Mechanical properties of steel
To present the essential engineering properties of structural steels, introducing their principal metallurgical characteristics.
This lecture provides a brief introduction to the crystalline nature of steels and the structure-sensitivity of the material properties. It explains the effect of dislocations on mechanical strength and demonstrates the use of the tensile stress/strain curve as the main means of characterising strength. It introduces the concept of Poisson's ratio, multi-axial stress states, strain hardening, and the influences of temperature and strain rate. It describes the metallurgical and mechanical means of improving strength. It introduces the concept of hardness.
1.1 The 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.
1.2 Structure-sensitive and structure-insensitive properties
Before embarking on an examination of the properties of interest, the meaning of structure-sensitivity and structure-insensitivity, in the context of material properties, must be clarified.
Structure-insensitive properties are those which are not influenced significantly by changes in microstructure or macrostructure. It is recognised that many of the physical properties of a material, e.g. elastic modulus, bulk density, specific heat, and coefficient of thermal expansion, do not vary other than by small amounts from specimen to specimen of a given material, even if the different specimens have been subjected to very different working and/or heat treatment processes. This insensivity is present despite the fact that these processes may have produced quite substantial microstructural and macrostructural modifications. On the other hand, most of the mechanical properties are very dependent on these modifications. Thus, for instance, the yield strength, ductility, and fracture strength are seen to be structure-sensitive.Read more