I RON AND S TEEL The transition from the Bronze Age to the Iron Age was ushered in by two developments: Growing scarcity of copper and/or tin Increased processing temperatures Iron and Steel Characteristics? What is the difference?

I RON VS S TEEL Bronze was an Alloy of Cu –Arsenic or Tin most common alloying impurity Steel is an Alloy of Iron –Carbon is the most common alloying impurity Iron and Carbon is probably the most useful combination in the history of Society Terms you should know –Forging – beating or hammering a material into shape –Casting – Pouring a liquid into a mold

F ORMS Low Carbon 0-0.2% –Wrought Iron –Pure, Ductile, typically about as strong as Bronze –High melting point (>1500°C) Medium Carbon % –Steel –Very strong, hard, forgable, –1000X harder than pure Fe –High melting point (>1400°C) High Carbon % –Cast Iron or Pig Iron –Low Melting Point, Brittle, Not Forgable –Can only be Cast

The Iron-Iron Carbide Phase Diagram Wrought Iron Steel Cast Iron

Why is Steel Strong? Two Important phases of iron-  - Ferrite (BCC)  - Austenite (FCC) How much carbon can you dissolve in the Iron? Ferrite very little (<0.02%) Austenite Lots (up to 2.1%) Why is that important? When you heat Steel to 1000°C what phase do you have? Austenite with lots of Carbon When you cool it what does the carbon do? It can precipitate into Carbide particles. Strains crystal makes dislocation motion difficult (hardens) Ferrite Austenite Steel

7 I RON -C ARBON (F E -C) P HASE D IAGRAM 2 important points - Eutectoid (B):  + Fe 3 C - Eutectic (A): L  + Fe 3 C Adapted from Fig. 9.24, Callister & Rethwisch 8e. Fe 3 C (cementite) L  (austenite)  +L+L  +Fe 3 C   +   (Fe) C, wt% C 1148ºC T(ºC)  727ºC = T eutectoid 4.30 Result: Pearlite = alternating layers of  and Fe 3 C phases 120  m (Adapted from Fig. 9.27, Callister & Rethwisch 8e.) 0.76 B    A L+Fe 3 C Fe 3 C (cementite-hard)  (ferrite-soft)

8 Fe 3 C (cementite) L  (austenite)  +L+L  + Fe 3 C  L+Fe 3 C  (Fe) C, wt% C 1148ºC T(ºC)  727ºC (Fe-C System) C0C H YPOEUTECTOID S TEEL Adapted from Figs and 9.29,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.) Adapted from Fig. 9.30, Callister & Rethwisch 8e. proeutectoid ferrite pearlite 100  m Hypoeutectoid steel  pearlite              

9 Fe 3 C (cementite) L  (austenite)  +L+L  + Fe 3 C  L+Fe 3 C  (Fe) C, wt% C 1148ºC T(ºC)  727ºC (Fe-C System) C0C H YPOEUTECTOID S TEEL        sr W  = s/(r + s) W  =(1 - W  ) R S  pearlite W pearlite = W  W  ’ = S/(R + S) W =(1 – W  ’ ) Fe 3 C Adapted from Figs and 9.29,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.) Adapted from Fig. 9.30, Callister & Rethwisch 8e. proeutectoid ferrite pearlite 100  m Hypoeutectoid steel

10 H YPEREUTECTOID S TEEL Fe 3 C (cementite) L  (austenite)  +L+L  +Fe 3 C  L+Fe 3 C  (Fe) C, wt%C 1148ºC T(ºC)  Adapted from Figs and 9.32,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.) (Fe-C System) 0.76 C0C0 Fe 3 C             Adapted from Fig. 9.33, Callister & Rethwisch 8e. proeutectoid Fe 3 C 60  m Hypereutectoid steel pearlite

11 Fe 3 C (cementite) L  (austenite)  +L+L  +Fe 3 C  L+Fe 3 C  (Fe) C, wt%C 1148ºC T(ºC)  H YPEREUTECTOID S TEEL (Fe-C System) 0.76 C0C0 pearlite Fe 3 C     x v VX W pearlite = W  W  = X/(V + X) W =(1 - W  ) Fe 3 C’ W  =(1-W  ) W  =x/(v + x) Fe 3 C Adapted from Fig. 9.33, Callister & Rethwisch 8e. proeutectoid Fe 3 C 60  m Hypereutectoid steel pearlite Adapted from Figs and 9.32,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in-Chief), ASM International, Materials Park, OH, 1990.)

