7-Engineering Alloys

C t diagram critical cooling rate the slowest rate

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Unformatted text preview: am. C - T DIAGRAM Critical cooling rate The slowest rate that will still produce a fully hardened martensitic structure. ANNEALING Full Anneal Hypo Hyper Microstructure "Heating & furnace cooling" "Heat in austenite region about 40ºC above the austenite - ferrite boundary." "Heat about 40ºC above eutectoid temperature." "Proeutectoid ferrite or cementite and coarse pearlite." ANNEALING Process Anneal" "Heat hypoeutectoid steel below the eutectoid temperature." " "Stress relief partially softens cold worked low carbon steel." ANNEALING Normalizing - austenitize and cool in still air." "Proeutecoid ferrite and fine pearlite for " "hypoeutectoid steels." " " " "Purposes:" " "Refine grain size" " "Reduce segregation" " " " "More economical than full " " "annealing." ANNEALING Spheroidizing - heat treat just below eutectoid temperature" " " " "Ferrite and spherical Fe3C." " "Purposes:" " "Improve isotropic properties" TEMPERING Changes the structure of martensite to make it softer, more ductile and increases toughness." " " Assists in obtaining equilibrium by heating below the eutectoid temperature." STAGES OF TEMPERING 1. "Epsilon-carbide formation below 200ºC." " 2. "Transformation of retained austenite between 200 and 300ºC." " 3. "Formation of iron carbide between 200 and 300ºC." TEMPERING AND MECHANICAL PROPERTIES Hardness decreases " " Strength decreases " " Ductility increases " " Toughness increases " Quenched and Tempered Steel Direct Quench to Tempering Temperature Typically very undesirable due to formation of bainite rather than tempered martensite. LOW ALLOY STEELS Plain carbon steels have limitations:" 1.  " 2.  " 3.  " 4.  cannot be strengthened beyond 100 ksi" not deep hardenable" low corrosion and oxid...
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