Concept and basis of steel metallurgy
To introduce important aspects of the metallurgy of steel required for the understanding of the engineering properties of steel.
This lecture commences with a discussion of the need for civil and structural engineers to have a basic knowledge of the metallurgy of steel. Then the crystalline nature of irons and steels is described together with the influence of grain size and composition on properties. The ability of iron to have more than one crystalline structure (its allotropy) and the properties of the principal crystalline forms of alloys of iron and carbon are discussed.
The metallurgy and properties of slowly cooled steels are reviewed, including the influence of grain size, rolling, subsequent heat treatment and inclusion shape, and distribution. Rapidly cooled steels are treated separately; a brief description of quenching and tempering is followed by a discussion of the influence of welding on the local thermal history. Hardenability, weldability, and control of cracking are briefly discussed. Finally the importance of manganese as an alloying element is introduced.
1.1 Why metallurgy for civil and structural engineers?
The engineering properties of steel, i.e. strength, ductility, and resistance against brittle fracture, depend on its crystalline structure, grain size, and other metallurgical characteristics.
These microstructural properties are dependent on the chemical composition and on the temperature-deformation history of the steel. Heat treatments that occur during welding may also have a large influence on the engineering properties.
When selecting steel for welded structures, it is important to have at least a basic knowledge of metallurgy. This knowledge is required especially when large and complicated structures are being designed, such as bridges, offshore structures, and high-rise buildings.
Selecting materials, welding processes, and welding consumables usually require consultation of 'real' metallurgists and welding specialists. A basic knowledge of metallurgy is essential for communication with these specialists.
Finally, a basic knowledge of metallurgy also enables civil and structural engineers to have a better understanding of the engineering properties of steel and the performance of welded structures.
1.2 The scope of lectures in Properties of steel chapter
Concept and basis of steel metallurgy lecture deals with the characteristics of iron-carbon alloys. Where possible, direct links are indicated to the engineering properties and weldability of steel. These subjects are covered in Manufacturing and forming processess lecture and Weldability of structural steels lecture respectively.
Concept and basis of steel metallurgy and Toughness and resistance to brittle fracture describe steelmaking and the forming of steel into plates and sections. The various processes for controlling the chemical composition and the different temperature-deformation treatments are discussed. Most of the underlying principles described in Concept and basis of steel metallurgy lecture are applied.
Steels are available in various grades and qualities. The grade designates the strength properties (yield strength and ultimate tensile strength), while the quality is mainly related to resistance against brittle fracture. Grades and qualities are explained in Steel grades and qualities lecture. A system for choosing the right quality according to Eurocode 3 (Annex C)  is presented. Some guidelines for the selection of steel grade are given.Read more