MSE-226 Engineering Materials

Transcription

1 MSE-226 Engineering Materials Lecture-2 IRON-IRON CARBIDE PHASE DIAGRAM

2 Classification of Metal Alloys Metal Alloys Ferrous Nonferrous Steels <1.4 wt% C Steels <1.4wt%C Cast Irons wt% C Cast Irons wt%c 1600 d 1400 T(ºC) L g austenite g +L 1148ºC 4.30 Eutectic: L+Fe 3 C a ferrite ºC Eutectoid: 0.76 g +Fe 3 C a +Fe 3 C Fe 3 C cementite (Fe) C o, wt% C 2

3 PURE IRON Carbon content < 0.1 % PURE IRON, T m =1535 o C Iron is an allotrophic metal, which means that it can exist in more than one type of lattice structure depending on the temperature Pure iron (ingot) is ductile and malleable. Small additions of other elements (C, Mn, Mo, Cr, Ni, etc.) enhance the properties significantly STEEL

4 STEELS Steels are iron-carbon alloys that may contain appreciable concentrations of other alloying elements,i.e. Cr, Mo, Co, W.

5 Classification of STEELS Steels are classified according to carbon content as follows; 1- Low carbon steels (<0.25 wt.% carbon) 2-Medium carbon steels (0.25 < wt%c < 0.6) 3-High carbon steels (0.6 < wt%c < 1.4) Subclasses also exist within each group according concentration of other alloying elements; 1- Plain carbon steels: Contain only residual concentrations of impurities other than carbon and a little Mn 2-Alloy steels: More alloying elements are intentionally added Examples: SAE 1020 steel: Plain carbon steel, low carbon steel SAE 1080 steel: Plain carbon steel, high carbon steel SAE 4340 steel: Alloy steel, medium carbon steel

6 Properties and Applications of STEELS 1- Low carbon steels (<0.25 wt.% carbon) Properties: Relatively soft and weak but have outstanding ductility and toughness. Machinable and weldable Typical applications: automobile body components, structural shapes (I-beams), sheets used in pipelines, buildings, 2-Medium carbon steels (0.25 < wt%c < 0.6) Properties: Can be heat treated to improve mechanical properties. Heat treated alloys are stronger than low carbon steels but sacrifice of ductility and toughness. Typical applications: Railways wheels and tracks, gears, crankshafts and structural applications required combination of high strength, wear resistance and toughness. 3-High carbon steels (0.6 < wt%c < 1.4) Properties: Hardest, strongest and yet least ductile class of steels. They are always used in hardened and tempered condition. Tool and die steels are high carbon steels which contain Cr, V, W and Mo as alloying elements. Alloying elements form very hard and wear resistance carbides (Cr 23 C 6, V 4 C 3 and WC) Applications: Drills, saw, lathe and planer tools

7 Iron-Iron Carbide Equilibrium Phase Diagram 1600 d T(ºC) 1400 L 1200 g g +L 1148ºC L+Fe 3 C 1000 austenite 4.30 Eutectic: a ºC g +Fe 3 C Fe 3 C ferrite Eutectoid: a +Fe 3 C cementite 400 (Fe) C o, wt% C Phases and phase mixtures present in iron alloys; Ferrite (α); Cementite (Fe 3 C); Pearlite (ferrite + cementite); Austenite (γ); d-ferrite; Ledeburite (austenite + cementite)

8 Definition and Properties of Phases 1) Ferrite : a-iron, Solid Solution, max. Carbon solubility 0.022%wt. at 727 o C BCC structure, SOFT 2) Cementite : Iron carbide(fe 3 C), contains 6.67% wt. C Orthorhombic structure, HARD and BRITTLE 3) Pearlite : Phase mixture (ferrite+cementite), Lamellar structure, contains ~0.8% wt. C Produced from austenite decomposition 4) Austenite : g-iron, Solid solution, stable at higher temperatures (>727 o C) Max. Carbon solubility is 2.14%wt. at 1147 o C, FCC structure HIGH TOUGHNESS 5) Ledeburite: Eutectic phase mixture(austenite+fe 3 C), seen in cast irons Contains 4.3 %wt. Carbon, forms at 1147 o C 6) d-ferrite : Solid solution, max. carbon solubility is 0.1%wt. At 1493 o C

9 Invariant reactions in Fe-Fe 3 C Phase diagram 1) At 1493 o C, 0.18 %wt C (PERITECTIC REACTION) Liquid(l, 0.5%C)+d-ferrite(d,0.1%C) cooling heating Austenite(g, 0.18%C) 2) At 1147 o C, 4.30 %wt C (EUTECTIC REACTION) Liquid(l, 4.30 %C) cooling heating Austenite(g, 2.14 %C) + Cementite(Fe 3 C,6.67%C) LEDEBURITE 3) At 727 o C, 0.77 %wt C (EUTECTOID REACTION) Austenite(g, 0.77 %C) cooling heating Ferrite(a, %C) + Cementite(Fe 3 C,6.67%C) PEARLITE

10 Carbon solubility in BCC-ferrite and FCC-Austenite BCC FCC Octahedral sites Tetrahedral sites

11 Alloying steel with alloying elements T eutectoid changes: C eutectoid changes:

12 Eutectoid Steel

13 Transformation of Austenite to pearlite in Eutectoid Steel under equilibrium conditions (very slow cooling or annealing) Upon very slow cooling transformation of austenite to pearlite occurs by diffusion of carbon atoms(time is required for carbon diffusion). So, this type of transformation is called DIFFUSIONAL (Time Dependent) TRANSFORMATION.

14 Microstructure of eutectoid steel (~0.77wt.%C) Pearlite (ferrite and cementite)

15 Hypoeutectoid Steel

16 Microstructure of Hypoeutectic Steel (~0.3wt.%C) X200 Primary ferrite Pearlite (ferrite and cementite layers cannot be resolved) X500

17 Hypereutectoid Steel

18 Microstructure of Hypereutectoid steel(1.4 wt%c) Pearlite (cementite+ferrite) Primary cementite!!! The microstructure of annealed hypereutectoid steel consists of coarse lamellar pearlite areas surrounded by a network of proeutectoid cementite. Because this proeutectic cementite network is brittle and tends to be a plane of weakness, annealing should never be a final heat treatment for hypereutectoid steels.

19 Microstructure-Property Relationship of Steels The mechanical properties of an alloy depend upon the properties of the phases and the way in which these phases are arranged to make up the structure. In steel ferrite is relatively soft with low tensile strength While cementite is hard with very low tensile strength Pearlite (ferrite + cementite) greater tensile strength than that of ferrite and cementite. The approximate tensile strength of annealed hypoeutectoid steels may be determined by the proportion of ferrite and pearlite present: Approximate tensile strength (psi) = [40,000 (wt.%ferrite) +120,000 (wt.% pearlite)] / 100

20 Effect of Carbon on Mechanical Properties

21 HOMEWORK-I (1) For an annealed (cooled in equilibrium conditions after austenitization) hypoeutectoid steel; (i) (ii) (iii) Determine the composition of the steel (C wt.%) if it contains 22% secondary ferrite at room temperature. Determine the amount of primary phase present in the steel. Draw the room temperature microstructure. (2) Why hypereutectoid steels shouldn t be annealed?