3 Engineering steels
Overview
The term engineering steels applies to a wide range of compositions that are
generally heat treated to produce high strength levels, i.e. tensile strengths greater
than 750 N/mm 2. These steels are subjected to high service stresses and are
typified by the compositions that are used in automotive engine and transmission
components, steam turbines, bearings, rails and wire ropes. As well as carbon
and low-alloy grades, engineering steels also embrace the maraging compositions
that are generally based on 18% Ni and which are capable of developing tensile
strengths greater than 2000 N/mm 2.
Engineering steels are concerned primarily with the generation of a particular
level of strength in a specific section size or ruling section. This introduces the
concept of hardenability which is concerned with the ease with which a steel can
harden in depth rather than the attainment of a specific level of hardness/strength.
In turn, this relates to the effects of alloying elements on hardenability and the
influence of cooling rate on a specific composition or section size. Much of the
information that is available today on hardenability concepts and the metallurgical
factors affecting hardenability was generated in the United States in the 1930s
with names such as Grossman, Bain, Grange, Jominy and Lamont featuring
prominently in the literature. This period also coincided with the introduction
of isothermal transformation diagrams which paved the way to the detailed
understanding of the decomposition of austenite and a qualitative indication of
hardenability.
Up until the late 1940s, engineering steels often contained substantial levels of
nickel and molybdenum, the concept being that these elements were required in
order to provide a good combination of strength and toughness. Whereas these
alloy additions certainly fulfilled this objective, what was to change in subsequent
years was the generation of quantitative data on the actual level of toughness that
was required in engineering components. This paved the way to the substitution of
nickel and molybdenum by cheaper elements such as manganese, chromium and
boron and the more economical use of alloy additions for particular hardenability
requirements. The theme of cost reduction was also pursued very vigorously in the
1970s and 1980s with the introduction of medium-carbon, micro-alloy steels for
automotive forgings. As illustrated later in the text, these steels offer the potential
of major savings over traditional quenched and tempered alloy grades through
lower steel costs, the elimination of heat treatment and improved machinability.
Steel cleanness and the reduction of non-metallic inclusions have also been
of major concern to users of engineering steels, particularly in applications with
the potential for failure by fatigue. Bearing steels are a typical example and
the fatigue performance of these steels has been improved progressively over
the years with the adoption of facilities such as vacuum degassing (1950s),