lap6

MIME 1650 Laboratory 6

Heat Treatment of Metal Alloys

Objective

The Student should learn the principles involved in the treatment of metal alloys, in

particular ferrous alloys. Practical application of hardness testing is also a key to this

experience.

Agenda - Safety

- Review of TTT Diagrams (can be found on page 130 of text)

- Laboratory 5 performance

Instructional Materials The fundamentals of heat treatment are based on phase transformations of the structure of

steel. The beginning point in understanding the heat treatment process is the phase

diagram shown below.

Fig. 1 Iron-iron carbide equilibrium phase diagram (iron-rich portion). (adapted from

Metal Progress Data Sheets, No. 38, American Society for Metals, Metals Park, Ohio, 1952.)

Note that steel changes phases at several critical temperatures depending on the alloy.

The austenite region defines steel at elevated temperatures. As the austenite cools, it will

form an alloy of ferrite and cementite. The exact temperature that this occurs is

determined by where the vertical line drawn for the alloy crosses the curves marked A1,

A3 and/or A1, 3 marked on the diagram above. Below these temperatures, the austenite is

unstable and will transform to martensite, bainite or perlite depending upon how the

austenite is cooled. Each of these alloys is a form of the combination of ferrite and

cementite but with different microstructure and physical properties.

The Time and rate of cooling to form these alloys depends on the carbon content as well

as any other alloying elements in the steel. The objective of heat-treating is to create in

the material the set of material properties desirable for the purpose the metal is used for.

These phase transformations alter the grain structure of an alloy, which directly affects

the alloy’s mechanical properties. The degree of treatment depends on a material’s

current condition, microstructure, and chemical composition. Mechanical properties

affected by heat treatment include strength, toughness and hardness.

Common treatment methods include annealing, quenching, tempering, and normalizing.

These methods work in conjunction with a material’s composition to yield reasonably

predictable results. Charts, which are useful while studying the effects of treatments, are

known as time-temperature-transformation (TTT) diagrams as shown below.

Fig 2 Isothermal transformation diagram for eutectoid steel.

This is a graph with temperature along the ordinate and time along the abscissa. This TTT

diagram applies to only one alloy of steel, in this case eutectoid of 0.8% carbon steel

(AISI 1080). Each different alloy of steel will have a separate TTT diagram. The critical

temperature line is taken from the A1, A3, Acm and/or A1, 3 curves of the phase diagram.

For this chart, the critical temperature is A1 at the eutectoid temperature, 727°C or

1340°F.

In standard heating practices, the steel is heated to this critical temperature to insure that

it is in austenite state. Then it is cool, usually continuously, (although this is not

necessary), such that the resulting metal forms pearlite, bainite or martensite.

The properties obtained deepened on how the steel is cooled, and the figure below

illustrates how hardness changes depending on the cooling curve. Curve A represents a

very slow cooling rate as it might be obtained by turning the heat off in the furnace and

letting it cool naturally. As a result the hardness obtained from curve A has a coarse

pearlitic structure with a HRC of 15. This will be a low strength steel, but it will have

considerable toughness. If the steel is air cooled as suggested by curve B, the pearlitic

microstructure is finer and the hardness is increased to HRC 30. The steel may be also

placed in oil to cool. This is called ‘oil quenching’ and results in a hardness of HRC 45. It

is likely in this case that the microstructure is either a fine-grained pearlite or a feathery

bainite/martensite structure. As a result, the nose of the pearlite transformation curves is

entirely missed and the result is a very hard but brittle martensite of HRC 65.

Fig. 3 Typical continuous cooling transformation curves

The martensite transformation region starts at a certain temperature (indicated by Ms) and

a finishes to complete the martensite transformation (indicated by Mf) at a much lower

temperatures. For certain alloys of steel, the martensite transformation end temperature

(Mf) may be below room temperature. As the martensite is formed, the grain structure

takes up more room than the FCC structure of austenite. The volume of the steel

increases as it changes into martensite. If conversion is incomplete (the temperature is

stopped above Mf), unstable austenite is retained in the steel, causing dimensional

instabilities. These instabilities can result in heat-treated parts be rejected for dimensional

errors. These alloys must be refrigerated to complete the martensite transformation and to

avoid retained austenite.

Using these charts, one can predict microstructure changes and resulting properties for

treatment methods. The temperature and rate of cooling are integral to heat treatment

processes. Below are examples of the common process that also rely upon the heat

treating process:

Annealing: After the initial heat treating and working, it may be necessary to restore the

original properties of the metal in order to increase ductility while reducing hardness and

strength. This can be accomplished by heating it above the critical temperature, followed

by very slow cooling.

Stress Relieving: Process by which stresses are removed from a material by heating to

just below the critical temperature and then slowly cooling.

Tempering: Martensite is very hard and brittle. It can soften and become more ductile

during tempering. This improvement is reached by raising the temperature to about half

of the critical temperature, and holding that temperature for an extended period of time.

Experimental Procedure A. Specimen Materials and Heat Treatment Parameters

• Water Hardening Drill Rod

1. Tool steel (specimen diameter= ¾”)

i. Composition: C 0.95-1.05%, Mg 0.30-0.40%, Si 0.10-0.25%

ii. Heat Treatment Parameters

• Oil Hardening Drill Rod

2. Tool steel (specimen diameter= 5/8”)

i. Composition: C 0.85-1.0%, Mg 1.0-0.40%, Si .5%(max.),

Cr .4-.6%, V (max.) 0.30

ii. Heat Treatment Parameters

1500 °F

T (°F)

t (min) 60 min.

Air Cooling

Water

Quenching

Hold Heating

• Air Hardening Drill Rod

3. Tool steel (specimen diameter= 1/2”)

i. Composition: C 0.95-1.05%, Mg 1.0% (Max), Cr 4.75-5.5%, Mo .90-1.4%, V

0.15-0.50%

ii. Heat Treatment Parameters

1500 °F

T (°F)

t (min) 60 min.

Air Cooling

Oil

Quenching

Hold Heating

Heating Hold

60 Minutes

T(°F)

t (min)

Air

Cooling

1500 °F

B. Procedure: 1. Measurement of Hardness (HRC).

a. Initial hardness of the specimens of tool steel. N.B. The hardness test surface must be first cleared of

the brittle, black oxide layer which may cover the

treated specimen. Remove this layer via belt sander.

b. Hardness after heat treatment of the specimens of tool steel.

2. Observe the changes in surface and color before and after treatment. 3. Heat treatment

c. Pre-heat the oven to specified temperature (for oil and water quenching process:1500°F, for air cooling: 1450

°F)

d. Using tongs and gloves, place three specimens into the oven; one for water quenching, one for air-cooling, and

one for oil cooling.

e. Begin timing 1 hour. f. Follow the heat treatment parameters shown in the

appropriate curve above.

g. When the time is up, use the tongs and gloves to take the specimen out. If water quenching, put the specimen

into the water immediately. If oil quenching, put the

specimen in water immediately. If air-cooling, place the

specimen on a heat-resistant block until thoroughly

cooled. DO NOT TOUCH THE SPECIMEN WITH

AN UNGLOVED HAND!!

Report Requirements 1. Design the data sheet to record the hardness data of all specimens. 2. Compare the hardness of each material before and after heat treatment.

Explain.

3. As the cooling rate increases, what can be said about the microstructure of the high carbon steel? Would you expect more or less pearlite in a high carbon

steel specimen, which was water quenched as compared to air quenched?

4. Name some practical applications of the various treatment processes.