of anneal heat treating may involve one or more of the following
To 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:
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).
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 membranes.
1. Effect of annealing cold-worked mild steel
2. Effect of annealing at 650°C on worked steel. Ferrite recrystallised.
Pearlite remains elongated (x600)
The 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 and panelling.
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 on:
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 then small.
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
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