Rory's Deck

Question 1

What does the term ‘heat treatment’ refer too?

Answer 1

This applies to several processes involving heating metals and alloys to controlled temperatures for specific periods and afterward cooling them at controlled rates. Heat treatment modifies the properties of an alloy without changing its basic shape.

Question 2

What is the most commonly used ferrous metal?

Answer 2

Course steel is the most commonly used ferrous metal.

Question 3

What are the important basic considerations in heat treatment?

Answer 3

• Time and temperature
• Uniformity of heating and cooling
• requirements for atmosphere control
• heating and cooling quenching rate
• the temperature at which heat treatment was performed relative to the critical temperature. Or otherwise, the temperature at which a phase change occurs

Question 4

What does high temperature heat treatment result in?

Answer 4

High-temperature heat treatment results in the recrystallization of the microstructure. Depending on the rate of cooling, results can vary with a wide range of property improvements such as
- increased uniformity of properties
- toughness and hardness
- an increase in mechanical properties such as yield and tensile strength.

Question 5

Why are heat treatment processes designed to achieve a wide variety of purposes?

Answer 5

Generally, heat treatment is undertaken to improve or modify the structure and properties, or acts to assist these processes as an intermediate step.

Question 6

What is annealing?

Answer 6

The aim of annealing heat treatment is to reduce the hardness of a material. This is done in order to facilitate further machining processes. It is commonly used after casting, forging, and rolling operations. This is done to increase ductility and to reduce internal stresses.

Question 7

How does annealing lead to reduced hardness

Answer 7

Annealing leads to reduced hardness through successive stages of
- Recovery
- Recrystallisation
- Grain Growth

Question 8

What does the technique of annealing involve?

Answer 8

Involves heating the metal to a suitable temperature, holding the metal at that temperature, then slowly cooling.

Question 9

What is the result of annealing?

Answer 9

Annealing results in the removal of stresses and the generation of new grain growths. This results in
- Softer materials
- a homogeneous microstructure
- improved machinability and formability
- a tougher, more ductile, and less brittle material
- the removal of residual stresses due to cold working or casting.

Question 10

What are the three types of annealing and what differs them?

Answer 10

Sub-critical annealing - is performed at a temperature above 820 degrees for hypo eutectoid steels with less than the eutectoid composition of 0.83%
Inter critical annealing - is undertaken between 750 and 800 degrees
Full (or subcritical) annealing - is usually between a temperature of 500 - 690 degrees.

Question 11

What is normalising?

Answer 11

Normalising consists of heating steel to a temperature, so that its structure becomes fully autunitic with all the carbon in solid solution. After this the steel is then allowed to cool in air.

Question 12

How does normalising transform grain structure?

Answer 12

On cooling, transformation from austenite to ferrite and pearlite occur, This process occurs at the austenite grain boundaries. Grain refinement can occur with each crossing of the phase boundary, as grain size reduces, this causes yield strength to increase. Grain size reduction will also increase tensile strength and toughness in the material.

Question 13

Why are quenching and tempering operations performed?

Answer 13

To increase the mechanical properties, for example tensile strength and yield strength. After the steels are held at temperature for an appropriate period of time, it is removed from the furnace and rapidly cooled/quenched.

Question 14

What variety of media is used to quench steel?

Answer 14

Depending on the steels chemistry and size, medias used may be
- Air
- Molten Lead
- Water
- Brine or oil
These variety of media allow for an amount of control over the rate of quench rate or heat extraction.

Question 15

What is the purpose of quenching?

Answer 15

Is to supress the formation of ferrite and cementite. To form a metastable phase that is known as martensite. Plain carbon steels require fast quenching rates to avoid the formation of ferrite and cementite. However when alloying elements such as boron, chromium, nickel… etc are introduced the quench rate can be reduced.

Question 16

How is ductility restored?

Answer 16

Quenching will exhibit steel with very high tensile, but it will also be brittle. To restore ductility it is subjected to tempering heat treatment. Tempering results in precipitation of cementite leading to a reduction in internal stresses, tensile strength and hardness. But improves ductility, impact resistance and dimensional stability.

Question 17

What are the manufacturing processes of non ferrous metals?

Answer 17

Alloying, annealing and solid solution hardening

Question 18

What are the most widely used non ferrous alloys?

Answer 18

Non-ferrous metals are exponentially diverse in nature. The most widely used non ferrous metals belong to the following groups
Nickel
Copper
Magnesium
Aluminium

Question 19

What typically occurs during the alloying of non ferrous materials.

Answer 19

There are two ways the alloying of a metal typically occurs, this depends on the atomic radius of the alloying metals relative to the base metal. If the atomic radius of the alloying or solute metal differs by no more than 15% a substitution of one or the other can occur within the metal. What is known as the substitutional solid solution is what occurs when an alloy has addition forms.

Question 20

What is solid solution hardening?

Answer 20

Solid solution hardening is a type of hardening that can be used to improve the strength of a pure metal. If rapid cooling takes place the precipitation of a second phase can be suppressed. However, if the alloy is unstable, Overtime or under low temperate heat treatment the second phase will precipitate evenly, as a fine dispersion. The alloy duralumin is a representation of a common precipitation-hardening alloy. The time and temperature of the aging need to be controlled. If precipitates grow too large they lose cohesion. This leads to a loss of strength. At this point, the alloy has been overaged. Dispersion strength and hardening are other examples of solid solution hardening.

Question 21

Annealing of non-ferrous metals

Answer 21

Annealing takes solutes back into a solution. It removes cold work stress and involves recrystallization of grain structure. Temperatures need to be heated to differ from metal to metal. Example - Wrought Brass = 425-700 degrees
99% Pure Aluminium = 420-1100 degrees

Question 22

How does strengthening occur in nickel alloys?

Answer 22

By a combination of solid solution hardening and dispersion hardening.

Question 23

What are the main heat treatments of alloys?

Answer 23

Quenching
Tempering
Annealing
Precipitation hardening

Question 24

How does heat treatment change an alloys grain structure?

Answer 24

Significant changes occur in alloys as a result of heat treatment. It causes changes in Grain growth, as high temperatures promote the diffusion of atoms within the alloy. This causes grain boundaries to migrate. This can affect an alloy’s mechanical properties. For example, the heat treatment of a nickel alloy will change its strength and ductility.

Question 25

How does heat treatment change cause recrystallisation in alloys?

Answer 25

Heat treatment induces recrystallisation in alloys that have experienced cold working or deformation. This induces the formation of new grains with more favourable crystallographic orientation. This helps to relieve internal stresses and restore the alloys formability and ductility.