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23 Cards in this Set
- Front
- Back
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Work hardening (cold forming)
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•Atoms can slip across one another
•Defects assist slip •Working moves defects and atoms so they won’t slip further |
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Alloying
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•Adding other atoms to host atoms
•If alloying atoms are near the size of the host atoms → alloying atoms can impede the motion of dislocation thus strengthen the metal |
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Composite
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•Strengthening metals by matrix composites
•Reinforcing metal by fibers impedes the dislocation motion |
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Heat Treatment
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•Heating and cooling cycles applied to metals to change their mechanical properties
•This process transforms the crystal structure of metals |
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How does alloying strengthen metals?
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Alloying strengthens metals by adding elements (or atoms) to distort crystals
•Distorted crystals can’t slip easily |
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What is the rationale behind H.T.?
Why it is applied only to metals and not to other materials? |
When metals are subjected to heating and rapid cooling cycles, their crystal structure can be transformed from one type such as BCC to another harder crystal type such as BCT.
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Definitions and reasons for HT
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oHeating and cooling cycles applied to metals to change their mechanical properties
•Hardening, toughening, reducing brittleness |
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What is the difference between equilibrium and non-equilibrium transformation?
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oEquilibrium
•When the phase transformation occurs at a slow cooling rate oNon-equilibrium •Rapid cooling |
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What is the difference between interstitial and substitutional solid solutions?
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o in interstitial format two kinds of atoms combine in one type of space lattice due to smaller size of carbon atoms compared to iron atoms
o when the size of solute atoms is near the size of the host atom (Cr. Vs Fe) then the solution will be in substitutional solution |
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What is the difference between single phase and multi-phase structures in steel?
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Single-phase: BCC, FCC, or HCP
Multi-phase: BCC combined with HCP |
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Ferrite
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•Trace of carbon (.02%)
•Very soft, easily deformed, magnetic |
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Austenite
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•More carbon
•Unstable, only exists above 1350 •Susceptible to atomic slip and has moderate strength |
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Pearlite
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•C = .8% and slow cooling, austenite → pearlite
•Two-phase structure: ferrite and cementite |
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Cementite
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•Increase in carbon (over .8%) extra carbon can’t be dissolved in iron
•Compound called cementite is formed •Hard and brittle •Depending on size and shape, impeded atomic slip |
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Martensite
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•Target product of HT
•Has BCT structure •Trapped extra carbon atoms results in strained structure •Very ____ •Must be ______ |
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Basic requirements for hardening of steel
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Sufficient carbon content
Sufficient heat (austenite temp zone) Sufficient heat time Quenching |
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Definition of Austenitizing and effect of quenching on carbon atoms
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Heating to temperature above 1350 F and keeping it at that temperature for sufficient amount of time
Quenching is rapid cooling casues carbon atoms to be trapped in iron crystal structure |
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Effect of quenching rate on hardness
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Rapid quenching → increases hardness
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Applications of selective (differential) hardening and diffusion hardening (case hardening)
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Selective hardening – hardening of selected area of a part
Case hardening – applied when there is insufficient carbon content |
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How steel can be hardened by carburizing? (U-shaped wire experiment)
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Steel is packed in charcoal granules to produce CO gas at temp above austenite line
CO gas reacts with surface of steel and diffuses the carbon atoms into steel |
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Annealing
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Involves raising temperature to austenite region, soaking, followed by slow cooling
•Recrystallization occurs •In cold rolling, metal becomes hard due to atomic slip and also brittle. •Used to soften a brittle cold-rolled steel. |
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Stress relieving
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•Applied to weldments and cold-rolled shapes
•Involves heating below austenite region (no recrystallization) •Thermally induces dislocations and defects to remove strains |
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Tempering
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•Low-temp HT applied immediately after quenching to avoid cracking and distortion.
•Improves toughness w/o significant reduction in hardness, reduces brittleness •Some of the carbon atoms are diffused from strained martensite structure and some ferrite structure develops. |
