Transformations - Applications Flashcards
What is annealing?
A general term, most frequently used for heat treatments in the high austenite region where diffusion is very quick even over long distances. High tempearture, for a long time.
What is normalizing?
Low austenite heat treating to produce a uniform fine-grained ferrite/pearlite structure.
Define spheroidizing
Heat treatment just below the eutectoid temperature - it makes a coarse pearlite structure with low strength.
What’s process/stress relief annealing?
Sub-austenitic heat treatment that induces recystalization or relaxes internal stresses via plastic flow or diffusion.
Define hardening (in the context of heat treatment)
Formation of martensite (i.e. quench from austenite)
Define tempering
A softening heat treatment done after hardening - it makes martensite less brittle and tough at the cost of some strength.
Why would you want to anneal a component?
To eliminate solute gradients, relieve residual stresses, refine the grain size, and/ord improve ductility/toughness
How does normalizing differ from annealing?
Normalizing is distinguished by a relatively fast cool to rom temperature - produces a finer grain size than annealing: gives a higher strength steel at the cost of some ductility/toughness.
What’s autenitizing?
Heating steel to the austenite region and allowing it transform completely to austenite.
Why might normalizing be preferable to annealing for a hypereutectic alloy?
Normalizing’s faster cool would prevent carbon from cementite that dissolved during austenisation from diffusing to form a brittle carbide network (which would severely reduce the steel’s toughness)
What is the Hall-Petch effect?
Strengthening a material by decreasing its grain size (i.e. grain boundaries impede dislocation movement)
Why is spheroidizing a subscritical anneal?
Because it takes place below the eutectoid temperature (727C) - there is no change in phase fractions.
How did spheroidizing get its name?
Heating steel for a long time (>24hrs) allows carbon to diffuse significantly and iron-carbide (cementite) to form into spheres (to reduce their surface-to-volume ratio).
What properties does spheroidizing achieve?
Low strength and hardness, substantially increased ductility (w.r.t. pearlite).
Why is martensite heavily distorted?
Because it is supersaturated with trapped carbon.
How does alloying steel with elements like Mn, Ni and Cr affect steel’s hardenability?
It improves it - these elements push the pearlite bay to longer times thereby making it easier to form martensite.
What is austempering?
Heating steel to the austenite phase, then quench and holding to form bainite.
What is case hardening used for?
Getting wear resisting benefits of martensite with the toughness of ferrite/pearlite in the same component.
Give two methods for surface hardening.
Heat and quench a low-hardenability steel, carburization
What is carburization?
Infiltrating carbon into a steel’s surface by heating the steel in a carbon-rich environment.
Give the expression for the diffusion coefficient for carburisation, D
D = D_{0}exp(-Q/RT)
What does the C_x in the expression pertaining to carburization stand for?
Carbon content at depth x
How is carburisation depth defined?
The depth where the carbon content is equal to the mean of the surface and nominal carbon contents
Give an expression for the case depth of a carburized steel, indicating the meaning of each variable
d = sqrt(Dt) d = case depth D = diffusion coeff. (D=D_0exp(-Q/RT)) t = carburization time
