7-Engineering Alloys

Atoms retain original neighbors carbon atoms are

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Unformatted text preview: etain original neighbors. Carbon atoms are trapped in the lattice. Martensite lattice is distorted. HARDNESS AND STRENGTH OF MARTENSITE Hardness Increases with carbon content which decreases ductility and toughness Other factors: •  dislocation density •  packet size •  lath size ISOTHERMAL DECOMPOSITION Rapid cooling to below the eutectoid temperature Transformation occurs with time at a constant temperature. STEPS IN ISOTHERMAL TRANSFORMATIONS 1.  Austenitize 2.  Rapidly cool to isothermal temperature 3.  Allow time to transform the specimen ISOTHERMAL TRANSFORMATIONS ISOTHERMAL DIAGRAMS By repeating the steps and using different transformation temperatures a C-curve relationship occurs. PEARLITE TRANSFORMATION Occurs between 723ºC and ~550ºC. Time and temperature dependent. Finer pearlite spacing occurs as temperature decreases. Maximum growth rate (fastest formation) occurs at the shortest time in the C-curve. BAINITE TRANSFORMATION Occurs between approximately 550ºC and 250ºC. Non-lamellar structure of ferrite and cementite. Intermediate between martensite and pearlite. Carbon transforms by diffusion. Austenite to ferrite by shear. NON - EUTECTOID ISOTHERMAL DIAGRAMS Hypoeutectoid Produces a proeutectoid ferrite C-curve ahead of the pearlite curve. Decreasing carbon decreases the time for transformation. Hypereutectoid Produces a proeutectoid cementite C-curve ahead of the pearlite curve. CONTINUOUS COOLING TRANSFORMATION DIAGRAMS In the real world, steel is generally continuously cooled from the austenitizing temperature to room temperature. The transformation of austenite occurs over a range of temperatures rather than a single isothermal temperature. Connection between Isothermal Diagram, Continuous Cooling Diagram, and Cooling Rate Curves C-T DIAGRAM The final microstructure, as a result of transformation over a range of temperatures, is very complex. The continuous cooling diagram transformation start and finish lines are shifted to longer times and lower temperatures in relation to the isothermal diagr...
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This document was uploaded on 02/27/2014 for the course NANO 108 at UCSD.

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