Prof. H. K. Khaira
• Steels can be heat treated to high hardness and
strength levels. The reasons for doing this are
obvious. Structural components subjected to high
operating stress need the high strength of a hardened
structure. Similarly, tools such as dies, knives, cutting
devices, and forming devices need a hardened
structure to resist wear and deformation.
• As-quenched hardened steels are so brittle that even
slight impacts may cause fracture. Tempering is a heat
treatment that reduces the brittleness of a steel
without significantly lowering its hardness and
strength. All hardened steels must be tempered
• Tempering consists of heating a hardened
steel to a temperature below eutectoid
temperature and keeping it at that
temperature for a specified time to reduce
brittleness followed by air cooling.
• The aim of tempering is to decrease
brittleness of hardened steel.
• The steel, after hardening, contains
martensite which is accicular and is very
brittle. It can not be used as such. Hence it is
essential to temper it to make it less brittle.
• During tempering, the martensite hardness
may also get reduced to some extent.
Properties of hardened steel
• Hardening produces matensite which is a super-saturated
solid solution of carbon in α (BCC) iron. Hence it is hard and
brittle. The fig. below shows variation of hardness of
martensite with carbon content.
• Tempering : a process of heating a
martensitic steel at a temp. below
the eutectoid temp. to make it
softer and more ductile.
: Fe3C particles precipitates from
the ’ phase → tempered
martensite → spheroidite
Properties of hardened steel
• Hardening also introduces high internal
stresses making steel brittle.
• Accicular nature of martensite also causes triaxial state of stress causing brittleness.
Changes During Tempering
• Martensite is a metastable phase. The
equilibrium phases are ferrite and cementite.
• During tempering, martensite changes to low
carbon martensite and then to equilibrium
phases (ferrite and cementite) resulting in
reduction in brittleness.
• The internal stresses also get reduced making
steel less brittle.
• Austenitizing : heating the
steels to a high enough
temperature until they
convert to at least partial
• Quenching: Media – brine
(salt water), fresh water, oil
• Tempering – Reheat to 200
- 550°C, decrease
hardness, regain ductility
Stages of tempering
The overlapping changes, which occur when high
carbon martensite is tempered, are divided into
Fourth stage or Secondary hardening
First Stage of Tempering
• First stage (50-200°C)
• Martensite breaks down to a low carbon martension
and transition precipitate known as ε-carbide (Fe2.4C)
• Second stage (205-305°C)
– Decomposition of retained austenite to bainite and
decrease in hardness.
• Third stage (250-500°C)
– Conversion of the aggregate of low carbon martensite
and ε - carbide into ferrite and cementite, which
gradually coarsens to give visible particles and rapid
Figure 11.29 Tempered martensite in steel (
500). (From ASM Handbook, Vol.
9, Metallography and Microstructure
(1985), ASM International Materials Park, OH
Fourth Stage of Tempering
• Secondary hardening ocures during fourth
stage of tempering.
• In some steels, the hardness increases instead
of decreasing during tempering. This is known
as secondary hardening.
• It ocurs due to precipitation of alloy carbides.
• Steels containing carbide forming alloying
elements show secondary hardening.
• It ocures during fourth stage of tempering.
• Steels containing W, Cr, V etc. show secondary
• High speed steel containing 18% W, 4% Cr and 1%
V show secondary hardening.
• High speed steels are used as tool steels. During
machining, the temperature of the tool tip
increases causing softening in other steels. But
High Speed Steels retain their hardness for a
longer time due to secondary hardening and
therefore can be used for longer time.
• Secondary hardening occurs during fourth stage of
tempering (400-700°C )
• Carbide changes in alloy steel at 400-700°C. In steels
containing one alloying addition, cementite forms first
and the alloy diffuses to it. When sufficiently
enriched, the Fe3C transforms to an alloy carbide.
• After further enrichment this carbide may be
superseded by another and this formation of transition
carbides may be repeated several times before the
equilibrium carbide forms. In chromium steel, changes
are: Fe3C→Cr7C3 → Cr23C6.
• In steels containing several carbide-forming elements
the reactions are often more complex, and the
carbides which decompose are not necessarily
followed by carbides based on the same alloy
• The transformation can also occur in situ by gradual
exchange of atoms without any appreciable hardening;
or by re-solution of existing iron carbides and fresh
nucleation of coherent carbide with considerable
hardening that counteracts the normal softening that
occurs during tempering.
• In some alloy steels, the hardness is maintained constant
up to about 500°C or in some cases it rises to a peak
followed by a gradual drop due to breakdown of coherence
and coalescence of the carbide particles. This agehardening process is known as secondary hardening and it
enhances high temperature creep properties of steel.
• Chromium, for an example, seems to stabilise the size of
the cementite particles over a range 200-500°C.
• Vanadium and molybdenum form a fine dispersion of
coherent precipitates (V4C3Mo2C) in a ferrite matrix with
considerable hardening. When over-ageing starts the V4C3
grows in the grain boundaries and also forms a
Widmanstätten pattern of plates within the grain