1 1

2 2  Austenite - The name given to the FCC crystal structure of iron.  Ferrite - The name given to the BCC crystal structure of iron that can occur as α or δ.  Cementite - The hard, brittle ceramic-like compound Fe 3 C that, when properly dispersed, provides the strengthening in steels.  Pearlite - A two-phase lamellar microconstituent, containing ferrite and cementite, that forms in steels cooled in a normal fashion or isothermally transformed at relatively high temperatures. Section 11.9 The Eutectoid Reaction

3 3 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The Fe- Fe 3 C phase diagram ( a portion of the Fe-C diagram). The vertical line at 6.67% C is the stoichiometric compound Fe 3 C.

4 4 Figure Growth and structure of pearlite: (a) redistribution of carbon and iron, and (b) photomicrograph of the pearlite lamellae (2000). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH )

5 5 Calculate the amounts of ferrite and cementite present in pearlite. Example 11.5 SOLUTION Since pearlite must contain 0.77% C, using the lever rule: Example 11.5 Phases and Composition of Pearlite

6 6 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The evolution of the microstructure of hypoeutectoid and hypoeutectoid steels during cooling. In relationship to the Fe-Fe 3 C phase diagram.

7 7  Controlling the Amount of the Eutectoid  Controlling the Austenite Grain Size  Controlling the Cooling Rate  Controlling the Transformation Temperature  TTT diagram - The time-temperature-transformation diagram describes the time required at any temperature for a phase transformation to begin and end.  Isothermal transformation - When the amount of a transformation at a particular temperature depends on the time permitted for the transformation. Section Controlling the Eutectoid Reaction

8 8

9 9 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The time-temperature-transformation (TTT) diagram for an eutectoid steel.

10 Figure (a) Upper bainite (gray, feathery plates) ( 600). (b) Lower bainite (dark needles) ( 400). (From ASM Handbook, Vol. 8, (1973), ASM International, Materials Park, OH )

11 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The effect of transformation temperature on the properties of an eutectoid steel.

12 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The time-temperature-transformation (TTT) diagram for an eutectoid steel.

13 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure (a) The unit cell of BCT martensite is related to the FCC austenite unit cell. (b) As the percentage of carbon increases, more interstitial sites are filled by the carbon atoms and the tetragonal structure of the martensite becomes more pronounced.

14  Martensite - A metastable phase formed in steel and other materials by a diffusionless, athermal transformation.  Displacive transformation - A phase transformation that occurs via small displacements of atoms or ions and without diffusion. Same as athermal or martensitic transformation.  Tempering - A low-temperature heat treatment used to reduce the hardness of martensite by permitting the martensite to begin to decompose to the equilibrium phases. Section The Martensitic Reaction and Tempering

15 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The effect of carbon content on the hardness of martensite in steels.

16 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure Effect of tempering temperature on the properties of and eutectoid steel.

17 Figure Tempered martensite in steel ( 500). (From ASM Handbook, Vol. 9, Metallography and Microstructure (1985), ASM International Materials Park, OH )

18 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure The eutectoid portion of the Fe-Fe 3 C phase diagram (for Problems 11.78, 11.86, 11.87, and 11.88)

19

20