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Steel Designers' Manual - 6th Edition (2003)
226 Applied metallurgy of steel
commonest impurities are sulphur and phosphorus, high levels of which lead to
reduced resistance to ductile fracture and the possibility of cracking problems in
welded joints. For weldable steels the sulphur and phosphorus levels must be kept
less than 0.050%, and with modern steel-making practice should now preferably be
less than 0.010%. They are not always harmful however and in cases where welding
or fracture toughness are not important, deliberate additions of sulphur may be
made up to about 0.15% to promote free machining qualities of steel, and small
additions of phosphorus may be added to non-weldable weathering grade steels.
Other elements which may occur as impurities and may sometimes have serious
detrimental effects in steels are tin, antimony and arsenic, which in certain steels
may promote a problem known as temper embrittlement, in which the elements
migrate to grain boundaries if the steel is held in a temperature range between about
500°C and 600°C for any length of time. At normal temperature steels in this condition
can have very poor fracture toughness, with failure occurring by inter-granular
fracture. It is particularly important to ensure that this group of tramp elements
is eliminated from low alloy steels.
Steels with a high level of dissolved gases, particularly oxygen and nitrogen, can
behave in a brittle manner. The level of dissolved gases can be controlled by addition
of small amounts of elements with a particular affinity for them so that the
element combines with the gas and either floats out in the liquid steel at high temperature
or remains as a distribution of solid non-metallic inclusions. A steel with
no such additions to control oxygen level is known as a rimming steel, but for most
structural applications the elements silicon and/or aluminium are added as deoxidants.
Aluminium also helps in controlling the free nitrogen level, which it is important
to keep to low levels in cases where the phenomenon of strain ageing
embrittlement may be important.
6.3 Heat treatment
6.3.1 Effect on microstructure and grain size
During the manufacture of steel the required chemical composition is achieved
while it is in the liquid state at high temperature. As the steel cools, it solidifies at
the melting temperature at about 1350°C, but substantial changes in structure take
place during subsequent cooling and may also be affected by further heat treatments.
If the steel is cooled slowly, it is able to take up the equilibrium type of lattice
crystal structure and microstructure appropriate to the temperature and chemical
composition.
These conditions can be summarized on a phase or equilibrium diagram for the
particular composition; the equilibrium diagram for the iron–iron carbide system is
shown in Fig. 6.3. Essentially this is a diagram of temperature against percentage of
carbon by weight in the iron matrix. At 6.67% carbon, an inter-metallic compound
called cementite is formed, which is an extremely hard and brittle material. At the