12 E XAMPLE P ROBLEM For a 99.6 wt% Fe-0.40 wt% C steel at a temperature just below the eutectoid, determine the following: a)The compositions of Fe 3 C and ferrite (  ). b)The amount of cementite (in grams) that forms in 100 g of steel. c)The amounts of pearlite and proeutectoid ferrite (  ) in the 100 g.

13 S OLUTION TO E XAMPLE P ROBLEM b)Using the lever rule with the tie line shown a) Using the RS tie line just below the eutectoid C  = wt% C C Fe 3 C = 6.70 wt% C Fe 3 C (cementite) L  (austenite)  +L+L  + Fe 3 C  L+Fe 3 C  C, wt% C 1148ºC T(ºC) 727ºC C0C0 R S C Fe C 3 CC Amount of Fe 3 C in 100 g = (100 g)W Fe 3 C = (100 g)(0.057) = 5.7 g

14 S OLUTION TO E XAMPLE P ROBLEM ( CONT.) c) Using the VX tie line just above the eutectoid and realizing that C 0 = 0.40 wt% C C  = wt% C C pearlite = C  = 0.76 wt% C Fe 3 C (cementite) L  (austenite)  +L+L  + Fe 3 C  L+Fe 3 C  C, wt% C 1148ºC T(ºC) 727ºC C0C0 V X CC CC Amount of pearlite in 100 g = (100 g)W pearlite = (100 g)(0.512) = 51.2 g

F ORMS OF IRON Wrought Iron –Very little carbon (<0.2%) –Difficult to slow the dislocations since carbon is dissolved –Soft, malleable, easily wrought Steel –Medium carbon ( %) –Can dissolve carbon in Austenite but it wants to precipitate upon cooling –Properties depend on how fast you cool –Strain from Carbides greatly strengthens the steel –Must control both the amount of carbon and the tempering Cast Iron –High Carbon ( %) –Forms lots of carbide phase –Make the material brittle (too much of a good thing)

W HY IS THE H EAT TREATMENT IMPORTANT ? For Steel if you heat it to 1000°C make Austenite –Slow Cool If you cool slowing make a mixture of Alpha iron and carbide particles Give it time for the Carbon to form particles Natural Composite Slightly stronger than Bronze –Fast Cool or Quench Not enough time to transform to Alpha Iron Form Martensite (very hard phase of iron and carbon) Excess Carbon strains martensite (brittle) –Tempering Heat again to move carbon around and restore ductility

17 T RANSFORMATIONS & U NDERCOOLING For transf. to occur, must cool to below 727ºC (i.e., must “undercool”) Eutectoid transf. (Fe-Fe 3 C system):  + Fe 3 C 0.76 wt% C wt% C 6.7 wt% C Fe 3 C (cementite) L  (austenite)  +L+L  +Fe 3 C  L+Fe 3 C  (Fe) C, wt%C 1148ºC T(ºC)  ferrite 727ºC Eutectoid: Equil. Cooling: T transf. = 727 º C TT Undercooling by T transf. < 727  C Adapted from Fig. 9.24,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.)

