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IE370Lecture5 - 5.1 Introduction Chapter 05 Heat Treatment...

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9/13/2008 1 Chapter 05: Heat Treatment 5.1 Introduction s Heat treatments are processes of controlled heating and cooling to purposefully alter a material’s structure and properties s Changes in properties can be introduced with no change in shape s Heat treatments are integrated with other processes to obtain effective results 5.2 Processing Heat Treatments s Most heat treatments are thermal processes that increase strength s Processing heat treatments are used to prepare the material for fabrication s Equilibrium phase diagrams are often used to predict resulting structures s Annealing is a common heat treatment process b May be used to reduce strength and hardness b Removes residual stresses b Improves toughness b Restores ductility b Refines grain size Annealing s Full annealing b Hypoeutectoid steels are heated to convert the grain structure to homogenous single-phase austenite, then control cooled s The cooling results in coarse pearlite with excess ferrite s Results in soft and ductile steel b Hypereutectoid steels undergo a similar process but the structure will be coarse pearlite with excess cementite b Full anneals are time and energy consuming processes
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9/13/2008 2 Normalizing s Normalizing is heating steel to a temperature higher than in annealing b More cost effective than annealing b Cooled in air s Normalizing vs. annealing b In normalizing, cooling will be different in different locations b Properties will vary between the surface and interior in normalized steel b Lower cost of normalizing is justified if uniform properties are needed Additional Heat Treatments s Process anneal b Recrystallization is induced after a material has been cold worked to reduce strain hardening effects b Induces a change in size, shape, and distribution s Stress-relief anneal b Reduces residual stresses in casting, welded assemblies, and cold-formed products b Materials are heated and then slow cooled s Spheroidization b Objective is to produce a structure in which all of the cementite is in the form of small spheroids or globules dispersed throughout a ferrite matrix Heat Treatments for Nonferrous Metals s Nonferrous metals do not have significant phase transitions s Heat treated for three purposes b Produce a uniform, homogenous structure b Provide a stress relief b Induce recrystallization s In castings that have been cooled too rapidly, homogenization can be achieved by heating to moderate temperatures and then holding 5.3 Heat Treatments Used to Increase Strength s Six mechanisms for increasing strength b Solid-solution strengthening s Base metal dissolves other atoms as substitutional solutions or interstitial solutions b Strain hardening s Increases strength by plastic deformation b Grain size refinement s Metals with smaller grains tend to be stronger b Precipitation hardening s Strength is obtained from a nonequilibrium structure b Dispersion hardening s Dispersing second-phase particles through a base material b Phase transformations s Heated to form a single phase at an elevated temperature
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IE370Lecture5 - 5.1 Introduction Chapter 05 Heat Treatment...

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