Materials (Steel & Iron)

Iron & Steel:

(the following informative notes on materials are all from 'Materials for Builders & Architects' by Arthur Lyons)

Iron & Steel: Ferrous metals that are a part of the many different metals used in construction. These metals are defined as metals in which iron predominates. Iron has been used for quite a while in construction compared to steel.

Manufacture of Steel: Involves a sequence of operations which include the making of pig iron, conversion to steel and casting.

Pig iron is produced from iron ore, coke and limestone. These materials are heated in a blast furnace with the aid of hot air and oxygen: this process creates iron. The molten iron is heavy and falls to the bottom, while the limestone forms a slag that floats on its surface. This slag purifies the iron. At this stage, the iron still has a high content of carbon and impurities.

There are two processes to make steel: the basic oxygen process and the electric air process. The first one creates bulk quantities of steel rather rapidly. The second is used to create high-quality and stainless steels, but takes longer.

Carbon contents of ferrous metals: The amount of carbon in iron influences its physical properties. Lowering the amount of carbon generally produces better quality iron or steel.

Wrought iron & Cast Iron: Cast iron is weak in tension and strong in compression and can be easily cast into different shapes. This is all rather similar with steel. However, the chemistry of iron and steel is different. The carbon contents in cast iron are over 2% while steel has between 0.05% and 1.7% carbon and different amounts affect its strength. Additionally alloys can be added to steel to give it differing qualities. Another way of making steel stronger or less likely to brittle or more manageable is to treat it with heat by a cycle of heating a cooling.

Heat treatment: There are four different ways of ‘heat treatment’ – Hardening, Annealing & pressurising, Tempering and Carburising. Hardening involves rapid cooling from a high temperature forming hard and brittle steel. Annealing & normalising facilitates cold working and machining processes by a cycle of slower reheating and cooling. Tempering reduces brittleness by reheating to a moderate temperature. Carburising essentially increases the carbon content in steel and hardens its outer surface.

Structural steel: There are different types of structural steels used in construction. This needs to be protected from fire, since steel rapidly loses its strength when the temperature rises above 600C.

Fire protection: Because it is important to protect steel from fire, several methods have been developed. These include: application of intumescent paints or sprayed coatings, boarded systems, preformed casings, masonry and/or concrete and water-filled systems. These are rather self-explanatory: they prevent heat and fire from affecting the steel long enough. Unfortunately these solutions are not fool-proof, since it is only a question of time until the steel is hot enough and starts rapidly losing strength.