Page 61 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

A Quick Guide to Welding and Weld Inspection

means a reduction in toughness with low impact strength.
The lower critical temperature (LCT) is the temperature
below which the austenite forms into ferrite and cementite.

Normalising
When an annealed part is removed from the furnace and
allowed to cool in air, it is called ‘normalising’. It is often
used for hardenable steels to regain toughness after high heat
input processes have formed large grain structures. The steel
is heated to just above its upper critical temperature, held for
a specified period and then allowed to cool in air. Small
grains are formed, which give a harder and much tougher
metal with normal tensile strength rather than the maximum
ductility achieved by annealing.
Hardening (quenching)
Hardening of steel is achieved by heating the alloy to above
its upper critical temperature until it is a fully austenitic
structure and then cooling it rapidly with forced air, oil,
water or brine. Upon being rapidly cooled, a portion of
austenite (dependent on alloy composition) will transform
into martensite. Martensite is very hard and strong but too
brittle for most applications. It must therefore be subjected to
a process called tempering, which will temper the martensite
into a very strong and tough structure. Most applications
require that quenched parts be tempered, to impart some
toughness and further ductility, although some yield strength
is lost.
Tempering
Tempering of steel is used to transform a hard and brittle
martensitic structure into a tougher, more ductile structure.
There is always a trade-off between ductility and brittleness,
and the precise control of time and temperature during the
tempering process is necessary to achieve a structure with the
correct balance of these mechanical properties. The steel is
normally tempered after thermal hardening by heating
between 150 and 650 8C (300 and 1200 8F) dependent upon

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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 46 of 48