Thermomechanical- Intro Flashcards
Hot working definition
Deformation under conditions of temperature and strain rate such that restoration processes occur substantially during deformation to allow large strains at essentially zero strain hardening. The restoration processes are dynamic recovery and dynamic recrystallisation
Hot working applications and disadvantages
Applications: primary metal forming processes for large deformation at low loads and microstructural control capabilities.
Disadvantages: difficulty in controlling shape tolerances and surface finish because of oxidation, means the hot working stage is the semi-finished stage
Difference for cold working
No dynamic restoration so limited strain due to hardening and loss of ductility.
Advantages and requirements for cold working
Requires annealing cycle to restore formability (static recovery and RX).
Excellent shape tolerances and surface finish so used for secondary forming processes
Features of warm working
In between hot and cold. No dynamic RX but some dynamic recovery. Cooler than hot so dimensional tolerances and surface finish are good. Becoming popular in forging, particularly automotive parts, also more in rolling recently
Workability definition
Term sometimes applied to the general suitability of an alloy for metal forming. More accurately applied to the extent to which the alloy can be deformed in a specific metal working process without the formation of cracks
How is thermomechanical processing distinct from classical hot or warm working?
In that we exploit both the ease of shaping and the opportunity to modify the microstructure to the benefit of the final product properties.
Hall-Petch relationship and meaning of terms
σ0=σi+kd^-1/2
σ0 is yield stress
σi is friction stress (overall resistance of crystal lattice to dislocation movement)
k is locking parameter (relative hardening contribution of GBs)
d is grain diameter
What produces heterogeneity in the properties?
Chemical inhomogeneity
Example order of processing steps
Casting
Preheating
Hot rolling
Coiling
Cold rolling
Softening annealing
Why study TMP?
Greater understanding and control: reduced variability in properties, reduced tolerances in design and manufacture, e.g weight saving valuable in transport applications.
Improved properties with simpler alloys: less use of scarce resources and simpler recycling.
Reduce process steps: e.g warm forge instead of hot and cold anneal.
Difference in stress-strain relationship between conventional and ultra fine ferrite
Both almost vertical at start. Ultra fine ferrite stays like this for higher stresses. Conventional then has curve then flat. UFF just straight to flat
