solution of carbon dissolved in BCC α-iron. Carbon solubility wt.% max. at 723 C, 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. Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 14

8 Austenite (γ) An interstitial solid solution of carbon dissolved in FCC γ-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. Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 15 Cementite () An interstitial intermetallic compound of iron carbide with an orthorhombic structure. Its chemical formula is 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. cemen nite Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 16

9 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: 120,000 psi TS, 20 % elong. in 2 inch, RC 20 hardness. lite pearl Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 17 ledeburite Ledeburite 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 wt. transformed % carbonladeburite. Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 18

10 Carbon Solubility in Iron Austenite rrite FCC structure BCC structure 4 iron atoms/unit cell 2 iron atoms/unit cell Dense atomic packing (74 %) Loose atomic packing (68 %) Iron expands when changes from to ferrite during cooling. Why does absorb more carbon than ferrite? Radius of an average carbon atom is 0.70 (x10-8 ) cm. Radius of maximum void in FCC 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 has a greater solubility of C atoms than ferrite Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 19 Classification of Iron Carbon Alloys C Steels are -C alloys that pass through zone during cooling liquid Temperature, ferrite ferrite + pearlite hypo - eutectoid eutectoid 0.8 STEELS hyper - eutectoid pearlite + cementite 2.0 hypo - eutectic eutectic 4.3 CAST IRONS hyper - eutectic Rashid, DMME, BUET, 2011 MME 291, Lec 07: Iron Iron Carbide Phase Diagram P 20

11 Next Class MME 291: Lecture 08 Development of Structure in Steels