18 T HE F E -F E 3 C E UTECTOID T RANSFORMATION Coarse pearlite  formed at higher temperatures – relatively soft Fine pearlite  formed at lower temperatures – relatively hard Transformation of austenite to pearlite: Adapted from Fig. 9.15, Callister & Rethwisch 8e.        pearlite growth direction Austenite (  ) grain boundary cementite (Fe 3 C) Ferrite (  )  For this transformation, rate increases with [T eutectoid – T ] (i.e.,  T). Adapted from Fig , Callister & Rethwisch 8e. 675ºC (  T smaller) 0 50 y (% pearlite) 600ºC (  T larger) 650ºC 100 Diffusion of C during transformation      Carbon diffusion

19 Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.) G ENERATION OF I SOTHERMAL T RANSFORMATION D IAGRAMS The Fe-Fe 3 C system, for C 0 = 0.76 wt% C A transformation temperature of 675ºC T = 675ºC y,y, % transformed time (s) %pearlite 100% 50% Austenite (stable) T E (727ºC) Austenite (unstable) Pearlite T(ºC) time (s) isothermal transformation at 675ºC Consider:

20 Eutectoid composition, C 0 = 0.76 wt% C Begin at T > 727ºC Rapidly cool to 625ºC Hold T (625ºC) constant (isothermal treatment) Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) A USTENITE - TO -P EARLITE I SOTHERMAL T RANSFORMATION %pearlite 100% 50% Austenite (stable) T E (727ºC) Austenite (unstable) Pearlite T(ºC) time (s)      

time (s) T(ºC) Austenite (stable) 200 P B TETE 0% 100% 50% A A B AINITE : A NOTHER F E -F E 3 C T RANSFORMATION P RODUCT Bainite: -- elongated Fe 3 C particles in  -ferrite matrix -- diffusion controlled Isothermal Transf. Diagram, C 0 = 0.76 wt% C Adapted from Fig , Callister & Rethwisch 8e. Adapted from Fig , Callister & Rethwisch 8e. (Fig from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.)  Fe 3 C (cementite) 5  m  (ferrite) 100% bainite 100% pearlite

22 Spheroidite: -- Fe 3 C particles within an  -ferrite matrix -- formation requires diffusion -- heat bainite or pearlite at temperature just below eutectoid for long times -- driving force – reduction of  -ferrite/Fe 3 C interfacial area S PHEROIDITE : A NOTHER M ICROSTRUCTURE FOR THE F E -F E 3 C S YSTEM Adapted from Fig , Callister & Rethwisch 8e. (Fig copyright United States Steel Corporation, 1971.) 60  m  (ferrite) (cementite) Fe 3 C

23 Martensite: --  (FCC) to Martensite (BCT) Adapted from Fig , Callister & Rethwisch 8e. (Fig courtesy United States Steel Corporation.) Adapted from Fig , Callister & Rethwisch 8e. M ARTENSITE : A N ONEQUILIBRIUM T RANSFORMATION P RODUCT Martensite needles Austenite 60  m x x x x x x potential C atom sites Fe atom sites Adapted from Fig , Callister & Rethwisch 8e. Isothermal Transf. Diagram  to martensite (M) transformation.. -- is rapid! (diffusionless) -- % transf. depends only on T to which rapidly cooled time (s) T(ºC) Austenite (stable) 200 P B TETE 0% 100% 50% A A M + A 0% 50% 90%

24  (FCC)  (BCC) + Fe 3 C M ARTENSITE F ORMATION slow cooling tempering quench M (BCT) Martensite (M) – single phase – has body centered tetragonal (BCT) crystal structure Diffusionless transformation BCT if C 0 > 0.15 wt% C BCT  few slip planes  hard, brittle

25 T EMPERED M ARTENSITE tempered martensite less brittle than martensite tempering reduces internal stresses caused by quenching Adapted from Fig , Callister & Rethwisch 8e. (Fig copyright by United States Steel Corporation, 1971.) tempering decreases TS, YS but increases %RA tempering produces extremely small Fe 3 C particles surrounded by  Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.) 9  m YS(MPa) TS(MPa) Tempering T (ºC) %RA TS YS %RA Heat treat martensite to form tempered martensite

