Page 60 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Materials and Their Weldability

softening of the metal by removal of crystal defects and the
internal stresses that they cause. The second stage is
recrystallisation, where new grains replace those deformed
by internal stresses. Further annealing after recrystallisation
will lead to grain growth, which starts to coarsen the
microstructure and may cause poor mechanical properties
such as a loss of strength and toughness. In work-hardened
non-ferrous metals, annealing is used to recrystallise work-
hardened grains and the cooling rate is not always critical.
For steel, there are two basic kinds of annealing: full
annealing and subcritical annealing.
Full annealing
This is commonly carried out on large castings. The steel is
heated to around 50 8C above its upper critical temperature
(UCT) and held in a furnace for sufficient time to allow the
temperature to become uniform throughout the steel. It is
then slowly cooled, causing grain growth. The tensile
strength will not be particularly improved but toughness
and ductility will increase. The upper critical temperature
(UCT) of plain carbon steels ranges from 723 to 910 8C
depending on carbon content, so the actual annealing
temperature will be dependent upon the carbon content of
the steel. Above the UCT the steel structure will be an
austenite structure.
Subcritical annealing
Subcritical annealing methods are used to increase the
machinability of high carbon steels or for softening
worked-hardened mild steels to allow further cold work to
be applied. The steel is heated to a temperature above which
recrystallisation will take place but below the lower critical
temperature (LCT) of 723 8C. This recrystallises the distorted
ferrite grains so that the structure becomes softer again. The
recrystallisation temperature and time held at temperature
will be dependent on the carbon content of the steel.
Annealing generally puts a metal, or alloy, into its most
ductile condition. In steels the resultant large grain structure

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Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200904QG Welding chap4.3d Page 45 of 48