Prepared by Prof. Naman M. Dave Assistant Professor,

Mechanical Engg. Dept. Gandhinagar Institute of Technology.

MATERIAL SCIENCE & METALLURGY 2131904

Chapter 8 Heat Treatment

Please do not blindly follow the presentation files only, refer it just as reference material.

More concentration should on class room work and text book-reference books.

Introduction In engineering, a successful product depends upon its design

and selection of MOC (Material of Construction). If MOC is the best suitable for functions that a product has to

perform, it will make the product performance also the best. Properties of material are dependent upon its crystal structure

and micro-structure. For knowing the micro-structure of a material there are graphs

(Temperature vs. Composition) known as phase diagrams. Phase diagrams are constructed based on a very important

assumption viz. the heating rate or the cooling rate of the metal or alloy is very slow. This is called equilibrium cooling.

Suppose the micro-structure and the properties attained by very slow cooling rate are not up to the mark then ... ???

The answer to this question is non-equilibrium cooling or fast cooling of a metal or an alloy should be done from a temperature above “critical temperature” so as to achieve desired properties. This is known as Heat Treatment.

Prof. Naman M. Dave

Definition • “Heat Treatment is controlled heating and cooling of metals to alter

their mechanical properties without changing the product shape.”

Objectives • Steels are heat treated for one of the following purposes: • 1) Softening : Softening is done to reduce strength or hardness,

remove residual stresses, improve toughness, restore ductility, refine grain size of the steel. Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed, such as in a cold-rolling operation or wiredrawing

• 2) Hardening : Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques.

• Material modification • Relieving Internal Stresses • Improve machinability

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• Cause relief of internal stresses developed during cold working, welding, casting, forging etc.

• Harden and strengthen metals. • Improve machinability. • Change grain size. • Soften metals for further (cold) working as in wire

drawing or cold rolling. • Improve ductility and toughness. • Increase, heat, wear and corrosion resistance of

materials. • Improve electrical and magnetic properties. • Homogenies the structure; to remove coring or

segregation. • Spheroidize tiny particles, such as those of Fe3C in

steel, by diffusion.

Purpose of Heat Treatment

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A Typical Heat Treatment Cycle Heat Treatment is done in following manner...

1) The Steel is heated above a specific temperature called “Critical” or “Austenitizing” temperature.

2) After reaching the Austenitizing Temperature, the steel is held at that temperature for some time known as “holding” or “soaking” period. Soaking results in the formation of homogenous austenite throughout the entire cross section.

3) Steel with homogenous austenite is cooled to room temperature. The cooling rate depends upon the properties required. If necessary, the steel may be re-heated to a temperature below lower critical temperature (A1) and cooled again (as in the case of “Tempering “ heat treatment).

Prof. Naman M. Dave

Step-1 : Deciding the temperature of heating / holding / soaking / Austenitizing; This can be done with the help of Iron-Carbon diagram as shown below. Step-2 : Deciding the time of holding / soaking / Austenitizing depending on the maximum thickness of casting Step-3 : Deciding the cooling rate required to attain a specific micro- structure. This decision can be taken only with the help of a T.T.T. or C.C.T. Diagram

Steps for Heat Treatment Cycle

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Time in region indicates amount of microconstituent!

Cont. Cooling Transformation (C.C.T.) Dig. Slow Cooling Rate

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Cooling Rate, R, is Change in Temp / Time °C/s

Cont. Cooling Transformation (C.C.T.) Dig. Medium Cooling Rate

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This steel is very hardenable… 100% Martensite in ~ 1 minute of cooling!

Cont. Cooling Transformation (C.C.T.) Dig. Fast Cooling Rate

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Time-Temp.-Transformation (T.T.T.) Diagram

A) Introduction Iron-Carbon diagram predicts the micro-structure of final

product of Steels or Cast Irons after first stage manufacturing (casting).

T.T.T. diagram predicts the micro-structure of final product for Steels or Cast Irons expected after Heat Treatment.

There are different T.T.T. Diagrams for Hypo-eutectoid, Eutectoid and Hyper-Eutectoid Steels and similarly there are different T.T.T. Diagrams for different types of Cast Irons.

This diagram is also known as C-Curve, S-Curve or Baine’s Curve or Iso-Thermal Transformation diagram.

It is known as ‘C’ or ‘S’ curve due to the shape of curves. It is known as Baine’s Curve from the name of metallurgist. It is known as Iso-thermal transformation diagram because it

shows the relationship between temperature and time for iso-thermal transformation / decomposition of austenite which in turn decides the resultant properties of steel.

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Time-Temp.-Transformation (T.T.T.) Diagram

B) Steps to construct T.T.T. diagram 1. Prepare a large no. of small specimens cut from the same

steel bar e.g. Eutectoid Steel bar. 2. Place the specimens in a furnace or molten salt bath

maintained at just above the austenitizing temperature. The specimens are held at this temperature for long enough time to form complete austenite.

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Time-Temp.-Transformation (T.T.T.) Diagram

3. The specimens are then quickly transferred to molten salt baths kept at constant temperatures below lower critical temperature (A1) e.g. There can be 4 baths - 700OC, 500OC, 400OC & 250OC.

4. A given specimen is allowed to react iso-thermally for a certain time, and then it is quenched (rapidly cooled) in cold water. This quenching results in formation of “martensite” from austenite. The time for iso-thermal reaction of all specimens in a same bath (e.g. 700 OC bath) is kept different e.g. 2 s, 4 s, 6 s, 8 s and so on upto say 15 hrs. In other words it means each sample is brought out of a bath after every 2 s and then it is quenched.

