Engineeringsteels 201particular element can lead to segregation at the casting stage which may not
be removed completely on subsequent processing and heat treatment. Therefore
components of large section size, which require a high level of hardenability to
achieve through-hardening, will be based on multiple alloying additions, some
of which may also be incorporated to confer tempering resistance.
The carbon content of engineering steels is important in that it controls
the strength of martensitic structures and also contributes significantly to the
hardenability of the steels. Very broadly, the carbon content of most engineering
steels falls into two categories, namely a level of around 0.2% in carburizing
grades and about 0.4% in through-hardening grades. In the former, the carbon
content in the surface regions of a component is raised to about 0.8% by
gaseous diffusion in the austenitic state and on oil quenching, a high-carbon
martensitic case is developed on a lower-carbon martensitic core. Such a process
therefore develops a duplex microstructuie in which a hard, fatigue-resistant
case is supported by a lower-strength, ductile core. Carburized steels are used
extensively in automotive transmission components, such as gears and back
axles, which must be produced to very accurate dimensions in order to avoid
misalignment, overloading and premature failure. This introduces the problem
of distortion, namely the changes in shape that accompany heat treatment and
transformation and which are exacerbated by the large temperature gradients
developed under fast cooling rates from the austenitic state.
Having produced a martensitic microstructure on quenching from a temperature
about 20~ above Ac3, engineering steels are tempered in order to provide a good
combination of strength and ductility/toughness. The tempering temperatures may
be around 200~ in carburized components, which introduces stress relief, and
up to 650~ in the higher-carbon, through-hardening grades which produces a
substantial change in structure. In turn, this invokes consideration of the effect
of alloying elements on tempering resistance and, in particular, the benefit that
will be obtained from adding expensive elements, such as vanadium and molyb-
denum. These elements produce more stable carbides than iron, manganese and
chromium, retarding the degeneration of the martensitic structure. Vanadium and
molybdenum therefore feature prominently in ferritic creep-resisting grades for
power generation applications and also in high-speed cutting steels, in order to
provide high strength during elevated temperature service.
Nickel may be regarded as a common alloying element in engineering steels
but, in fact, it has relatively little effect on hardenability. Additionally, it has
little affinity for carbon and therefore is ineffective in retarding the tempering
of martensite. However, nickel is perceived to be a toughening agent and is
incorporated in some of the higher-strength, martensitic grades, notably maraging
steels, and these will be discussed later in this chapter.
Engineering steels are used for the production of bearings in the carburized
condition or from through-hardened steels of the 1% C-Cr type. In this respect,
detailed consideration must be given to steel cleanness in order to alleviate the
adverse effects of non-metallic inclusions on fatigue resistance. Bearing steels
are therefore subjected to stringent deoxidation practices and may be vacuum
degassed or vacuum melted in order to eliminate hard, angular inclusions which
are particularly detrimental.