Development of Structures in Iron– Carbon Alloys MME 293: Lecture 9 Development of Structures in Iron– Carbon Alloys Department of MME BUET, Dhaka

Today’s Topics  The critical temperature lines  Microstructural development during slow cooling of steels  Classification of steels  Effect of carbon and other alloying elements on structure and properties of steels Reference: 1. SH Avner, Introduction to Physical Metallurgy, 2nd Ed., Ch. 7, pp.236-243. 2. WD Callister, Jr. Materials Science and Engineering An Introduction,

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

The Critical Temperature Lines Temperature, °C wt. % carbon 0.80 2.00 1130 910 723 Upper Critical Temperatures where transformation of austenite begins during cooling A3  g-a allotropic transformation Acm  precipitation of Fe3C from g Upper-critical Temperature Line A3 Acm g g+Fe3C Lower Critical Temperatures where austenite transformation ends during cooling by forming pearlite isothermally g+a A1 A3,1 Lower-critical Temperature Line For a particular alloy, both upper- and lower-critical temperatures change during heating and cooling. Critical temperature line goes down on cooling, while it goes up on heating. The variation in temperature depends on cooling and heating rate. For slower rates, the two temperatures approach each other. It is found that in actual practice the critical line on heating and the critical line on cooling are not occur at same temperature. The critical line on heating is always higher than the critical line on cooling. The former is denoted by Ac and the later is denoted by Ar.  A for arret (means arrest), C for chauffage (means heating), R for Refroidissement (means cooling), e for equilibrium. If extremely slow rates of heating or cooling are employed then critical temperatures are nearly equal i.e. Ac1 = Ar1 = Ae1 a+P P+Fe3C Steel portion of Fe-Fe3C diagram (d-zone omitted)

Slow Cooling of Steels Eutectoid steels (0.80 % C) Temperature, °C wt. % carbon 0.80 2.00 1130 910 723 Austenite Ferrite + Pearlite Pearlite + Cementite Austenite + Ferrite When an eutectoid steel is cooled slowly, all austenite grains transforms into the eutectoid mixture pearlite, a lamellar or layered structure of ferrite and cementite.  The layers of alternating phases in pearlite are formed in the same way as layered structure of eutectic is formed: by redistribution C atoms between ferrite and cementite by atomic diffusion. 

Slow Cooling of Steels Eutectoid steels (0.80 % C) Austenite (0.80 % C) cannot change into ferrite (0.025 % C) until some of its carbon atoms come out of solution. Therefore the first attempt of transformation is the precipitation of carbon atoms out of austenite to form plates of cementite (6.67 % C). Pearlite microstructure containing alternate layers of ferrite (white) and cementite (black) In the areas immediately adjacent to cementite, the iron is depleted of carbon, and the atoms rearrange themselves to form ferrite. Thus, thin layers of ferrite are formed on each sides of cementite plate. This process continues until all austenite change into pearlite.

Slow Cooling of Steels Hypoeutectoid steels (< 0.80 % C)  Temperature, °C wt. % carbon 0.80 2.00 1130 910 723 Austenite Ferrite + Pearlite Pearlite + Cementite Austenite + Ferrite Allotropic transformation of austenite (FCC) to ferrite (BCC) starts at A3 line at the grain boundaries of austenite.  0.20 Since ferrite can dissolve very little carbon, the extra carbon comes out of solution and the remaining austenite becomes richer in carbon. The carbon content of g gradually moves down and to the right along A3 line. A3 Proeutectoid Ferrite   A1 At 723 C (A1 line), the remaining g grains containing 0.80 % carbon undergoes eutectoid reaction and forms pearlite.

Slow Cooling of Steels Hypereutectoid steels (> 0.80 % C) The Acm line is a solvus line along which carbon solubility of austenite decreases with T. Below Acm, the extra carbon comes out of g along the grain boundary as Fe3C precipitates.  Temperature, °C wt. % carbon 0.80 2.00 1130 910 723 Austenite Ferrite + Pearlite Pearlite + Cementite + Cementite  1.0 Acm Proeutectoid Cementite During their growth, Fe3C combine each other to form one single grain (just like joining two soap bubbles).   A3,1 As Fe3C precipitates, carbon content of the remaining g gradually moves down and to the left along Acm line. At 723 C (A3,1 line), the remaining g containing 0.80 % carbon undergoes eutectoid reaction and forms pearlite.

Slow Cooling of Steels Variation in steel structures with carbon content Dead soft steel (C0.01%) Ferrite grains only Low carbon steel (C0.1%) Mostly ferrite with a few pearlite Medium carbon steel (C0.4%) Almost equal ferrite and pearlite Eutectoid steel (C0.8%) Pearlite grains only Hypereutectoid steel (C0.9%) Pearlite grains surrounded by cementite network

Problem For an Fe-0.35C alloy at a temperature just below the eutectoid, determine the following: [a] The fraction of ferrite and cementite phases [b] The fraction of ferrite and pearlite [c] The fraction of eutectoid ferrite

Solution [a] Apply lever rule for a tie line that extends all the way across the Ferrite + Cementite phase field. Then, WF = 100 (6.67 – 0.35) / (6.67 – 0.025) = 95 % WFe3C = 100 – 95 = 5 % [b] Use tie line that extends only up to the Ferrite + Pearlite phase field. Then, WF = 100 (0.80 – 0.35) / (0.80 – 0.025) = 58 % WP = 100 – 58 = 42 % [c] The amount of ferrite calculated in [a] is the total (i.e., proeutectoid plus eutectoid) ferrite content, while that calculated in [b] is the proeutectoid ferrite content only. Thus, The eutectoid ferrite content = 95 – 58 = 37 %.

Classification of Steels Steelmaking process basic oxygen process, electric process, etc. De-oxidation practice rimmed, capped, semi-killed, killed Product form wire, bar, plate, sheet, strip, tubing, or structural shapes Finishing method cold drawn, cold rolled, hot rolled, extruded, etc. Application tool steels, bearing steels, spring steels, etc. Metallography hypoeutectoid, eutectoid, hypereutectoid Composition plain carbon, low alloy, high alloy, high-strength-low-alloy (HSLA)

Classification of Steels Based on carbon content Dead soft (0.03–0.10 % C) Steel wires, rivets, chain, sheet, strip, welded pipe Mild (0.10–0.35 % C) Steel rolled plate, structural shapes, gears, forgings Medium carbon (0.35–0.65 % C) Steel connecting rods, crane hooks, shafts, axles, gears, rotors, rails High carbon (0.65–2.0 % C) Steel screw drivers, saws, drills, dies, hammers, wrenches, punches, chisels

The Influence Of Other Alloying Elements Additions of other alloying elements (Cr, Ni, Ti, etc.) bring about rather dramatic changes in the binary iron–iron carbide phase diagram. One of the important changes is the shift in position of the eutectoid with respect to temperature and to carbon concentration. Thus, other alloy additions alter not only the temperature of the eutectoid reaction but also the relative fractions of pearlite and the proeutectoid phase that form.