Heat Treatment & Materials — Comprehensive Study Pack Flashcards

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TTT diagram

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A chart showing the start and finish times of isothermal transformations of austenite at different temperatures. It identifies regions for pearlite, bainite and martensite formation and guides heat-treatment schedules.

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Martensite

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A hard, supersaturated phase formed by the diffusionless transformation of austenite during rapid cooling. Martensite is characterized by high hardness and internal stresses and is tempered to improve toughness.

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Bainite

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A microstructure formed at intermediate isothermal temperatures between pearlite and martensite; it offers a combination of high hardness and good toughness. Bainite can be classified as upper or lower depending on transformation temperature.

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Tempering

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A heat treatment performed after quenching to reduce residual stresses and brittleness of martensite by heating below A1. Different tempering temperatures adjust hardness, ductility, and precipitation of carbides.

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Austempering

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An isothermal heat treatment that produces bainite by quenching into a bath and holding at a temperature where bainite forms. It yields a tough, moderately hard microstructure with reduced distortion compared to martensite formation.

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Normalizing

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Heating steel above the austenitizing temperature (A3 or A1 depending on composition) followed by air cooling to produce a fine, uniform grain structure. It improves mechanical properties and refines cast or forged parts.

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Spheroidizing

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A soft-annealing process that converts lamellar carbides into spheroidal particles by prolonged heating just below A1. This improves machinability and ductility of high-carbon steels.

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Annealing

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A family of heat treatments aimed at softening material, relieving stresses, and improving ductility by heating and slow cooling. Types include full anneal, stress-relief, recrystallization, and spheroidizing.

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Precipitation hardening

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Strengthening by solution treating an alloy, quenching, and ageing to precipitate fine particles that impede dislocation motion. Widely used in Al and some Cu/Ni alloys to raise yield and tensile strength.

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Carbide

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A compound of carbon with a metal (e.g., TiC, V4C3, Cr7C3) that is extremely hard and wear-resistant. Carbides improve cutting and wear resistance but can reduce toughness if present as large, brittle phases.

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Hardenability

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A measure of how deep into a section martensite can form during quenching; influenced by carbon and alloying elements and section size. Measured using the Jominy end-quench test and Burns diagrams for material selection.

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Jominy test

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A standardized end-quench test where one end of a cylindrical specimen is water-quenched and hardness is measured vs. distance. The resulting curve indicates steel hardenability and guides heat-treatment choices.

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Carburizing

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A thermochemical surface treatment that diffuses carbon into low-carbon steel at high temperature to form a hard, wear-resistant case while retaining a tough core. Typical processes: gas, pack, or plasma carburizing followed by quenching and tempering.

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Nitriding

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Introduction of nitrogen into the surface to form hard nitrides (e.g., FeN) at temperatures below A1, producing a hard case without quenching. It yields good wear and fatigue resistance with minimal distortion.