Heat Treatments - Part 1

Question 1

What is a heat treatment?

Answer 1

The controlled heating and cooling of metals with the purpose of altering their physical and mechanical properties without changing the component’s shape.

Heat treatment typically improve component performance by increasing strength, hardness, ductility etc.

Question 2

What are the purposes of heat treatments?

Hint: 5 given.

Answer 2

• Increase strength and hardness
• Enhance ductility
• Grain size refinement
• Improve toughness and machinability
• Relieve internal stresses

Question 3

Name the three classifications of heat treatments of steels.

Answer 3

• Hardening
• Softening
• Conditioning

Question 4

List the two types of hardening and state what hardening processes achieve.

Answer 4

Direct and Diffusion hardening.

Hardening increases strength and wear resistance.

Question 5

How and why is martensite tempered?

Answer 5

Tempering reduces the internal stresses caused by quenching.

It is done by holding at a temp below 723C (eutectoid temp) which allows relaxation of the structure. Some carbon comes out of the solution.

Some strength and hardness is sacrificed for increased toughness.

Question 6

What is hardenability? and how is it measured?

Answer 6

The measure of the ability to form martensite. The Jominy end quench test is used to measure hardenability.

Question 7

What crystal structures do martensite, pearlite and austenite have?

Answer 7

Martensite - Body Centered Tetragonal
Pearlite - Body Centered Cubic
Austenite - Face Centered Cubic

Question 8

List the 6 possible quenching mediums

Answer 8

1. Water
2. Salt Water
3. Oil
4. Cryogenic quench
5. Air
6. Switch furnace off

Question 9

Describe Air, Oil and Water in terms of the severity of quench and the hardness produced.

Answer 9

Air - low severity, low hardness
Oil - moderate severity, moderate hardness
Water - high severity, high hardness

Question 10

What is the effect of increased surface area to volume ratio on quenching?

Answer 10

When the surface area to volume ratio increases, the cooling rate and hardness throughout the interior increases.

Question 11

What structure is formed when eutectoid steel is cooled in water, oil, air or a furnace?

Answer 11

Water - martesite
Oil - very fine pearlite
Air - fine pearlite
Furnace - coarse pearlite

Question 12

What are the 4 types of selective austenitising and quenching (type of direct hardening)?

Answer 12

1. Flame
2. Induction
3. Laser
4. Electron beam

Question 13

What is flame hardening?

Answer 13

Localised heating with gas flames. The depth of the treatment depends on heat transfer of fuel gas/oxygen mixture.
Often integrated with a quenching spray.
Complex shapes can be processed but it can be costly and difficult to control.

Question 14

What is induction hardening?

Answer 14

Induced eddy currents heat the surface if the steel very rapidly. This is quickly followed by jets of water to quench the component. A hard outer layer is created with a soft core.

Depends on the frequency of the current and the shape of the coils used to produce the current.

Question 15

What is laser heat treatment?

Answer 15

Used for local heat treatments for preheating or post weld treatment.

Question 16

What is electron beam heat treatment?

Answer 16

The heat source is a beam of high-energy electrons. The surface of the steel can be hardened very precisely both in depth and in location.

Question 17

State and define three diffusion hardening treatments.

Answer 17

1. Carburising - adding carbon to the surface of steel
2. Nitriding - adding nitrogen to the surface of steel
3. Carbonitriding - carbon and nitrogen are diffused into the surface of steel.

Question 18

What is the atmosphere and various temperatures for the carburising, nitriding and carbonitriding processes?

Answer 18

Atmosphere: methane or propane+ammonia

Temperatures:
Carburising - 850-900C
Nitriding - 600C (approx 100 hrs)
Carbonitriding - 815-900C