Strengthening of steel Ferritic, pearlitic, bainitic and martensitic steels

Various hardening mechanisms can be used to increase the stress level required for plastic deformation of metal alloys. Most of these mechanisms are based on the displacement of dislocations in the crystal lattice and their interactions with various elements of the microstructure [M 4 340] . Steels provide an illuminating illustration of the various ways in which different microstructures can be developed, leading to improved mechanical properties, i.e. increased yield strength, ultimate tensile strength or hardness. Several driving forces have led to the need to increase the strength of steels, such as, for example, lightening for equivalent performance with a smaller structural size (thickness), or safety, or improving the service life of structures. These different objectives can be combined.

Alongside increased strength, it is important to maintain, or even improve, other properties of use, such as ductility (a material's ability to undergo plastic deformation without fracture) or toughness (a material's ability to resist fracture in the presence of cracks). This remains a challenge for the development of steels, since an increase in strength is generally accompanied by a drop in ductility.

This article focuses on the links between microstructure and mechanical strength when the steel's major constituent is ferritic, pearlitic, bainitic or martensitic. Readers interested in the properties of other steel families may wish to consult the article on austenitic and multiphase steels [M 4 342] . We begin by presenting the various constituents that can be found in steels and their mechanical properties. Several examples of steels, developed from these constituents and their combinations, illustrate the relationship between microstructure and mechanical behavior.

A glossary and a table of symbols are provided at the end of the article....