Applied Metallurgy

What is Steel?

 

According to European standard EN 10 020, steel is a material which contains by weight more iron than any other single element, having a carbon content generally less than 2% and containing other elements (Figure 1). A limited number of chromium steels may contain more than 2% of carbon, but 2% is the usual dividing line between steel and cast iron.

Steel Wire

Due to its high strength, its good machineability and its high economic efficiency, steel is one of the most important construction materials. By changes in the chemical composition and in the production conditions, it is possible to vary steel properties over a wide range and the steel manufacturer is able to adapt the properties to the specific requirements of users.

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The Phenomenon of Brittle Failure

A material is generally said to be brittle if it cannot be deformed to any appreciable degree prior to fracture. This behaviour does not imply that the ultimate tensile strength measured on a smooth specimen during a tensile test is low. On the contrary, the opposite phenomenon is usually observed. Hardening treatments which aim to increase strength are usually accompanied by a dramatic degradation of ductility and tend to enhance brittleness.

Brittle Failure Curve

Brittleness is neither an absolute nor a simple concept. As a rule, the susceptibility to brittle behaviour in a given material is increased by:

  • the lower the temperature to which it is exposed.
  • the more rapid the loading to which it is submitted.
  • the more disturbed the stress distribution it experiences.

Brittleness is influenced by ductility, i.e. the capacity of a material to strain plastically, and by strain-hardenability, i.e. the property of developing a higher strength while undergoing plastic deformation.

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Methodologies for Steel Selection

 

Selection of the right steel quality for a structure is a matter of major significance as regards both the safety and the economy of constructional steelwork. This posting surveys procedures which have been proposed for this purpose and presents the new rules which are included in Annex C of Eurocode 3. All these express, as a function of extreme service conditions applicable to a structure, a toughness level specified in terms of performance in the Charpy V test that the selected steel should fulfil, with a transition temperature at the level of 28J for instance.

Selection of the right steel quality for a structure is a matter of major significance as regards both the safety and the economy of constructional steelwork.

Numerous comparisons between the output of different procedures are reported which, on the one hand highlight their consistency, while on the other hand the possible sources of discrepancies among the various material requirements determined using these procedures. Such procedures are based on fracture mechanics concepts such as those of the Stress Intensity Factor, the Crack Tip Opening Displacement or the Full Yield Criterion. As an introduction the lecture reviews the main aspects of resistance to brittle failure, with reference to basic documents which the reader may find it useful to consult for more detail.

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Engineering Properties of Steels

Nature of Metals

Metallic bonding is a consequence of the metal atoms giving up valence electrons to a ‘free electron gas‘. Metallic structures at the atomic level are then envisaged as almost close-packed arrays of metal ions surrounded by the electron gas. The bonding is, in most cases, non-directional. As a consequence the common metallic crystal structures are face-centred cubic, e.g. Cu, Al, Ni, or body-centred cubic, e.g. Fe. (Some metals exist with a hexagonal close-packed structure, e.g. Zn, Cd, but these are not commonly used for structural applications.)

Metals (and alloys) with cubic structures exhibit four characteristic metallic properties, namely:

  • good ductility (or malleability).
  • high thermal conductivity.
  • high electrical conductivity.
  • metallic lustre.

Ductility is a consequence of the lack of directionality in the bonding of the atoms and the close-packed nature of the crystal structures which normally allows profuse crystallographic slip to occur under stress. The non-directionality in the bonding also allows thermal vibrations to be readily transmitted from one vibrating atom to its neighbours, hence the high thermal conductivity. The existence of free electrons provides for high electrical conductivity. These free electrons are also responsible for metallic lustre since incident light of a wide range of wavelengths can be readily absorbed and re-radiated.

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