Prof. Naman M. Dave

Prof. Naman M. Dave

Time-Temp.-Transformation (T.T.T.) Diagram

5. After cooling, the micro-structures are observed.

6. A graph of Time in seconds on X-axis vs. “% of Transformed Austenite” on Y-axis is plotted for one salt bath say 700OC.

This is one curve. Similarly there are no. Of curves depending on no. Of salt baths say 500OC, 400OC, 250OC, etc. It should be noted that each curve has a start point (S) and a finish point (F). Start indicates the time when the austenite started transforming to other phases whereas F indicates the time of 100% completion of austenite transformation (to other phases)

Prof. Naman M. Dave

Prof. Naman M. Dave

Prof. Naman M. Dave

Prof. Naman M. Dave

Prof. Naman M. Dave

Cooling Curve 1: • very slow cooling rate, within a furnace • typical of conventional annealing. • Transformation product is coarse pearlite with low hardness (Rc15) Cooling Curve 2: • Isothermal transformation • Salt bath • Harder then CC1

Cooling Curve 3: • Transformation will start at 3 with the formation of coarse pearlite

and finish at 4, • with the formation of medium pearlite Since there is a greater

temperature difference between point 3 and 4 than there is between 1 and 2,

• the structure will show a greater variation in the fineness of pearlite and a smaller proportion of coarse pearlite as compared to that of curve-a curve-b involves a faster cooling rate than curve a (annealing) and may be considered typical of normalizing.

Cooling Curve 4: • This curve is typical of a slow oil quench and the microstructure

will be a mixture of medium and fine pearlite Prof. Naman M. Dave

Cooling Curve 5: • This curve is typical of an intermediate cooling rate and austenite

will start to transform (at point 5) to fine pearlite as Ms line is crossed,

• the remaining austenite will transform to martensite. • The final structure at room temperature will thus consist of

martensite and fine pearlite.

Cooling Curve 6: • This curve is typical of a drastic quench, the substance remains

austenitic until the Ms line is reached, and changes to martensite between the Ms, and Mf lines.

Cooling Curve 6”(6-8): • It is possible to form I00% pearlite or 100% martensite by

continuous cooling, but it is not possible to form 100% Bainite. • cooling curve-6-8 obtains a bainitic structure, by cooling rapidly

enough to miss the nose of curve and then holding in the temperature range at which bainite is formed until transformation is complete'

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Cooling Curve 7: • This curve is tangent to the nose of TTT curve. • The cooling rate associated with this curve is approximate

critical cooling rate for this steel. • Any cooling rate equal to or faster than this CCR (cooling rate

(CC-6)) will form only martensite and any cooling rate slower than CCR (CC- 1 to 4) will form some softer transformation products such as pearlite or bainite.

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Critical Cooling Rate

Various factors affecting the critical cooling rate are: 1. Composition of steel 2. Temperature of hardening 3. Purity of steel

Prof. Naman M. Dave

Prof. Naman M. Dave

AUSTENITE GRAIN SIZE AND GRAIN SIZE CONTROL

• The grain size of steel nominally refers to the austenite grain size i.e., the size of the austenite grains before the steel is cooled to room temperature.

• Grain size is a very important factor in relation to strength, usefulness and other physical properties of steel and it is also very important in developing fundamental theories of metallic behavior.

Importance of grain size Fine grains • increase impact toughness; • improve machining finishes; and • mitigate quenching cracks, distortion in quenching and surface

decarburization. Coarse austenite grains • raise hardenability, tensile strength as normalized and creep

strength; and • improve rough machinability.

Prof. Naman M. Dave

Prof. Naman M. Dave

Grain size control

Nature and extent of deoxidizers Chemical composition of steel The method of manufacture of steel Alloying elements Metallic and non-metallic inclusions Heat treatment processes

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Temper Brittleness After being quenched and tempered steels lose impact resistance and become brittle, If, after tempering, they are slowly cooled or held for longer time in temperature range 600-300 C.

Example: Steel with 3.5%Ni 1.5%Cr Oil hardened: 810-850 C Tempering: 150-650 C

Suffer Temper brittleness when tempered in the range: 250-400 C Low resistance in Izod Test

Prevention of Temper Embrittlement • Reduction of harmful impurities in steel. • Accelerated cooling from the temperature of high-temperature

tempering (above 600 C); • Add small amount of molybdenum (0.2-0.37%); and • Subjecting the metal to high temperature thermo-mechanical

treatment.

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Over Heating of steel If steel is heated well above the upper critical temperature large austenite grains form. Steel develops undesirable coarse grains and if cooled slowly to room temperature, it lacks both ductility and resistance to shock; but it is not damaged. “overheated steel”

The grain structure of the overheated steel can be corrected by 1. Suitable Heat Treatment 2. Mechanical work 3. A combination of the above two

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Burning of Steels • A severely overheated steel • Indicated by the presence of a light etching network outlining the

austenite grain boundaries, when the steel is etched with alcoholic solution of nitric acid, and of a dark-etching network when picric acid is used.

• The structure in a burnt steel, revealed by etching with nitro sulphuric acid and ammonium nitrate are the reverse of those obtained in the overheated steel,

• Burning can occur at a temperature well below the solidus of an alloy- of the same chemical composition

Prof. Naman M. Dave

Prof. Naman M. Dave

Thank

You Prof. Naman M. Dave