Lesson 4 - Quenching And Tempering Of Steels

Question 1

What is quenching?

Answer 1

Rapid cooling methods using liquids

Question 2

List of liquids used for cooling from fastest to slowest cooling rates.

Answer 2

Brine (agitated)
Brine
Water (agitated)
Water
Oil

Question 3

What is an equilibrium process? And why is quenching not one?

Answer 3

Equilibrium condition can only be achieved when the process is very slow, allowing the system time to reach thermodynamic equilibrium. (Hence why time isn’t in a phase diagram)
Heat treatments such as quenching involve the rapid change of temperature and thus are ‘non-equilibrium’ processes. (So heat treatments can produce new phases)

Question 4

How does Quenching work?

Answer 4

• For any given steel, there is a critical cooling rate over which the equilibrium transformation of austenitic to ferrite/cementite/perlite will not happen
• When the temp drops below A1 = 723’C, the austenite tends to transform to ferrite and cementite but both phases are unstable (due to non-equilibrium condition)
• Cooling becomes so fast that carbon cannot diffuse out of the iron crystal structure and be redistributed. At a threshold temp. A new microstructure (martensite) is formed.

Question 5

How can the martensite transformation begin and end?

Answer 5

When the temperature drops to a starting temperature threshold (Ms) which will end when a finishing temperature threshold (Mf) is crossed. Both Ms and Mf decrease with wt%C.

Question 6

what is a TTT diagram used for?

Answer 6

A Time-Temperature-Transformation (TTT) diagram is used to visualise time-dependent transformations.

Question 7

What can martensite lead to in the worst case?

Answer 7

Cracking as is very hard and brittle

Question 8

What is cooling-rate affected by?

Answer 8

• The thickness of the steel section that is being quenched.
• Thin sections cool rapidly, whilst thicker sections cool slowly

Question 9

What are some issues with quenching?

Answer 9

• Can Cause distortion and internal stresses, leading to cracking
• Make the metal harder than necessary, reducing workability
• Cause the metal to become brittle and reduce ductility and toughness

Question 10

What do post-quenching treatments do?

Answer 10

• Relieve internal stresses
• Redistribute carbon and improve mechanical strength

Question 11

What are two common types of post quenching heat treatments (tempering)?

Answer 11

• Austempering
• Martempering

Question 12

What happens during Austempering?

Answer 12

• The steel, although cooled very rapidly, does not actually pass through the Ms temperature line
• The steel is held at a temperature slightly above Ms, typically for a period of 2 hours plus, then the steel is cooled to room temperature.

Question 13

What are the results of Austempering?

Answer 13

• All stresses from quenching are relieved
• Another transformation takes place to produce Bainite
• The nearer the Austempering temperature to Ms, the finer the bainite microstructure (thus, the higher its strength)
• Austempering gives low distortion and a good balance between toughness and stiffness (flexibility)

Question 14

What happens during Martempering?

Answer 14

• Martensite transformation has been completed
• This tempering treatment relieves stresses and minimises distortion
• The microstructure after Martempering process contains martensite, bainite and pearlite.
• Martempering is useful when distortion control is critical