Metals- Deformation at Different Temperatures

Question 1

Warm working

Answer 1

Temperature in between hot and cold working. No dynamic recrystallisation but some dynamic recovery. Cooler than hot rolling so good dimensional tolerances and surface finish. Becoming popular in forging, particularly for automotive parts, and more in rolling recently

Question 2

Dynamic recrystallisation

Answer 2

Occurs during deformation. Leads to dislocation annihilation

Question 3

Static recrystallisation

Answer 3

Occurs after deformation. Leads to dislocation annihilation

Question 4

Dislocations generation and annihilation for no work hardening

Answer 4

Dislocation generation rate = dislocation annihilation/rearrangement rate

Question 5

Ways of increasing flow stress for given strain

Answer 5

For the work hardening: increase the strain rate, decrease the temperature, increase the solute content

Question 6

What is thermomechanical processing (TMP)?

Answer 6

More sophisticated version of classical hot or warm working such to exploit both the ease of shaping and the opportunity to modify the microstructure to the benefit of the final product properties. Produces the starting microstructure for the remainder of the metal’s processing such as cold working and annealing

Question 7

What does TMP start with?

Answer 7

As-cast microstructure. Typically large grain size and chemical segregation (gives heterogeneity in the properties)

Question 8

Hall-Petch relationship

Answer 8

σ0=σi+kd^-1/2
σ0 is yield stress
σi is friction stress
k is locking parameter
d is grain diameter
Means smaller grains give stronger metal

Question 9

Traditional hot rolling operation of steel

Answer 9

Start at 1200°C austenite. Hot rolling while it cools a bit to give coarse recrystallised austenite grains. Cools to 400°C to give coarse ferrite grains. Heat again until austenite grains. Cool to give fine ferrite grains. Expensive process involving 2 heating stages

Question 10

Controlled rolling (hot) of steel operation

Answer 10

Same as traditional but have delay after first hot rolling while it cools a little then hot roll again to give deformed austenite grains. Normal cooling from then gives fine ferrite grains. Accelerated cooling (using forced air or water) gives finer ferrite grains (down to 5μm)

Question 11

Why study TMP

Answer 11

Greater understanding and control means reduced variability in product properties so reduced tolerances in design and manufacture.
Can improve properties of simpler alloys so use less scarce resources and simpler recycling.
Can reduce processing steps e.g warm forging instead of hot, cold and anneal

Question 12

Ultra fine ferrite

Answer 12

Has much greater flow stress for given strain than conventional ferrite

Question 13

Generic production route

Answer 13

Casting, pre-working treatments, hot working, cold working, post-processing, finishing