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Ch. 05 - Chapter 05 Heat Treatment 5.1 Introduction Heat...

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1/15/2010 1 Chapter 05: Heat Treatment 5.1 Introduction Heat treatments are processes of controlled heating and cooling to purposefully alter a material’s structure and properties Changes in properties can be introduced with no change in shape Heat treatments are integrated with other processes to obtain effective results
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1/15/2010 2 5.2 Processing Heat Treatments Most heat treatments are thermal processes that increase strength Processing heat treatments are used to prepare the material for fabrication Equilibrium phase diagrams are often used to predict resulting structures Annealing is a common heat treatment process May be used to reduce strength and hardness Removes residual stresses Improves toughness Restores ductility Refines grain size Annealing Full annealing Hypoeutectoid steels are heated to convert the grain structure to homogenous single-phase austenite, then control cooled The cooling results in coarse pearlite with excess ferrite Results in soft and ductile steel Hypereutectoid steels undergo a similar process but the structure will be coarse pearlite with excess cementite Full anneals are time and energy consuming processes
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1/15/2010 3 Normalizing Normalizing is heating steel to a temperature higher than in annealing More cost effective than annealing Cooled in air Normalizing vs. annealing In normalizing, cooling will be different in different locations Properties will vary between the surface and interior in normalized steel Lower cost of normalizing is justified if uniform properties are needed Additional Heat Treatments Process anneal Recrystallization is induced after a material has been cold worked to reduce strain hardening effects Induces a change in size, shape, and distribution Stress-relief anneal Reduces residual stresses in casting, welded assemblies, and cold-formed products Materials are heated and then slow cooled Spheroidization 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
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