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Steel Designers' Manual - 6th Edition (2003)
Manufacture and effect on properties 233
A second stage heat treatment to temperatures below the austenitizing range is
frequently applied, known as tempering. This has the effect of giving more time for
the transformation processes which were previously curtailed to develop further,
and can permit changes in the precipitation of carbides, allowing them to merge
together and develop into larger or spheroidal forms. These thermally activated
events are highly dependent on temperature and time for particular compositions.
The net effect of tempering is to soften previously hardened structures and make
them tougher and more ductile.
Both plain carbon and low alloy steels can be supplied in the quenched and tempered
condition for plates and engineering sections to particular specifications. The
term ‘hardenability’ is used to describe the ability of steel to form martensite to
greater depths from the surface, or greater section sizes. There are, therefore,
practical limits of section thickness or size at which particular properties can be
obtained.
In BS 970 (some sections of which have been replaced by BS EN standards as
part of a phased transition), a range of compositions of engineering steels is given,
together with the choice of heat treatments and limiting section sizes for which different
properties can be supplied. The heat treatment condition is represented by a
letter in the range P to Z. The more commonly supplied conditions are in the range
P to T. It should be noted that the term ‘hardenability’ does not refer to the absolute
hardness level which can be achieved, but to the ability to develop uniform hardening
throughout the cross section. Cooling rates vary at different positions in the
cross section as heat is conducted away in a quenching operation from the surface.
It is sometimes necessary to apply heat treatment to components or structures
after fabrication, particularly when they have been welded. The aim is mainly to
relieve residual stresses but heat treatment may also be required to produce controlled
metallurgical changes in the regions where undesirable effects of welding
have occurred. Applications at high temperatures may also lead to metallurgical
changes taking place in service. It is vitally important that where any form of heat
treatment is applied the possible metallurgical effects on the particular type of steel
are taken into account.
Heat treatments are sometimes applied to produce controlled changes in shape
or correction of distortion and again temperatures and times involved in these heat
treatments must be carefully chosen and controlled for the particular type of steel
being used.
6.4 Manufacture and effect on properties
6.4.1 Steelmaking
Manufacture of steel takes place mainly in massive integrated steelworks. The first
stage starts with iron ore and coke, which are mixed and heated to produce a sinter.
This mixture then has limestone added to form the burden or raw material fed into
a blast furnace. Reactions which take place at high temperature in the blast furnace