purpose of anneal heat treating may involve one or more
of the following aims:
soften the steel and to improve machinability.
To relieve internal stresses induced by some previous
treatment (rolling, forging, uneven cooling).
To remove coarseness of grain.
The treatment is applied to forgings, cold-worked sheets
and wire, and castings. The operation consists of:
heating the steel to a certain temperature,
"soaking" at this temperature for a time sufficient
to allow the necessary changes to occur,
cooling at a predetermined rate.
It is not always necessary to heat the steel into the critical
range. Mild steel products which have to be repeatedly cold
worked in the processes of manufacture are softened by annealing
at 500° to 650°C for several hours. This is known
as "process" or "close" annealing, and
is commonly employed for wire and sheets. The recrystallisation
temperature of pure iron is in the region of 500°C consequently
the higher temperature of 650°C brings about rapid recrystallisation
of the distorted ferrite Since mild steel contains only a
small volume of strained pearlite a high degree of softening
is induced. As shown, Fig. 1b illustrates the structure formed
consisting of the polyhedral ferrite with elongated pearlite
(see also Fig. 2).
annealing induces greater ductility at the expense of strength,
owing to the tendency of the cementite in the strained pearlite
to "ball-up" or spheroidise, as illustrated in Fig.
1c. This is known as "divorced pearlite". The ferrite
grains also become larger, particularly if the metal has been
cold worked a critical amount. A serious embrittlement sometimes
arises after prolonged treatment owing to the formation of
cementitic films at the ferrite boundaries. With severe forming
operations, cracks are liable to start at these cementite
1. Effect of annealing cold-worked mild steel
2. Effect of annealing at 650°C on worked steel. Ferrite
recrystallised. Pearlite remains elongated (x600)
modern tendency is to use batch or continuous annealing
furnaces with an inert purging gas. Batch annealing usually
consists of 24-30 hrs 670°C, soak 12 hrs, slow cool
4-5 days. Open coil annealing consists in recoiling loosely
with controlled space between wraps and it reduces stickers
and discoloration. Continuous annealing is used for thin
strip (85% Red) running at about 400 m/min. The cycle
is approximately up to 660°C 20 sec, soak and cool
30-40 sec. There is little chance for grain growth and
it produces harder and stiffer strip; useful for cans
reduced" steel is formed by heavy reduction (~50%) after
annealing but it suffers from directionality. This can be
eliminated by heating between 700-920°C and rapidly quenching.
Anneal and Normalising Treatments
For steels with less than 0,9% carbon both treatments consist
in heating to about 25-50°C above the upper critical point
indicated by the Fe-Fe3C equilibrium diagram (Fig. 3). For
higher carbon steels the temperature is 50°C above the
lower critical point.
3. Heat-treatment ranges of steels
annealing and hardening temperatures are:
temperatures allow for the effects of slight variations in
the impurities present and also the thermal lag associated
with the critical changes. After soaking at the temperature
for a time dependent on the thickness of the article, the
steel is very slowly cooled. This treatment is known as full
annealing, and is used for removing strains from forgings
and castings, improving machinability and also when softening
and refinement of structure are both required.
differs from the full annealing in that the metal is allowed
to cool in still air. The structure and properties produced,
however, varying with the thickness of metal treated. The
tensile strength, yield point, reduction of area and impact
value are higher than the figures obtained by annealing.
Consider the heating of a 0,3% carbon steel. At the lower
critical point (Ac1) each "grain" of pearlite changes
to several minute austenite crystals and as the temperature
is raised the excess ferrite is dissolved, finally disappearing
at the upper critical point (Ac3), still with the production
of fine austenite crystals. Time is necessary for the carbon
to become uniformly distributed in this austenite. The properties
obtained subsequently depend on the coarseness of the pearlite
and ferrite and their relative distribution. These depend
the size of the austenite grains; the smaller their size the
better the distribution of the ferrite and pearlite.
b) the rate of cooling through the critical range, which affects
both the ferrite and the pearlite.
the temperature is raised above Ac3 the crystals increase
in size. On a certain temperature the growth, which is rapid
at first, diminishes. Treatment just above the upper critical
point should be aimed at, since the austenite crystals are
cooling slowly through the critical range, ferrite commences
to deposit on a few nuclei at the austenite boundaries. Large
rounded ferrite crystals are formed, evenly distributed among
the relatively coarse pearlite. With a higher rate of cooling,
many ferrite crystals are formed at the austenite boundaries
and a network structure of small ferrite crystals is produced
with fine pearlite in the centre.
Burnt and Underannealed Structures
When the steel is heated well above the upper critical temperature
large austenite crystals form. Slow cooling gives rise to
the Widmanstätten type of structure, with its characteristic
lack of both ductility and resistance to shock. This is known
as an overheated structure, and it can be refined by reheating
the steel to just above the upper critical point. Surface
decarburisation usually occurs during the overheating.
the Second World War, aircraft engine makers were troubled
with overheating (above 1250°C) in drop-stampings made
from alloy steels. In the hardened and tempered condition
the fractured surface shows dull facets. The minimum overheating
temperature depends on the "purity" of the steel
and is substantially lower in general for electric steel than
for open-hearth steel. The overheated structure in these alloy
steels occurs when they are cooled at an intermediate rate
from the high temperature. At faster or slower rates the overheated
structure may be eliminated. This, together with the fact
that the overheating temperature is significantly raised in
the presence of high contents of MnS and inclusions, suggests
that this overheating is conected in some way with a diffusion
and precipitation process, involving MnS. This type of overheating
can occur in an atmosphere free from oxygen, thus emphasising
the difference between overheating and burning.
the steel approaches the solidus
temperature, incipient fusion and oxidation take place at
the grain boundaries. Such a steel is said to be burnt and
it is characterised by the presence of brittle iron oxide
films, which render the steel unfit for service, except as
scrap for remelting.
to heat treating of steel