Steels_ Metallurgy and Applications, Third Edition

200 Steels: Metallurgy and Applications

argon shrouding of the molten stream (1960s) and vacuum steelmaking (1970s).

However, major improvements in cleanness have also been obtained in bulk

steelmaking processes for bearing steels with the introduction of secondary steel-

making facilities.

The machining of automotive components can account for up to 60% of the

total cost and therefore major effort has been devoted to the development of

engineering steels with improved machinability. In the main, these developments

have been focused on the traditional resulphurized grades but with the addition of

elements such as calcium and tellurium for sulphide shape control and improved

transverse properties.

In summary, the author's overall perception of this sector has been one of

continuing effort to achieve cost reduction, initially through the use of cheaper

alloying elements but latterly via the concept of lower through costs and involving

a reduction in the cost of heat treatment and machining.

Underlying metallurgical principles

Whereas strip and structural steels are based primarily on ferrite or
ferrite-pearlite microstructures, engineering grades are generally heat treated to
provide high strengths via the generation of lower-temperature transformation
products, such as bainite or martensite. In order to form such products, the alloy
content of the steels must be high enough to suppress the formation of ferrite
and pearlite, under the cooling conditions employed from the austenitic state.
Although water quenching would reduce the critical alloy content required to
achieve a martensitic structure in a particular section size, this might generate
severe internal stresses or quench cracking in a component and therefore oil
quenching is generally employed.
In their simplest form, engineering steels are based on C-Mn compositions
which are only effective in producing a martensitic structure in small section
sizes. Such steels are described as having low hardenability in the context of
through-hardening but can develop high levels of surface hardening through
induction heating and quenching which develops a martensitic structure in the
outer fibres of the component. However, engineering steels are generally produced
via the basic electric arc furnace in which the addition of large amounts of scrap
steel introduces a substantial amount of residual elements, such as copper and
nickel. Such elements can be present at total levels of about 0.5% and therefore
contribute significantly to the hardenability of carbon or low-alloy engineering
grades.
For larger section sizes, steels with higher alloy content and hardenability
are required in order to generate a substantially martensitic structure. However,
whereas most of the common alloying elements will increase hardenability and
promote the formation of martensite, due consideration must be given to the cost
of these elements and, more particularly, to their specific cost in achieving a
given increment in hardenability. Thus elements such as manganese, chromium
and boron may be used in preference to nickel and molybdenum, provided that
hardenability is the prime consideration. However, the excessive addition of one