26 S UMMARY OF P OSSIBLE T RANSFORMATIONS Adapted from Fig , Callister & Rethwisch 8e. Austenite (  ) Pearlite (  + Fe 3 C layers + a proeutectoid phase) slow cool Bainite (  + elong. Fe 3 C particles) moderate cool Martensite (BCT phase diffusionless transformation) rapid quench Tempered Martensite (  + very fine Fe 3 C particles) reheat Strength Ductility Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends

S O HOW DO YOU MAKE I RON ? Start with –Iron Oxide (either Fe 2 O 3 or Fe 3 O 4 ), –Limestone (CaCO 3, Oyster Shells) and –Carbon (Charcoal) Layer these and light on fire and use bellows to get hot 2C +O 2 => 2CO Fe 2 O 3 +3CO => 2 Fe +3CO 2 (makes Iron) Now have to get rid of sand from iron oxide CaCO 3 => CaO +CO 2 (make lime) CaO + SiO 2 => CaSiO 3 (glass slag) Get a mixture called a Bloom

E ARLY W ROUGHT I RON : B LOOMERIES Bloom Iron –Make a Bloom by heating the mixture we just described –Beat on the bloom with a hammer to separate the iron from the slag –Left with pure wrought (low carbon) iron How do you add carbon to make steel? –Melting point too high until 1700’s –Heat Iron in Carbon rich air – Carborization –Carbon diffuses into the iron making outer layer of steel –Skilled craftsmen

C AST I RON M AKING (B LAST F URNACE ) Method –Same ingredients (Iron Ore, Charcoal and Limestone) –Heat hotter and collect liquid iron from bottom –Cast Iron is also called Pig Iron –Chinese skipped Bloomery stage and just made cast iron from ancient blast furnaces –Not seen in Europe until 1500’s Now you have too much carbon –Use Finery Forge to remelt pig iron, –oxidize carbon and silicon –Make bloom and beat it, create wrought (bar) iron

B LAST F URNACE This course contains copyright materials that are used solely for instructional purposes within this course. Copying, printing or distribution for other purposes is prohibited Early Blast Furnace

H ISTORY OF I RON AND S TEEL 3500BC Beads in Ancient Egypt for iron –From Meteor (nickel content) First Iron Production 3000BC Syria and Mesopotamia Hittites mass produced Iron BC –Bloom Iron in small batch process –Use water power to beat blooms China starts Iron age later around 700BC –Used the first blast furnaces to make cast iron –Also used Finery forges to make wrought iron 300BC

H ISTORY C ONTINUED By 1400’s needs increased –Church bells, cannons etc –Blast furnace developed (Cast Iron) By 1500’s Japan is making the strongest Steel in the world –Samuri Sword take a week just make the iron –Use Magnetite (black sand Fe2O3) and charcoal –Broad range of carbon, separate it by feel and sound –Make the center from low carbon ductile steel –Then add high carbon hard/brittle steel to outside –Create a composite and the best steel for several hundred years

H ISTORY C ONTINUED 1588 Queen Elizabeth limits use of Timber –By 1700’s have a timber famine (charcoal) Why not use Coal? –Too much Sulfur weakens steel –Coke the coal, Drives off impurities (thanks to beer) 1709 Darby England –Low sulfur coke –Cast iron become cheap –Start of Industrial revolution Over 200 years income increases 10X Population increases 6X Machine based economy Free market, rule of law, no trade barriers

H ISTORY CONTINUED Puddling –Invented in 1784 –Stir Molten Pig (Cast) iron in oxidizing atmosphere –Observe it “come to nature” –Gather in a puddling ball Replaced by the Bessemer Furnace 1855 –Blow Air through the melt –Rapidly remove carbon and silicon –Initially didn’t work tough to remove just the right amount –Instead decided to remove it all and add carbon back –Steel very fast (in 20 minutes) and efficient Today we use 1950’s Process –Basic Oxygen Process (BOP) –Inject pure Oxygen

THE IRON AGE Bloom Iron video – – – Iron – Steel –