When describing cross sectional distortion of cold-formed steel members the term fluidity can from time to time be observed. This might rattle most people’s logic, as steel is a well-known solid. It is not that structural steel exists both in solid and liquid state simultaneously during cold forming, but rather that when a stress great enough to yield a member, is applied to that member through a bending moment, steel members display fluid like characteristics in the form of flow. This is commonly seen in wall thinning and thickening and or a reduction in the cross section of a member. To explain how this is possible it is necessary to have a working knowledge of steel and ultimately steel’s base metal, iron (Fe), on the molecular level.
Steel is an alloy or a mixture of Iron and Carbon with other elements working as additives to alter the chemistry and enhance certain properties and or characteristics of the steel. Iron, steel’s base metal is the most common element on earth. Pure it is relatively soft and not very strong but its interaction with carbon makes it one of today’s most utilized materials. When we speak about the cold forming of steel and how during that process steel displays fluid/flow like characteristics; we must strongly consider steels’ base metal, iron. When Iron starts solidifying from liquid state it does so in crystalline form; meaning that at the molecular level, iron atoms arrange themselves in a lattice type structure. The bonds formed between iron atoms are the key to the fluid/flow like characteristics that steel display when under stress. Metallic bonds unlike covalent or ionic bonds (where two atoms share one electron or one atom takes an electron away from another atom to form a bond) share their valance electron over the entire lattice structure of atoms, known as a delocalization of the valance electron. This is important, as this type of bonding allows for iron to be malleable meaning it can be shaped; it also allows for iron to be ductile meaning it can be stretched and drawn. These fluid/flow like characteristics are types of plastic deformation. The delocalized sea of electrons acts like a bonding blanket encompassing the iron lattice structure, keeping the lattice bonded but allowing the atoms within the lattice structure to move or slip without breaking bonds. This dislocation or slippage of the lattice structure is considered to be a linear defect of the iron crystal. But this defect in the iron crystal is not a bad thing in the sense of the word, but rather is what makes plastic deformation even possible. The ease of movement of these dislocations is what determines the material’s strength and yield. Any obstruction or defect within the lattice structure makes slippage/plastic deformation more difficult; even the slippage of dislocations make plastic deformation more difficult. Dislocation of slip planes create other dislocations that obstruct the movement of dislocations, this is known as work hardening.
Steel comes into play in all of this when you add a percentage of carbon to the iron mixture. As said before pure iron is rather soft, displaying flow/fluid like characteristics in the form of malleability and ductility but ultimately it lacks the strength one would seek in a building/construction material. Add 0.12-2.0% of carbon into the mix and iron becomes the material we know today as steel. As iron and carbon are heated the lattice structure of iron changes and rearranges on itself. These different forms of lattice structures are known as allotropes. The allotropy of iron is extremely important as these allotropic formations, when heated to certain temperatures, interact with carbon in different ways. Through substitution, this allows the carbon atom to join the lattice structure. It is this positioning of the carbon atom within the lattice structures that gives steel its strength and toughness. To a certain extent increasing the carbon content within the alloy steel mixture will increase strength and hardness of the material. But in general it is the hardenability of the steel alloy mixture that is increased, as making steel harder is generally achieved through heat treating and cooling times which either allows or prevents the carbon atom to escape or diffuse from the lattice structure. Add too much carbon to the mixture and the material gets harder but less tough as the material cannot absorb energy efficiently and the material becomes brittle and susceptible to cracking. Ultimately, it must be thought that when bending/forming i.e. plastically deforming steel members we are really deforming the slip planes/dislocations of the iron/carbon molecular lattice; and that the carbon atoms placement within the lattice structure prohibits the slippage of the dislocations to a certain degree, making steel harder and stronger than its base metal, iron. Now of course in the steel we know today there are many other elements added into the mixture to assist and enhance certain characteristics like hardness, toughness, stiffness, etc. but carbon is the major alloying element that makes iron the material we know today as steel. Many of steels attributes including those that display fluid/flow like characteristics can be attributed to steels base metal iron and its metallic bonds.