Chapter 10 – Heat Treatment of Steels

Chapter 10 – Heat Treatment of Steels -...

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Chapter 10 – Heat Treatment of Steels Heat Treating – defined as the controlled heating and cooling of metals for the primary purpose of altering their properties (strength, ductility, hardness, toughness, machinability, etc) Can be done for Strengthening Purposes (converting structure to martensite) Can be done for Softening and Conditioning Purposes (annealing, tempering, etc.) First, a basic review of metallurgy!
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1.5 The Nature of Metals: Characterized by: 1. Valence electrons of 1,2 or 3 – see periodic table 2. Primary bonding between electrons called metallic bonding: Valence electrons not “bonded” to particular atom but shared and free to drift through the entire metal 3. Properties include: good conductors of electricity and heat, not transparent, quite strong yet deformable!
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Crystalline structures (i.e. metals) atoms are arranged in unit cells – 4 common cells shown above
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How do Metal Crystals Fail?? Answer: Slip due to dislocations
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How does crystal Structure FCC, BCC, HCP effect: Strength?? Ductility/Toughness?? Stiffness??
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Theoretical Strength of Metal Strength, Su should be approximately E/10 if based on atomic bond. E/10 = 3,000 ksi for steel >>> actual Su which is between approximately 30 ksi to 200 ksi Why????? DEFECTS!!!
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Types of Defects: Surface Defects Grain boundaries Point Defects Vacancy, substitutional (atom replaces host), interstitial (atom squeezes in between host) , impurity Line Defects Edge dislocations, screw dislocations = good defect!
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Defects in crystals. (a) Vacancies–missing atoms. (b) Foreign (solute) atom on interstitial and substitutional sites. (c) Line Defect = A dislocation–an extra half-plane of atoms. (d) Grain boundaries. Alloying and heat treating Little impact on strength Course GB = weak, Fine GB = strong and ductile Greatest impact on strength and ductility!!
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What is the most significant defect? Answer: The line defect (edge dislocation or screw dislocation)
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(a) Making a dislocation by cutting, slipping and rejoining bonds across a slip plane. (b) The atom configuration at an edge dislocation in a simple cubic crystal. The configurations in other crystal structures are more complex but the principle remains the same. Line Defects – How metals fail: Slip due to line defects (aka dislocations)
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An initially perfect crystal is shown in (a). The passage of the dislocation across the slip plan, shown in the sequence (b), (c) and (d), shears the upper part of the crystal over the lower part by the slip vector b . When it leaves the crystal has suffered a shear strain γ . Slip due to line defects (aka dislocations)
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A screw dislocation. The slip vector b is parallel to the dislocation line S S .
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Dislocation motion causes extension Millions of dislocations produce the noticeable “yield marks” seen below in a simple tensile specimen:
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How to Strengthen Metals: Key: prevent dislocations from moving through crystal Key: prevent dislocations from moving through crystal
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This note was uploaded on 01/30/2011 for the course EMSE 103 taught by Professor Ggh during the Spring '10 term at Case Western.

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Chapter 10 – Heat Treatment of Steels -...

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