Chapter 9 Heat Treatment

Question 1

What are heat treatments associated with in Metals and alloys

Answer 1

Annealing
Normalising
Solution treatment
Hardening

Question 2

What are heat treatments associated with in glasses

Answer 2

Stress reliving

Tempering

Question 3

What does annealing consist of

Answer 3

Heating the metal to a given temperature
Holding it for a set period of time at that temperature
Slow cooking to room temperature

Question 4

How does annealing remove the effects of strain hardening via recovery and recrystallisation

Answer 4

Decreasing yield strength
Increasing ductility
Increasing electrical conductivity

Question 5

What is the annealing point of glades

Answer 5

The point at which annealing takes place

Question 6

What does relieve tensile stresses in glasses do

Answer 6

It reduces the probability of fracture which would have been a result from forming or cooling from the forming temperature

Question 7

What does normalising consist of

Answer 7

Heating the material to a given temperature
Holding it for a set period of time at the temperature
Slow cooling to room temperature in still air

Question 8

What is normalising used to eliminate

Answer 8

Residual stresses from casting, forming or welding ( carried out after rough machining of castings and forgings but before final machining )

Question 9

What does faster air cooling restrict and why can it be excessive

Answer 9

It restricts grain growth and it can be excessive because the metal becomes less ductile

Question 10

What is solution treatment carried out on

Answer 10

Precipitation hardening alloys especially non ferrous alloys e.g. Al based alloys

Question 11

Describe the process of solution treatment

Answer 11

The alloy is heated to high temperature
It then soaks for a short period of time
This dissolves the precipitates to form a solid solution
The alloy is quenched or air cooled to retain the copper in a solid solution

Question 12

What are the different types of hardening processes

Answer 12

Strain hardening
Precipitation hardening
Quench hardening
Tempering

Question 13

What does strain hardening do

Answer 13

Strain hardening involves cold forming operations in which their is an increase in strength of metals by strain hardening. It is the only way to harden non ferrous metals and alloys. Involves a decrease in ductility.

Question 14

Describe the process of precipitation hardening (with reference to Duralumin)

Answer 14

Copper will slowly combine with aluminium to produce a very fine distribution of hard precipitates. Increases strength and decreases ductility. In aluminium alloys the process is slow at room temperature- natural ageing

Question 15

What is artificial ageing

Answer 15

The process in which precipitation hardening is fastened by heating the alloy to a temperature well below that for solution treatment

Question 16

What is quench hardening

Answer 16

The heat treatment of steels in which the crystal structure changes

Iron —> increasing temperature
Bcc–> fcc—>bcc

Question 17

What does quench hardening consist of

Answer 17

Holding medium, high or alloy carbon steels steels at high temperatures (900c) to dissolve carbides in the FCC structure. Then cooling rapidly in brine, water or oil. This gives the structure a high hardness but with no ductility.

Question 18

When is tempering carried out and what is it used to do to steel

Answer 18

It is carried out after quench hardening and it is used to increase the toughness of steel (accompanied by a decrease in hardness)

Question 19

What heat is the quenched hardened steel heated to

Answer 19

The tempering temperature is relatively low at 205 - 650

Question 20

In quench hardening what does hardness achieved depend on

Answer 20

Composition ( carbon content )
Quench rate ( brine > water > oil

Question 21

Describe the process of tempering glass

Answer 21

Glass is heated to softening point
Glass is cooled rapidly with jets of cold air
This places the surface of the glass into compression
This increases the strength by 4x