Lex 06

MME-292

Arman HussainLecturerDGCE, BUET

Metallic Materials Sessional

Lecture 06 Heat Treatment of SteelCourtesy: Prof. A.K.M.B. Rashid

Department of MME, BUET

September 9, 2014

The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram

In their simplest form, steels are alloys of Iron (Fe) and Carbon (C).

Heat Treatment Fundamentals

Purpose of Heat Treatment

• To obtain desired structure and properties• To increase strength• To improve toughness• To increase hardness• To improve ductility• To refine grain size• To homogenize the structure• To releaf internal stresses developed dring cold

working or heavy annealing

Steels can be heat treated because of the structural changes that can

take place within solid iron-carbon alloys

Heat Treatment of Carbon Steel

Tempering

Hardening

Increases the hardness of steel by 2-3 times

Normalizing

Increases Strength of steel suitable for many

engineering applications

Annealing

A softening treatment applied to steel (may or may not be cold worked) to increase ductility at the expense of hardness and

strength

A specialized treatment applied to hardened steel to recover strength at the cost of

lower hardness

Heat Treatment Fundamentals

Heat Treatment of Carbon Steel

Soaking temperature is usually 50 to 100 C above upper critical temperature (A3) for hypo-eutectoid steel

and usually 50 to 100 C above lower critical

temperature (A3,1) for hyper-eutectoid steel

Heat Treatment Fundamentals

Annealing of Steels

Normalising of Steels

Heat Treatment of Carbon Steel

Full Austenite

Annealing

Normalizing

Austenite + Ferrite

At 720 C

At Room Temperature

Heat Treatment of Carbon Steel

Microstructure: Annealed Mild Steel

Microstructure: Normalized Mild Steel

Heat Treatment of Carbon Steel

MARTENSITE needles

white regions are RETAINED

AUSTENITEVery high hardness

Above Rc 55 Microstructure: Quenched Low carbon Steel

From Austenite zone steel is directly quenched in to a

water or oil bath

Full Austenite

Heat Treatment of Carbon Steel

Structural changes during tempering

Retained Austenite is

gone. At high temperature Martensite becomes

unstable and breaks down to

cementite globules in

ferrite matrix

Heat Treatment of Carbon Steel

Quenched Low carbon Steel Quenched and tempered Low carbon Steel