2.
Normalizing
• Generally is the process in which the metal is heated to a higher
temperature and then removed from the furnace for air cooling. As it
is mentioned in the first lecture that this process is applicable for
ferrous metals only.
• A heat treatment process consisting of austenitizing at temperatures of
30–80˚C above the AC3 transformation temperature followed by slow
cooling (usually in air)
• The aim of which is to obtain a fine-grained, uniformly distributed,
ferrite–pearlite structure and good hardness and ductility.
• Normalizing is applied mainly to unalloyed and low-alloy
hypoeutectoid steels
• For hypereutectoid steels the austenitizing temperature is 30–80˚C
above the AC1 or ACm transformation temperature
In hypo-eutectoid steels normalizing is done 50C above the annealing temperature.
In hyper-eutectoid steels normalizing done above Acm → due to faster cooling
cementite does not form a continuous film along GB.
Normalizing – Heating and Cooling
a, Heating; b, holding at austenitizing temperature; c, air cooling; d, air or furnace cooling
2. Normalizing
NORMALIZING
n
910C z a tio Acm
Nor al i
mal o r m
A3 iz atio N
n
723C
A1
T
Wt% C
0.8 %
Normalizing also improves the ductility without reducing the hardness and strength
To reduce segregation in casting or forgings
Purposes
Refine grain structure prior to hardening
To remove the internal stresses induced by welding, casting, forging,
forming. To harden the steel slightly
Quenching
• Depending on how fast steel must be quenched, the
heat treater will determine type of quenching required:
– Water (most severe)
– Oil
– Molten Salt
– Gas/ Air (least severe)
– Many phases in between,, Ex: add water/polymer to water
reduces quench time! Adding 10% sodium hydroxide or salt
will have twice the cooling rate.
HARDENING
The sample is heated above A3 | Acm to cause Austenization. The sample is then
quenched at a cooling rate higher than the critical cooling rate (i.e. to avoid the nose
of the CCT diagram).
The quenching process produces residual strains (thermal, phase transformation).
Heat above A3 | Acm → Austenization → Quench (higher than critical cooling rate)
The transformation to Martensite is usually not complete and the sample will have
some retained Austenite.
The Martensite produced is hard and brittle and tempering operation usually follows
hardening. This gives a good combination of strength and toughness.
910C ning Acm
Har rde
A3
den
ing Ha
723C Full Annealing
A1
T
Wt% C
0.8 %
Hardening Methods:
- One of the prerequisites 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.
Hardening Methods
a-Direct Hardening b- Selective
Hardening/Diffusion
Treatment Hardening
Techniques
From hardening of the sample
The surface of is affected by the quenching medium and experiences the best
possible cooling rate. The interior of the sample is cooled by conduction through
the (hot) sample and hence experiences a lower cooling rate. This implies that
different parts of the same sample follow different cooling curves on a CCT
diagram and give rise to different microstructures.
This gives to a varying hardness from centre to circumference. Critical diameter
(dc) is that diameter, which can be through hardened (i.e. we obtain 50% Martensite
and 50% pearlite at the centre of the sample).
Schematic showing variation
in cooling rate from surface
to interior leading to
different microstructures
Schematic of Jominy End Quench Test
Jominy hardenability test Variation of hardness along a Jominy bar
(schematic for eutectoid steel)
How to increase hardenability?
Hardenability should not be confused with the ability to obtain high
hardness. A material with low hardenability may have a higher
surface hardness compared to another sample with higher
hardenability.
A material with a high hardenability can be cooled relatively slowly
to produce 50% martensite (& 50% pearlite). A material with a high
hardenability has the ‘nose’ of the CCT curve ‘far’ to the right (i.e. at
higher times). Such a material can be through hardened easily.
Hardenability of plain carbon steel can increased by alloying with most elements
(it is to be noted that this is an added advantage as alloying is usually done to
improve other properties).
However, alloying gives two separate ‘C-curves’ for Pearlitic and Bainitic
transformations.
This implies that the ‘nose’ of the Bainitic transformation has to be avoided to get
complete Martensite on quenching.
