202 Steels: Metallurgy and ApplicationsAlthough the bulk of steels for engineering applications are based on tempered
martensitic structures, there are notable examples in which the required strength
is developed by aircooling from the austenitic region in order to generate a
predominantly pearlitic microstructure. These include rail steels, micro-alloy
automotive forgings and high-carbon, wire rod. Again these materials will be
discussed in more detail, later in the text, but in general they are based on
high carbon contents with manganese levels up to about 1.5% which augments
carbon in the formation of pearlite. Very clearly, such steels involve completely
different concepts, in terms of underlying metallurgy, from those present in
martensitic grades and the strengthening mechanisms are those involving grain
size, volume fraction of ferrite and pearlite, and the interlamellar spacing of
pearlite. Consideration must also be given to solid solution hardening effects due
to manganese, silicon and free nitrogen. The structure-property relationships of
these medium-carbon, pearlitic grades are therefore similar to those involved in
the lower-carbon, strip and structural grades, but of course with greater emphasis
on the contribution from the higher volume fraction of pearlite. Micro-alloy
forging grades are also similar in concept and indeed represent an extension to
high-strength, low alloy (HSLA) grades of strip and structural steels, albeit with
a higher carbon content and higher volume fraction of pearlite. Medium carbon,
micro-alloy forging grades generally incorporate an addition of 0.05-0.20% V
which is soluble at the reheating temperature and which results in the precipitation
of vanadium carbonitride in both the proeutectoid ferrite and the ferrite lamellae
of the pearlite, on cooling from the forging operation. This results in tensile
strengths in the range 800-1100 N/ram 2 over a wide range of section size,
which are comparable to those achieved in conventional, quenched and tempered
martensitic grades.
Machining is a very important stage in the production of most engineering
components and, in many automotive transmission parts, can account for up
to 60% of total production costs. Due consideration must therefore be given to
optimizing the machinability of engineering steels, consistent with other property
requirements. Whereas normalizing or carbide-spheroidizing heat treatments
may be applied in order to provide an easily machined microstructure, most
engineering components must be machined in the fully heat-treated, high-strength
condition. In this case, small amounts of elements such as sulphur and lead may be
added to the steels which improve the machining performance very dramatically.
This derives from the presence of MnS inclusions or discrete particles of lead
which exude into the tool-chip interface, acting as a lubricant and also forming
a protective deposit on the tool.
High sulphur additions impair the transverse ductility of steels, particularly
when MnS is present as long elongated inclusions. However, inclusion-modifying
agents, such as calcium, can be added to the steel which improve the transverse
properties. Lead is generally present as globular particles or as tails to the MnS
inclusions. As such, it causes little detriment to the mechanical properties but
major hygiene precautions must be taken during steelmaking to minimize lead
fume and the associated toxicity effects.