IT 208Chapter 121 Change of Condition Chapter 12.

IT 208 Chapter 12 1

Change of Condition

Chapter 12

IT 208 Chapter 12 2

Competencies

Describe the different methods of softening steel Describe the different methods of hardening steel Describe the difference between Martensite and

Austinsite

IT 208 Chapter 12 3

Phase Diagram

Is a graphical means of representing the phases of a metal allow system as a function of composition and temperature

Discuss the Water phase system (O/H) Discuss the Copper-nickel Alloy system (O/H)

IT 208 Chapter 12 4

Phase Diagram Discuss the Tin-Lead Allow system

• widely used in soldering for making electrical connection. The addition of two solid phases alpha (α) and beta (β).

• Alpha phase is a solid solution of tin in lead• Beta phase is solid solution of lead in tin that occurs only at

elevated temperatures around 200 degrees C Between these solid solutions lies a mixture of the two solid

phases, (α) + (β). Two liquidus lines that begin at the melting points of the pure

metals and meet at a composition of 61.9% Sn. • Point is called the eutectic composition for the tin-lead

system

IT 208 Chapter 12 5

Phase Diagram (Tin-Lead Allow system)

A eutectic alloy is a particular composition in an allow system for which the solidus and liquidus are at the same temperature.

The corresponding eutectic temperature, the melting point of the eutectic composition is 183 deg C

Eutectic temperature is always the lowest melting point for an alloy system.

IT 208 Chapter 12 6

Discuss Iron-Carbon Phase Diagram

Steels with less than 0.3 % carbon cannot be hardened effectively, while the maximum effect is obtained at about 0.7 % due to an increased tendency to retain austenite in high carbon steels

The ferrous metals of engineering importance are alloys of iron and carbon.

These alloys divide into two major groups; steel and cast iron.

IT 208 Chapter 12 7

Iron-Carbon Phase

1. Pure iron melts at 1539 degrees C (2827 deg F) during the rise in temperature from ambient, it undergoes several solid phase transformationsa. Starting at room temperature the phase is alpha iron

or ferrite. With less than 0.025% carbon at temperatures below 894 deg C

b. At 912 degrees C, ferrite transforms to gamma iron, called austenite. With less than 2% carbon

c. At 1394 degrees C, austenite transforms to delta iron, which remains until melting occurs at 1539 degrees C

IT 208 Chapter 12 8

Iron-Carbon Phase

2. Solubility limits of carbon in iron are low in the ferrite phase – only about 0.022% at 723 deg C. Austenite can dissolve up to about 2.1 % carbon at 1130 deg C. The difference in solubility between alpha and gamma leads to opportunities for strengthening by heat treatment

3. The eutectoid point is the lowest temperature at which austenite can exist (722 deg C).a. Eutectoid – the temperature and composition (0.77 - 0.81%

Carbon) at which a single-phase solid goes directly, on cooling, to a two-phase solid. Steels below 0.77% Carbon are considered hypoeutectoid steels those above up to 2.1% are considered hypereutectoid steels.

b. The eutectoid composition of the Iron-Carbon system is called pearlite.

IT 208 Chapter 12 9

Iron-Carbon Phase

4. Even without head treatment, the strength of iron increases dramatically as carbon content increases, and we enter the region in which the metal is called steel. More precisely, steel is defined as an iron-carbon

alloy containing from 0.02% to 2.1 % carbon.

5. Another prominent phase in the iron-alloy system. Is Fe3C also known as cementite. Which is a metallic compound of iron and carbon that is hard and brittle Carbon content of about 6.7%.

IT 208 Chapter 12 10

Iron-Carbon Phase

6. Above a carbon content of 2.1% up to about 4% or 5% is defined as cast iron

7. If sufficient time is allowed for cooling of the austenite it will revert completely to pearlite however, if the steel is cooled quickly from the

austenite, martensite is formed Martensitic steel has Rockwell C hardness of about

66 and pearlite is very soft in comparison.


Discuss the TTT curve (12-11)

Three major categories of heat treatments• Methods of softening steels

• Methods of hardening steels

• Methods of modifying the properties of steels


Methods of Softening Steels

Annealing is the softening of a metal to its softest possible condition. For steels, the metal must be heated into the austenitic range and cooled very slowly.

Normalizing is a heat treatment used to give steel an even GRAIN size. It is used prior to machining or other heat treatments.


Methods of Hardening Steels

Can be done by flame, induction, electron beams, and laser beam

Quenching is the rapid cooling of a metal to harden it.

Cryogenics, or deep freezing • done to make sure there is no retained Austenite

during quenching.

• When steel is at the hardening temperature, there is a solid solution of Carbon and Iron, known as Austenite.



• The amount of Martensite formed at quenching is a function of the lowest temperature encountered.

• At any given temperature of quenching there is a certain amount of Martensite and the balance is untransformed Austenite. This untransformed austenite is very brittle and can cause loss of strength or hardness, dimensional instability, or cracking.



• Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100 % Martensite at room temperature.

• High carbon and high alloy steels have retained Austenite at room temperature. To eliminate retained Austenite, the temperature has to be lowered.

• In Cryogenic treatment the material is subject to deep freeze temperatures of as low as -185°C (-301°F), but usually -75°C (-103°F) is sufficient.

• The Austenite is unstable at this temperature, and the whole structure becomes Martensite.



Surface Hardening If steel is hardened all the way through the part, it will be brittle.

In parts that have wearing surfaces such as gear teeth, shafts, lathe beds, and cams, only the surface of the part should be hardened so as to leave the inside soft and ductile. • Flame hardening is widely used in deep hardening for

large substrates. • Induction hardening is suitable for small parts in

production lines.

These processes are applicable only to steels that have sufficient carbon and alloy content to allow quench hardening.



Case Hardening If low-carbon steel is used and toughness is need in

the workpiece, its surface cannot be significantly hardened. Therefore a process to add carbon or nitrogen to the surface is done.• Done by carburizing, nitriding, carbonitriding or

cyaniding• These elements diffuses into the outer layers of the

steel to increase hardness. • The steel surface can then be hardened by

QUENCHING.


Hardness

Is a function of the Carbon content of the steel. Requires a change in structure from the body-

centered cubic structure found at room temperature to the face-centered cubic structure found in the Austenitic region.

Steel is heated to Autenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure.


Hardenability

The ease with which full hardness can be achieved throughout the material.

A measure of the depth of full hardness achieved• Is related to the type and amount of alloying elements.

• Different alloys, which have the same amount of Carbon content, will achieve the same amount of maximum hardness; however, the depth of full hardness will vary with the different alloys.

The reason to alloy steels is not to increase their strength, but increase their hardenability


Methods Of Modifying The Properties Of Steels

Tempering – is the removal of internal stresses in a metal by heating the part back to a temperature between 200 – 1200 deg F, for an appropriate time based on part size and desired tempering.

Spheroidizing – done by heating the steel to a temperature just under 1300 deg F and held for a period based on size. Grains will be changed into small spheres


Methods Of Modifying The Properties Of Steels

Martempering • steel is quenched from the austenitic temperature to

just above the MARTENSITE start temperature

• held there for a few seconds to a few minutes

• and then quenched.

• It is used to provide an even-sized martensite throughout the part.


Austempering – • steel is quenched to just above the MARTENSITE

start temperature

• held there for several hours before lowering the temperature to room conditions.

• The grain structure of the steel will be entirely bainitic

• Bainite has some of the hardness properties of martensite and some of the toughness properties of pearlite

IT 208Chapter 121 Change of Condition Chapter 12.

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