Heat Treatment

Question 1

Explain annealing

Answer 1

Where a material is exposed to a elevated temp for an extended period of time then slowly cooled to room temp

Question 2

Why is the annealing cooling process slow

Answer 2

To avoid warping/cracking

Question 3

What is the purpose of annealing

Answer 3

To relive internal stress
Increase ductility, toughness and softness
Produce specific microstructure

Question 4

What is process annealing

Answer 4

Used to soften and increase ductility of a previously strain hardened metal.
Used during fabrication that require plastic deformation to prevent fracture

Question 5

What is stress relief annealing

Answer 5

Stresses may develop, distortion and warping

The sample is heated and held to attain uniform temp before cooling

Question 6

What is normalising

Answer 6

Sample heated to at least 50 degrees above the upper critical temp.
After sufficient time has been allowed for alloy to completely transform to austenite, the treatment is terminated by cooling in air

Question 7

Purpose of normalising

Answer 7

Used to decrease the average grain size and produce a more uniform and desirable size distribution
Fine grains are tougher

Question 8

What is full annealing

Answer 8

Sample heated to form austenite, 50 degrees above upper critical temp
Alloy then furnace cool which takes several hours
Forms coarse Pearlite that is relatively soft and ductile
Full annealing time consuming yet forms microstructure having small grains and a uniform grain structure

Question 9

What is spherodising

Answer 9

Prolonged heating to 15-25 hours just below the eutectoid temp
Results in soft spherodite structure

Question 10

What is quenching

Answer 10

Heating to austenite temp and then cooling fast enough to avoid the formation of ferrite, Pearlite or bainite to Obtain pure martensite

Question 11

What does martensite hardness depend on

Answer 11

Carbon content of the steel

Question 12

What is tempering

Answer 12

Reheating martensite steels to temps between 150-509 to force some carbide precipitation

Question 13

Why do we temper

Answer 13

Increases the ductility and toughness of martensite

Hardness and strength is lost

Question 14

What is martempering

Answer 14

Rapid cooling and held until the inner and outer core of the steel are the same temp. Then cooled

Question 15

Low carbon steels

Answer 15

0.1  - 0.25 % C
Pro eutectoid ferrite and Pearlite 
High ductility and formability 
Low strength, can’t be strengthened 
Pipes, sheets panels, wires

Question 16

Medium carbon steel

Answer 16

0.25 - 0.55 % C
Good combination of strength and ductility
Strengthened by treatment
Bolts, gears and cranes

Question 17

High strength low allow steels

Answer 17

Carbon content reduced
Loss of strength compensated by increasing Mn content and by microalloying
Large welded structures, oil rigs, ocean liners

Question 18

High carbon steel

Answer 18

0.6 - 0.8 % C
Eutectoid Pearlite, strengthened and hardened by treatment
High strength moderate toughness

Question 19

Tool steels

Answer 19

0.8 - 1.2 % C 
Pro eutectoid cementite and Pearlite 
High hardness and low toughness 
Hammers, knives and tools 
Poor weldability and machinability

Question 20

Name the three basic classes of stainless steel

Answer 20

Ferritics 
Chromium and iron alloy 
12 - 25 % C 
Cheap 
Martensitics 
Iron chromium alloy 
Hard cutting materials
Austensitics 
Iron chromium alloy 
Corrosion resistant

Question 21

Cast irons

Answer 21

Content between 2- 5.3% C

Tertiary alloy system with the third element being silicon

Question 22

White cast iron

Answer 22

Relatively less carbon and silicon
Cool rapidly and do not have enough silicon the cast iron solidifies as white
Cementite makes the alloy hard and brittle
Intermediate product for producing malleable irons
Slurry pipe elbows

Question 23

Grey cast irons

Answer 23

2-3% silicon
Cool the iron reasonably slow and silicon will cause carbon to form as graphite flakes
Adding more silicon gets carbon out of austenite and into flakes
Cheap, castable
Unsuitable for taking tensile loads

Question 24

Malleable cast iron

Answer 24

Heat white cast iron above critical line, convert carbide to graphite which will produce a rough climb of graphite
Stronger, more ductile than gray cast iron
Expensive
Gear box casing

Question 25

Nodular or ductile cast irons

Answer 25

By adding Mg or rare elements to the molten liquid just before casting
Produces nodules of graphite instead of flakes but ends with matrix of either ferrite or Pearlite
Stronger and tougher than gray irons
Expensive