Thermal Treatments

Question 1

What is the effect of heating?

Answer 1

-Strengthening effects due to grain size reduction, precipitation and strain hardening can be eliminated by and elevated temperature heat treatment
-Solid solution strengthening is unaffected by heat treatment

Question 2

What is the effect of heating on stress strain behaviour

Answer 2

• Yield strength and TS decrease with increasing test temperature
• %EL increases with increasing test temperature
• This happens because vacancies help dislocations move past obstacles

Question 3

What defects are vacancy atoms?

Answer 3

Point defects

Question 4

What defects are dislocations?

Answer 4

Line defects

Question 5

What defects are grain boundaries?

Answer 5

Area defects

Question 6

What is the effect of heating after cold work?

Answer 6

• One hour treatment at the annealing temperature decreases TS and increases %EL
• Effects of cold work are reversed

Question 7

How many annealing stages are there?

Answer 7

There are three annealing stages

Question 8

Explain the first annealing stage

Answer 8

The first annealing stage is RECOVERY
- Dislocations are moving in the absence of externally applies stresses
- annihilation of dislocations reduce dislocation density

Question 9

Explain the second annealing stage

Answer 9

Second stage is RECRYSTALLISATION
- New grains are formed
- Have a small dislocation density
- are small
- consume cold worked grains
- TR = Recrystallisation temperature
- Hot work if above TR
- Cold work if below TR

Question 10

Explain the third annealing stage

Answer 10

The third stage is GRAIN GROWTH
- Grain growth does not need to be proceeded by recovery and recrystallisation.
- it may occur in all polycrystalline metals and ceramics
- The mech properties at room temp of rind grained metals are usually superior to those of coarse grained ones

Question 11

Where are dislocations observed?

Answer 11

Dislocations are observed primarily in metals and alloys

Question 12

How is strength increased?

Answer 12

Strength is increased by making dislocation motion difficult

Question 13

What are the ways to increase strength?

Answer 13

• Decreases grain size
• solid solution strengthening
• precipitate strengthening
• Cold work

Question 14

What does heating (annealing) do?

Answer 14

Heating (annealing) can reduce dislocation density and increase grain size, this decreases the strength

Question 15

Explain deformation mechanics for ceramic materials

Answer 15

• Plastic deformation does not occur by the motion of dislocations because there is no regular atomic structure
• instead they deform by viscous flow in the same manner that liquids deform

Question 16

Explain drawing in plastic polymers

Answer 16

• Drawing stretches the polymer prior to use and aligns chains in the stretching direction
• This increases E in the stretching
  direction
• increases TS in the stretching
  direction
• Decreases ductility

Question 17

Explain annealing after drawing for plastic polymers

Answer 17

• Annealing (after drawing)
  
  • Decreases chain alignment
  • reverses effects of drawing

Question 18

Explain annealing for undrawn plastic polymers

Answer 18

• Annealing (undrawn)
  
  • Increases crystallite size and perfection
  • Increases yield strength and TS, reduces %EL